IEC 60738-1:2006

IEC 60738-1 ®
Edition 3.1 2009-07
CONSOLIDATED VERSION
INTERNATIONAL
STANDARD
QC 440000
Thermistors – Directly heated positive temperature coefficient –
Part 1: Generic specification
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IEC 60738-1 ®
Edition 3.1 2009-07
CONSOLIDATED VERSION
INTERNATIONAL
STANDARD
QC 440000
Thermistors – Directly heated positive temperature coefficient –
Part 1: Generic specification
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
CP
ICS 31.040.30 ISBN 978-2-88910-269-3
– 2 – 60738-1 © IEC:2006+A1:2009(E)

CONTENTS
FOREWORD.5

1 Scope.7

2 Normative references .7

3 Terms and definitions .8

4 Units and symbols .16

5 Preferred values.17

5.1 Climatic categories.17
5.2 Marking .17
5.3 Spacings .18
6 Quality assessment procedures.18
6.1 General .18
6.2 Primary stage of manufacture.19
6.3 Subcontracting .19
6.4 Structurally similar components.19
6.5 Qualification approval procedures .20
6.6 Rework and repair .27
6.7 Release for delivery .27
6.8 Certified test records of released lots .28
6.9 Delayed delivery.28
6.10 Alternative test methods .28
6.11 Manufacture outside the geographical limits of IECQ NSIs .28
6.12 Unchecked parameters.28
7 Test and measurement procedures.28
7.1 General .28
7.2 Standard conditions for testing .29
7.3 Drying and recovery .29
7.4 Visual examination and check of dimensions.30
7.5 Zero-power resistance.30
7.6 Temperature coefficient of resistance.31
7.7 Insulation resistance (for insulated types only) .31
7.8 Voltage proof (for insulated types only) .32

7.9 Resistance/temperature characteristic.32
7.10 Dissipation factor at U (δ) .33
max
7.11 Response time by ambient temperature change (t ) .34
a
7.12 Response time by power change (t ).34
p
7.13 Thermal time constant by ambient temperature change (τ ) .35
a
7.14 Thermal time constant by cooling (τ ).35
c
7.15 Robustness of terminations .37
7.16 Solderability .38
7.17 Resistance to soldering heat .39
7.18 Rapid change of temperature .40
7.19 Vibration.40
7.20 Bump .40
7.21 Shock.41

60738-1 © IEC:2006+A1:2009(E) – 3 –

7.22 Climatic sequence .41

7.23 Damp heat, steady state.42

7.24 Endurance.43

7.25 Tripping current and tripping time .47

7.26 Non-tripping current .47

7.27 Residual current .47

7.28 Surface temperature.48

7.29 Inrush current.49

7.30 Mounting (for surface mount thermistors only) .49

7.31 Shear (adhesion) test .50

7.32 Substrate bending test .51

Annex A (normative) Interpretation of sampling plans and procedures as described in
IEC 60410 for use within the IEC quality assessment system for electronic components
(IECQ) .52
Annex B (informative) Mounting for electrical measurements (except surface mount
types) .53
Annex C (informative) Mounting for temperature measurements .56

Figure 1 – Typical resistance-temperature characteristic and definitions for PTC
thermistors (at zero power) .10
Figure 2 – Typical R-TNF characteristic for PTC thermistors in sensor applications .11
Figure 3 – Typical current/voltage characteristic for PTC thermistors .11
Figure 4 – I against t at U .14
in dc
Figure 5 – I against t at U .15
in rms
Figure 6 – Dissipation factor test circuit .33
Figure 7 – Temperature gradient.34
Figure 8 – Circuit for measurement of thermal time constant by cooling.36
Figure 9 – Circuit for endurance at maximum operating temperature and maximum
voltage.45
Figure 10 – Circuit for surface temperature measurement.48
Figure 11 – Measuring circuit.49
Figure B.1 – Example of a preferred mounting method for thermistors without wire
terminations.53

Figure B.2 – Example of a preferred mounting method for thermistors with wire
terminations.54
Figure B.3 – Example of a preferred mounting method for surface mount thermistors .55
Figure C.1 – Example of a preferred mounting method for temperature measurement
on cylindrical heating elements .56

Table 1 – Creepage distances and clearances .18
Table 2 – Fixed sample size test schedule for qualification approval of thermistors for
current limitation – Assessment level EZ.22
Table 3 – Fixed sample size test schedule for qualification approval of thermistors for
use as heating elements – Assessment level EZ.23
Table 4 – Fixed sample size test schedule for qualification approval of thermistors for
inrush current application – Assessment level EZ .24
Table 5 – Fixed sample size test schedule for qualification approval of thermistors for
use as temperature sensing elements, Assessment level EZ .25

– 4 – 60738-1 © IEC:2006+A1:2009(E)

Table 6 – Quality conformance inspection for lot-by-lot inspection .26

Table 7 – Quality conformance inspection for periodic testing .27

Table 8 – Tensile force .37

Table 9 – Number of cycles per climatic category .42

60738-1 © IEC:2006+A1:2009(E) – 5 –

INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
THERMISTORS – DIRECTLY HEATED POSITIVE

TEMPERATURE COEFFICIENT –
Part 1: Generic specification
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
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transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
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expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.

This consolidated version of the official IEC Standard and its amendment has been
prepared for user convenience.
IEC 60738-1 edition 3.1 contains the third edition (2006-04) [documents 40/1651/FDIS
and 40/1730/RVD] and its amendment 1 (2009-05) [documents 40/1940/CDV and 40/1999/
RVC].
A vertical line in the margin shows where the base publication has been
modified by amendment 1.
International Standard IEC 60738-1 has been prepared by IEC technical committee 40:
Capacitors and resistors for electronic equipment.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

– 6 – 60738-1 © IEC:2006+A1:2009(E)

The QC number that appears on the front cover of this publication is the specification number

in the IEC Quality Assessment System for Electronic Components (IECQ).

IEC 60738 consists of the following parts, under the general title Thermistors – Directly

heated positive step-function coefficient:

Part 1: Generic specification
Part 1-1: Blank detail specification – Current limiting application – Assessment level EZ

Part 1-2: Blank detail specification – Heating element application – Assessment level EZ

Part 1-3: Blank detail specification – Inrush current application – Assessment level EZ

Part 1-4: Blank detail specification – Sensing application – Assessment level EZ
The committee has decided that the contents of the base publication and its amendments will
remain unchanged until the maintenance result date indicated on the IEC web site under
"http://webstore.iec.ch" in the data related to the specific publication. At this date,
the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
60738-1 © IEC:2006+A1:2009(E) – 7 –

THERMISTORS – DIRECTLY HEATED POSITIVE

TEMPERATURE COEFFICIENT –
Part 1: Generic specification
1 Scope
This part of IEC 60738 describes terms and methods of test for positive step-function
temperature coefficient thermistors, insulated and non-insulated types typically made from
ferro-electric semi-conductor materials.
It establishes standard terms, inspection procedures and methods of test for use in detail
specifications for Qualification Approval and for Quality Assessment Systems for electronic
components.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60027 (all parts), Letter symbols to be used in electrical technology
IEC 60050 (all parts), International Electrotechnical Vocabulary (IEV)
IEC 60062, Marking codes for resistors and capacitors
IEC 60068-1:1988, Environmental testing – Part 1: General and guidance
Amendment 1 (1992)
IEC 60068-2-1:1990, Environmental testing – Part 2: Tests – Tests A: Cold
Amendment 1 (1993)
Amendment 2 (1994)
IEC 60068-2-2:1974, Environmental testing – Part 2: Tests – Tests B: Dry heat
Amendment 1 (1993)
IEC 60068-2-6, Environmental testing – Part 2: Tests – Test Fc: Vibration (sinusoidal)
IEC 60068-2-11, Environmental testing – Part 2: Tests – Test Ka: Salt mist
IEC 60068-2-13, Environmental testing – Part 2: Tests – Test M: Low air pressure
IEC 60068-2-14:1984, Environmental testing – Part 2: Tests – Test N: Change of temperature
Amendment 1 (1986)
IEC 60068-2-20:1979, Environmental testing – Part 2: Tests – Test T: Soldering
Amendment 2 (1987)
IEC 60068-2-21, Environmental testing – Part 2-21: Tests – Test U: Robustness of
terminations and integral mounting devices

– 8 – 60738-1 © IEC:2006+A1:2009(E)

IEC 60068-2-27, Environmental testing – Part 2: Tests – Test Ea and guidance: Shock

IEC 60068-2-29, Environmental testing – Part 2: Tests – Test Eb and guidance: Bump

IEC 60068-2-30:2005, Environmental testing – Part 2: Tests – Test Db: Damp heat, cyclic

(12 h + 12-hour cycle)
IEC 60068-2-45:1980, Environmental testing – Part 2: Tests – Test XA and guidance –

Immersion in cleaning solvents

IEC 60068-2-58, Environmental testing – Part 2-58: Tests – Test Td: Test methods for

solderability, resistance to dissolution of metallization and to soldering heat of surface
mounting devices (SMD)
IEC 60068-2-78, Environmental testing – Part 2-78: Tests – Test Cab: Damp heat, steady
state
IEC 60294, Measurement of the dimensions of a cylindrical component having two axial
terminations
IEC 60410, Sampling plans and procedures for inspection by attributes
IEC 60617 (all parts) [DB] : Graphical symbols for diagrams
IEC 60717, Method for determination of the space required by capacitors and resistors with
unidirectional terminations
IEC 61249-2-7, Materials for printed boards and other interconnecting structures – Part 2-7:
Reinforced base materials clad and unclad – Epoxide woven E-glass laminated sheet of
defined flammability (vertical burning test), copper-clad
IEC 61760-1, Surface mounting technology – Part 1: Standard method for the specification of
surface mounting components (SMDs)
IEC QC 001002-3, Rules of Procedure of the IEC Quality Assessment System for Electronic
Components (IECQ) – Part 3: Approval procedures
ISO 1000, SI units and recommendations for the use of their multiples and of certain other
units
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
type
group of components having similar design features and the similarity of whose manufacturing
techniques enables them to be grouped together either for qualification approval or for quality
conformance inspection
They are generally covered by a single detail specification
NOTE Components described in several detail specifications, may, in some cases, be considered as belonging to
the same type but they are generally covered by a single detail specification.
—————————
“DB” refers to the IEC on-line database.

