IEC 61163-2:2020

Reliability stress screening - Part 2: Components

IEC 61163-2:2020 provides guidance on RSS techniques and procedures for electrical, electronic, and mechanical components. This document is procedural in nature and is not, and cannot be, exhaustive with respect to component technologies due to the rapid rate of developments in the component industry. This document is:
a) intended for component manufacturers as a guideline;
b) intended for component users as a guideline to negotiate with component manufacturers on RSS requirements;
c) intended to allow the planning of an RSS process in house to meet reliability requirements or to allow the re-qualification of components for specific, upgraded, environments;
d) intended as a guideline to sub-contractors who provide RSS as a service.
This document is not intended to provide test plans for specific components or for delivery of certificates of conformance for batches of components. The use of bi-modal Weibull analysis to select and optimize an RSS process without having to estimate the reliability and life time of all items is described. This second edition cancels and replaces the first edition published in 1998. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
a) this version of the document is a complete rewrite and restructure from the previous version.

Déverminage sous contraintes - Partie 2: Composants

IEC 61163-2:2020 fournit des recommandations relatives aux techniques et procédures de RSS pour composants électriques, électroniques et mécaniques. Le présent document est de nature procédurale. En raison de l'évolution rapide de l'industrie des composants, elle n'est pas, ni ne peut être, exhaustive au regard de la technologie des composants. Le présent document est prévu pour:
a) les fabricants de composants, en tant que ligne directrice;
b) les utilisateurs de composants, en tant que ligne directrice pour la négociation relative aux exigences de RSS avec les fabricants de composants;
c) permettre la planification d'un processus de RSS interne, destiné à satisfaire aux exigences de fiabilité, ou pour admettre la requalification des composants pour des environnements spécifiques mis à niveau;
d) les sous-traitants qui proposent le RSS comme un service, en tant que ligne directrice.
Le présent document n'a pas pour but de fournir des plans d'essai pour soumettre des composants spécifiques à l'essai ni de délivrer des certificats de conformité pour des lots de composants. L'usage d'une analyse bimodale de Weibull visant à choisir et optimiser un processus de RSS, sans avoir à évaluer la fiabilité ni la durée de vie de toutes les entités, fait l'objet d'une description. Cette deuxième édition annule et remplace la première édition parue en 1998. Cette édition constitue une révision technique. Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
a) cette version du document a fait l'objet d'une refonte intégrale et a été réorganisée par rapport à la version précédente.

General Information

• Status: Published
• Publication Date: 10-Mar-2020
• ICS: 03.120.01 - Quality in general
• ICS: 31.020 - Electronic components in general
• Technical Committee: TC 56 - Dependability
• Drafting Committee: MT 12 - TC 56/MT 12
• Current Stage: PPUB - Publication issued
• Start Date: 11-Mar-2020
• Completion Date: 20-Mar-2020

Relations

• Revises: IEC 61163-2:1998 - Reliability stress screening - Part 2: Electronic components
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC 61163-2 ®
Edition 2.0 2020-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Reliability stress screening –
Part 2: Components
Déverminage sous contraintes –
Partie 2: Composants
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IEC 61163-2 ®
Edition 2.0 2020-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Reliability stress screening –

Part 2: Components
Déverminage sous contraintes –

Partie 2: Composants
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 03.120.01; 31.020 ISBN 978-2-8322-7910-6

– 2 – IEC 61163-2:2020 © IEC 2020
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Description of reliability stress screening (RSS) . 8
5 Types of RSS . 10
5.1 General . 10
5.2 Constant stress screening . 10
5.3 Step stress screening . 10
5.4 Highly accelerated stress screening (HASS) . 10
6 Managing RSS . 11
6.1 Planning . 11
6.2 Termination of RSS . 12
7 Design of RSS . 12
7.1 General . 12
7.2 Physics of failure . 12
7.3 Common screening procedures . 13
7.4 Characteristics of a well-designed screening procedure . 14
7.5 Screening evaluation . 14
7.6 Selection of samples . 14
7.7 Setting the duration of RSS . 15
8 Managing an RSS programme . 15
8.1 Resources . 15
8.2 Monitoring during RSS . 16
9 Analysis for RSS . 16
9.1 General . 16
9.2 Cost benefit analysis . 16
9.3 Identifying early failures . 16
9.4 Analysis of the outputs of RSS . 17
Annex A (informative) Data analysis . 18
A.1 Symbols . 18
A.2 Weibull analysis . 18
A.3 Design of a reliability stress screening . 19
Annex B (informative) Examples of applications of reliability stress screening
processes . 23
B.1 General . 23
B.2 Transformers . 23
B.3 Connectors . 25
Bibliography . 28

