IEC 60738-1:2022

IEC 60738-1 ®
Edition 4.0 2022-10
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 4.0 2022-10
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 978-2-8322-5731-9

– 2 – IEC 60738-1:2022 © IEC 2022
CONTENTS
FOREWORD . 7
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 10
4 General requirements . 19
4.1 Units and symbols. 19
4.2 Preferred values . 20
4.2.1 Climatic categories . 20
4.2.2 Spacings . 20
4.3 Marking . 20
4.3.1 General . 20
4.3.2 Coding . 21
5 General provisions for measurements and test method . 21
5.1 General . 21
5.2 Standard conditions for testing . 21
5.2.1 Standard atmospheric conditions for testing . 21
5.2.2 Error of measurement . 21
5.3 Drying and recovery . 21
5.3.1 Drying . 21
5.3.2 Recovery . 22
5.3.3 Shock test severities . 22
5.3.4 Vibration severities . 22
5.4 Mounting (for surface mount thermistors only) . 22
5.4.1 General . 22
5.4.2 Printed wiring board and land pattern . 23
5.4.3 Wave soldering method . 23
5.4.4 Reflow soldering method . 23
6 Electrical tests and measurements . 24
6.1 Zero-power resistance . 24
6.1.1 General . 24
6.1.2 Measurement procedures . 24
6.1.3 Requirements . 25
6.2 Temperature coefficient of resistance . 25
6.2.1 General . 25
6.2.2 Requirements . 25
6.3 Insulation resistance (for insulated types only) . 25
6.3.1 General . 25
6.3.2 Test methods . 25
6.3.3 Applied voltage . 26
6.3.4 Requirements . 26
6.4 Voltage proof (for insulated types only) . 26
6.4.1 General . 26
6.4.2 Test methods . 26
6.4.3 Test voltage . 26
6.4.4 Requirements . 26
6.5 Resistance/temperature characteristic . 26

6.5.1 Test methods . 26
6.5.2 Requirements . 27
6.6 Dissipation factor at U (δ) . 27
max
6.6.1 General . 27
6.6.2 Preconditioning . 27
6.6.3 Test circuit . 27
6.6.4 Test methods . 27
6.6.5 Requirements . 28
6.7 Response time by ambient temperature change (t ) . 28
a
6.7.1 General . 28
6.7.2 Preconditioning . 28
6.7.3 Test procedures . 28
6.7.4 Measurement procedures . 29
6.7.5 Requirements . 29
6.8 Response time by power change (t ) . 29
p
6.8.1 Mounting . 29
6.8.2 Test methods . 29
6.9 Thermal time constant by ambient temperature change (τ ) . 30
a
6.10 Thermal time constant by cooling (τ ) . 30
c
6.10.1 Mounting . 30
6.10.2 Test methods . 30
6.10.3 Requirements . 32
6.11 Tripping current and tripping time. 32
6.12 Non-tripping current . 32
6.13 Residual current. 32
6.14 Surface temperature . 33
6.15 Inrush current . 34
6.15.1 Measuring circuit . 34
6.15.2 Measuring method . 34
6.16 Energy by discharge . 35
6.16.1 Measuring circuit . 35
6.16.2 Measuring method . 35
7 Mechanical test and measurements . 36
7.1 Visual examination and check of dimensions . 36
7.1.1 Visual examination . 36
7.1.2 Marking . 36
7.1.3 Dimensions (gauging) . 36
7.1.4 Dimensions (detail) . 36
7.2 Robustness of terminations . 36
7.2.1 General . 36
7.2.2 Test Ua – Tensile . 36
7.2.3 Test Ub – Bending . 37
7.2.4 Test Uc – Torsion . 37
7.2.5 Visual examination . 37
7.2.6 Final measurements and requirements . 37
7.3 Vibration . 37
7.4 Shock . 38
7.5 Shear (adhesion) test . 38

