IEC 61709:2011

IEC 61709 ®
Edition 2.0 2011-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electric components – Reliability – Reference conditions for failure rates and
stress models for conversion
Composants électriques – Fiabilité – Conditions de référence pour les taux
de défaillance et modèles de contraintes pour la conversion

All rights reserved. Unless otherwise specified, 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 either IEC or
IEC's member National Committee in the country of the requester.
If you have any questions about IEC copyright or have an enquiry about obtaining additional rights to this publication,
please contact the address below or your local IEC member National Committee for further information.

Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite
ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie
et les microfilms, sans l'accord écrit de la CEI ou du Comité national de la CEI du pays du demandeur.
Si vous avez des questions sur le copyright de la CEI ou si vous désirez obtenir des droits supplémentaires sur cette
publication, utilisez les coordonnées ci-après ou contactez le Comité national de la CEI de votre pays de résidence.

IEC Central Office
3, rue de Varembé
CH-1211 Geneva 20
Switzerland
Email: inmail@iec.ch
Web: www.iec.ch
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigenda or an amendment might have been published.
 Catalogue of IEC publications: www.iec.ch/searchpub
The IEC on-line Catalogue enables you to search by a variety of criteria (reference number, text, technical committee,…).
It also gives information on projects, withdrawn and replaced publications.
 IEC Just Published: www.iec.ch/online_news/justpub
Stay up to date on all new IEC publications. Just Published details twice a month all new publications released. Available
on-line and also by email.
 Electropedia: www.electropedia.org
The world's leading online dictionary of electronic and electrical terms containing more than 20 000 terms and definitions
in English and French, with equivalent terms in additional languages. Also known as the International Electrotechnical
Vocabulary online.
 Customer Service Centre: www.iec.ch/webstore/custserv
If you wish to give us your feedback on this publication or need further assistance, please visit the Customer Service
Centre FAQ or contact us:
Email: csc@iec.ch
Tel.: +41 22 919 02 11
Fax: +41 22 919 03 00
A propos de la CEI
La Commission Electrotechnique Internationale (CEI) est la première organisation mondiale qui élabore et publie des
normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.

A propos des publications CEI
Le contenu technique des publications de la CEI est constamment revu. Veuillez vous assurer que vous possédez
l’édition la plus récente, un corrigendum ou amendement peut avoir été publié.
 Catalogue des publications de la CEI: www.iec.ch/searchpub/cur_fut-f.htm
Le Catalogue en-ligne de la CEI vous permet d’effectuer des recherches en utilisant différents critères (numéro de référence,
texte, comité d’études,…). Il donne aussi des informations sur les projets et les publications retirées ou remplacées.
 Just Published CEI: www.iec.ch/online_news/justpub
Restez informé sur les nouvelles publications de la CEI. Just Published détaille deux fois par mois les nouvelles
publications parues. Disponible en-ligne et aussi par email.
 Electropedia: www.electropedia.org
Le premier dictionnaire en ligne au monde de termes électroniques et électriques. Il contient plus de 20 000 termes et
définitions en anglais et en français, ainsi que les termes équivalents dans les langues additionnelles. Egalement appelé
Vocabulaire Electrotechnique International en ligne.
 Service Clients: www.iec.ch/webstore/custserv/custserv_entry-f.htm
Si vous désirez nous donner des commentaires sur cette publication ou si vous avez des questions, visitez le FAQ du
Service clients ou contactez-nous:
Email: csc@iec.ch
Tél.: +41 22 919 02 11
Fax: +41 22 919 03 00
IEC 61709 ®
Edition 2.0 2011-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electric components – Reliability – Reference conditions for failure rates and
stress models for conversion
Composants électriques – Fiabilité – Conditions de référence pour les taux
de défaillance et modèles de contraintes pour la conversion

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XD
ICS 31.020 ISBN 978-2-88912-551-7