60738-1 © IEC:2006+A1:2009(E) – 9 –

3.2
style
variation within a type having specific nominal dimensions and characteristics

3.3
thermistor
thermally sensitive semiconducting resistor which exhibits a significant change in electrical

resistance with a change in body temperature

3.4
positive temperature coefficient thermistor

thermistor, the resistance of which increases with its increasing temperature throughout the
useful part of its characteristic
3.5
positive step-function temperature coefficient thermistor
PTC
thermistor which shows a step-like increase in its resistance when the increasing temperature
reaches a specific value
A PTC thermistor will show secondary effects which are to be taken into account
3.6
zero-power resistance
R
T
value of the resistance of a PTC thermistor, at a given temperature, under conditions such
that the change in resistance due to the internal generation of heat is negligible with respect
to the total error of measurement
NOTE Any resistance value of a PTC thermistor is dependent on the value and the mode of the applied voltage
(a.c. or d.c.) and, when an a.c. source is used, on the frequency (see 3.8 and 3.9).
3.7
nominal zero-power resistance
R
n
d.c. resistance value of a thermistor measured at a specified temperature, preferably at 25 °C,
with a power dissipation low enough that any further decrease in power will result only in a
negligible change in resistance. Zero-power resistance may also be measured using a.c. if
required by the detail specification
3.8
voltage dependency
secondary effect, exhibiting a decreasing resistance with increasing voltage across the

thermistor when measured at a constant body temperature
3.9
frequency dependency
secondary effect exhibiting a substantial decrease of the positive temperature coefficient of
the thermistor with increasing frequency
3.10
resistance/temperature characteristics
relationship between the zero-power resistance of a thermistor and the temperature of the
thermo-sensitive element when measured under specified reference conditions (see Figure 1)
NOTE PTC thermistors may have more than one resistance/temperature characteristic specified. The zero-power
resistance of the resistance/temperature characteristics can be measured using a pulse voltage (U ) higher than
pulse
1,5 V, which is specified in the detail specification. The right curve in Figure 1 shows the typical
resistance/temperature characteristic when using the pulse voltage (U ).
pulse
– 10 – 60738-1 © IEC:2006+A1:2009(E)

Log R
R
p
≤ 1,5 V
d.c.
U
pulse
R
b
R
R
min
T T T
Rmin b p T  °C
Linear scale
IEC  402/06
Figure 1 – Typical resistance-temperature characteristic
and definitions for PTC thermistors (at zero power)

60738-1 © IEC:2006+A1:2009(E) – 11 –

(logarithmic scale)
R  Ω
4 000
1 330
R
min
–20 0 T  °C
TNF – 20
TNF – 5
TNF
TNF + 5
TNF + 15
IEC  403/06
Figure 2 – Typical R-TNF characteristic for PTC thermistors in sensor applications
3.11
current/voltage characteristic
relationship in still air at 25 °C (unless otherwise stated) between the applied voltage (d.c.
and/or a.c.) at the thermistor terminations and the current under steady-state conditions (see
Figure 3)
Log I
T = constant
ambiant
I
res
U
Log U
max
IEC  404/06
NOTE 1 U will be specified by the manufacturer.
max
NOTE 2 The breakdown voltage is the value beyond which the voltage-handling capability of the thermistor no
longer exhibits its characteristic property
Figure 3 – Typical current/voltage characteristic for PTC thermistors

– 12 – 60738-1 © IEC:2006+A1:2009(E)

3.12
nominal functioning temperature

T
NF
nominal temperature at the steep part of the resistance temperature characteristic at which

the system controlled by the thermistor is designed to operate

NOTE T is exclusively defined for PTC resistors in sensor applications.
NF
3.13
switching temperature
T
b
temperature at which the step-like function commences

3.14
minimum resistance
R
min
minimum value of the zero-power resistance/temperature characteristic (see Figures 1 and 2)
3.15
resistance at switching temperature
R
b
value of the zero-power resistance corresponding to the switching temperature defined as R
b
= 2 × R . As an alternative definition R = 2 × R can be used. If this definition is used, this
min b 25
shall be explicitly stated in the detail specification
3.16
temperature for minimum resistance
T
Rmin
temperature at which R occurs
min
3.17
temperature
T
p
temperature, higher than T , in the PTC part of the resistance/temperature characteristic for
b
which a minimum value R of the zero-power resistance is specified
p
3.18
resistance
R
p
zero-power resistance at temperature T measured at maximum voltage or a voltage specified
p
in the detail specification and given as a minimum value
NOTE The measurement should be made under such conditions that a change in resistance due to internal

generation of heat is negligible with respect to the total error of measurement. The applied voltage and the
characteristics of any pulse used should be given in the detail specification; when applying the maximum voltage,
the maximum overload current may not be exceeded.
3.19
average temperature coefficient of resistance at a stated voltage
a
R
rate of change of resistance with temperature expressed as %/K
It is calculated from the formula:
R
p
α = × ln
R
()TT− R
pb b
where T exceeds T by a minimum of 10 K.
p b
60738-1 © IEC:2006+A1:2009(E) – 13 –

The temperatures T and T are to be given if applicable and the measurement conditions for
p b
R and R should be the same, unless otherwise specified in the detail specification
b p
NOTE The detail specification may specify the measurement of the temperature coefficient of resistance in a
narrow temperature range where its value is a maximum, together with a suitable test method.

3.20
upper category temperature
UCT
maximum ambient temperature for which a thermistor has been designed to operate

continuously at zero power
3.21
lower category temperature
LCT
minimum ambient temperature for which a thermistor has been designed to operate
continuously at zero power
3.22
maximum voltage
U
max
maximum a.c. or d.c. voltage which may be continuously applied to the thermistor without
exceeding the maximum overload current
3.23
operating temperature range at maximum voltage
range of ambient temperatures at which the thermistor can operate continuously at the
maximum voltage without exceeding the maximum overload current
3.24
isolation voltage (applicable only to insulated thermistors)
maximum peak voltage which may be applied under continuous operating conditions between
any of the thermistor terminations and any conducting surface
3.25
maximum overload current
I
mo
value of current for the operating temperature range, which shall not be exceeded
NOTE It may be necessary to limit the current through the thermistor by the use of a series resistor R .
s
3.26
residual current
I
res
value of current in the thermistor at a specified ambient temperature (preferably 25 °C) under
steady-state conditions. The applied voltage is the maximum voltage unless otherwise
specified (see Figure 3)
3.27
tripping current
I
t
lowest current which will cause the thermistor to trip to a high resistance condition at a
specified temperature (preferably 25 °C) and within a time to be specified in the detail
specification
3.28
maximum non-tripping current
I
max nt
maximum current at a specified ambient temperature (preferably 25 °C), which the thermistor
will conduct indefinitely in its low-resistance condition

– 14 – 60738-1 © IEC:2006+A1:2009(E)

3.29
inrush current
I
in
current occurring during the transient period from the moment of switching to the steady-state

condition
3.30
peak inrush current
I
in p
peak inrush current is the maximum value of current during the transient period (see Figure 4)

3.31
minimum peak inrush current
I
in p min
lowest specified value of the peak of the inrush current
3.32
maximum peak inrush current
I
in p max
maximum specified value of the peak inrush current
I
in
I
in p max
I
in p min
t
IEC  405/06
Figure 4 – I against t at U
in dc
3.33
peak-to-peak inrush current (for a.c. conditions only)
I
in pp
value of the inrush current measured between adjacent positive and negative peaks (see
Figure 5)
3.34
minimum peak-to-peak inrush current
I
in pp min
lowest specified value of the peak to peak inrush current
3.35
maximum peak to peak inrush current
I
in pp max
maximum specified value of the peak to peak inrush current

60738-1 © IEC:2006+A1:2009(E) – 15 –

I
in
I
in pp min
I t
in pp max
IEC  406/06
Figure 5 – I against t at U
in rms
3.36
peak inrush power
P
in p
peak power (U × I ) measured at the maximum peak value of the current occurring during
in p
the transient period from the moment of switching to the steady-state operating condition
measured under specified conditions of ambient temperature, voltage and circuit
3.37
maximum peak inrush power
P
in p max.
maximum peak power which can occur during the transient period before the thermistor
reaches its steady-state operating condition
NOTE In the detail specification it should be specified whether the “0-peak value” or the “r.m.s. value” of the
voltage should be taken.
3.38
maximum power
P
max.
maximum power is the power (U × I ) which can be dissipated continuously by the
max. res
thermistor when the maximum voltage is applied under specified conditions of ambient
temperature, circuit and thermal dissipation when thermal equilibrium is obtained
NOTE If the power is supplied by an a.c. source then the voltage and current should be measured with true r.m.s.
meters.
3.39
dissipation factor
δ
quotient (in W/K) of the change in power dissipation in the thermistor and the resultant
change of the body temperature under specified ambient conditions (temperature, medium)
3.40
thermal resistance
R
th
quotient (in K/W) of the temperature difference between the thermistor and its ambient and
the power dissipated by the thermistor under specified ambient conditions (temperature,
medium)
NOTE "Dissipation factor" and "thermal resistance" are mutually reciprocal.

