IEC 60749-43:2017

IEC 60749-43 ®
Edition 1.0 2017-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Semiconductor devices – Mechanical and climatic test methods –
Part 43: Guidelines for IC reliability qualification plans

Dispositifs à semiconducteurs – Méthodes d'essais mécaniques et climatiques –
Partie 43: Lignes directrices concernant les plans de qualification de la fiabilité
des CI
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IEC 60749-43 ®
Edition 1.0 2017-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Semiconductor devices – Mechanical and climatic test methods –

Part 43: Guidelines for IC reliability qualification plans

Dispositifs à semiconducteurs – Méthodes d'essais mécaniques et climatiques –

Partie 43: Lignes directrices concernant les plans de qualification de la fiabilité

des CI
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 31.080.01 ISBN 978-2-8322-4471-5

– 2 – IEC 60749-43:2017 © IEC 2017
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Product categories and applications . 8
5 Failure . 9
5.1 Failure distribution . 9
5.2 Early failure . 10
5.2.1 Description . 10
5.2.2 Early failure rate . 11
5.2.3 Screening . 14
5.3 Random failure . 17
5.3.1 Description . 17
5.3.2 Mean failure rate . 17
5.4 Wear-out failure . 20
5.4.1 Description . 20
5.4.2 Wear-out failure rate . 20
6 Reliability test . 23
6.1 Reliability test description . 23
6.2 Reliability test plan . 23
6.2.1 Procedures for creating a reliability test plan . 23
6.2.2 Estimation of the test time required to confirm the TDDB from the
number of test samples . 26
6.2.3 Estimation of the number of samples required to confirm the TDDB from
the test time. 27
6.3 Reliability test methods . 28
6.4 Acceleration models for reliability tests . 31
6.4.1 Arrhenius model . 31
6.4.2 V-model: . 32
6.4.3 Absolute water vapor pressure model . 32
6.4.4 Coffin-Manson model . 32
7 Stress test methods . 32
8 Supplementary tests . 33
9 Summary table of assumptions . 34
10 Summary . 36
Bibliography . 37

Figure 1 – Bathtub curve . 10
Figure 2 – Failure process of IC manufacturing lots during the early failure period . 11
Figure 3 – Weibull conceptual diagram of the early failure rate . 12
Figure 4 – Example of a failure ratio: α (in hundreds) and the number of failures for
CL of 60 % . 14
Figure 5 – Screening and estimated early fail rate in Weibull diagram . 15
Figure 6 – Bathtub curve setting the point immediately after production as the origin . 16

Figure 7 – Bathtub curve setting the point after screening as the origin. 17
Figure 8 – Conceptual diagram of calculation method for the mean failure rate from
the exponential distribution . 18
Figure 9 – Conceptual diagram of calculation method for the mean failure rate as an
extension of early failure . 19
Figure 10 – Conceptual diagram of the wear-out failure . 21
Figure 11 – Conceptual diagram describing the concept of the acceleration test . 21
Figure 12 – Concept of the reliability test in a Weibull diagram (based on sample size) . 25
Figure 13 – Concept of the reliability test in a Weibull diagram (based on test time) . 28
Figure 14 – Difference in sampling sizes according to the m value (image) . 29

Table 1 – Examples of product categories . 9
–6
Table 2 – Cumulative failure probability 0,1 % over 10 years [×10 ] for the third, fifth
and seventh years . 25
Table 3 – Major reliability (life) test methods and purposes . 30
Table 4 – Examples of the number of test samples and the test time in typical
reliability (life) test methods . 31
Table 5 – LTPD sampling table for acceptance number Ac = 0 . 33
Table 6 – Major reliability (strength) test methods and purposes . 33
Table 7 – Supplementary tests . 34
a
Table 8 – Accelerating factors, calculation formulae and numerical values . 35

– 4 – IEC 60749-43:2017 © IEC 2017
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SEMICONDUCTOR DEVICES –
MECHANICAL AND CLIMATIC TEST METHODS –