– 2 – 61709 © IEC:2011
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 9
3 Terms, definitions and symbols . 10
3.1 Terms and definitons . 10
3.2 Symbols . 12
4 Context and conditions . 13
4.1 Failure modes . 13
4.2 Operating profile considerations . 14
4.3 Storage conditions . 14
4.4 Environmental conditions . 14
5 Generic reference conditions and stress models . 16
5.1 Recommended generic reference conditions . 16
5.2 Generic stress models . 17
5.2.1 General . 17
5.2.2 Stress factor for voltage dependence, π . 18
U
5.2.3 Stress factor for current dependence, π . 18
I
5.2.4 Stress factor for temperature dependence, π . 18
T
5.2.5 Environmental application factor, π . 20
E
5.2.6 Other factors of influence . 21
6 Specific reference conditions and stress models. 21
6.1 Integrated semiconductor circuits . 21
6.1.1 Reference conditions . 21
6.1.2 Stress factors . 23
6.2 Discrete semiconductors . 27
6.2.1 Reference conditions . 27
6.2.2 Stress factors . 28
6.3 Optoelectronic components . 32
6.3.1 Reference conditions . 32
6.3.2 Stress factors . 34
6.4 Capacitors . 38
6.4.1 Reference conditions . 38
6.4.2 Stress factors . 38
6.5 Resistors and resistor networks . 41
6.5.1 Reference conditions . 41
6.5.2 Stress factors . 42
6.6 Inductors, transformers and coils . 43
6.6.1 Reference conditions . 43
6.6.2 Stress factors . 43
6.7 Microwave devices . 44
6.7.1 Reference conditions . 44
6.7.2 Stress factors . 45
6.8 Other passive components . 45
6.8.1 Reference conditions . 45

61709 © IEC:2011 – 3 –
6.8.2 Stress factors . 45
6.9 Electrical connections. 45
6.9.1 Reference conditions . 45
6.9.2 Stress factors . 46
6.10 Connectors and sockets . 46
6.10.1 Reference conditions . 46
6.10.2 Stress factors . 46
6.11 Relays . 46
6.11.1 Reference conditions . 46
6.11.2 Stress factors . 47
6.12 Switches and push-buttons . 49
6.12.1 Reference conditions . 49
6.12.2 Stress factors . 50
6.13 Signal and pilot lamps . 51
6.13.1 Reference conditions . 51
6.13.2 Stress factors . 51
Annex A (normative) Failure modes of components . 53
Annex B (informative) Failure rate prediction . 55
Annex C (informative) Considerations for the design of a data base on failure rates . 65
Annex D (informative) Potential sources of failure rate data and methods of selection . 68
Annex E (informative) Overview of component classification . 74
Annex F (informative) Examples . 86
Bibliography . 88

Figure 1 – Selection of stress regions in accordance with current and voltage-operating
conditions . 48
Figure 2 – Selection of stress regions in accordance with current and voltage-operating
conditions . 50
Figure B.1 – Stress profile . 59
Figure B.2 – Averaging failure rates . 60

Table 1 – Basic environments . 15
Table 2 – Values of environmental parameters for basic environments . 15
Table 3 – Recommended reference conditions for environmental and mechanical
stresses . 17
Table 4 – Environmental application factor, π . 20
E
Table 5 – Memory . 21
Table 6 – Microprocessors and peripherals, microcontrollers and signal processors . 22
Table 8 – Analog integrated circuits (IC) . 23
Table 9 – Application-specific ICs (ASICs) . 23
Table 10 – Constants for voltage dependence . 24
Table 11– Factor π for digital CMOS-family ICs . 24
U
Table 12 – Factor π for bipolar analog ICs . 24
U
Table 13 – Constants for temperature dependence . 24

