IEC 62506:2013
(Main)Methods for product accelerated testing
Methods for product accelerated testing
IEC 62506:2013 provides guidance on the application of various accelerated test techniques for measurement or improvement of product reliability. Identification of potential failure modes that could be experienced in the use of a product/item and their mitigation is instrumental to ensure dependability of an item. The object of the methods is to either identify potential design weakness or provide information on item dependability, or to achieve necessary reliability/availability improvement, all within a compressed or accelerated period of time. This standard addresses accelerated testing of non-repairable and repairable systems. It can be used for probability ratio sequential tests, fixed duration tests and reliability improvement/growth tests, where the measure of reliability may differ from the standard probability of failure occurrence. This standard also extends to present accelerated testing or production screening methods that would identify weakness introduced into the product by manufacturing error, which could compromise product dependability. Keywords: test techniques for measurement or improvement of product reliability
Méthodes d'essais accélérés de produits
La CEI 62506:2013 fournit des recommandations pour l'application de diverses techniques d'essais accélérés permettant de mesurer ou d'améliorer la fiabilité des produits. L'identification des modes de défaillance potentiels qui pourraient être rencontrés lors de l'utilisation d'un produit/entité donné(e) et la manière d'y remédier contribuent à assurer la sûreté de fonctionnement d'une entité. L'objectif est soit d'identifier les faiblesses potentielles de la conception et fournir des informations sur la sûreté de fonctionnement de l'entité, soit d'atteindre l'amélioration nécessaire de la fiabilité/disponibilité, dans les deux cas en comprimant ou en accélérant la période d'essai requise. La présente norme couvre les essais accélérés de systèmes non réparables et réparables. Elle peut être utilisée pour des essais progressifs à taux de probabilité, des essais à durée fixe et des essais d'amélioration/croissance de la fiabilité, lorsque la mesure de la fiabilité peut être différente de la probabilité normalisée d'occurrence de défaillance. La présente norme décrit également des méthodes d'essais accélérés ou de déverminage de la production qui permettraient d'identifier les faiblesses induites par une éventuelle erreur de fabrication du produit et qui risqueraient de ce fait d'en compromettre la sûreté de fonctionnement. Mots clés: techniques d'essais permettant de mesurer ou d'améliorer la fiabilité des produits
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IEC 62506 ®
Edition 1.0 2013-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Methods for product accelerated testing
Méthodes d'essais accélérés de produits
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IEC 62506 ®
Edition 1.0 2013-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Methods for product accelerated testing
Méthodes d'essais accélérés de produits
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XD
ICS 03.120.01; 21.020 ISBN 978-2-83220-861-8
– 2 – 62506 © IEC:2013
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms, definitions, symbols and abbreviations . 9
3.1 Terms and definitions . 9
3.2 Symbols and abbreviated terms . 11
4 General description of the accelerated test methods. 12
4.1 Cumulative damage model . 12
4.2 Classification, methods and types of test acceleration . 14
4.2.1 General . 14
4.2.2 Type A: qualitative accelerated tests . 15
4.2.3 Type B: quantitative accelerated tests . 15
4.2.4 Type C: quantitative time and event compressed tests . 16
5 Accelerated test models . 17
5.1 Type A, qualitative accelerated tests . 17
5.1.1 Highly accelerated limit tests (HALT) . 17
5.1.2 Highly accelerated stress test (HAST) . 21
5.1.3 Highly accelerated stress screening/audit (HASS/HASA) . 21
5.1.4 Engineering aspects of HALT and HASS . 22
5.2 Type B and C – Quantitative accelerated test methods . 23
5.2.1 Purpose of quantitative accelerated testing . 23
5.2.2 Physical basis for the quantitative accelerated Type B test methods . 23
5.2.3 Type C tests, time (C ) and event (C ) compression . 24
1 2
5.3 Failure mechanisms and test design . 26
5.4 Determination of stress levels, profiles and combinations in use and test –
stress modelling . 27
5.4.1 General . 27