– 4 – IEC 60738-1:2022 © IEC 2022
7.5.1 Test conditions . 38
7.5.2 Test methods . 38
7.5.3 Requirements . 38
7.6 Substrate bending test . 38
7.6.1 Mounting . 38
7.6.2 Initial measurements . 39
7.6.3 Test procedures . 39
7.6.4 Measurements . 39
7.6.5 Requirements . 39
7.6.6 Final inspection and requirements . 39
8 Environmental and climatic tests . 39
8.1 Rapid change of temperature . 39
8.2 Climatic sequence. 39
8.2.1 General . 39
8.2.2 Initial measurements . 39
8.2.3 Dry heat . 40
8.2.4 Damp heat (cyclic), first cycle . 40
8.2.5 Cold. 40
8.2.6 Low air pressure . 40
8.2.7 Damp heat (cyclic), remaining cycles . 40
8.2.8 Final measurements . 40
8.3 Damp heat, steady state . 41
8.4 Endurance . 41
8.4.1 Endurance at room temperature (cycling) . 41
8.4.2 Endurance at upper category temperature . 42
8.4.3 Endurance at maximum operating temperature and maximum voltage . 43
8.4.4 Cold environment electrical cycling . 44
8.4.5 Thermal runaway . 45
9 Test related to component assembly . 45
9.1 Solderability . 45
9.1.1 General . 45
9.1.2 Thermistors with leads . 45
9.1.3 Surface mount thermistors . 46
9.2 Resistance to soldering heat . 46
9.2.1 General . 46
9.2.2 Test conditions . 46
9.2.3 The period of recovery . 47
9.2.4 Requirements for except surface mount thermistors . 47
9.2.5 Requirements for surface mount thermistors . 47
10 Quality assessment procedures . 47
Annex A (informative) Mounting for electrical measurements . 48
A.1 Mounting for thermistors without wire terminations . 48
A.2 Mounting for thermistors with wire terminations . 48
A.3 Mounting for surface mount thermistors . 49
Annex B (informative) Mounting for temperature measurements . 51
Annex Q (normative) Quality assessment procedures . 52
Q.1 General . 52
Q.2 Applicability of qualification approval . 52

Q.3 Primary stage of manufacture . 52
Q.4 Subcontracting . 52
Q.5 Structurally similar components . 52
Q.5.1 General . 52
Q.5.2 For electrical tests . 53
Q.5.3 For environmental tests . 53
Q.5.4 For visual inspection . 53
Q.5.5 For endurance tests . 53
Q.6 Qualification approval procedures . 53
Q.6.1 Eligibility for qualification approval . 53
Q.6.2 Application for qualification approval . 53
Q.6.3 Test procedure for qualification approval . 53
Q.6.4 Qualification approval based on the fixed sample size procedure. 54
Q.6.5 Granting of qualification approval . 59
Q.6.6 Maintenance of qualification approval . 59
Q.6.7 Quality conformance inspection . 60
Q.7 Rework and repair. 61
Q.7.1 Rework . 61
Q.7.2 Repair . 61
Q.8 Release for delivery . 61
Q.8.1 General . 61
Q.8.2 Release for delivery under qualification approval before the completion
of group B tests . 62
Q.9 Certified test records of released lots . 62
Q.10 Delayed delivery . 62
Q.11 Alternative test methods . 62
Q.12 Manufacture outside the geographical limits . 62
Q.13 Unchecked parameters . 62
Annex X (informative) Cross-references to IEC 60738-1:2009 . 63
Bibliography . 65

Figure 1 – Typical resistance-temperature characteristic and definitions for PTC
thermistors (at zero power) . 12
Figure 2 – Typical R-TNF characteristic for PTC thermistors in sensor applications . 13
Figure 3 – Typical current/voltage characteristic for PTC thermistors . 14
Figure 4 – I against t at U . 17
in dc
Figure 5 – I against t at U . 17
in RMS
Figure 6 – Dissipation factor test circuit . 28
Figure 7 – Temperature gradient . 29
Figure 8 – Circuit for measurement of thermal time constant by cooling . 30
Figure 9 – Circuit for surface temperature measurement . 33
Figure 10 – Measuring circuit . 34
Figure 11 – Measuring circuit . 35
Figure 12 – Circuit for endurance at maximum operating temperature and maximum
voltage . 44
Figure A.1 – Example of a preferred mounting method for thermistors without wire
terminations . 48

– 6 – IEC 60738-1:2022 © IEC 2022
Figure A.2 – Example of a preferred mounting method for thermistors with wire
terminations . 49
Figure A.3 – Example of a preferred mounting method for surface mount thermistors . 50
Figure B.1 – Example of a preferred mounting method for temperature measurement
on cylindrical heating elements . 51