Figure A.1 – Estimation of η and β . 18
Figure A.2 – Nomograph of the cumulative binomial distribution (Larson) . 20
Figure A.3 – Example of a Weibull plot . 21

Figure B.1 – Weibull plot of the bump screening . 25
Figure B.2 – Weibull plot of the pull test . 27

Table 1 – Common screening types and typical defect types precipitated by RSS . 13
Table A.1 – RSS test results . 21
Table A.2 – Screening results for weak populations . 22

– 4 – IEC 61163-2:2020 © IEC 2020
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
RELIABILITY STRESS SCREENING –

Part 2: Components
FOREWORD
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rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61163-2 has been prepared by IEC technical committee 56:
Dependability.
This second edition cancels and replaces the first edition published in 1998. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) this version of the document is a complete rewrite and restructure from the previous version.
The text of this International Standard is based on the following documents:
FDIS Report on voting
56/1875/FDIS 56/1887/RVD
Full information on the voting for the approval of this International Standard can be found in the
report on voting indicated in the above table.

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61163 series, published under the general title Reliability stress
screening, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://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.
– 6 – IEC 61163-2:2020 © IEC 2020
INTRODUCTION
Although first developed to stabilize the parameters of manufactured components (burn-in),
reliability stress screening (RSS) can be used to remove from a component population the
weaker components. This can be done at times where the manufacturing processes for
components are difficult to control or for other reasons such as where the components need to
be selected (re-qualified) to operate in harsher than usual operating conditions. This is also
done where more narrow specifications are required for the application and no alternative
courses of action are available.
The use of RSS is normally only a temporary measure when early failures need to be avoided
under a specific set of conditions as outlined above.
RSS is an effective tool in identifying and removing flaws due to poor component design and
manufacturing deficiencies.
RELIABILITY STRESS SCREENING –

Part 2: Components
1 Scope
This part of IEC 61163 provides guidance on RSS techniques and procedures for electrical,
electronic, and mechanical components. This document is procedural in nature and is not, and
cannot be, exhaustive with respect to component technologies due to the rapid rate of
developments in the component industry.
This document is:
a) intended for component manufacturers as a guideline;
b) intended for component users as a guideline to negotiate with component manufacturers on
RSS requirements;
c) intended to allow the planning of an RSS process in house to meet reliability requirements
or to allow the re-qualification of components for specific, upgraded, environments;
d) intended as a guideline to sub-contractors who provide RSS as a service.
This document is not intended to provide test plans for specific components or for delivery of
certificates of conformance for batches of components.
The use of bi-modal Weibull analysis to select and optimize an RSS process without having to
estimate the reliability and life time of all items is described.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
screen
conditions, for example stress level and duration, used for the removal of non-conforming items
from a population
3.2
screening
process carried out to detect and remove non-conforming items, or those susceptible to early
life failure
Note 1 to entry: Screening may employ representative or elevated stresses.

– 8 – IEC 61163-2:2020 © IEC 2020
[SOURCE: IEC 60050-192:2015, 192-09-11, modified – Deletion of “test” in the term,
replacement of “test” with "process" in the definition and replacement of “The test” with
"Screening" in the Note 1 to entry.]
3.3
RSS
reliability stress screening
process for detecting flaws by applying environmental and/or operational stresses to precipitate
them as detectable failures
Note 1 to entry: RSS is designed with the intention of precipitating flaws into detectable failures. An ageing process
designed specifically with the intention of stabilizing parameters is not an RSS process and is therefore outside the
scope of this document.
Note 2 to entry: This note applies to the French language only.
[SOURCE: IEC 60050-192:2015, 192-09-19, modified – Addition of Note 1 to entry.]
3.4
flaw
imperfection that could result in failure
Note 1 to entry: An imperfection in this case is a physical characteristic of the component that leads to a failure to
perform in a required way.
[SOURCE: IEC 60050-192:2015, 192-04-03, modified – Addition of Note 1 to entry.]
3.5
early life failure period
infant mortality period
time interval of early life during which the instantaneous failure intensity of a repairable item,
or the instantaneous failure rate of a non-repairable item, decreases significantly with time
Note 1 to entry: What is considered “significant” will depend upon the application.
[SOURCE: IEC 60050-192:2015, 192-02-28]
3.6
weak item
item which has a high probability of failure in the early life period due to a flaw
3.7
weak population
subset of the total population of items made up of only weak items
3.8
strong population
subset of the total population of items made up of non-weak items
4 Description of reliability stress screening (RSS)
The process of RSS is used to detect flaws in a population of items, usually components,
leading to the subsequent removal of these flawed items from the population. The removal of
such components facilitates rapid achievement of the reliability level expected for the population
over the useful life.
This can often happen when problems with items are identified and it takes time to fix the design
or the production process for the item but the existing items need to be used immediately. This
is typically a sorting exercise where the RSS is used to fail the items with problems so they can
be identified in the population or batches.