Part 43: Guidelines for IC reliability qualification plans

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
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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
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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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
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60749-43 has been prepared by IEC technical committee 47:
Semiconductor devices.
The text of this International Standard is based on the following documents:
FDIS Report on voting
47/2389/FDIS 47/2406/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 60749 series, published under the general title Semiconductor
devices – Mechanical and climatic test methods, 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.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 6 – IEC 60749-43:2017 © IEC 2017
INTRODUCTION
This document provides guidelines for semiconductor IC vendors in the preparation of
detailed reliability test plans for device qualification. Such plans are intended to be prepared
before commencing qualification tests and after consultation with the user of their
semiconductor integrated circuit product.
The guideline gives some examples for creating reliability qualification test plans to determine
appropriate reliability test conditions based on the quality standards demanded in use
conditions for each application of semiconductor integrated circuits. Categories are set for
automotive applications and for general applications as a target of reliability. The grade for
automotive use is further classified into two grades according to applications. The guideline
assumes annual operating hours, useful life, etc. for each grade, and defines the verification
methods for early failure rate and wear-out failure to propose appropriate reliability tests, and
at the same time, presents concepts to properly ensure the quality of semiconductor
integrated circuits using screening techniques which are designed to reduce the early failure
rate.
Note that the test conditions and the values of acceleration factors presented in this guideline
are shown to provide examples of calculations for obtaining reliability test conditions in order
to verify the required quality standards, and are not designed to define the standards to
ensure reliability of semiconductor integrated circuits.
NOTE Qualification tests are tests in which the semiconductor vendor takes account of the reliability required by
its product users.
SEMICONDUCTOR DEVICES –
MECHANICAL AND CLIMATIC TEST METHODS

