IEC 60738-1:1998

INTERNATIONAL
IEC
STANDARD
60738-1
QC 440000
Second edition
1998-11
Thermistors –
Directly heated positive step-function
temperature coefficient –
Part 1:
Generic specification
Thermistances à basculement à coefficient de température
positif à chauffage direct –
Partie 1:
Spécification générique
Reference number
Numbering
As from 1 January 1997 all IEC publications are issued with a designation in the
60000 series.
Consolidated publications
Consolidated versions of some IEC publications including amendments are

available. For example, edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the
base publication, the base publication incorporating amendment 1 and the base

publication incorporating amendments 1 and 2.

Validity of this publication
The technical content of IEC publications is kept under constant review by the IEC,
thus ensuring that the content reflects current technology.
Information relating to the date of the reconfirmation of the publication is available
in the IEC catalogue.
Information on the subjects under consideration and work in progress undertaken by
the technical committee which has prepared this publication, as well as the list of
publications issued, is to be found at the following IEC sources:
• IEC web site*
• Catalogue of IEC publications
Published yearly with regular updates
(On-line catalogue)*
• IEC Bulletin
Available both at the IEC web site* and as a printed periodical
Terminology, graphical and letter symbols
For general terminology, readers are referred to IEC 60050: International Electro-
technical Vocabulary (IEV).
For graphical symbols, and letter symbols and signs approved by the IEC for
general use, readers are referred to publications IEC 60027: Letter symbols to be
used in electrical technology, IEC 60417: Graphical symbols for use on equipment.
Index, survey and compilation of the single sheets and IEC 60617: Graphical symbols
for diagrams.
* See web site address on title page.

INTERNATIONAL
IEC
STANDARD
60738-1
QC 440000
Second edition
1998-11
Thermistors –
Directly heated positive step-function
temperature coefficient –
Part 1:
Generic specification
Thermistances à basculement à coefficient de température
positif à chauffage direct –
Partie 1:
Spécification générique
 IEC 1998  Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission 3, rue de Varembé Geneva, Switzerland
Telefax: +41 22 919 0300 e-mail: inmail@iec.ch IEC web site http://www.iec.ch
Commission Electrotechnique Internationale
PRICE CODE
X
International Electrotechnical Commission
For price, see current catalogue

– 2 – 60738-1 © IEC:1998(E)
CONTENTS
Page
FOREWORD. 4

Clause
1 General. 5

1.1 Scope. 5
1.2 Normative references. 5
2 Technical data. 6
2.1 Units and symbols. 6
2.2 Definitions. 6
2.3 Preferred values. 13
2.4 Marking. 14
3 Quality assessment procedures. 14
3.1 General. 14
3.2 Primary stage of manufacture. 15
3.3 Subcontracting. 15
3.4 Structurally similar components. 15
3.5 Qualification approval procedures . 15
3.6 Rework and repair. 24
3.7 Release for delivery . 24
3.8 Certified test records of released lots . 24
3.9 Delayed delivery. 24
3.10 Alternative test methods. 24
3.11 Manufacture outside the geographical limits of IECQ NSIs. 25
3.12 Unchecked parameters. 25
4 Test and measurement procedures . 25
4.1 General. 25
4.2 Standard conditions for testing . 25
4.3 Drying and recovery . 26
4.4 Visual examination and check of dimensions . 26

4.5 Zero-power resistance. 27
4.6 Temperature coefficient of resistance. 27
4.7 Insulation resistance (for insulated types only). 28
4.8 Voltage proof (for insulated types only). 29
4.9 Resistance/temperature characteristic. 29
δ
4.10 Dissipation factor at U ( ) . 29
max.
4.11 Response time by ambient temperature change (t ) . 30
a
4.12 Response time by power change (t ) . 31
p
4.13 Thermal time constant by ambient temperature change (τ ) . 32
a
4.14 Thermal time constant by cooling (τ ) . 32
c
4.15 Robustness of terminations . 34