RSS can also be used to sort items to meet specific operating conditions or functional
parameters where it is used to select items that meet a requirement higher than what was
originally specified from a batch that was lower than what was originally specified, for example
screening components for temperature stability or other factors that affect reliability.
Typically RSS is initiated in response to one or more of the following situations:
– customer requirements specify the use of screening;
– field performance identifies an issue with early product failures;
– the production process generates a concern for latent defects;
– to reduce the uncertainty with the introduction of a new product or process;
– to select, from a selection of different components performing the same function but with
different technologies/techniques;
– some items need to be screened to meet a tighter or increased specification.
The RSS method is achieved by applying specific environmental or operating conditions to
stress the population of items. This applied stress, or combination of stresses, will often have
environmental and operating conditions in excess of the stress at normal operating conditions.
The stresses usually used are temperature, humidity, vibration, acceleration, electrical stress
and similar conditions. A screening may have one or more conditions set at higher than normal
levels.
The screening takes places at the item level, which is usually at component level but may
include some large packages containing multiple components. RSS of products is covered by
[1] .
The screening will cause flawed components to fail quickly and so be identified in the
population. These components are then removed from the population. The remaining
components are then referred to as having been screened and the process is similar to sorting,
where the RSS is used to split the population into two distinct sets, one that has been failed by
the screening and one that has not. In some cases, a sample from a batch is screened to
determine whether a lot contains weak components.
NOTE 1 If a screening strength is too high then non-flawed components can also fail and in fact an extremely strong
screening could fail the entire population. It can also degrade them without failure but reduce their useful life. For
this reason, it is important that a screening procedure is carefully designed according to the physics and materials
of the components undergoing the screening and the reasons for the screening.
RSS should not be used as a normal procedure to assure the reliability of individual
components. The RSS method can, however, improve the actual reliability of a population or
system by removing flawed components that are more likely to cause failure.
The cost of performing RSS should be carefully evaluated and the screening only undertaken
if the potential benefits outweigh the cost.
If early failures are caused by the assembly processes for the finished item including the
component, and its handling (ESD damage, contamination, etc.) RSS will not be effective and
so should not be done. However, it may be possible to perform RSS of the finished item [1].
NOTE 2 The use of RSS is inappropriate if there are no early failures. The failures can be reduced if needed using
other methods like design changes [5]. Early failures can be identified using the techniques in [6].
NOTE 3 The use of RSS is inappropriate if the relevant failures can be detected without operating the item over
time. Failure detection at zero operating time is carried out by parametric measurement or the use of non-invasive
techniques like X-ray, scanning acoustic microscope (SAM) and similar methods.
_____________
Numbers in square brackets refer to the Bibliography.