– 16 – 60738-1 © IEC:2006+A1:2009(E)

3.41
heat capacity
C
th
energy (in J) the thermistor needs to increase its body temperature by 1 K

3.42
response time
a) response time by ambient temperature change (t )
a
time (in s) required by a thermistor to change its temperature between two defined

conditions when subjected to a change in ambient temperature

b) response time by power change (t )
p
time (in s) required by a thermistor to change its temperature between two defined
conditions of power input
3.43
thermal time constant
thermal time constant (ideal) for a thermistor is the product of its heat capacity and its thermal
resistance
a) thermal time constant by ambient temperature change (τ )
a
time required for a thermistor to respond to 63,2 % of an external step change in ambient
temperature
b) thermal time constant by cooling (τ )
c
time required for a thermistor to cool by 63,2 % of its temperature excess, due to electrical
heating, in still air
3.44
insulated thermistors
thermistors capable of meeting the requirements of the insulation-resistance and voltage-
proof tests when specified in the test schedule
3.45
surface temperature
T
s
temperature under defined conditions at the boundary layer between two media
3.46
final surface temperature
T
sf
temperature reached under defined conditions when thermal equilibrium is established

4 Units and symbols
Units, graphical symbols, letter symbols and terminology shall, whenever possible, be taken
from the following publications:
IEC 60027
IEC 60050
IEC 60617
ISO 1000
When further items are required they shall be derived in accordance with the principles of the
documents listed above.
60738-1 © IEC:2006+A1:2009(E) – 17 –

5 Preferred values
5.1 Climatic categories
The thermistors are classified into climatic categories according to the general rules given in

the annex to IEC 60068-1. The detail specification shall prescribe the appropriate category.

5.1.1 Bump test severities
The test severity given in the detail specification shall preferably be the following:

Test Eb (IEC 60068-2-29)
Acceleration: 400 m/s
Number of bumps: 4 000 total
Thermistors shall be mounted by their normal means, in such a manner that there shall be no
parasitic vibration.
5.1.2 Shock test severities
Test severities given in detail specifications shall preferably be the following:
Test Ea (IEC 60068-2-27)
Pulse shape: Half-sine
Acceleration: 500 m/s
Pulse duration: 11 ms
Severity: Three successive shocks in each axis direction per specimen
Separate specimens to be used for each axis (six shocks total per
specimen)
NOTE The shock and bump tests are normally specified as alternatives.
5.1.3 Vibration severities
Test severities given in the detail specifications shall preferably be selected from the
following:
Test Fc (IEC 60068-2-6)
Frequency Total number of
Hz sweep cycles
10 to   55
3 × 24
10 to  500
3 × 10
10 to 2 000
3 ×  8
5.2 Marking
5.2.1 General
The following shall be clearly marked on the thermistor in the following order of precedence
as space permits.
a) Values of the primary characteristics appropriate to the application of the thermistor to be
specified in the detail specification. When these values are coded (including colour
coding), details shall be given in the detail specification or type designation.
b) Manufacturer's name and/or trade mark.

– 18 – 60738-1 © IEC:2006+A1:2009(E)

c) Date of manufacture.
d) The number of the detail specification and style.

The package containing the thermistors shall be clearly marked with all the information listed

above.
Any additional marking shall be so applied that no confusion can arise.

5.2.2 Coding
Where coding for the date is used, the method shall be selected from those given in

IEC 60062.
5.3 Spacings
5.3.1 The spacings between parts of opposite polarity of a thermistor shall be not less than
the applicable values specified in Table 1.
Table 1 – Creepage distances and clearances
Environment Operating voltage
DCV or Vrms
≤ 32 > 32 and ≤ 50 > 50 and ≤ 300 > 300 and ≤ 600
Sealed or encapsulated Exception 1 0,4 mm 0,8 mm 0,8 mm
Clean Exception 1 1,0 mm 1,0 mm 1,0 mm
Normal and dirty Exception 1 1,0 mm 1,5 mm 1,5 mm

This applies to both creepage distances and clearances.
Exception 1: as to the creepage distances and clearances for PTC thermistors with a rated
operating voltage of less than 32 V, this value meets the intention of these requirements when
the thermistor is confirmed to have the maximum voltage specified in the detail specification.
5.3.2 An insulating barrier or liner that is used to provide spacings shall be not less than
0,71 mm.
Exception 2: a barrier or liner that is used in conjunction with at least 50 % of required
through-air spacing shall be not less than 0,33 mm thickness.

Exception 3: a barrier or liner having a thickness of less than that specified above complies
with the requirements when it is investigated and found to be rated for the intended conditions
of use, and found to be mechanically and electrically equivalent to the barrier or liner
specified above.
6 Quality assessment procedures
6.1 General
When these documents are being used for the purposes of a full quality assessment system
such as the IEC Quality Assessment System for Electronic Components (IECQ), compliance
with 6.5 is required.
60738-1 © IEC:2006+A1:2009(E) – 19 –

When these documents are used outside such quality assessment systems for purposes such
as design proving or type testing, the procedures and requirements of 6.5.1 and 6.5.3b) may

be used, but, if used, the tests and parts of tests shall be applied in the order given in the test

schedules.
Before thermistors can be qualified according to the procedures of this subclause, the

manufacturer shall obtain the approval of his organization in accordance with the provisions of

IEC QC 001002-3.
The method for the approval of thermistors of assessed quality given in 6.5 is qualification

approval according to the provisions of Clause 3 of IEC QC 001002-3.

6.1.1 Applicability of qualification approval
Qualification approval is appropriate for a standard range of thermistors manufactured to
similar design and production processes and conforming to a published detail specification.
The programme of tests defined in the detail specification for the appropriate assessment and
performance levels applies directly to the subfamily of thermistors to be qualified, as
prescribed in 6.5 and the relevant blank detail specification.
6.2 Primary stage of manufacture
The primary stage of manufacture is defined as the initial mixing process of the ingredients.
6.3 Subcontracting
If subcontracting of the primary stage of manufacture and/or subsequent stages is employed it
shall be in accordance with 4.2.2 of IEC QC 001002-3.
The blank detail specification may restrict subcontracting in accordance with 4.2.2.2 of
IEC QC 001002-3.
6.4 Structurally similar components
Thermistors may be grouped as structurally similar for the purpose of forming inspection lots
provided that the following requirements are met.
– They shall be produced by one manufacturer on one site using essentially the same
design, materials, processes and methods.
– The sample taken shall be determined from the total lot size of the grouped devices.

– Structurally similar devices should preferably be included in one detail specification but
the details of all claims to structural similarity shall be declared in the qualification
approval test reports.
6.4.1 For electrical tests, devices having the same electrical characteristics may be grouped
provided that the element determining the characteristics is similar for all the devices
concerned.
6.4.2 For environmental tests, devices having the same encapsulation, basic internal
structure and finishing processes may be grouped.
6.4.3 For visual inspection (except marking), devices may be grouped if they have been
made on the same production line, have the same dimensions, encapsulation and external
finish.
– 20 – 60738-1 © IEC:2006+A1:2009(E)

This grouping may also be used for robustness of terminations and soldering tests where it is

convenient to group devices with different internal structures.

6.4.4 For endurance tests, thermistors may be grouped if they have been made on the same

production line using the same design and differing only in electrical characteristics. If it can

be shown that one type from the group is more heavily stressed than the others then tests on

this type may be accepted for the remaining members of the group.

6.5 Qualification approval procedures

6.5.1 Eligibility for qualification approval

The manufacturer shall comply with 3.1.1 of IEC QC 001002-3.
6.5.2 Application for qualification approval
The manufacturer shall comply with 3.1.3 of IEC QC 001002-3.
6.5.3 Test procedure for qualification approval
One of the two following procedures shall be followed.
a) The manufacturer shall produce test evidence of conformance to the specification
requirements on three inspection lots for lot-by-lot inspection taken in as short a time as
possible and one lot for periodic inspection. No major changes in the manufacturing
process shall be made in the period during which the inspection lots are taken.
Samples shall be taken from the lots in accordance with IEC 60410 (see Annex A). Normal
inspection shall be used, but when the sample size would give acceptance on zero non-
conformances, additional specimens shall be taken t
...