5.4.2 Step-by-step procedure . 27
5.5 Multiple stress acceleration methodology – Type B tests . 27
5.6 Single and multiple stress acceleration for Type B tests . 30
5.6.1 Single stress acceleration methodology . 30
5.6.2 Stress models with stress varying as a function of time – Type B
tests . 37
5.6.3 Stress models that depend on repetition of stress applications –
Fatigue models . 38
5.6.4 Other acceleration models – Time and event compression. 40
5.7 Acceleration of quantitative reliability tests . 40
5.7.1 Reliability requirements, goals, and use profile . 40
5.7.2 Reliability demonstration or life tests . 42
5.7.3 Testing of components for a reliability measure . 47
5.7.4 Reliability measures for components and systems/items . 48
5.8 Accelerated reliability compliance or evaluation tests . 48
5.9 Accelerated reliability growth testing . 50
5.10 Guidelines for accelerated testing . 50
5.10.1 Accelerated testing for multiple stresses and the known use profile . 50
5.10.2 Level of accelerated stresses . 51
62506 © IEC:2013 – 3 –
5.10.3 Accelerated reliability and verification tests . 51
6 Accelerated testing strategy in product development . 51
6.1 Accelerated testing sampling plan . 51
6.2 General discussion about test stresses and durations . 52
6.3 Testing components for multiple stresses . 53
6.4 Accelerated testing of assemblies . 53
6.5 Accelerated testing of systems . 53
6.6 Analysis of test results . 53
7 Limitations of accelerated testing methodology . 53
Annex A (informative) Highly accelerated limit test (HALT) . 55
Annex B (informative) Accelerated reliability compliance and growth test design . 59
Annex C (informative) Comparison between HALT and conventional accelerated
testing . 74
Annex D (informative) Estimating the activation energy, E . 75
a
Annex E (informative) Calibrated accelerated life testing (CALT) . 77
Annex F (informative) Example on how to estimate empirical factors . 79
Annex G (informative) Determination of acceleration factors by testing to failure . 84
Bibliography . 87
Figure 1 – Probability density functions (PDF) for cumulative damage, degradation,
and test types . 13
Figure 2 – Relationship of PDFs of the product strength vs. load in use . 18
Figure 3 – How uncertainty of load and strength affects the test policy . 19
Figure 4 – PDFs of operating and destruct limits as a function of applied stress . 20
Figure 5 – Line plot for Arrhenius reaction model . 34
Figure 6 – Plot for determination of the activation energy . 35
Figure 7 – Multiplier of the test stress duration for demonstration of required reliability
for compliance or reliability growth testing . 45
Figure 8 – Multiplier of the duration of the load application for the desired reliability . 46
Figure B.1 – Reliability as a function of multiplier k and for combinations of parameters
a and b . 61
Figure B.2 – Determination of the multiplier k . 64
Figure B.3 – Determination of the growth rate . 73
Figure D.1 – Plotting failures to estimate the activation energy E . 76
a
Figure F.1 – Weibull graphical data analysis . 81
Figure F.2 – Scale parameter as a function of the temperature range . 82
Figure F.3 – Probability of failure as a function of number of cycles ∆T = 50 °C . 83
Figure G.1 – Weibull plot of the three data sets . 85
Figure G.2 – Scale parameters’ values fitted with a power line . 86
Table 1 – Test types mapped to the product development cycle . 14
Table A.1 – Summary of HALT test results for a DC/DC converter . 56
Table A.2 – Summary of HALT results from a medical system . 57
Table A.3 – Summary of HALT results for a Hi-Fi equipment . 58
Table B.1 – Environmental stress conditions of an automotive electronic device . 63
– 4 – 62506 © IEC:2013
Table B.2 – Product use parameters . 67
Table B.3 – Assumed product use profile . 71
Table B.4 – Worksheet for determination of use times to failures . 72
Table B.5 – Data for reliability growth plotting . 73
Table C.1 – Comparison between HALT and conventional accelerated testing . 74
Table F.1 − Probability of failure of test samples A and B . 80
Table F.2 – Data transformation for Weibull plotting . 80
Table G.1 – Voltage test failure data for Weibull distribution . 84
62506 © IEC:2013 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
METHODS FOR PRODUCT ACCELERATED TESTING
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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International Standard IEC 62506 has been prepared by IEC technical committee 56:
Dependability.