– 10 – IEC 61163-2:2020 © IEC 2020
NOTE 4 Using RSS to upgrade component population specifications can lead to problems, for example a logistical
problem can occur when similarly screened components are not available at a later date. This can be mitigated by
performing RSS on enough components for the repair of the system over its entire service life or by ensuring that
the system documentation is sufficient to control component procurement so that all replacement components be
similarly screened (see [7]).
Sometimes it is necessary to carry out other actions beyond RSS in order to meet the
requirements and many of the principles of reliability growth described in [5] apply. Typically,
changes in the design, the manufacturing processes or in the components' use may have to be
made. It also may be necessary to adopt a failure mode avoidance strategy that can remove
the causes of the failures or at least deal with them when they occur, for example via
redundancy.
In some cases, the stress screening will not give the results that are expected and in those
cases, further investigation is required to understand what has happened. This can happen
when a stress applied has effects that were not predicted in the initial physics of failure analysis
(see 7.2). In these cases, a redesign of the stresses applied to be more specific will be
necessary.
Some examples of the application of RSS are given in Annex B.
5 Types of RSS
5.1 General
There are a number of types of RSS: constant stress screening, step stress screening, and
highly accelerated stress screening (HASS).
The purpose of all of these screening types is to cause relevant failures to occur in the item.
Such relevant failures are those that would have prevented the item from achieving its reliability
requirements in service.
5.2 Constant stress screening
A constant stress screening is a screening procedure where a constant environmental and/or
operational stress is used for the duration of the process.
5.3 Step stress screening
Step stress screening is a screening procedure where environmental and/or operational
stresses are changed at planned intervals, usually increasing in strength for the duration of the
process.
Step stress screening is often used to shorten process times, and to give some idea of likely
failures rates at different stress levels. For this reason it is sometimes used in the RSS planning
phase to select those levels.
5.4 Highly accelerated stress screening (HASS)
Highly accelerated stress screening (HASS) is a screening procedure used in conjunction with
a highly accelerated limit test (HALT, see IEC 62506 [2], [3]). A HALT is needed before a HASS
screening procedure can be started.
NOTE HALT uses very high stress levels, typically high and low temperature, rapid temperature change and
mechanical vibration or mechanical shock. HALT is performed on a small sample of items. The output of the HALT
is typically the high and low operational limits for example the temperatures when the item stops functioning, but
recovers function once the item is brought back to normal operating temperature. Further, the HALT identifies the
destruction limits, the temperatures where the item fails permanently i.e. it does not recover as the temperature is
brought back to normal. In some cases, the limits cannot be found within the temperature range relevant for the
technology of the item. This limit information is used as the basis for setting up a HASS procedure.

HASS, unlike HALT, is intended to be an on-going process either performed on the whole
production (100 % screening) or on a sample from the production or from a batch.
The HASS process is typically set up as a rapid temperature change between the upper
operating limit reduced by some amount and the lower temperature limit plus the same amount.
If no operating limits have been identified, a level as high as appropriate for the item’s
technology is chosen. Normally the screening strength of the HASS screening is adjusted by
increasing or decreasing the number of temperature cycles. The number of cycles in the HASS
can be determined and kept optimized using the procedure described in Clause A.3 and [11].
HASS normally stays within the items' operational limits to allow continuous monitoring of the
function of the item but operational limits can be exceeded where the items under HASS are
not monitored during the screening. The stress levels should stay below the destruction limit
for good items. The items should then be tested for function after the HASS. Since HASS is
resource and time consuming, requiring specialist equipment, HASS is often performed on a
sample from the production.
6 Managing RSS
6.1 Planning
In order to plan RSS, the objectives need to be identified and these objectives will be slightly
different according to the underlying reasons for carrying out RSS. The objectives of the RSS
will affect the type of screening applied (see 7.3).
The plan should define the objectives and success criteria of the RSS.
The RSS plan should encompass all aspects of the RSS process and is integrated into the
overall manufacturing test plans. The plan should be a useful tool for identifying the resources
that are required for conducting RSS.
As a minimum, the RSS plan needs to contain the following items:
– overall process flow chart;
– a schedule identifying dates for the completion of RSS procedures and the beginning of
RSS;
– which stresses will be employed;
– how the screening procedures will be applied to the item, i.e., RSS parameters, sequence
of screening procedures, combination of screening procedures;
– the data collection process and methods of monitoring of items during the RSS;
– screening facilities and equipment used;
– methods for modifying the RSS based on item failures;
– decision-making ground rules applicable to failures;
– subcontractor/supplier RSS requirements;
– RSS organization and management responsibilities;
– personnel and their responsibilities for the various functions within the RSS process.
The RSS process flow should be a closed loop operation, i.e. one that includes feedback of
results, in order to be successful.
Deliverables of the RSS process are
• failure reports;
• throughput data;
– 12 – IEC 61163-2:2020 © IEC 2020
• RSS performance data.
These deliverables are reviewed for trends and screening effectiveness.
6.2 Termination of RSS
Termination of RSS is normally justified by a substantial amount of failure-free observational
data from the process, suggesting that the weak components have been removed or reduced
significantly.
RSS may be accepted as a permanent process only in cases where components have to be
screened for performance against a particular component parameter (e.g. actual operating
temperature limit), where the standard components from the supplier does not fulfil the
requirements for the use of the component, or it is a contractual obligation.
7 Design of RSS
7.1 General
Designing an effective screening is an empirical process that can adapt standard RSS
processes. This is not to say that certain screening procedures cannot be reasonably effective
for similar item configurations, however, as a rule, an individual screening procedure should be
tailored to the item being screened, considering both the characteristics of the item and the
anticipated defect types.
An effective screening is one that delivers the required failures as quickly as possible without
causing other failure mechanisms that would not normally occur in use. The screening also
needs to be economically viable. There is often a trade-off between the cost and effectiveness
of the screening. A process of optimisation may need to be undertaken to choose the most
effective set of screening procedures.
The particular set of screening conditions will require a certain application time before results
are seen. The stress level will control how long this application time needs to be and, in general,
higher stresses require shorter time.
NOTE The use of the maximum stress level that is appropriate is possible but verification that the chosen
combination of stress types and stress levels does not reduce the life time of the strong components is important.
This can, for example, be done by exposing a sample of components to 10 times the duration of the planned RSS
screening.
Under some circumstances, it is beneficial to use step stress screening [8] where the failures
at each stress level are examined and the dwell times at each level are shortened as necessary.
This is the case when there are no failures at lower stress levels, perhaps because they are not
set up to be harsh enough initially to precipitate failure, or perhaps some other type of failure
is occurring that requires further investigation.
7.2 Physics of failure
Before any screening procedure can be defined, it is necessary to have a detailed
understanding of the way in which the items under consideration can fail. To do this, information
on the failure modes will need to be gathered. Failure mode information is available from a
number of sources, the most reliable of which being the item manufacturer.
NOTE 1 Only failure modes that are significant to the overall objective are considered.
Each failure mode that is considered can arise due to a number of different failure mechanisms,
and these are based on the basic physics of the item being considered.
Each failure mechanism will have a number of contributory factors that will cause it to occur at
a particular rate. Each of these contributory factors will have certain levels that need to be