– 4 – 61709 © IEC:2011
for ICs (without EPROM; FLASH-EPROM; OTPROM; EEPROM;
Table 14 – Factor π
T
EAROM) . 26
Table 15 – Factor π for EPROM, FLASH-EPROM, OTPROM, EEPROM, EAROM. 26
T
Table 16 – Transistors common, low frequency. 27
Table 17 – Transistors, microwave, e.g. RF >800 MHz. 27
Table 18 – Diodes . 28
Table 19 – Power semiconductors . 28
Table 20 − Constants for voltage dependence of transistors . 29
Table 21 – Factor π for transistors . 29
U
Table 22 – Constants for temperature dependence of discrete semiconductors . 29
Table 23 – Factor π for transistors, reference and microwave diodes . 31
T
Table 24 – Factor for diodes (without reference and microwave diodes) and power
π
T
semiconductors . 31
Table 25 – Optoelectronic semiconductor signal receivers . 32
Table 26 – LEDs, IREDs, laser diodes and transmitter components . 33
Table 27 – Optocouplers and light barriers. 33
Table 28 – Passive optical components . 34
Table 29 – Transceiver, transponder and optical sub-equipment . 34
Table 30 – Constants for voltage dependence of phototransistors . 35
Table 31 – Factor π for phototransistors . 35
U
Table 32 – Constants for current dependence of LEDs and IREDs . 35
Table 33 – Factor π for LEDs and IREDs . 35
I
Table 34 – Constants for temperature dependence of optoelectronic components . 36
Table 35 – Factor π for optical components . 37
T
Table 36 – Capacitors . 38
Table 37 – Constants for voltage dependence of capacitors . 39
Table 38 – Factor π for capacitors . 39
U
Table 39 – Constants for temperature dependence of capacitors . 40
Table 40 – Factor π for capacitors . 41
T
Table 41 – Resistors and resistor networks . 42
Table 42 – Constants for temperature dependence of resistors . 42
Table 43 – Factor π for resistors . 43
Τ
Table 44 – Inductors, transformers and coils . 43
Table 45 – Constants for temperature dependence of inductors, transformers and coils . 43
Table 46 – Factor π for inductors, transformers and coils . 44
Τ
Table 47 – Microwave devices . 44
Table 48 – Other passive components . 45
Table 49 – Electrical connections. 46
Table 50 – Connectors and sockets . 46
Table 51 – Relays . 47
Table 52 – Factor π for low current relays. 48
ES
61709 © IEC:2011 – 5 –
Table 53 – Factor π for general purpose relays . 48
ES
Table 54 – Factor π for automotive relays . 49
ES
Table 55 – Constants for temperature dependence of relays . 49
Table 56 – Facteur π for relays . 49
T
Table 57 – Switches and push-buttons . 50
Table 58 – Factor π for switches and push-buttons for low electrical stress . 51
ES
Table 59 – Factor π for switches and push-buttons for higher electrical stress . 51
ES
Table 60 – Signal and pilot lamps . 51
Table 61 – Factor π for signal and pilot lamps . 52
U
Table A.1 – Failure modes – Integrated circuits (ICs)(digital) . 53
Table A.2 – Failure modes – Transistors, diodes, optocouplers . 53
Table A.3 – Failure modes – Capacitors . 54
Table A.4 – Failure modes – Resistors, inductive devices, relays . 54
Table C.1 – Reliability prediction database attributes . 66
Table D.1 – Sources of reliability data (in alphabetical order) . 70
Table E.1 – Classification tree (IEC 61360). 75

– 6 – 61709 © IEC:2011
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTRIC COMPONENTS –
RELIABILITY –
REFERENCE CONDITIONS FOR FAILURE RATES
AND STRESS MODELS FOR CONVERSION

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 itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
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 61709 has been prepared by IEC technical committee 56:
Dependability.
This second edition cancels and replaces the first edition, published in 1996 and constitutes a
technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
– the addition of a number of component types and the updating of models for a large
number of component types;
– the addition of annexes on reliability prediction, sources of failure rate data and
component classification information.

61709 © IEC:2011 – 7 –
The text of this standard is based on the following documents:
FDIS Report on voting
56/1422/FDIS 56/1431/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.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication 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.
– 8 – 61709 © IEC:2011
INTRODUCTION
This International Standard is intended for the reliability prediction of components as used in
equipment and is aimed at organizations that have their own data and describes how to state
and use that data in order to perform reliability predictions.
It can also be used to allow an organization to set up a failure rate database and describes
the reference conditions for which field failure rates should be stated. The reference
conditions adopted in this standard are typical of the majority of applications of components in
equipment however when components operate under other conditions the users may consider
stating these conditions as their reference conditions.
Using the presented stress models allows extrapolation of failure rates to other operating
conditions which in turn permits the prediction of failure rates at assembly level. This allows
estimation of the effect of design changes or changes in the environmental conditions on
component reliability. Reliability prediction is most useful in the early design phase of
electrical equipment. It can be used, for example, to identify potential reliability problems, the
planning of logistic support strategies and the evaluation of designs.
The stress models contained herein are generic and are as simple as possible while still being
comparable with more complex equations contained in other models.
This standard does not contain failure rates, but it describes how they can be stated and
used. This approach allows a user to select the most relevant and up to date failure rates for
the prediction from a source that they select. This standard also contains information on how
to select the data that can be used in the presented models.