The text of this standard is based on the following documents:
FDIS Report on voting
56/1503/FDIS 56/1513/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.
– 6 – 62506 © IEC:2013
The committee has decided that the contents of this publication will remain unchanged until
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related to the specific publication. At this date, the publication will be
• reconfirmed,
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IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
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62506 © IEC:2013 – 7 –
INTRODUCTION
Many reliability or failure investigation test methods have been developed and most of them
are currently in use. These methods are used to either determine product reliability or to
identify potential product failure modes, and have been considered effective as
demonstrations of reliability:
– fixed duration,
– sequential probability ratio,
– reliability growth tests,
– tests to failure, etc.
Such tests, although very useful, are usually lengthy, especially when the product reliability
that has to be demonstrated was high. The reduction in time-to-market periods as well as
competitive product cost, increase the need for efficient and effective accelerated testing.
Here, the tests are shortened through the application of increased stress levels or by
increasing the speed of application of repetitive stresses, thus facilitating a quicker
assessment and growth of product reliability through failure mode discovery and mitigation.
There are two distinctly different approaches to reliability activities:
– the first approach verifies, through analysis and testing, that there are no potential failure
modes in the product that are likely to be activated during the expected life time of the
product under the expected operating conditions;
– the second approach estimates how many failures can be expected after a given time
under the expected operating conditions.
Accelerated testing is a method appropriate for both cases, but used quite differently. The first
approach is associated with qualitative accelerated testing, where the goal is identification of
potential faults that eventually might result in product field failures. The second approach is
associated with quantitative accelerated testing where the product reliability may be estimated
based on the results of accelerated simulation testing that can be related back to the use of
the environment and usage profile.
Accelerated testing can be applied to multiple levels of items containing hardware or software.
Different types of reliability testing, such as fixed duration, sequential test-to-failure, success
test, reliability demonstration, or reliability growth/improvement tests can be candidates for
accelerated methods. This standard provides guidance on selected, commonly used
accelerated test types. This standard should be used in conjunction with statistical test plan
standards such as IEC 61123, IEC 61124, IEC 61649 and IEC 61710.
The relative merits of various methods and their individual or combined applicability in
evaluating a given system or item, should be reviewed by the product design team (including
dependability engineering) prior to selection of a specific test method or a combination of
methods. For each method, consideration should also be given to the test time, results
produced, credibility of the results, data required to perform meaningful analysis, life cycle
cost impact, complexity of analysis and other identified factors.
– 8 – 62506 © IEC:2013
METHODS FOR PRODUCT ACCELERATED TESTING
1 Scope
This International Standard provides guidance on the application of various accelerated test
techniques for measurement or improvement of product reliability. Identification of potential
failure modes that could be experienced in the use of a product/item and their mitigation is
instrumental to ensure dependability of an item.
The object of the methods is to either identify potential design weakness or provide
information on item dependability, or to achieve necessary reliability/availability improvement,
all within a compressed or accelerated period of time. This standard addresses accelerated
testing of non-repairable and repairable systems. It can be used for probability ratio
sequential tests, fixed duration tests and reliability improvement/growth tests, where the
measure of reliability may differ from the standard probability of failure occurrence.