reached before the mechanism will start and the mechanism will run at different rates as the
levels of these contributory factors change.
EXAMPLE Corrosion will often lead to open circuit failure as conductors corrode away and corrosion has a particular
set of contributory factors that will be present in order to occur, such as moisture, and ionic contaminants.
In order to create an effective screening, the failure mechanisms need to be known so that the
screening conditions can induce the required failure mechanisms to occur at a sufficient rate
so that the relevant failure mode is precipitated in a short period.
NOTE 2 Arbitrary application of screening stresses is not an effective strategy since the stresses are specifically
directed to the problems that are identified.
NOTE 3 It is likely that the most effective screening conditions can require a mix of screening conditions and that
the order in which the screening conditions are applied can be significant.
It may be possible to use simulation to model the effect of a screening before applying it to the
item so that the screening effectiveness can be judged. To do this, physics of failure models, if
available, can be used.
In addition, a number of empirical equations have been developed that allow the effect of stress
to be modelled in a general way. Some of the general equations are described in [9] and more
specific ones for electrical component types can be found in [10].
7.3 Common screening procedures
There are a number of common types of screening procedures which have been used with some
degree of success. These common screening procedures (stresses) are shown in Table 1.
Table 1 – Common screening types and typical defect types precipitated by RSS
Stress Defect types precipitated
Thermal cycling Component parameter drift
Hermetic seal failure
Poor thermal coefficient matches
Stress relaxation
Loosening of connections or parts
Cracks
Vibration Particle contamination
Defective oscillator crystals
Poorly bonded internal parts
Poorly secured high-mass parts
Mechanical flaw
Loosening of connections or parts
Part mounting issues
Combined thermal cycling and vibration All mechanisms under vibration and thermal cycling
Interaction between mechanisms
High voltage Shorted connections
Humidity Sealing properties
Hygroscopic contamination
Circuit stability
Corrosion
High temperature Performance degradation
Chemical reaction
– 14 – IEC 61163-2:2020 © IEC 2020
Stress Defect types precipitated
Acceleration Cracks
Mechanical defects
Gas pressure test Leaks and hermetic failure
Power cycling In-rush current response
Circuit transients
EXAMPLE “Combined thermal cycling and vibration”: The defects listed occur for two basic reasons: certain defect
types are susceptible to both temperature and vibration stresses and a synergistic effect exists between temperature
cycling and random vibration. The synergistic effect can manifest itself when temperature cycling and random
vibration stresses are applied either simultaneously (preferred method), or in sequence. If applied in sequence, best
results are most often achieved by the sequence of thermal cycling-vibration-thermal cycling.
7.4 Characteristics of a well-designed screening procedure
An effective screening will
– precipitate relevant flaws rapidly;
– not induce new types of flaw;
– precipitate adequate proportion of inherent flaws;
– not initiate design changes based on false alarms;
– not consume a significant percentage of item life (minimize consumption of useful life).
7.5 Screening evaluation
The approach taken during screening conditions development is to subject a small sample of
items to the screening conditions in an experimental situation to ensure that the flaws are being
stimulated to failure.
In this evaluation, the items are exposed to the selected screening for a sufficient time so that
a relevant number of flaw related failures may occur. The number of failures that need to be
observed during this evaluation to show that it could be an effective screening is related to the
assumed size of the weak population within the sample and the required confidence levels. If
the expected number of failures is not generated by the evaluation in the expected time, then
the stress levels can be modified. Bi-modal Weibull analysis can be used to analyse and
evaluate a screening [11]. Note that this screening evaluation can also be used to set the
duration of the RSS.
7.6 Selection of samples
Sampling is a standard statistical control process that enables the user to establish the
consistency or quality of output items on the basis of testing of item samples.
If RSS is intended to remove all weak components then the sample shall be 100 % (complete),
in that all components to be used need to be subjected to the screening. However, screening
100 % of the components does not guarantee that 100 % of flaws will be identified.
The advantage of a 100 % screening is that the confidence that all weak components have been
screened away is high, hence it is generally recommended to start with a 100 % screening of
the first batches and when the numbers of weak components are reduced to go to screening of
batches using samples.
If the purpose of the screening is to detect with a stated confidence that a problem is present
in a given batch or production line, then a sampling procedure needs to be selected based on
the risk of non-detection of a given percentage of non-conforming items.