60738-1 © IEC:1998(E) – 3 –
Clause Page
4.16 Soldering. 35

4.17 Rapid change of temperature . 35

4.18 Vibration. 35

4.19 Bump. 36

4.20 Shock. 36

4.21 Climatic sequence. 37

4.22 Damp heat, steady state. 38

4.23 Endurance. 38
4.24 Tripping current and tripping time . 41
4.25 Non-tripping current. 42
4.26 Residual current. 42
4.27 Surface temperature. 42
4.28 Inrush current. 43
4.29 Mounting (for surface mount thermistors only) . 44
4.30 Shear (adhesion) test. 45
4.31 Substrate bending test . 45
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) . 46
Annex B (informative) Mounting for electrical measurements (except surface
mount types) . 47
Annex C (informative) Mounting for temperature measurements . 49

– 4 – 60738-1 © IEC:1998(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION

__________
THERMISTORS –
DIRECTLY HEATED POSITIVE STEP-FUNCTION

TEMPERATURE COEFFICIENT –
Part 1: Generic specification
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the 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, the IEC publishes International Standards. 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. The 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 the 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 National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60738-1 has been prepared IEC technical committee 40: Capacitors
and resistors for electronic equipment.
This second edition cancels and replaces the first edition published in 1982 and constitutes a
technical revision.
The text of this standard is based on the following documents:

FDIS Report on voting
40/1080/FDIS 40/1096/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
Annex A forms an integral part of this standard.
Annexes B and C are for information only.
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).

60738-1 © IEC:1998(E) – 5 –
THERMISTORS –
DIRECTLY HEATED POSITIVE STEP-FUNCTION

TEMPERATURE COEFFICIENT –
Part 1: Generic specification
1 General
1.1 Scope
This International Standard prescribes 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.
1.2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this International Standard. At the time of publication, the editions
indicated were valid. All normative documents are subject to revision, and parties to
agreements based on this International Standard are encouraged to investigate the possibility
of applying the most recent editions of the normative documents indicated below. Members of
IEC and ISO maintain registers of currently valid International Standards.
In the case of IEC 60068 publications, the referenced edition shall be used, regardless of any
subsequent new edition(s) and amendment(s).
IEC 60027-1:1992, Letter symbols to be used in electrical technology – Part 1: General
IEC 60050: International Electrotechnical Vocabulary
IEC 60062:1992, Marking codes for resistors and capacitors
IEC 60068-1:1988, Environmental testing – Part 1: General and guidance
IEC 60068-2-1:1990, Environmental testing – Part 2: Tests – Tests A: Cold
IEC 60068-2-2:1974, Environmental testing – Part 2: Tests – Tests B: Dry heat

IEC 60068-2-3:1969, Environmental testing – Part 2: Tests – Test Ca: Damp heat, steady state
IEC 60068-2-6:1995, Environmental testing – Part 2: Tests – Test Fc and guidance: Vibration
(sinusoidal)
IEC 60068-2-11:1981, Environmental testing – Part 2: Tests – Test Ka: Salt mist
IEC 60068-2-13:1983, 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
IEC 60068-2-20:1979, Environmental testing – Part 2: Tests – Test T: Soldering
IEC 60068-2-21:1983, Environmental testing – Part 2: Tests – Test U: Robustness of terminations
and integral mounting devices
– 6 – 60738-1 © IEC:1998(E)
IEC 60068-2-27:1987, Environmental testing – Part 2: Tests – Test Ea and guidance: Shock

IEC 60068-2-29:1987, Environmental testing – Part 2: Tests – Test Eb and guidance: Bump

IEC 60068-2-30:1980, Environmental testing – Part 2: Tests – Test Db and guidance: Damp

heat, cyclic (12 + 12-hour cycle)

IEC 60068-2-45:1980, Environmental testing – Part 2: Tests – Test XA and guidance:

Immersion in cleaning solvents

IEC 60249-2-4:1987, Base materials for printed circuits – Part 2: Specifications – Specification
No. 4: Epoxide woven glass fabric copper-clad laminated sheet, general purpose grade
IEC 60294:1969, Measurement of the dimensions of a cylindrical component having two axial
terminations
IEC 60410:1973, Sampling plans and procedures for inspection by attributes
IEC 60617: Graphical symbols for diagrams
IEC QC 001002-3:1998, Rules of procedure of the IEC Quality Assessment System for
Electronic Components (IECQ) – Part 3: Approval procedures
IEC QC 001003: Guidance documents
ISO 1000:1992, SI units and recommendations for the use of their multiples and of certain
other units
2 Technical data
2.1 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
2.2 Definitions
When further items are required they shall be derived in accordance with the principles of the
documents listed above.
2.2.1
type
a 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.