This standard also extends to present accelerated testing or production screening methods
that would identify weakness introduced into the product by manufacturing error, which could
compromise product dependability.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60068 (all parts), Environmental testing
IEC 60300-3-1:2003, Dependability management – Part 3-1: Application guide – Analysis
techniques for dependability – Guide on methodology
IEC 60300-3-5, Dependability management – Part 3-5: Application guide – Reliability test
conditions and statistical test principles
IEC 60605-2, Equipment reliability testing – Part 2: Design of test cycles
IEC 60721 (all parts), Classification of environmental conditions
IEC 61014:2003, Programmes for reliability growth
IEC 61164:2004, Reliability growth – Statistical test and estimation methods
IEC 61124:2012, Reliability testing – Compliance tests for constant failure rate and constant
failure intensity
IEC 61163-2, Reliability stress screening – Part 2: Electronic components
IEC 61649:2008, Weibull analysis
IEC 61709, Electronic components – Reliability – Reference conditions for failure rates and
stress models for conversion
62506 © IEC:2013 – 9 –
IEC 61710, Power law model – Goodness-of-fit tests and estimation methods
IEC 62303, Radiation protection instrumentation – Equipment for monitoring airborne tritium
IEC/TR 62380, Reliability data handbook – Universal model for reliability prediction of
electronics components, PCBs and equipment
IEC 62429, Reliability growth – Stress testing for early failures in unique complex systems
3 Terms, definitions, symbols and abbreviations
For the purposes of this document, the term and definitions given in IEC 60050-191:____, as
well as the following, apply.
NOTE Symbols for reliability, availability, maintainability and safety measures follow those of
IEC 50060-191:1990, where available.
3.1 Terms and definitions
3.1.1
item
subject being considered
Note 1 to entry: The item may be an individual part, component, device, functional unit, equipment, subsystem, or
system.
Note 2 to entry: The item may consist of hardware, software, people or any combination thereof.
Note 3 to entry: The item is often comprised of elements that may each be individually considered. See "sub-
item", definition 191-41-02 and "indenture level", definition 191-41-05.
Note 4 to entry: IEC 60050-191:1990, first edition, identified the term “entity” as a synonym, which is not true for
all applications.
Note 5 to entry: The definition for item given in the first edition is a description rather than a definition. This new
definition provides meaningful substitution throughout this standard. The words of the former definition form the
new note 1.
[SOURCE: IEC 60050-191:—, definition 191-41-01] [1]
3.1.2
step stress
step stress test
test in which the applied stress is increased, after each specified interval, until failure occurs
or a predetermined stress level is reached
Note 1 to entry: The ‘intervals’ could be specified in terms of number of stress applications, durations, or test
sequences.
Note 2 to entry: The test should not alter the basic failure modes, failure mechanisms, or their relative
prevalence.
[SOURCE: IEC 60050-191:—, definition 191-49-10]
3.1.3
acceleration factor
ratio between the item failure distribution characteristics or reliability measures (e.g. failure
intensities) of an item when it is subject to stresses in expected use and those the item
acquires when the higher level stresses are applied for achieving a shorter test duration
—————————
Figures in square brackets refer to the Bibliography.
– 10 – 62506 © IEC:2013
Note 1 to entry: For a test to be effectively accelerated, the acceleration factor is >1.
Note 2 to entry: When the failure distribution Poisson is assumed with constant failure rate, then the acceleration
factor corresponds to the ratio of time under stress in use vs. time under increased stress in test.
3.1.4
highly accelerated limit test
HALT
test or sequence of tests intended to identify the most likely failure modes of the product in a
defined stress environment
Note 1 to entry: HALT is sometimes spelled out as the highly accelerated life test (as it was originally named in
error). However, as a non-measurable accelerated test, it does not provide information on life duration, but on the
magnitude of stress which represents the limit of the design.
3.1.5
highly accelerated stress test
HAST
test where applied stresses are considerably increased in order to reduce duration of their
application
3.1.6
highly accelerated stress screening
HASS
screening intended to identify latent defects in a product caused by manufacturing process or
control errors
3.1.7
highly accelerated stress audit
HASA
process monitoring tool where a sample from a production lot is tested to detect potential
weaknesses in a product caused by manufacturing
3.1.8
activation energy
E
a
empirical factor for estimating the acceleration caused by a change in absolute temperature
Note 1 to entry: Activation energy is usually measured in electron volts per degree Kelvin.