Table 1 – Creepage distances and clearances . 20
Table 2 – Test severities of vibration . 22
Table 3 – Tensile force . 37
Table 4 – Number of cycles per climatic category . 40
Table A.1 – Recommended land dimensions . 50
Table Q.1 – Fixed sample size test schedule for qualification approval of thermistors
for current limitation – Assessment level EZ . 56
Table Q.2 – Fixed sample size test schedule for qualification approval of thermistors
for use as heating elements – Assessment level EZ . 57
Table Q.3 – Fixed sample size test schedule for qualification approval of thermistors
for inrush current application – Assessment level EZ . 58
Table Q.4 – Fixed sample size test schedule for qualification approval of thermistors
for use as temperature sensing elements – Assessment level EZ . 59
Table Q.5 – Quality conformance inspection for lot-by-lot inspection . 60
Table Q.6 – Quality conformance inspection for periodic testing . 61
Table X.1 – Cross-references to the previous edition . 63

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
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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
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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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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.
IEC 60738-1 has been prepared by IEC technical committee 40: Capacitors and resistors for
electronic equipment. It is an International Standard.
This fourth edition cancels and replaces the third edition published in 2006 and
Amendment 1:2009. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Restructured completely to comply with ISO/IEC Directives; categorization and
reorganization of test methods into these categories;
b) Some wordings, figures and references have been revised.

– 8 – IEC 60738-1:2022 © IEC 2022
The text of this International Standard is based on the following documents:
Draft Report on voting
40/2969/FDIS 40/2977/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
A list of all parts in the IEC 60738 series, published under the general title Thermistors – Directly
heated positive temperature coefficient, can be found on the IEC website.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document 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.

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 documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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) (available at
www.electropedia.org)
IEC 60062, Marking codes for resistors and capacitors
IEC 60068-1:2013, Environmental testing – Part 1: General and guidance
IEC 60068-2-1:2007, Environmental testing – Part 2-1: Tests – Test A: Cold
IEC 60068-2-2:2007, Environmental testing – Part 2-2: Tests – Test B: Dry heat
IEC 60068-2-6, Environmental testing – Part 2-6: Tests – Test Fc: Vibration (sinusoidal)
IEC 60068-2-11, Environmental testing – Part 2-11: Tests – Test Ka: Salt mist
IEC 60068-2-13, Environmental testing – Part 2-13: Tests – Test M: Low air pressure
IEC 60068-2-14, Environmental testing – Part 2-14: Tests – Test N: Change of temperature
Amendment 1 (1986)
IEC 60068-2-20, Environmental testing – Part 2-20: Tests – Test Ta and Tb: Test methods for
solderability and resistance to soldering heat of devices with leads
IEC 60068-2-21, Environmental testing – Part 2-21: Tests – Test U: Robustness of terminations
and integral mounting devices
IEC 60068-2-27, Environmental testing – Part 2-27: Tests – Test Ea and guidance: Shock

– 10 – IEC 60738-1:2022 © IEC 2022
IEC 60068-2-30, Environmental testing – Part 2-30: Tests – Test Db: Damp heat, cyclic
(12 h + 12-hour cycle)
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 with axial terminations
IEC 60617, Graphical symbols for diagrams (available at http://std.iec.ch/iec60617)
IEC 60717, Method for determination of the space required by capacitors and resistors with
unidirectional terminations
IEC 61193-2, Quality assessment systems – Part 2: Selection and use of sampling plans for
inspection of electronic components and packages
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)
ISO 80000-1, Quantities and units – Part 1: General
3 Terms and definitions
For the purposes of this document and subordinate specifications, the following terms and
definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1
type
group of components having similar design features and whose similar manufacturing
techniques enable them to be grouped together either for qualification approval or for quality
conformance inspection
Note 1 to entry: A type is generally covered by a single detail specification.
Note 2 to entry: Components described in several detail specifications can, in some cases, be considered as
belonging to the same type, but they are generally covered by a single detail specification.
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 whose resistance 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
Note 1 to entry: A PTC thermistor will show secondary effects which are to be considered.
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 1 to entry: Any resistance value of a PTC thermistor is dependent on the value and the mode of the applied
voltage (AC or DC) and, when an AC source is used, on the frequency (see 3.8 and 3.9).
3.7
nominal zero-power resistance
R
n
DC 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
Note 1 to entry: Zero-power resistance can also be measured using AC 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 when the frequency increases
3.10
resistance/temperature characteristic
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 1 to entry: PTC thermistors can 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
pulse
than 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
– 12 – IEC 60738-1:2022 © IEC 2022

Figure 1 – Typical resistance-temperature characteristic
and definitions for PTC thermistors (at zero power)

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 (DC or
AC) at the thermistor terminations and the current under steady-state conditions (see Figure 3)

– 14 – IEC 60738-1:2022 © IEC 2022

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
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 1 to entry: T is exclusively defined for PTC resistors in sensor applications
...