60738-1 © IEC:1998(E) – 7 –
2.2.2
style
variation within a type having specific nominal dimensions and characteristics

2.2.3
thermistor
a thermally sensitive semiconducting resistor which exhibits a significant change in electrical

resistance with a change in body temperature

2.2.4
positive temperature coefficient thermistor

a thermistor, the resistance of which increases with its increasing temperature throughout the
useful part of its characteristic
2.2.5
positive step-function temperature coefficient thermistor (PTC)
a thermistor which shows a step-like increase in its resistance when the increasing tempera-
ture reaches a specific value
A PTC thermistor will show secondary effects which have to be taken into account.
2.2.6
zero-power resistance (R )
T
the 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 2.2.8 and 2.2.9).
2.2.7
nominal zero-power resistance (R )
n
the d.c. resistance value of a thermistor measured at a specified temperature, preferable 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.
2.2.8
voltage dependency
a secondary effect, exhibiting a decreasing resistance with increasing voltage across the
thermistor when measured at a constant body temperature
2.2.9
frequency dependency
a secondary effect exhibiting a substantial decrease of the positive temperature coefficient of
the thermistor with increasing frequency
2.2.10
resistance/temperature characteristics
the relationship between the zero-power resistance of a thermistor and the temperature of the
thermosensitive element when measured under specified reference conditions (see figure 1)
NOTE – PTC thermistors may have more than one resistance/temperature characteristic specified.

– 8 – 60738-1 © IEC:1998(E)
Log R
R
p
≤1,5 V
d.c.
U
pulse
R
NF
R
b
R
R
min.
T T T T
Rmin. b NF p T (°C)
Linear scale
IEC  1 566/98
Figure 1 – Typical resistance-temperature characteristic
and definitions for PTC thermistors (at zero-power)
2.2.11
current/voltage characteristic
the 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 2)
Log I
T = constant
ambiant
I
res
Log U
U
max.
IEC  1 567/98
NOTE 1 – U will be specified by the manufacturer.
max.
NOTE 2 – The breakdown voltage is the value beyond which the thermistor's voltage handling capability no longer
exhibits its characteristic property.
Figure 2 – Typical current/voltage characteristic for PTC thermistors

60738-1 © IEC:1998(E) – 9 –
2.2.12
nominal functioning temperature (T )
NF
the nominal temperature at the steep part of the resistance temperature characteristic at which

the system controlled by the thermistor, is designed to operate

2.2.13
switching temperature (T )
b
the temperature at which the step-like function commences

2.2.14
minimum resistance (R )
min.
the minimum value of the zero-power resistance/temperature characteristic (see figure 1)
2.2.15
resistance at switching temperature (R )
b
the value of the zero-power resistance corresponding to the switching temperature
It is defined as R = 2 × R . As an alternative definition R = 2 × R can be used. If this
b min b 25
definition is used, this shall be explicitly stated in the detail specification.
2.2.16
temperature for minimum resistance (T )
Rmin.
that temperature at which R occurs
min.
2.2.17
temperature T
p
that temperature, higher than T , in the PTC part of the resistance/temperature characteristic
b
for which a minimum value R of the zero-power resistance is specified
p
2.2.18
resistance R
p
the zero-power resistance at temperature T measured at maximum voltage or a voltage
p
specified in the detail specification and given as a minimum value
NOTE – The measurement should be made under such conditions that any 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.
2.2.19
average temperature coefficient of resistance at a stated voltage (αα )
R
the 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
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.

– 10 – 60738-1 © IEC:1998(E)
2.2.20
upper category temperature (UCT)
the maximum ambient temperature for which a thermistor has been designed to operate

continuously at zero power
2.2.21
lower category temperature (LCT)
the minimum ambient temperature for which a thermistor has been designed to operate