3.1.9
event compression
increasing stress repetition frequency to be considerably higher than it is in the field
3.1.10
time compression
removal of exposure time at low or deemed non damaging stress levels from a test for
purpose of acceleration
3.1.11
precipitation screen
screening profile to precipitate, through failure, conversion of latent into permanent faults
3.1.12
detection screen
low stress level exposure to detect intermittent faults
62506 © IEC:2013 – 11 –
3.2 Symbols and abbreviated terms
Symbol/
Abbreviation Description
R(t) reliability as a function of time; probability of survival past the time t
NOTE 1 IEC 60050-191:1990, definition 191-12-01 uses the general symbol . Time may be substituted by
( )
R t ,t
1 2
cycles, measure of distance, etc.
failure rate as a function of time
λ(t)
NOTE 2 In reliability growth testing, the same symbol normally used for the instantaneous failure rate can be
used for variable failure intensity.
HALT highly accelerated limit test
HASS highly accelerated stress screening test
HAST highly accelerated stress test
HASA highly accelerated stress audit
λ(S) failure rate as a function of a stress
UUT unit under test
A acceleration, acceleration factor
A overall acceleration in a test
test
ADT accelerated degradation testing
DSL design specification limit
RTL reliability test level
SL specification limit
DL destruct limit
LDL lower destruct limit
UDL upper destruct limit
OL operating limit
UOL upper operating limit
LOL lower operating limit
SPRT sequential probability ratio test
RG reliability growth
URTL upper reliability test limit
LRTL lower reliability test limit
THB temperature humidity bias test
TTF time to failure
MTBF mean operating time between failures
MTTF mean time to failure
AF acceleration factor
FIT failure to time
CALT calibrated accelerated life testing
ADT accelerated degradation test
t start of a period of in determination of product destruct life rest
t duration of a predetermined time, e. g. life
L
SPRT sequential probability ratio tests
– 12 – 62506 © IEC:2013
4 General description of the accelerated test methods
4.1 Cumulative damage model
Accelerated testing of any type is based on the cumulative damage principle. The stresses of
the product in its life cause progressive damage that accumulates throughout the product life.
This damage may or may not result in a product’s failure in the field.
The strategy of any type of accelerated testing is to produce, by increasing stress levels
during testing, cumulative damage equivalent to that expected in the product’s life for the type
of expected stress. Determination of product destruct limits, without reliability estimation,
provides information on whether there exists a sufficient margin between those destruct limits
and product specification limits, thus providing assurance that the product will survive its
predetermined life period without failure related to that specific stress type. This technique
may or may not necessarily quantify a probability of product survival for its life, just assurance
that the necessary adjustments in product strength would help eliminate such failure in
product use. Where sufficient margins are determined unrelated to the probability of survival,
the type of test is qualitative. In tests where this probability of survival is determined, the
magnitude of the stress is correlated to the probability that the product would survive that
stress type beyond the predetermined life, and this test type is quantitative.
Figure 1 depicts the principle of cumulative damage in both qualitative and quantitative
accelerated tests.
In Figure 1, for simplicity, all stresses, operating limits, destruct limits, etc. are shown as
absolute values. The specification values for an item are usually given in both extremes,
upper and lower, thus the upper and lower (or low) specification limit, USL and LSL with the
corresponding design limits (DSL), UDL and LDL, the upper and lower operating limits, UOL
and LOL, and also the reliability test limits, URTL and LRTL. The rationale is that the opposite
(negative stresses, may also cause cumulative damage probably with a differently failure
mechanism, thus the relationship between the expected and specified limits can be illustrated
in the same manner as for the high or positive stress. As an example, cold temperature
extremes might produce the same or different failure modes in a product. To avoid clutter, the
positive and the negative thermal or any other stresses are not separately shown in Figure 1,
thus the magnitudes of stresses are either positive or negative, and thus represented as
absolute values only as upper or lower limits.