IEC 60738-1 ®
Edition 3.0 2006-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Thermistors – Directly heated positive temperature coefficient –
Part 1: Generic specification
Thermistances – Coefficient de température positif à chauffage direct –
Partie 1: Spécification générique

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IEC 60738-1 ®
Edition 3.0 2006-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Thermistors – Directly heated positive temperature coefficient –

Part 1: Generic specification
Thermistances – Coefficient de température positif à chauffage direct –

Partie 1: Spécification générique

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XA
ICS 31.040.30 ISBN 978-2-83221-336-0

– 2 – 60738-1  IEC:2006
CONTENTS
FOREWORD . 5

1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Units and symbols . 16
5 Preferred values . 17
5.1 Climatic categories . 17
5.2 Marking . 17
5.3 Spacings . 18
6 Quality assessment procedures . 18
6.1 General . 18
6.2 Primary stage of manufacture . 19
6.3 Subcontracting . 19
6.4 Structurally similar components . 19
6.5 Qualification approval procedures . 20
6.6 Rework and repair . 27
6.7 Release for delivery . 27
6.8 Certified test records of released lots . 28
6.9 Delayed delivery . 28
6.10 Alternative test methods . 28
6.11 Manufacture outside the geographical limits of IECQ NSIs . 28
6.12 Unchecked parameters . 28
7 Test and measurement procedures . 28
7.1 General . 28
7.2 Standard conditions for testing . 29
7.3 Drying and recovery . 29
7.4 Visual examination and check of dimensions . 30
7.5 Zero-power resistance . 30
7.6 Temperature coefficient of resistance . 31
7.7 Insulation resistance (for insulated types only) . 31
7.8 Voltage proof (for insulated types only) . 32
7.9 Resistance/temperature characteristic . 32
7.10 Dissipation factor at U (δ) . 33
max
7.11 Response time by ambient temperature change (t ) . 34
a
7.12 Response time by power change (t ) . 34
p
7.13 Thermal time constant by ambient temperature change (τ ) . 35
a
7.14 Thermal time constant by cooling (τ ). 35
c
7.15 Robustness of terminations . 37
7.16 Solderability . 38
7.17 Resistance to soldering heat . 39
7.18 Rapid change of temperature . 40
7.19 Vibration. 40
7.20 Bump . 40
7.21 Shock . 41

60738-1  IEC:2006 – 3 –
7.22 Climatic sequence . 41
7.23 Damp heat, steady state . 42
7.24 Endurance . 43
7.25 Tripping current and tripping time . 46
7.26 Non-tripping current . 47
7.27 Residual current . 47
7.28 Surface temperature . 47
7.29 Inrush current . 48
7.30 Mounting (for surface mount thermistors only) . 49
7.31 Shear (adhesion) test . 50
7.32 Substrate bending test . 50

Annex A (normative) Interpretation of sampling plans and procedures as described in
IEC 60410 for use within the IEC quality assessment system for electronic
components (IECQ) . 51
Annex B (informative) Mounting for electrical measurements (except surface mount
types) . 52
Annex C (informative) Mounting for temperature measurements . 55

Figure 1 – Typical resistance-temperature characteristic and definitions for PTC
thermistors (at zero power) . 10
Figure 2 – Typical R-TNF characteristic for PTC thermistors in sensor applications . 11
Figure 3 – Typical current/voltage characteristic for PTC thermistors . 11
Figure 4 – I against t at U . 14
in dc
Figure 5 – I against t at U . 15
in rms
Figure 6 – Dissipation factor test circuit . 33
Figure 7 – Temperature gradient . 34
Figure 8 – Circuit for measurement of thermal time constant by cooling . 36
Figure 9 – Circuit for endurance at maximum operating temperature and maximum
voltage . 45
Figure 10 – Circuit for surface temperature measurement . 48
Figure 11 – Measuring circuit . 48
Figure B.1 – Example of a preferred mounting method for thermistors without wire
terminations . 52
Figure B.2 – Example of a preferred mounting method for thermistors with wire
terminations . 53
Figure B.3 – Example of a preferred mounting method for surface mount thermistors . 54
Figure C.1 – Example of a preferred mounting method for temperature measurement
on cylindrical heating elements . 55

Table 1 – Creepage distances and clearances . 18
Table 2 – Fixed sample size test schedule for qualification approval of thermistors for
current limitation – Assessment level EZ . 22
Table 3 – Fixed sample size test schedule for qualification approval of thermistors for
use as heating elements – Assessment level EZ . 23
Table 4 – Fixed sample size test schedule for qualification approval of thermistors for
inrush current application – Assessment level EZ . 24
Table 5 – Fixed sample size test schedule for qualification approval of thermistors for
use as temperature sensing elements, Assessment level EZ . 25

– 4 – 60738-1  IEC:2006
Table 6 – Quality conformance inspection for lot-by-lot inspection . 26
Table 7 – Quality conformance inspection for periodic testing . 27
Table 8 – Tensile force . 37
Table 9 – Number of cycles per climatic category . 42

60738-1  IEC:2006 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
THERMISTORS – DIRECTLY HEATED POSITIVE
TEMPERATURE COEFFICIENT –
Part 1: Generic specification
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60738-1 has been prepared by IEC technical committee 40:
Capacitors and resistors for electronic equipment.
This third edition cancels and replaces the second edition published in 1998 and constitutes a
minor revision. The changes with respect to the previous edition refer to the tables, figures
and references.
This bilingual version (2014-01) corresponds to the monolingual English version, published in
2006-04.
The text of this standard is based on the following documents:
FDIS Report on voting
40/1651/FDIS 40/1730/RVD
– 6 – 60738-1  IEC:2006
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
The French version of this standard has not been voted upon.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
IEC 60738 consists of the following parts, under the general title Thermistors – Directly
heated positive step-function coefficient:
Part 1: Generic specification
Part 1-1: Blank detail specification – Current limiting application – Assessment level EZ
Part 1-2: Blank detail specification – Heating element application – Assessment level EZ
Part 1-3: Blank detail specification – Inrush current application – Assessment level EZ
Part 1-4: Blank detail specification – Sensing application – Assessment level EZ
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
60738-1  IEC:2006 – 7 –
THERMISTORS – DIRECTLY HEATED POSITIVE
TEMPERATURE COEFFICIENT –
Part 1: Generic specification
1 Scope
This part of IEC 60738 describes terms and methods of test for positive step-function
temperature coefficient thermistors, insulated and non-insulated types typically made from
ferro-electric semi-conductor materials.
It establishes standard terms, inspection procedures and methods of test for use in detail
specifications for Qualification Approval and for Quality Assessment Systems for electronic
components.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60027 (all parts), Letter symbols to be used in electrical technology
IEC 60050 (all parts), International Electrotechnical Vocabulary (IEV)
IEC 60062, Marking codes for resistors and capacitors
IEC 60068-1:1988, Environmental testing – Part 1: General and guidance
Amendment 1 (1992)
IEC 60068-2-1:1990, Environmental testing – Part 2: Tests – Tests A: Cold
Amendment 1 (1993)
Amendment 2 (1994)
IEC 60068-2-2:1974, Environmental testing – Part 2: Tests – Tests B: Dry heat
Amendment 1 (1993)
IEC 60068-2-6, Environmental testing – Part 2: Tests – Test Fc: Vibration (sinusoidal)
IEC 60068-2-11, Environmental testing – Part 2: Tests – Test Ka: Salt mist
IEC 60068-2-13, Environmental testing – Part 2: Tests – Test M: Low air pressure
IEC 60068-2-14:1984, Environmental testing – Part 2: Tests – Test N: Change of temperature
Amendment 1 (1986)
IEC 60068-2-20:1979, Environmental testing – Part 2: Tests – Test T: Soldering
Amendment 2 (1987)
IEC 60068-2-21, Environmental testing – Part 2-21: Tests – Test U: Robustness of
terminations and integral mounting devices

– 8 – 60738-1  IEC:2006
IEC 60068-2-27, Environmental testing – Part 2: Tests – Test Ea and guidance: Shock
IEC 60068-2-29, Environmental testing – Part 2: Tests – Test Eb and guidance: Bump
IEC 60068-2-30:2005, Environmental testing – Part 2: Tests – Test Db: Damp heat, cyclic
(12 h + 12-hour cycle)
IEC 60068-2-45:1980, Environmental testing – Part 2: Tests – Test XA and guidance –
Immersion in cleaning solvents
IEC 60068-2-58, Environmental testing – Part 2-58: Tests – Test Td: Test methods for
solderability, resistance to dissolution of metallization and to soldering heat of surface
mounting devices (SMD)
IEC 60068-2-78, Environmental testing – Part 2-78: Tests – Test Cab: Damp heat, steady
state
IEC 60294, Measurement of the dimensions of a cylindrical component having two axial
terminations
IEC 60410, Sampling plans and procedures for inspection by attributes
IEC 60617 (all parts) [DB] : Graphical symbols for diagrams
IEC 60717, Method for determination of the space required by capacitors and resistors with
unidirectional terminations
IEC 61249-2-7, Materials for printed boards and other interconnecting structures – Part 2-7:
Reinforced base materials clad and unclad – Epoxide woven E-glass laminated sheet of
defined flammability (vertical burning test), copper-clad
IEC 61760-1, Surface mounting technology – Part 1: Standard method for the specification of
surface mounting components (SMDs)
IEC QC 001002-3, Rules of Procedure of the IEC Quality Assessment System for Electronic
Components (IECQ) – Part 3: Approval procedures
ISO 1000, SI units and recommendations for the use of their multiples and of certain other
units
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
type
group of components having similar design features and the similarity of whose manufacturing
techniques enables them to be grouped together either for qualification approval or for quality
conformance inspection
They are generally covered by a single detail specification
NOTE Components described in several detail specifications, may, in some cases, be considered as belonging to
the same type but they are generally covered by a single detail specification.
—————————
“DB” refers to the IEC on-line database.