continuously at zero power
2.2.22
maximum voltage (U )
max.
the maximum a.c. or d.c. voltage which may be continuously applied to the thermistor without
exceeding the maximum overload current
2.2.23
operating temperature range at maximum voltage
the range of ambient temperatures at which the thermistor can operate continuously at the
maximum voltage without exceeding the maximum overload current
2.2.24
isolation voltage (applicable only to insulated thermistors)
the maximum peak voltage which may be applied under continuous operating conditions
between any of the thermistor terminations and any conducting surface
2.2.25
maximum overload current (I )
mo
the 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
2.2.26
residual current (I )
res
the 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 2)
2.2.27
tripping current (I )
t
the 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
2.2.28
maximum non-tripping current (I )
max. nt
the maximum current at a specified ambient temperature (preferably 25 °C), which the thermistor
will conduct indefinitely in its low resistance condition
2.2.29
inrush current (I )
in
the current occurring during the transient period from the moment of switching to the steady-
state condition
2.2.30
peak inrush current (I )
in p
the peak inrush current is the maximum value of current during the transient period (see figure 3)

60738-1 © IEC:1998(E) – 11 –
2.2.31
minimum peak inrush current (I )
in p min.
the lowest specified value of the peak of the inrush current

2.2.32
maximum peak inrush current (I )
in p max.
the maximum specified value of the peak inrush current

I
in
I
in p max.
I
in p min.
t
IEC  1 568/98
Figure 3 – I against t at U
in dc
2.2.33
peak-to-peak inrush current (I ) (for a.c. conditions only)
in pp
the value of the inrush current measured between adjacent positive and negative peaks (see
figure 4)
2.2.34
minimum peak-to-peak inrush current (I )
in pp min.
the lowest specified value of the peak-to-peak inrush current
2.2.35
maximum peak to peak inrush current (I )
in pp max.
the maximum specified value of the peak-to-peak inrush current
I
in
I
in pp min.
t
I
in pp max.
IEC  1 569/98
Figure 4 – I against t at U
in rms
– 12 – 60738-1 © IEC:1998(E)
2.2.36
peak inrush power (P )
in p
the 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

2.2.37
maximum peak inrush power (P )
in p max.
the maximum peak power which can occur during the transient period before the thermistor

reaches its steady-state operating condition

2.2.38
maximum power (P )
max.
the 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.
2.2.39
dissipation factor (δδ)
the 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)
2.2.40
thermal resistance (R )
th
the 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.
2.2.41
heat capacity (C )
th
the energy (in J) the thermistor needs to increase its body temperature by 1 K
2.2.42
response time
a) Response time by ambient temperature change (t )
a
The time (in seconds) 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
The time (in seconds) required by a thermistor to change its temperature between two
defined conditions of power input.
2.2.43
thermal time constant
the 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
The 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
The time required for a thermistor to cool by 63,2 % of its temperature excess, due to
electrical heating, in still air.

60738-1 © IEC:1998(E) – 13 –
2.2.44
insulated thermistors
thermistors capable of meeting the requirements of the insulation resistance and voltage proof

tests when specified in the test schedule

2.2.45
surface temperature (T )
s
the temperature under defined conditions at the boundary layer between two media

2.2.46
final surface temperature (T )
sf
the temperature reached under defined conditions when thermal equilibrium is established
2.3 Preferred values
2.3.1 Climatic categories
The thermistors are classified into climatic categories according to the general rules given in
the appendix to IEC 60068-1. The detail specification shall prescribe the appropriate category.
2.3.2 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.
2.3.3 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.
2.3.4 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
– 14 – 60738-1 © IEC:1998(E)
2.4 Marking
2.4.1 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;
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.
2.4.2 Coding
Where coding for the date is used, the method shall be selected from those given in IEC 60062.
3 Quality assessment procedures
3.1 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 3.5 is required.
When these documents are used outside such quality assessment systems for purposes such
as design proving or type testing, the procedures and requirements of 3.5.1 and 3.5.3 b) 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 section, the
manufacturer shall obtain the approval of his organisation in accordance with the provisions of
IEC QC 001002-3.
The method for the approval of thermistors of assessed quality given in 3.5 is qualification
approval according to the provisions of clause 3 of IEC QC 001002-3.
3.1.1 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 3.5 and the relevant blank detail specification.

60738-1 © IEC:1998(E) – 15 –
3.2 Primary stage of manufacture

The primary stage of manufacture is defined as the initial mixing process of the ingredients.

3.3 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.
3.4 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.
3.4.1 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.
3.4.2 For environmental tests, devices having the same encapsulation, basic internal
structure and finishing processes may be grouped.
3.4.3 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.
This grouping may also be used for robustness of terminations and soldering tests where it is
convenient to group devices with different internal structures.
3.4.4 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.