61709 © IEC:2011 – 9 –
ELECTRIC COMPONENTS –
RELIABILITY –
REFERENCE CONDITIONS FOR FAILURE RATES
AND STRESS MODELS FOR CONVERSION

1 Scope
This International Standard gives guidance on how failure rate data can be employed for
reliability prediction of electric components in equipment.
Reference conditions are numerical values of stresses that are typically observed by
components in the majority of applications. Reference conditions are useful since they are the
basis of the calculation of failure rate under any conditions by the application of stress models
that take into account the actual operating conditions. Failure rates stated at reference
conditions allow realistic reliability predictions to be made in the early design phase.
The stress models described herein are generic and can be used as a basis for conversion of
the failure rate data at these reference conditions to actual operating conditions when needed
and this simplifies the prediction approach. Conversion of failure rate data is only permissible
within the specified functional limits of the components.
This standard also gives guidance on how a database of component failure data can be
constructed to provide failure rates that can be used with the included stress models.
Reference conditions for failure rate data are specified, so that data from different sources
can be compared on a uniform basis. If failure rate data are given in accordance with this
International Standard then no additional information on the specified conditions is required.
This standard does not provide base failure rates for components – rather it provides models
that allow failure rates obtained by other means to be converted from one operating condition
to another operating condition.
The prediction methodology described in this standard assumes that the parts are being used
within its useful life. The methods in this standard have a general application but are
specifically applied to a selection of component types as defined in Clause 6 and Clause E.2.
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 60050-191, International Electrotechnical Vocabulary – Part 191: Dependability and
quality of service
IEC 60605-6, Equipment reliability testing – Part 6: Tests for the validity and estimation of the
constant failure rate and constant failure intensity
IEC 60721-3-3, Classification of environmental conditions – Part 3: Classification of groups of
environmental parameters and their severities – Section 3: Stationary use at weather
protected locations
– 10 – 61709 © IEC:2011
IEC 60721-3-4, Classification of environmental conditions – Part 3: Classification of groups of
environmental parameters and their severities – Section 4: Stationary use at non-
weatherprotected locations
IEC 60721-3-5, Classification of environmental conditions – Part 3: Classification of groups of
environmental parameters and their severities – Section 4: Ground vehicle installations
IEC 60721-3-7, Classification of environmental conditions – Part 3: Classification of groups of
environmental parameters and their severities – Section 7: Portable and non-stationary use
3 Terms, definitions and symbols
3.1 Terms and definitons
For the purpose of this document, the terms and definitions of IEC 60050-191, as well as the
following terms and definitions apply.
3.1.1
electric component
component with conductive terminals through which voltages or currents may be applied or
delivered
[IEC 61360-1:2009, 2.18]
NOTE The term electric component includes the commonly used terms “electronic component”, “electrical
component” and “electro-mechanical component”.
3.1.2
failure (of an item)
loss of ability to perform as required
NOTE 1 When the loss of ability is caused by a pre-existing latent fault, the failure occurs when a particular set of
circumstances is encountered.
NOTE 2 A failure of an item is an event that results in a fault in that item, which is a state.
3.1.3
failure mode
manner in which failure occurs
NOTE A failure mode may be defined by the function lost or the state transition that occurred.
3.1.4
instantaneous failure rate
failure rate
limit, if it exists, of the ratio of the conditional probability that the instant of a failure of a non-
repairable item occurs within time interval (t, t + ∆t) to ∆t when ∆t tends to zero, given that it
has not failed within time interval (0,t)
NOTE 1 The instantaneous failure rate, λ(t), is expressed by the formula:
1 F(t + ∆t)− F(t) f (t)
λ(t) = lim =
∆t R(t) R(t)
∆t→0
where F(t) and f(t) are respectively the distribution function and the probability density of the failure instant, and
where R(t) is the reliability function, related to the reliability R(t , t ) by R(t) = R(0, t).
1 2
NOTE 2 See IEC 61703.
61709 © IEC:2011 – 11 –
NOTE 3 Other terms used for instantaneous failure rate are “hazard function”, “hazard rate” and “force of
mortality” (abbreviation FOM).
NOTE 4 In this standard λ(t) is assumed to be constant over time.
3.1.5
reference conditions
stresses selected so as to correspond to the majority of applications and usage of
components in equipment
NOTE Stresses include electrical stress, temperature and environmental conditions
3.1.6
reference failure rate
failure rate stated under reference conditions given in this standard
NOTE The reference failure rate is specific to the component, i.e. it includes the effect of complexity, technology
of the casing, dependence on manufacturers and the manufacturing process, etc.
3.1.7
duty cycle
specified sequence of operating condition
[IEC 60050-151:2001, 151-16-02]
NOTE The duty cycle states whether components are continuously or intermittently stressed during their
operation. Continuous duty means operation for a long duration with constant or changing loads (e.g. process
controls, telephone switch). Intermittent duty means operation with constant or changing loads during up state (e.g.
numerical controls for machinery, road traffic signals).
3.1.8
prediction
computation process used to obtain the predicted value of a quantity
NOTE The term "prediction" may also be used to denote the predicted value of a quantity.
3.1.9
component
constituent part of a device which cannot be physically divided into smaller parts without
losing its particular function
[IEC 60050:2001, 151-11-21]
3.1.10
equipment
single apparatus or set of devices or apparatuses, or the set of main devices of an
installation, or all devices necessary to perform a specific task
NOTE Examples of equipment are a power transformer, the equipment of a substation, or measuring equipment.
[IEC 60050-151:2001, 151-11-25]
3.1.11
useful life
time interval, from first use until user requirements are no longer met, due to economics or
obsolescence
3.1.12
drift
difference between the final value of a characteristic at the end of a specified period and the
initial value, all other operating conditions being held constant