60738-1  IEC:2006 – 9 –
3.2
style
variation within a type having specific nominal dimensions and characteristics
3.3
thermistor
thermally sensitive semiconducting resistor which exhibits a significant change in electrical
resistance with a change in body temperature
3.4
positive temperature coefficient thermistor
thermistor, the resistance of which increases with its increasing temperature throughout the
useful part of its characteristic
3.5
positive step-function temperature coefficient thermistor
PTC
thermistor which shows a step-like increase in its resistance when the increasing temperature
reaches a specific value
A PTC thermistor will show secondary effects which are to be taken into account
3.6
zero-power resistance
R
T
value of the resistance of a PTC thermistor, at a given temperature, under conditions such
that the change in resistance due to the internal generation of heat is negligible with respect
to the total error of measurement
NOTE Any resistance value of a PTC thermistor is dependent on the value and the mode of the applied voltage
(a.c. or d.c.) and, when an a.c. source is used, on the frequency (see 3.8 and 3.9).
3.7
nominal zero-power resistance
R
n
d.c. resistance value of a thermistor measured at a specified temperature, preferably at 25 °C,
with a power dissipation low enough that any further decrease in power will result only in a
negligible change in resistance. Zero-power resistance may also be measured using a.c. if
required by the detail specification
3.8
voltage dependency
secondary effect, exhibiting a decreasing resistance with increasing voltage across the
thermistor when measured at a constant body temperature
3.9
frequency dependency
secondary effect exhibiting a substantial decrease of the positive temperature coefficient of
the thermistor with increasing frequency
3.10
resistance/temperature characteristics
relationship between the zero-power resistance of a thermistor and the temperature of the
thermo-sensitive element when measured under specified reference conditions (see Figure 1)
NOTE PTC thermistors may have more than one resistance/temperature characteristic specified. The zero-power
resistance of the resistance/temperature characteristics can be measured using a pulse voltage (U ) higher than
pulse
1,5 V, which is specified in the detail specification. The right curve in Figure 1 shows the typical
resistance/temperature characteristic when using the pulse voltage (U ).
pulse
– 10 – 60738-1  IEC:2006
Log R
R
p
≤ 1,5 V
d.c.
U
pulse
R
b
R
R
min
T T T
Rmin b p T  °C
Linear scale
IEC  402/06
Figure 1 – Typical resistance-temperature characteristic
and definitions for PTC thermistors (at zero power)

60738-1  IEC:2006 – 11 –
(logarithmic scale)
R  Ω
4 000
1 330
R
min
–20 0 T  °C
TNF – 20
TNF – 5
TNF
TNF + 5
TNF + 15
IEC  403/06
Figure 2 – Typical R-TNF characteristic for PTC thermistors in sensor applications
3.11
current/voltage characteristic
relationship in still air at 25 °C (unless otherwise stated) between the applied voltage (d.c.
and/or a.c.) at the thermistor terminations and the current under steady-state conditions (see
Figure 3)
Log I
T = constant
ambiant
I
res
U
Log U
max
IEC  404/06
NOTE 1 U will be specified by the manufacturer.
max
NOTE 2 The breakdown voltage is the value beyond which the voltage-handling capability of the thermistor no
longer exhibits its characteristic property.
Figure 3 – Typical current/voltage characteristic for PTC thermistors

– 12 – 60738-1  IEC:2006
3.12
nominal functioning temperature
T
NF
nominal temperature at the steep part of the resistance temperature characteristic at which
the system controlled by the thermistor is designed to operate
NOTE T is exclusively defined for PTC resistors in sensor applications.
NF
3.13
switching temperature
T
b
temperature at which the step-like function commences
3.14
minimum resistance
R
min
minimum value of the zero-power resistance/temperature characteristic (see Figures 1 and 2)
3.15
resistance at switching temperature
R
b
value of the zero-power resistance corresponding to the switching temperature defined as R
b
= 2 × R . As an alternative definition R = 2 × R can be used. If this definition is used, this
min b 25
shall be explicitly stated in the detail specification
3.16
temperature for minimum resistance
T
Rmin
temperature at which R occurs
min
3.17
temperature
T
p
temperature, higher than T , in the PTC part of the resistance/temperature characteristic for
b
which a minimum value R of the zero-power resistance is specified
p
3.18
resistance
R
p
zero-power resistance at temperature T measured at maximum voltage or a voltage specified
p
in the detail specification and given as a minimum value
NOTE The measurement should be made under such conditions that a change in resistance due to internal
generation of heat is negligible with respect to the total error of measurement. The applied voltage and the
characteristics of any pulse used should be given in the detail specification; when applying the maximum voltage,
the maximum overload current may not be exceeded.
3.19
average temperature coefficient of resistance at a stated voltage
a
R
rate of change of resistance with temperature expressed as %/K
It is calculated from the formula:
R
p
α = × ln
R
(T − T ) R
p b b
where T exceeds T by a minimum of 10 K.
p b
60738-1  IEC:2006 – 13 –
The temperatures T and T are to be given if applicable and the measurement conditions for
p b
R and R should be the same, unless otherwise specified in the detail specification
b p
NOTE The detail specification may specify the measurement of the temperature coefficient of resistance in a
narrow temperature range where its value is a maximum, together with a suitable test method.
3.20
upper category temperature
UCT
maximum ambient temperature for which a thermistor has been designed to operate
continuously at zero power
3.21
lower category temperature
LCT
minimum ambient temperature for which a thermistor has been designed to operate
continuously at zero power
3.22
maximum voltage
U
max
maximum a.c. or d.c. voltage which may be continuously applied to the thermistor without
exceeding the maximum overload current
3.23
operating temperature range at maximum voltage
range of ambient temperatures at which the thermistor can operate continuously at the
maximum voltage without exceeding the maximum overload current
3.24
isolation voltage (applicable only to insulated thermistors)
maximum peak voltage which may be applied under continuous operating conditions between
any of the thermistor terminations and any conducting surface
3.25
maximum overload current
I
mo
value of current for the operating temperature range, which shall not be exceeded
NOTE It may be necessary to limit the current through the thermistor by the use of a series resistor R .
s
3.26
residual current
I
res
value of current in the thermistor at a specified ambient temperature (preferably 25 °C) under
steady-state conditions. The applied voltage is the maximum voltage unless otherwise
specified (see Figure 3)
3.27
tripping current
I
t
lowest current which will cause the thermistor to trip to a high resistance condition at a
specified temperature (preferably 25 °C) and within a time to be specified in the detail
specification
3.28
maximum non-tripping current
I
max nt
maximum current at a specified ambient temperature (preferably 25 °C), which the thermistor
will conduct indefinitely in its low-resistance condition

– 14 – 60738-1  IEC:2006
3.29
inrush current
I
in
current occurring during the transient period from the moment of switching to the steady-state
condition
3.30
peak inrush current
I
in p
peak inrush current is the maximum value of current during the transient period (see Figure 4)
3.31
minimum peak inrush current
I
in p min
lowest specified value of the peak of the inrush current
3.32
maximum peak inrush current
I
in p max
maximum specified value of the peak inrush current

I
in
I
in p max
I
in p min
t
IEC  405/06
Figure 4 – I against t at U
in dc
3.33
peak-to-peak inrush current (for a.c. conditions only)
I
in pp
value of the inrush current measured between adjacent positive and negative peaks (see
Figure 5)
3.34
minimum peak-to-peak inrush current
I
in pp min
lowest specified value of the peak to peak inrush current
3.35
maximum peak to peak inrush current
I
in pp max
maximum specified value of the peak to peak inrush current

60738-1  IEC:2006 – 15 –
I
in
I
in pp min
I t
in pp max
IEC  406/06
Figure 5 – I against t at U
in rms
3.36
peak inrush power
P
in p
peak power (U × I ) measured at the maximum peak value of the current occurring during
in p
the transient period from the moment of switching to the steady-state operating condition
measured under specified conditions of ambient temperature, voltage and circuit
3.37
maximum peak inrush power
P
in p max.
maximum peak power which can occur during the transient period before the thermistor
reaches its steady-state operating condition
NOTE In the detail specification it should be specified whether the “0-peak value” or the “r.m.s. value” of the
voltage should be taken.
3.38
maximum power
P
max.
maximum power is the power (U × I ) which can be dissipated continuously by the
max. res
thermistor when the maximum voltage is applied under specified conditions of ambient
temperature, circuit and thermal dissipation when thermal equilibrium is obtained
NOTE If the power is supplied by an a.c. source then the voltage and current should be measured with true r.m.s.
meters.
3.39
dissipation factor
δ
quotient (in W/K) of the change in power dissipation in the thermistor and the resultant
change of the body temperature under specified ambient conditions (temperature, medium)
3.40
thermal resistance
R
th
quotient (in K/W) of the temperature difference between the thermistor and its ambient and
the power dissipated by the thermistor under specified ambient conditions (temperature,
medium)
NOTE "Dissipation factor" and "thermal resistance" are mutually reciprocal.