3.5 Qualification approval procedures
3.5.1 Eligibility for qualification approval
The manufacturer shall comply with 3.1.1 of IEC QC 001002-3.
3.5.2 Application for qualification approval
The manufacturer shall comply with 3.1.3 of IEC QC 001002-3.

– 16 – 60738-1 © IEC:1998(E)
3.5.3 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 to meet the sample size requirements
to give acceptance on one non-conforming item.
b) The manufacturer shall produce test evidence to show conformance to the specification
requirements on one of the fixed sample size test schedules given in 3.5.4.
The specimens taken to form the sample shall be selected at random from current
production or as agreed with the National Supervising Inspectorate (NSI).
For the two procedures the sample sizes shall be of comparable order. The test conditions and
requirements shall be the same.
3.5.4 Qualification approval on the basis of the fixed sample size procedure
The fixed sample size test schedules for qualification approval given hereafter are appropriate
to the intended application of the thermistor which is to be approved. The schedules provide
information on the test grouping and sampling and acceptance criteria. The conditions of test
and the end of test requirements shall be identical to those specified in the related blank detail
specification for the lot-by-lot and periodic tests.
Tests
The complete series of tests specified in the relevant table are required for the approval of
thermistors covered by one detail specification. The tests of each group shall be carried out in
the order given.
The whole sample shall be subjected to the tests of group "0" and then divided for the other
groups.
Non-conforming specimens during the tests of group "0" shall not be used for the other groups.
"One non-conforming item" is counted when a thermistor has not satisfied the whole or a part

of the tests of a group.
The approval is granted when the number of non-conformances does not exceed the specified
number of permissible non-conforming items for each group or subgroup and the total number
of permissible non-conformances.
The following list applies to each test schedule developed in tables 1 to 4:
1 Clause number references are to clauses in this specification.
2 a) Where the test schedule of a blank detail specification omits a test, that test may be
omitted from the corresponding fixed sample size schedule in this specification.

60738-1 © IEC:1998(E) – 17 –
b) Where additional tests are specified in a detail specification, that test shall be included
in the corresponding fixed sample size schedule, either by its addition to an existing
group or, by the addition of another group. In the former case there shall be no change

in the number of specimens to be tested or in the acceptance criteria. In the latter case

the number of specimens to be tested and the acceptance criteria shall be comparable

to those already specified.
3 In these tables:
n is the sample size
c is the acceptance criterion (permitted number of non-conforming items)

D indicates a destructive test

ND indicates a non-destructive test.
4 The temperature at which the zero-power resistance shall be measured is the temperature
specified in the detail specification. This temperature shall be stated, where required, in
the test schedule.
5 Data for conditions of test are defined in the detail specification.
6 The additional specimens are to permit substitution for incidents not attributable to the
manufacturer. The specimens may be used to replace non-conforming specimens which
occur as a result of a test in a group which is identified as being “destructive”. Where a
specimen is used for this purpose, it shall be subjected to those tests in the group to which
the non-conforming item had already been subjected, before proceeding with the
remaining tests in the group.
7 The specimens used for this group may, at the discretion of the manufacturer, be used for
any subsequent group which is identified as being "destructive".
8 10 samples from group 0 test samples have to be chosen, 5 having the lowest zero power
resistance of the sample shall be used for group 1A, 5 having the highest zero power
resistance of the sample for group 1B.
9 The soldering – solderability and soldering – resistance to soldering heat tests shall only
be applied where the thermistor has terminations which are appropriate for soldering.
10 Where the terminations are stated to be suitable for printed wiring applications, the
appropriate test conditions in IEC 60068 shall apply.
11 The thermistors shall be mounted by their normal means.
12 The bump test and the shock test are to be alternatives. The test selected in the detail
specification shall be used.
13 The detail specification shall specify which of the endurance tests in groups 5, 6 and 7 for
table 1 or table 4, in groups 5 and 6 for table 2 or groups 4 and 5 for table 3, respectively,
are appropriate to the construction and application of the thermistor.