– 12 – 61709 © IEC:2011
NOTE The use of the term "drift" to refer to the immediate change of a characteristic in direct response to
changed operating conditions (for example, temperature) is deprecated.
[IEC 60747-1:2006, 3.6.1, modified]
3.1.13
virtual temperature
internal equivalent temperature (of a semiconductor device)
theoretical temperature which is based on a simplified representation of the thermal and
electrical behaviour of the semiconductor device
[IEC 60050-521: 2002, 521-05-14, modified]
3.1.14
virtual (equivalent) junction temperature
virtual temperature of the junction of a semiconductor device
[IEC 60050-521:2002, 521-05-15]
NOTE The virtual temperature is not necessarily the highest temperature in the device.
3.2 Symbols
In this standard, the following symbols are used:
S number of operating cycles per hour
E activation energy of a failure process

a
I operating current
op
I rated current
rat
I reference current
ref
P operating power dissipation
op
P rated power dissipation
rat
P reference power dissipation
ref
R(t ,t ) reliability between two times t and t

1 2 1 2
R thermal resistance
th
R thermal resistance (to the environment)
th,amb
S reference number of operating cycle per hour

ref
T ambient temperature in Kelvin
amb
T reference ambient temperature in Kelvin
T reference temperature in Kelvin
ref
U operating voltage
op
U rated voltage
rat
U reference voltage
ref
t fraction of time with part stress for an assembly

p
t fraction of time spent idle for an assembly

i
t fraction of time with full stress for an assembly

f
β shape parameter of the Weibull distribution
∆T actual self-heating in degrees Celsius
reference self-heating in degrees Celsius
∆T
ref
ambient temperature in degrees Celsius
θ
amb
– for capacitors the actual capacitor temperature;
– for discrete semiconductors and optoelectronic components the actual junction

61709 © IEC:2011 – 13 –
temperature;
– for inductors the actual winding temperature;
– for integrated circuits (ICs) the actual virtual (equivalent) junction temperature;
– for other electric components the actual ambient temperature;