– 16 – 60738-1  IEC:2006
3.41
heat capacity
C
th
energy (in J) the thermistor needs to increase its body temperature by 1 K
3.42
response time
a) response time by ambient temperature change (t )
a
time (in s) required by a thermistor to change its temperature between two defined
conditions when subjected to a change in ambient temperature
b) response time by power change (t )
p
time (in s) required by a thermistor to change its temperature between two defined
conditions of power input
3.43
thermal time constant
thermal time constant (ideal) for a thermistor is the product of its heat capacity and its thermal
resistance
a) thermal time constant by ambient temperature change (τ )
a
time required for a thermistor to respond to 63,2 % of an external step change in ambient
temperature
b) thermal time constant by cooling (τ )
c
time required for a thermistor to cool by 63,2 % of its temperature excess, due to electrical
heating, in still air
3.44
insulated thermistors
thermistors capable of meeting the requirements of the insulation-resistance and voltage-
proof tests when specified in the test schedule
3.45
surface temperature
T
s
temperature under defined conditions at the boundary layer between two media
3.46
final surface temperature
T
sf
temperature reached under defined conditions when thermal equilibrium is established
4 Units and symbols
Units, graphical symbols, letter symbols and terminology shall, whenever possible, be taken
from the following publications:
IEC 60027
IEC 60050
IEC 60617
ISO 1000
When further items are required they shall be derived in accordance with the principles of the
documents listed above.
60738-1  IEC:2006 – 17 –
5 Preferred values
5.1 Climatic categories
The thermistors are classified into climatic categories according to the general rules given in
the annex to IEC 60068-1. The detail specification shall prescribe the appropriate category.
5.1.1 Bump test severities
The test severity given in the detail specification shall preferably be the following:
Test Eb (IEC 60068-2-29)
Acceleration: 400 m/s
Number of bumps: 4 000 total
Thermistors shall be mounted by their normal means, in such a manner that there shall be no
parasitic vibration.
5.1.2 Shock test severities
Test severities given in detail specifications shall preferably be the following:
Test Ea (IEC 60068-2-27)
Pulse shape: Half-sine
Acceleration: 500 m/s
Pulse duration: 11 ms
Severity: Three successive shocks in each axis direction per specimen
Separate specimens to be used for each axis (six shocks total per
specimen)
NOTE The shock and bump tests are normally specified as alternatives.
5.1.3 Vibration severities
Test severities given in the detail specifications shall preferably be selected from the
following:
Test Fc (IEC 60068-2-6)
Frequency Total number of
Hz sweep cycles
10 to   55
3 × 24
10 to  500
3 × 10
10 to 2 000
3 ×  8
5.2 Marking
5.2.1 General
The following shall be clearly marked on the thermistor in the following order of precedence
as space permits.
a) Values of the primary characteristics appropriate to the application of the thermistor to be
specified in the detail specification. When these values are coded (including colour
coding), details shall be given in the detail specification or type designation.
b) Manufacturer's name and/or trade mark.

– 18 – 60738-1  IEC:2006
c) Date of manufacture.
d) The number of the detail specification and style.
The package containing the thermistors shall be clearly marked with all the information listed
above.
Any additional marking shall be so applied that no confusion can arise.
5.2.2 Coding
Where coding for the date is used, the method shall be selected from those given in
IEC 60062.
5.3 Spacings
5.3.1 The spacings between parts of opposite polarity of a thermistor shall be not less than
the applicable values specified in Table 1.
Table 1 – Creepage distances and clearances
Environment Operating voltage
DCV or Vrms
≤ 32 > 32 and ≤ 50 > 50 and ≤ 300 > 300 and ≤ 600
Sealed or encapsulated Exception 1 0,4 mm 0,8 mm 0,8 mm
Clean Exception 1 1,0 mm 1,0 mm 1,0 mm
Normal and dirty Exception 1 1,0 mm 1,5 mm 1,5 mm

This applies to both creepage distances and clearances.
Exception 1: as to the creepage distances and clearances for PTC thermistors with a rated
operating voltage of less than 32 V, this value meets the intention of these requirements when
the thermistor is confirmed to have the maximum voltage specified in the detail specification.
5.3.2 An insulating barrier or liner that is used to provide spacings shall be not less than
0,71 mm.
Exception 2: a barrier or liner that is used in conjunction with at least 50 % of required
through-air spacing shall be not less than 0,33 mm thickness.
Exception 3: a barrier or liner having a thickness of less than that specified above complies
with the requirements when it is investigated and found to be rated for the intended conditions
of use, and found to be mechanically and electrically equivalent to the barrier or liner
specified above.
6 Quality assessment procedures
6.1 General
When these documents are being used for the purposes of a full quality assessment system
such as the IEC Quality Assessment System for Electronic Components (IECQ), compliance
with 6.5 is required.
60738-1  IEC:2006 – 19 –
When these documents are used outside such quality assessment systems for purposes such
as design proving or type testing, the procedures and requirements of 6.5.1 and 6.5.3b) may
be used, but, if used, the tests and parts of tests shall be applied in the order given in the test
schedules.
Before thermistors can be qualified according to the procedures of this subclause, the
manufacturer shall obtain the approval of his organization in accordance with the provisions of
IEC QC 001002-3.
The method for the approval of thermistors of assessed quality given in 6.5 is qualification
approval according to the provisions of C
...

IEC 60738-1 ®
Edition 3.1 2009-07
CONSOLIDATED VERSION
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Thermistors – Directly heated positive temperature coefficient –
Part 1: Generic specification
Thermistances – Coefficient de température positif à chauffage direct –
Partie 1: Spécification générique

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IEC 60738-1 ®
Edition 3.1 2009-07
CONSOLIDATED VERSION
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Thermistors – Directly heated positive temperature coefficient –

Part 1: Generic specification
Thermistances – Coefficient de température positif à chauffage direct –

Partie 1: Spécification générique

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 31.040.30 ISBN 978-2-8322-1370-4

IEC 60738-1 ®
Edition 3.1 2009-07
CONSOLIDATED VERSION
REDLINE VERSION
VERSION REDLINE
colour
inside
Thermistors – Directly heated positive temperature coefficient –
Part 1: Generic specification
Thermistances – Coefficient de température positif à chauffage direct –
Partie 1: Spécification générique

– 2 – 60738-1  IEC:2006+A1:2009

CONTENTS
FOREWORD . 5

1 Scope . 7

2 Normative references . 7

3 Terms and definitions . 8

4 Units and symbols . 16

5 Preferred values . 17

5.1 Climatic categories . 17
5.2 Marking . 17
5.3 Spacings . 18
6 Quality assessment procedures . 18
6.1 General . 18
6.2 Primary stage of manufacture . 19
6.3 Subcontracting . 19
6.4 Structurally similar components . 19
6.5 Qualification approval procedures . 20
6.6 Rework and repair . 27
6.7 Release for delivery . 27
6.8 Certified test records of released lots . 28
6.9 Delayed delivery . 28
6.10 Alternative test methods . 28
6.11 Manufacture outside the geographical limits of IECQ NSIs . 28
6.12 Unchecked parameters . 28
7 Test and measurement procedures . 28
7.1 General . 28
7.2 Standard conditions for testing . 29
7.3 Drying and recovery . 29
7.4 Visual examination and check of dimensions . 30
7.5 Zero-power resistance . 30
7.6 Temperature coefficient of resistance . 31
7.7 Insulation resistance (for insulated types only) . 31
7.8 Voltage proof (for insulated types only) . 32

7.9 Resistance/temperature characteristic . 32
7.10 Dissipation factor at U (δ) . 33
max
7.11 Response time by ambient temperature change (t ) . 34
a
7.12 Response time by power change (t ) . 34
p
7.13 Thermal time constant by ambient temperature change (τ ) . 35
a
7.14 Thermal time constant by cooling (τ ). 35
c
7.15 Robustness of terminations . 37
7.16 Solderability . 38
7.17 Resistance to soldering heat . 39
7.18 Rapid change of temperature . 40
7.19 Vibration. 40
7.20 Bump . 40
7.21 Shock . 41

60738-1  IEC:2006+A1:2009 – 3 –

7.22 Climatic sequence . 41

7.23 Damp heat, steady state . 42

7.24 Endurance . 43

7.25 Tripping current and tripping time . 47

7.26 Non-tripping current . 47

7.27 Residual current . 47

7.28 Surface temperature . 48

7.29 Inrush current . 49

7.30 Mounting (for surface mount thermistors only) . 49

7.31 Shear (adhesion) test . 50

7.32 Substrate bending test . 51

Annex A (normative) Interpretation of sampling plans and procedures as described in
IEC 60410 for use within the IEC quality assessment system for electronic
components (IECQ) . 52
Annex B (informative) Mounting for electrical measurements (except surface mount
types) . 53
Annex C (informative) Mounting for temperature measurements . 56

Figure 1 – Typical resistance-temperature characteristic and definitions for PTC
thermistors (at zero power) . 10
Figure 2 – Typical R-TNF characteristic for PTC thermistors in sensor applications . 11
Figure 3 – Typical current/voltage characteristic for PTC thermistors . 11
Figure 4 – I against t at U . 14
in dc
Figure 5 – I against t at U . 15
in rms
Figure 6 – Dissipation factor test circuit . 33
Figure 7 – Temperature gradient . 34
Figure 8 – Circuit for measurement of thermal time constant by cooling . 36
Figure 9 – Circuit for endurance at maximum operating temperature and maximum
voltage . 45
Figure 10 – Circuit for surface temperature measurement . 48
Figure 11 – Measuring circuit . 49
Figure B.1 – Example of a preferred mounting method for thermistors without wire
terminations . 53

Figure B.2 – Example of a preferred mounting method for thermistors with wire
terminations . 54
Figure B.3 – Example of a preferred mounting method for surface mount thermistors . 55
Figure C.1 – Example of a preferred mounting method for temperature measurement
on cylindrical heating elements . 56

Table 1 – Creepage distances and clearances . 18
Table 2 – Fixed sample size test schedule for qualification approval of thermistors for
current limitation – Assessment level EZ . 22
Table 3 – Fixed sample size test schedule for qualification approval of thermistors for
use as heating elements – Assessment level EZ . 23
Table 4 – Fixed sample size test schedule for qualification approval of thermistors for
inrush current application – Assessment level EZ . 24
Table 5 – Fixed sample size test schedule for qualification approval of thermistors for
use as temperature sensing elements, Assessment level EZ . 25

– 4 – 60738-1  IEC:2006+A1:2009

Table 6 – Quality conformance inspection for lot-by-lot inspection . 26

Table 7 – Quality conformance inspection for periodic testing . 27

Table 8 – Tensile force . 37

Table 9 – Number of cycles per climatic category . 42

60738-1  IEC:2006+A1:2009 – 5 –

INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
THERMISTORS – DIRECTLY HEATED POSITIVE

TEMPERATURE COEFFICIENT –
Part 1: Generic specification
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.