– 18 – 60738-1 © IEC:1998(E)
Table 1 – Fixed sample size test schedule for qualification approval of thermistors

for current limitation, assessment level EZ

Group Test Subclause of D Number of Permissible number
No. (see list item 5) this standard or specimens of non-conforming
ND items
nc
0 Visual examination 4.4.1 ND 80 + 5 0

Marking 4.4.2 (see list item 6)
Dimensions (gauging) 4.4.3
Zero-power resistance R 4.5
n
Voltage proof 4.8
1 Temperature coefficient of 4.6 D 10 0
resistance (if specified) (see list item 7
and 8)
1A Tripping current and tripping time 4.24 5
Residual current 4.26 (list item 8)
1B Non-tripping current 4.25 5
(list item 8)
Soldering – Solderability 4.16.1
(see list items 9 and 10)
2A Soldering – Resistance to soldering 4.16.2 5
heat (see list items 9 and 10)
Robustness of terminations 4.15
2B Rapid change of temperature 4.17 5
Vibration 4.18
Bump or shock (see list item 12) 4.19 or 4.20
2 Climatic sequence 4.21 D 10 0
3 Thermal time constant by cooling 4.14 ND 10 0
(if specified)
4 Dimensions (detail) 4.4.4 ND 10 0
Dissipation factor at U
.
max 4.10
(if specified)
5 Endurance at room temperature 4.23.1 D 10 0
(cycling)(see list item 13)
6 Endurance at upper category 4.23.2 D 10 0
temperature (see list item 13)
7 Endurance at max. operating 4.23.3 D 10 0
temperature and max. voltage
(see list item 13)
8 Damp heat, steady state 4.22 D 10 0

60738-1 © IEC:1998(E) – 19 –
Table 2 – Fixed sample size test schedule for qualification approval of thermistors

for use as heating elements, assessment level EZ

Group Test Subclause of D Number of Permissible
No. (see list item 5) this standard or specimens number of non-
ND conforming items
nc
0 Visual examination 4.4.1 ND 70 + 5 0
Marking 4.4.2 (see list item 6)
Dimensions (gauging) 4.4.3
Zero-power resistance R 4.5
n
4.8
Voltage proof
Temperature coefficient of 4.6 D10 0
resistance (if specified) (see list item 7)
Final surface temperature 4.27
(if specified)
1 Maximum inrush power (if specified) 4.28
Residual current (if specified)
Soldering – Solderability 4.26
(see list items 9 and 10) 4.16.1
2A Soldering – Resistance to soldering 4.16.2 5
heat (see list items 9 and 10)
Robustness of terminations 4.15
2B Rapid change of temperature 4.17 5
Vibration (if specified, not for non- 4.18
insulated types)
Bump or shock (see list item 12) 4.19 or 4.20
2 Climatic sequence 4.21 D 10 0
3 Thermal time constant by cooling 4.14 ND 10 0
(if specified)
4 Dimensions (detail) 4.4.4 ND 10 0
Dissipation factor at U (if
.
max
4.10
specified)
5 Endurance at room temperature 4.23.1 D 10 0
(cycling)(see list item 13)
6 Endurance at max. operating 4.23.3 D 10 0
temperature and max. voltage
(see list item 13)
7 Damp heat, steady state 4.22 D 10 0

– 20 – 60738-1 © IEC:1998(E)
Table 3 – Fixed sample size test schedule for qualification approval of thermistors

for inrush current application, assessment level EZ

Group Test Subclause of D Number of Permissible number
No. (see list item 5) this standard or specimens of non-conforming
ND items
n c
0 Visual examination 4.4.1 ND 60 + 5 0

Marking 4.4.2 (see list item 6)

Dimensions (gauging) 4.4.3
Zero-power resistance R 4.5
n
Voltage proof 4.8
1 Inrush current 4.28 D10 0
Residual current 4.26 (see list item 7)
Soldering – Solderability (see list 4.16.1
items 9 and 10)
2A Soldering – Resistance to soldering 4.16.2 5
heat (see list items 9 and 10)
Robustness of terminations 4.15
2B Rapid change of temperature 4.17 5
Vibration 4.18
Bump or shock (see list item 12) 4.19 or 4.20
2 Climatic sequence 4.21 D 10 0
3 Dimensions (detail) (see list item 7) 4.4.4 ND 10 0
4 Endurance at room temperature 4.23.1 D 10 0
(cycling) (see list item 13)
5 Endurance at max. operating 4.23.3 D 10 0
temp
...