62506 © IEC:2013 – 13 –
1,00
0,90
Requirement level
0,80
Reliability test level
0,70
Design specification level
Operating level/HALT
0,60
Destruct level
0,50
0,40
0,30
0,20
0,10
0,00
0,0 2,5 5,0 7,5 10,0 12,5 15,0 17,5 20,0 22,5 25,0 27,5 30,0 32,5 35,0 37,5 40,0 42,5 45,0
Cumulative
damage
ReqL DSL OL DL
RTL
Stress level
(t )
L IEC 1378/13
Figure 1 – Probability density functions (PDF) for cumulative damage,
degradation, and test types
The graph in Figure 1 shows the required strength of a product regarding a stress for the
duration of its lifetime, from beginning of life (e.g. time when the product is made), t through
the end of life, t . The strength and stresses in tests are also assumed to have a Gaussian
L
distribution.
The different types of accelerated tests can now be illustrated using Figure 1 as a conceptual
model.
Functional testing is carried out within the range of the requirement specification and at the
level of the specification. In this area no failures should occur during the test; design is
validated to allow operation within the upper and lower specification limits. Accelerated testing
of Type B and C (4.2.3 and 4.2.4), i.e. accelerated degradation testing (ADT) or cumulative
damage testing can be illustrated as the distance between the design specification level
(DSL) and the level where the reliability demonstration test should be performed (RTL). When
the degradation reduces the performance below the requirement specifications the product
can be declared as failed, if this behaviour is defined as a failure. When testing the product at
time t no failures should be expected for stress levels up to and including the design
specification level (DSL).
The product design specification should take into consideration certain degradation during the
product’s life which is resultant from the cumulative damage of the stresses expected in life,
thus its limit is the design specification limit (DSL) which is higher than the requirement limit
(RL) in order to provide the necessary margin. After product degradation resultant from the
cumulative damage caused by expected stresses, the reliability test provides information on
the existing margin between the test level (the remaining strength) and the requirement. This
margin is a measure of reliability at the end of required period, t .
L
The ultimate strength of the design is considerably higher than the design specifications and
this is the level determined in the qualitative accelerated test where the goal is to identify
Stress PDF
– 14 – 62506 © IEC:2013
design weaknesses which could compromise product reliability, i.e. the weaknesses that
could occur in the product’s life span, as the product degrades. Thus, the strength in the
qualitative test is demonstrated at operating limit (OL).
The destruct limit is above (beyond) the operating limit, and is denoted as DL. This is where a
permanent failure is observed. If OL or DL are close to the DSL or standard deviation of the
OL or DL distributions are high, then the test will indicate a potential weakness in the design
as indicated in Figure 1.
Product reliability is a function of time, usually predetermined life time, t .
L
The cumulative normal distribution of the margin (difference of stress means divided by their
common standard deviation) between the specified strength (use conditions) which is
represented by the requirement and the reliability test level (RTL) determines product
reliability. The test level and its duration are chosen so as to cause cumulative damage during
testing corresponding to the degradation due to cumulative damage in the product’s life span.
The calculated value, produces product required reliability, which is then a quantitative
measure.
A summary of listed tests and the mapping of their applications to the product life cycle is
presented in Table 1.