This consolidated version of the official IEC Standard and its amendment has been prepared
for user convenience.
IEC 60738-1 edition 3.1 contains the third edition (2006-04) [documents 40/1651/FDIS and
40/1730/RVD] and its amendment 1 (2009-05) [documents 40/1940/CDV and 40/1999/RVC].
In this Redline version, a vertical line in the margin shows where the technical content is
modified by amendment 1. Additions and deletions are displayed in red, with deletions being
struck through. A separate Final version with all changes accepted is available in this
publication.
– 6 – 60738-1  IEC:2006+A1:2009

International Standard IEC 60738-1 has been prepared by IEC technical committee 40:
Capacitors and resistors for electronic equipment.

The French version of this standard has not been voted upon.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

IEC 60738 consists of the following parts, under the general title Thermistors – Directly

heated positive step-function coefficient:

Part 1: Generic specification
Part 1-1: Blank detail specification – Current limiting application – Assessment level EZ
Part 1-2: Blank detail specification – Heating element application – Assessment level EZ
Part 1-3: Blank detail specification – Inrush current application – Assessment level EZ
Part 1-4: Blank detail specification – Sensing application – Assessment level EZ
The committee has decided that the contents of the base publication and its amendment will
remain unchanged until the stability date indicated on the IEC web site under
"http://webstore.iec.ch" in the data related to the specific publication. At this date, the
publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
60738-1  IEC:2006+A1:2009 – 7 –

THERMISTORS – DIRECTLY HEATED POSITIVE

TEMPERATURE COEFFICIENT –
Part 1: Generic specification
1 Scope
This part of IEC 60738 describes terms and methods of test for positive step-function
temperature coefficient thermistors, insulated and non-insulated types typically made from
ferro-electric semi-conductor materials.
It establishes standard terms, inspection procedures and methods of test for use in detail
specifications for Qualification Approval and for Quality Assessment Systems for electronic
components.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60027 (all parts), Letter symbols to be used in electrical technology
IEC 60050 (all parts), International Electrotechnical Vocabulary (IEV)
IEC 60062, Marking codes for resistors and capacitors
IEC 60068-1:1988, Environmental testing – Part 1: General and guidance
Amendment 1 (1992)
IEC 60068-2-1:1990, Environmental testing – Part 2: Tests – Tests A: Cold
Amendment 1 (1993)
Amendment 2 (1994)
IEC 60068-2-2:1974, Environmental testing – Part 2: Tests – Tests B: Dry heat
Amendment 1 (1993)
IEC 60068-2-6, Environmental testing – Part 2: Tests – Test Fc: Vibration (sinusoidal)
IEC 60068-2-11, Environmental testing – Part 2: Tests – Test Ka: Salt mist
IEC 60068-2-13, Environmental testing – Part 2: Tests – Test M: Low air pressure
IEC 60068-2-14:1984, Environmental testing – Part 2: Tests – Test N: Change of temperature
Amendment 1 (1986)
IEC 60068-2-20:1979, Environmental testing – Part 2: Tests – Test T: Soldering
Amendment 2 (1987)
IEC 60068-2-21, Environmental testing – Part 2-21: Tests – Test U: Robustness of
terminations and integral mounting devices

– 8 – 60738-1  IEC:2006+A1:2009

IEC 60068-2-27, Environmental testing – Part 2: Tests – Test Ea and guidance: Shock

IEC 60068-2-29, Environmental testing – Part 2: Tests – Test Eb and guidance: Bump

IEC 60068-2-30:2005, Environmental testing – Part 2: Tests – Test Db: Damp heat, cyclic

(12 h + 12-hour cycle)
IEC 60068-2-45:1980, Environmental testing – Part 2: Tests – Test XA and guidance –

Immersion in cleaning solvents

IEC 60068-2-58, Environmental testing – Part 2-58: Tests – Test Td: Test methods for

solderability, resistance to dissolution of metallization and to soldering heat of surface
mounting devices (SMD)
IEC 60068-2-78, Environmental testing – Part 2-78: Tests – Test Cab: Damp heat, steady
state
IEC 60294, Measurement of the dimensions of a cylindrical component having two axial
terminations
IEC 60410, Sampling plans and procedures for inspection by attributes
IEC 60617 (all parts) [DB] : Graphical symbols for diagrams
IEC 60717, Method for determination of the space required by capacitors and resistors with
unidirectional terminations
IEC 61249-2-7, Materials for printed boards and other interconnecting structures – Part 2-7:
Reinforced base materials clad and unclad – Epoxide woven E-glass laminated sheet of
defined flammability (vertical burning test), copper-clad
IEC 61760-1, Surface mounting technology – Part 1: Standard method for the specification of
surface mounting components (SMDs)
IEC QC 001002-3, Rules of Procedure of the IEC Quality Assessment System for Electronic
Components (IECQ) – Part 3: Approval procedures
ISO 1000, SI units and recommendations for the use of their multiples and of certain other
units
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
type
group of components having similar design features and the similarity of whose manufacturing
techniques enables them to be grouped together either for qualification approval or for quality
conformance inspection
They are generally covered by a single detail specification
NOTE Components described in several detail specifications, may, in some cases, be considered as belonging to
the same type but they are generally covered by a single detail specification.
—————————
“DB” refers to the IEC on-line database.

60738-1  IEC:2006+A1:2009 – 9 –

3.2
style
variation within a type having specific nominal dimensions and characteristics

3.3
thermistor
thermally sensitive semiconducting resistor which exhibits a significant change in electrical

resistance with a change in body temperature

3.4
positive temperature coefficient thermistor

thermistor, the resistance of which increases with its increasing temperature throughout the
useful part of its characteristic
3.5
positive step-function temperature coefficient thermistor
PTC
thermistor which shows a step-like increase in its resistance when the increasing temperature
reaches a specific value
A PTC thermistor will show secondary effects which are to be taken into account
3.6
zero-power resistance
R
T
value of the resistance of a PTC thermistor, at a given temperature, under conditions such
that the change in resistance due to the internal generation of heat is negligible with respect
to the total error of measurement
NOTE Any resistance value of a PTC thermistor is dependent on the value and the mode of the applied voltage
(a.c. or d.c.) and, when an a.c. source is used, on the frequency (see 3.8 and 3.9).
3.7
nominal zero-power resistance
R
n
d.c. resistance value of a thermistor measured at a specified temperature, preferably at 25 °C,
with a power dissipation low enough that any further decrease in power will result only in a
negligible change in resistance. Zero-power resistance may also be measured using a.c. if
required by the detail specification
3.8
voltage dependency
secondary effect, exhibiting a decreasing resistance with increasing voltage across the

thermistor when measured at a constant body temperature
3.9
frequency dependency
secondary effect exhibiting a substantial decrease of the positive temperature coefficient of
the thermistor with increasing frequency
3.10
resistance/temperature characteristics
relationship between the zero-power resistance of a thermistor and the temperature of the
thermo-sensitive element when measured under specified reference conditions (see Figure 1)
NOTE PTC thermistors may have more than one resistance/temperature characteristic specified. The zero-power
resistance of the resistance/temperature characteristics can be measured using a pulse voltage (U ) higher than
pulse
1,5 V, which is specified in the detail specification. The right curve in Figure 1 shows the typical
resistance/temperature characteristic when using the pulse voltage (U ).
pulse
– 10 – 60738-1  IEC:2006+A1:2009

Log R
R
p
≤ 1,5 V
d.c.
U
pulse
R
b
R
R
min
T T T
Rmin b p T  °C
Linear scale
IEC  402/06
Figure 1 – Typical resistance-temperature characteristic
and definitions for PTC thermistors (at zero power)

60738-1  IEC:2006+A1:2009 – 11 –

(logarithmic scale)
R  Ω
4 000
1 330
R
min
–20 0 T  °C
TNF – 20
TNF – 5
TNF
TNF + 5
TNF + 15
IEC  403/06
Figure 2 – Typical R-TNF characteristic for PTC thermistors in sensor applications
3.11
current/voltage characteristic
relationship in still air at 25 °C (unless otherwise stated) between the applied voltage (d.c.
and/or a.c.) at the thermistor terminations and the current under steady-state conditions (see
Figure 3)
Log I
T = constant
ambiant
I
res
U
Log U
max
IEC  404/06
NOTE 1 U will be specified by the manufacturer.
max
NOTE 2 The breakdown voltage is the value beyond which the voltage-handling capability of the thermistor no
longer exhibits its characteristic property.
Figure 3 – Typical current/voltage characteristic for PTC thermistors

– 12 – 60738-1  IEC:2006+A1:2009

3.12
nominal functioning temperature

T
NF
nominal temperature at the steep part of the resistance temperature characteristic at which

the system controlled by the thermistor is designed to operate

NOTE T is exclusively defined for PTC resistors in sensor applications.
NF
3.13
switching temperature
T
b
temperature at which the step-like function commences

3.14
minimum resistance
R
min
minimum value of the zero-power resistance/temperature characteristic (see Figures 1 and 2)
3.15
resistance at switching temperature
R
b
value of the zero-power resistance corresponding to the switching temperature defined as R
b
= 2 × R . As an alternative definition R = 2 × R can be used. If this definition is used, this
min b 25
shall be explicitly stated in the detail specification
3.16
temperature for minimum resistance
T
Rmin
temperature at which R occurs
min
3.17
temperature
T
p
temperature, higher than T , in the PTC part of the resistance/temperature characteristic for
b
which a minimum value R of the zero-power resistance is specified
p
3.18
resistance
R
p
zero-power resistance at temperature T measured at maximum voltage or a voltage specified
p
in the detail specification and given as a minimum value
NOTE The measurement should be made under such conditions that a change in resistance due to internal

generation of heat is negligible with respect to the total error of measurement. The applied voltage and the
characteristics of any pulse used should be given in the detail specification; when applying the maximum voltage,
the maximum overload current may not be exceeded.
3.19
average temperature coefficient of resistance at a stated voltage
a
R
rate of change of resistance with temperature expressed as %/K
It is calculated from the formula:
R
p
α = × ln
R
(T − T ) R
p b b
where T exceeds T by a minimum of 10 K.
p b
60738-1  IEC:2006+A1:2009 – 13 –

The temperatures T and T are to be given if applicable and the measurement conditions for
p b
R and R should be the same, unless otherwise specified in the detail specification
b p
NOTE The detail specification may specify the measurement of the temperature coefficient of resistance in a
narrow temperature range where its value is a maximum, together with a suitable test method.