Three major sampling methods are common: single sampling, double sampling, and chain
sampling (commonly known as Wald’s method). Choice of an appropriate sampling method
requires an estimation of a population proportion and its confidence interval. Whilst the Wald
method is commonly used for calculating the confidence interval, it is not without some flaws
and alternative procedures such as the Clopper-Pearson method or the Wilson Score method
should also be considered [4], [12], [13], [14]. For more details on sampling, see [15].
7.7 Setting the duration of RSS
Part of the setting of the duration of the RSS depends on the actual operational time to failure
of the types of failure observed. This may be known through field failures, lab tests, or estimated
by engineering judgement supplemented by tests analysed by the bi-modal Weibull methods
[11].
The duration of application of the screening should be determined after the selection of the
appropriate screening. The screening and its duration should also be evaluated with respect to
the effect on the remaining life on the items that pass the screening.
The screening duration will need to consider the screening time to failure of the weak items.
The initial duration of the RSS is determined by screening a sample of items at the planned
RSS stress levels as described in Clause A.2. The duration of the screening should be so long
that it is expected that all weak items in the sample have failed under the screening conditions
of increased stresses. A typical duration will be the equivalent of one year in the field, taking
into account the acceleration factor from the increased stresses.
The observed failures are then analysed using the bi-modal Weibull analysis as described in
Clause A.3 (see also IEC 61649 [11]). The idea of the bi-modal Weibull analysis is to plot the
observed failures and identify the weak population i.e. the items that would have failed early in
the field under normal operating conditions. The Weibull plot will indicate this by the levelling
out of the curve once the weak items have failed. The percentage of failures where the curve
levels out is an estimate of the percentage of the weak population. A few items from the strong
population may fail later in the screening procedure, and be plotted to the right of the levelling
off point, giving the curve an S-shape. But often no items from the strong population will fail so
that the levelling off is confirmed by the absence of failures for a long time compared to the
time between failures of the weak items. Since the x-axis of the Weibull plot is logarithmic this
can normally be easily detected visually. Otherwise the Bayes method described in [17] can be
used.
This analysis will estimate the percentage of weak items as well as the characteristic life of the
weak items. Based on this information, the duration of the RSS needed to screen out a specified
percentage,
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