Table 1 – Test types mapped to the product development cycle
TTypypee DDesesiiggnn IIntnteegrgraattiionon VVaalidlidaattioionn AAccepccepttananccee MMaanufnufaaccttururiingng SSerervviicceses
FFMEMECCAA
HHALALTT HHAASS/SS/HHAASSAA
AA
QQuualaliittatatiiveve
Maturity Building
Maturity Confirmation
RReleliiaabibilliittyy G Grrowowtth h RReelialiabbililitityy RReleliiaabibilliittyy P Prroduoduccttiion on
TeTesstt QQualualiiffiiccaattiioon Tn Tesestt AAccecceppttaanncece T Teestst
B & CB & C
Maturity Assessment
QQuuananttatatiiveve
TTyyppee B B/C/C : :
CComompoponennentt
PPrroodduucctt
BBrreakdeakdoowwnn
TTyyppee A A : : A Assessembmbllyy
ststrruuctctuurre e
TTyype Ape A:: C Comompoponentnent an/an/oror S Sububssyysstteemm
OOppppoorrttuunniittyy
TTyyppee B B/C/C : :
TTyyppee B B/C/C : S : Syysstetemm
AAssessembmbllyy
IEC 1379/13
Table 1 provides the users of this standard a synthesis in order to get a better understanding
of the different methods as and when required during the whole life cycle product.
4.2 Classification, methods and types of test acceleration
4.2.1 General
Based on the cumulative damage model, the information expected from the test and the
product use assumptions, the accelerated test methods may be divided into three groups:
• Type A: qualitative accelerated tests: for detection of failure mode and/or phenomenon;
• Type B quantitative accelerated tests: for prediction of failure distribution in normal use;
• Type C: quantitative time and event compression tests: for prediction of failure distribution
in normal use.
NOTE Both B and C types of test may lead to test time reduction. Type B test should be performed based on
particular failure mechanism, and generally it may be applied to lifetime acceleration. Type C test requires
62506 © IEC:2013 – 15 –
research of usage or specific conditions’ assumption before test . Type C test may be applied to failure rate
acceleration.
4.2.2 Type A: qualitative accelerated tests
Type A, accelerated tests, are designed to identify potential design weaknesses and also
weaknesses caused by the manufacturing process. They can therefore be induced at levels
considerably higher, than OL, as shown in Figure 1, i.e. The goal of this type of test is not to
quantify product reliability, but to induce or precipitate, during the test, the product’s overall
performance issues which are likely to take place in the field some time during the product’s
useful life and result in a product failure. Improvement of the product design or manufacturing
processes is executed to preclude those failures, producing a stronger or more robust
product, expected to be more reliable in the field even under extreme or repetitive stresses as
outlined in the design specifications.
Product development processes using this type of test increase product reliability through the
mitigation of failure modes and by increasing product robustness without demonstrating a
reliability target or measuring reliability improvement. These tests are often made with such
high stress levels that, ideally, failures should be observed (DL in Figure 1) well beyond
design specification limits. The purpose is to identify the failure modes, the weak links in the
design and the margin between the functional limits, operating limit (OL) and the destruct limit
(DL) in Figure 1. The margin between the specification limit and the operating limit ensures
that the weaknesses are identified in HALT and are not expected to occur as failures during
the expected product life, t .
L
4.2.3 Type B: quantitative accelerated tests
Type B tests use cumulative damage methods to determine product reliability projected to the
end of the expected product life. The necessary margin between the expected cumulative
damage and the requirement produces a reliability measure. These tests are then accelerated
to achieve the required cumulative damage in considerably shorter time than the product’s
expected life. Type B accelerated tests use quantifiable acceleration factors which are based
on the physics of specific failures (or failure modes) and provide a relationship between the
exposure time to the specific stresses during testing and in use environment. The failure, or
failure mode distribution, is determined from information gathered through separate
accelerated tests. Such test information provides the basis for a functional life model and can
be used to quantify test acceleration for various reliability calculations, as necessary and/or
applicable. In this way, product reliability can be estimated through estimation of the reliability
or probability of occurrence of individual failure modes for any level of expected stresses. If
needed for data analysis using other test types (e.g. reliability growth or reliability
demonstration tests), the determined test acceleration factor can be used to recalculate times
to failure data from accelerated tests so as to represent times to failure occurrences in the
use environment, and use the results for reliability calculations. In Figure 1, these tests are
shown as reliabi
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