3.20
upper category temperature
UCT
maximum ambient temperature for which a thermistor has been designed to operate

continuously at zero power
3.21
lower category temperature
LCT
minimum ambient temperature for which a thermistor has been designed to operate
continuously at zero power
3.22
maximum voltage
U
max
maximum a.c. or d.c. voltage which may be continuously applied to the thermistor without
exceeding the maximum overload current
3.23
operating temperature range at maximum voltage
range of ambient temperatures at which the thermistor can operate continuously at the
maximum voltage without exceeding the maximum overload current
3.24
isolation voltage (applicable only to insulated thermistors)
maximum peak voltage which may be applied under continuous operating conditions between
any of the thermistor terminations and any conducting surface
3.25
maximum overload current
I
mo
value of current for the operating temperature range, which shall not be exceeded
NOTE It may be necessary to limit the current through the thermistor by the use of a series resistor R .
s
3.26
residual current
I
res
value of current in the thermistor at a specified ambient temperature (preferably 25 °C) under
steady-state conditions. The applied voltage is the maximum voltage unless otherwise
specified (see Figure 3)
3.27
tripping current
I
t
lowest current which will cause the thermistor to trip to a high resistance condition at a
specified temperature (preferably 25 °C) and within a time to be specified in the detail
specification
3.28
maximum non-tripping current
I
max nt
maximum current at a specified ambient temperature (preferably 25 °C), which the thermistor
will conduct indefinitely in its low-resistance condition

– 14 – 60738-1  IEC:2006+A1:2009

3.29
inrush current
I
in
current occurring during the transient period from the moment of switching to the steady-state

condition
3.30
peak inrush current
I
in p
peak inrush current is the maximum value of current during the transient period (see Figure 4)

3.31
minimum peak inrush current
I
in p min
lowest specified value of the peak of the inrush current
3.32
maximum peak inrush current
I
in p max
maximum specified value of the peak inrush current

I
in
I
in p max
I
in p min
t
IEC  405/06
Figure 4 – I against t at U
in dc
3.33
peak-to-peak inrush current (for a.c. conditions only)
I
in pp
value of the inrush current measured between adjacent positive and negative peaks (see
Figure 5)
3.34
minimum peak-to-peak inrush current
I
in pp min
lowest specified value of the peak to peak inrush current
3.35
maximum peak to peak inrush current
I
in pp max
maximum specified value of the peak to peak inrush current

60738-1  IEC:2006+A1:2009 – 15 –

I
in
I
in pp min
I t
in pp max
IEC  406/06
Figure 5 – I against t at U
in rms
3.36
peak inrush power
P
in p
peak power (U × I ) measured at the maximum peak value of the current occurring during
in p
the transient period from the moment of switching to the steady-state operating condition
measured under specified conditions of ambient temperature, voltage and circuit
3.37
maximum peak inrush power
P
in p max.
maximum peak power which can occur during the transient period before the thermistor
reaches its steady-state operating condition
NOTE In the detail specification it should be specified whether the “0-peak value” or the “r.m.s. value” of the
voltage should be taken.
3.38
maximum power
P
max.
maximum power is the power (U × I ) which can be dissipated continuously by the
max. res
thermistor when the maximum voltage is applied under specified conditions of ambient
temperature, circuit and thermal dissipation when thermal equilibrium is obtained
NOTE If the power is supplied by an a.c. source then the voltage and current should be measured with true r.m.s.
meters.
3.39
dissipation factor
δ
quotient (in W/K) of the change in power dissipation in the thermistor and the resultant
change of the body temperature under specified ambient conditions (temperature, medium)
3.40
thermal resistance
R
th
quotient (in K/W) of the temperature difference between the thermistor and its ambient and
the power dissipated by the thermistor under specified ambient conditions (temperature,
medium)
NOTE "Dissipation factor" and "thermal resistance" are mutually reciprocal.

– 16 – 60738-1  IEC:2006+A1:2009

3.41
heat capacity
C
th
energy (in J) the thermistor needs to increase its body temperature by 1 K

3.42
response time
a) response time by ambient temperature change (t )
a
time (in s) required by a thermistor to change its temperature between two defined

conditions when subjected to a change in ambient temperature

b) response time by power change (t )
p
time (in s) required by a thermistor to change its temperature between two defined
conditions of power input
3.43
thermal time constant
thermal time constant (ideal) for a thermistor is the product of its heat capacity and its thermal
resistance
a) thermal time constant by ambient temperature change (τ )
a
time required for a thermistor to respond to 63,2 % of an external step change in ambient
temperature
b) thermal time constant by cooling (τ )
c
time required for a thermistor to cool by 63,2 % of its temperature excess, due to electrical
heating, in still air
3.44
insulated thermistors
thermistors capable of meeting the requirements of the insulation-resistance and voltage-
proof tests when specified in the test schedule
3.45
surface temperature
T
s
temperature under defined conditions at the boundary layer between two media
3.46
final surface temperature
T
sf
temperature reached under defined conditions when thermal equilibrium is established

4 Units and symbols
Units, graphical symbols, letter symbols and terminology shall, whenever possible, be taken
from the following publications:
IEC 60027
IEC 60050
IEC 60617
ISO 1000
When further items are required they shall be derived in accordance with the principles of the
documents listed above.
60738-1  IEC:2006+A1:2009 – 17 –

5 Preferred values
5.1 Climatic categories
The thermistors are classified into climatic categories according to the general rules given in

the annex to IEC 60068-1. The detail specification shall prescribe the appropriate category.

5.1.1 Bump test severities
The test severity given in the detail specification shall preferably be the following:

Test Eb (IEC 60068-2-29)
Acceleration: 400 m/s
Number of bumps: 4 000 total
Thermistors shall be mounted by their normal means, in such a manner that there shall be no
parasitic vibration.
5.1.2 Shock test severities
Test severities given in detail specifications shall preferably be the following:
Test Ea (IEC 60068-2-27)
Pulse shape: Half-sine
Acceleration: 500 m/s
Pulse duration: 11 ms
Severity: Three successive shocks in each axis direction per specimen
Separate specimens to be used for each axis (six shocks total per
specimen)
NOTE The shock and bump tests are normally specified as alternatives.
5.1.3 Vibration severities
Test severities given in the detail specifications shall preferably be selected from the
following:
Test Fc (IEC 60068-2-6)
Frequency Total number of
Hz sweep cycles
10 to   55
3 × 24
10 to  500
3 × 10
10 to 2 000
3 ×  8
5.2 Marking
5.2.1 General
The following shall be clearly marked on the thermistor in the following order of precedence
as space permits.
a) Values of the primary characteristics appropriate to the application of the thermistor to be
specified in the detail specification. When these values are coded (including colour
coding), details shall be given in the detail specification or type designation.
b) Manufacturer's name and/or trade mark.

– 18 – 60738-1  IEC:2006+A1:2009

c) Date of manufacture.
d) The number of the detail specification and style.

The package containing the thermistors shall be clearly marked with all the information listed

above.
Any additional marking shall be so applied that no confusion can arise.

5.2.2 Coding
Where coding for the date is used, the method shall be selected from those given in

IEC 60062.
5.3 Spacings
5.3.1 The spacings between parts of opposite polarity of a thermistor shall be not less than
the applicable values specified in Table 1.
Table 1 – Creepage distances and clearances
Environment Operating voltage
DCV or Vrms
≤ 32 > 32 and ≤ 50 > 50 and ≤ 300 > 300 and ≤ 600
Sealed or encapsulated Exception 1 0,4 mm 0,8 mm 0,8 mm
Clean Exception 1 1,0 mm 1,0 mm 1,0 mm
Normal and dirty Exception 1 1,0 mm 1,5 mm 1,5 mm

This applies to both creepage distances and clearances.
Exception 1: as to the creepage distances and clearances for PTC thermistors with a rated
operating voltage of less than 32 V, this value meets the intention of these requirements when
the thermistor is confirmed to have the maximum voltage specified in the detail specification.
5.3.2 An insulating barrier or liner that is used to provide spacings shall be not less than
0,71 mm.
Exception 2: a barrier or liner that is used in conjunction with at least 50 % of required
through-air spacing shall be not less than 0,33 mm thickness.

Exception 3: a barrier or liner having a thickness of less than that specified above complies
with the requirements when it is investigat
...