– for resistors the actual resistor element temperature;
operating temperature in degrees Celsius
θ
op
θ reference ambient temperature in degrees Celsius
θ reference temperature in degrees Celsius
ref
– for capacitors the reference temperature of the capacitor;

– for discrete semiconductors and optoelectronic components the reference
junction temperature;
– for inductors the reference temperature of the winding;
– for ICs the reference virtual (equivalent) junction temperature;

– for other electric components the reference temperature of the component;
– for resistors the reference temperature of the resistor element.
λ failure rate at full stress for an assembly:
f
λ failure rate at part stress for an assembly;
p
failure rate during time spent idle for an assembly;
λ
i
failure rate of a component;
λ
component
failure rate of a components failure mode;
λ
mode
failure rate of a system;
λ
S
failure rate under operating conditions;
λ
failure rate under reference conditions;
λ
ref
π current dependence factor;
I
π electrical stress dependence factor;
ES
environmental application factor;
π
E
stress factor for operating profile;
π
op
π switching rate dependence factor;
S
π temperature dependence factor;
T
voltage dependence factor.
π
U
4 Context and conditions
4.1 Failure modes
The characteristic preferred for reliability data of electric components is the (instantaneous)
failure rate. It is to be noted that, although it is often generically defined as failure, the exact
observed event that is measured is a failure mode.
In equipment a failure (mode) or functional loss is caused by a component failure mode where
that component failure mode is relevant to the application being carried out by the equipment.
It should be noted that a component has many features and only some may be used in the
specific application. A function loss at the equipment level occurs only when there is a loss of
the component feature that is used to support that function.
Furthermore a circuit requires the presence of component features according to what was
defined by the designer; this may not encompass the total feature set of the component and

– 14 – 61709 © IEC:2011
may not use a particular feature to its full capacity as defined by the data sheet in terms of
functional characteristics and ratings.
Handbooks usually define failure rate as an overall value, which includes all failure modes.
This implies that component failure rate can be considered as the sum of the failure rates of
all the modes, as follows:
n
λ = (λ )
(1)
component mode i
∑
i=1
where (λ ) is the component failure rate in which the failure mode i occurs and n is the
mode
i
number of failure modes.
Failure modes are listed in Annex A and more details about failures are contained in Annex B.
4.2 Operating profile considerations
One of the major factors affecting component reliability is operating profile. This will vary
according to the type of operation that is undertaken. This operation may be continuous over
time at a fixed level, continuous over time at a variable level or sporadic over time at either a
fixed level or a variable level. In some cases switch on and switch off could be significant and
of more importance than the steady state operational conditions. Careful consideration of the
operating profile is needed in order to fully understand how it affects the component reliability.
The operating profile can be considered to be based on calendar time or on the time of actual
operation or it can be cycle based (e.g. how many times an item is used).
4.3 Storage conditions
Components that are under storage conditions are not immune from failure. However the
stress models for environmental application factors in this standard may not apply since they
only deal with operating conditions. Different failure mechanisms may exist under storage
conditions that have not been considered in the models.
Storage conditions should be treated separately from operating conditions. They may affect
the components’ failure behaviour in later life.
4.4 Environmental conditions
The environment contributes to failure that occurs in the life of the equipment. As a
consequence the duration and intensity of environmental stresses should be included in the
operational model of the equipment.
A more severe environment may cause the failures to occur more frequently than one that is
less severe. There will usually be several aspects of the environment that will be pertinent to
a specific failure and all may need to be understood. The locality of the environment is also
important, for example on an aircraft the in-cabin and on-engine environments are very
different.
The environment may be described in terms of several types of parameters. IEC 60721-3-3
describes the environment in terms of
– climatic conditions,
– special climatic conditions,
– biological conditions,
– chemically active substances,

61709 © IEC:2011 – 15 –
– mechanically active substances,
– mechanical conditions (both static and dynamic).
All the above listed conditions are in general relevant to the equipment’s reliability (failure
rate) and it is reasonable to consider that the reliability of components, and therefore of the
equipment, decreases as the environmental stress increases (see the IEC 60721 series for a
detailed quantitative descriptions of the environmental parameters for each environment).
In this standard it is assumed that the climatic and mechanical conditions are the most
significant
...