Part 43: Guidelines for IC reliability qualification plans

1 Scope
This part of IEC 60749 gives guidelines for reliability qualification plans of semiconductor
integrated circuit products (ICs). This document is not intended for military- and space-related
applications.
NOTE 1 The manufacturer can use flexible sample sizes to reduce cost and maintain reasonable reliability by this
guideline adaptation based on EDR-4708, AEC Q100, JESD47 or other relevant document can also be applicable if
it is specified.
NOTE 2 The Weibull distribution method used in this document is one of several methods to calculate the
appropriate sample size and test conditions of a given reliability project.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60749-5, Semiconductor devices – Mechanical and climatic test methods – Part 5:
Steady-state temperature humidity bias life test
IEC 60749-6, Semiconductor devices – Mechanical and climatic test methods – Part 6:
Storage at high temperature
IEC 60749-15, Semiconductor devices – Mechanical and climatic test methods – Part 15:
Resistance to soldering temperature for through-hole mounted devices
IEC 60749-20, Semiconductor devices – Mechanical and climatic test methods – Part 20:
Resistance of plastic encapsulated SMDs to the combined effect of moisture and soldering
heat
IEC 60749-21, Semiconductor devices – Mechanical and climatic test methods – Part 21:
Solderability
IEC 60749-23, Semiconductor devices – Mechanical and climatic test methods – Part 23: High
temperature operating life
IEC 60749-25, Semiconductor devices – Mechanical and climatic test methods – Part 25:
Temperature cycling
IEC 60749-26, Semiconductor devices – Mechanical and climatic test methods – Part 26:
Electrostatic discharge (ESD) sensitivity testing – Human body model (HBM)
IEC 60749-28, Semiconductor devices – Mechanical and climatic test methods – Part 28:
Electrostatic discharge (ESD) sensitivity testing – Charged device model (CDM) – Device
level
– 8 – IEC 60749-43:2017 © IEC 2017
IEC 60749-29, Semiconductor devices – Mechanical and climatic test methods – Part 29:
Latch-up test
IEC 60749-42, Semiconductor devices – Mechanical and climatic test methods – Part 42:
Temperature and humidity storage
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
failure mode
style classification of a fault phenomenon which causes product failure
Note 1 to entry: Disconnection, a short circuit, occasional loss, abrasion, characteristic deterioration, etc. are
typical items considered as failure modes.
3.2
failure mechanism
physical, chemical or other process which has led to a failure
3.3
integrated circuit
IC
microcircuit in which all or some of the circuit elements are inseparably associated and
electrically interconnected so that it is considered to be indivisible for the purpose of
construction and commerce
4 Product categories and applications
Quality-related requirements, operating hours, and operating condition of ICs demanded in
the field depend on the applications of products in which they are used. As an example of
creating scientific test plans, their applications are broadly classified into three product
categories: Automotive Use A; Automotive Use B; and Consumer Use. Table 1 shows a list of
quality-related requirements according to each product category and the definition of their use
conditions.
Table 1 – Examples of product categories
Category Automotive Use A Automotive Use B Consumer Use
Criteria for Applications for automotive Applications for automotive Applications other than for
category use directly relating to safety. use not directly relating to automotive use. Industrial
(Failures can cause safety. applications shall be handled
accidents.) individually.
Examples of Powertrains, brakes, driving Navigation systems, car air- Home electronics, toys,
applications support systems, airbags conditioners, audio systems appliances
Annual 500 h (driving hours) 500 h (driving hours) Up to 8 760 h
operating hours
Differs depending on whether Differs among applications.
or not to work with KEY
ON/OFF.
Useful life 15 years (cumulative failure 15 years (cumulative failure Up to 10 years (cumulative
probability: 0,1 %) probability: 0,1 %) failure probability: 0,1 %)
Differs among applications.
Assumed Example of engine compartment
operating
T = 0 °C / T = 70 °C
T = −40 °C/ T = 125 °C
conditions a,min a,max
a,min a,max
T = 70°C/105°C (max.)
T = 100 °C/ T = 150 °C
j
(examples of j,typ j,max
conditions which
min. RH: 0 / max. RH: 100 %, RH = 10 (min.)/80 % (max.)
differ among
applications) RH (during 20 % power on )
RH (during 10 % driving) (during 70 % stop)
(during 60 % power off)
Example of interior environment
T = -40°C (min.)/85°C (max.)
a
T = 85°C (typ.)/125°C (max.)
j
RH = 0 (min.)/100 % (max.),
RH (during 10 % driving) (during 70 % stop)
-6 -6 -6
Early failure rate 1 × 10 or below per annum 50×10 or below per annum Up to 500 × 10 per annum
Differs among applications.
Random failure 10 FIT or below 50 FIT or below >50 FIT (typical)
rate
Differs among applications.
NOTE These are examples of application conditions and requirements that do not have to all be met to be
relevant for each use case.
5 Failure
5.1 Failure distribution
Failure distribution of ICs can be broadly divided into three regions: early failure portion
(e.g., t = 1 year), random failure portion, and wear-out failure portion. Figure 1 shows the
ELF
relationship between the field use time and the instantaneous failure rate (bathtub curve).
Failure distributions for each region are described in detail in 5.2 to 5.4.
Most early failures are screened within manufacturing processes of IC vendors. However, ICs
not fully screened can expose problems in a relatively short period after their operation starts
in the field.
Random failure has been considered to achieve a certain failure rate with respect to time, but
actually, it is appropriate to consider as an extension of the early failure region where the
failure rate continues to decline. Potentially induced failures outside of the supplier’s control,

– 10 – IEC 60749-43:2017 © IEC 2017
such as ESD and EOS, should not be included in the failure rate calculations unless a total
fail rate that includes these types of fail modes is intended.
Wear-out failure is a failure which occurs due to the end of life of IC components such as
transistors and interconnections, and indicates the life of the ICs themselves. Wear-out failure
is a failure which depends on the usage load profile (time windows may be different). The
number of failures increases with time, and every IC will eventually cause a failure beyond the
intended design life of the part. Wear-out failures are not considered in the same manner,
because they have a totally different mechanism and therefore also a different mathematical
description (failure distribution). Therefore, it is important to prevent this failure during the
durable period. For ICs, the time to reach the cumulative failure probability of 0,1 % over the
design life of the part in the given application is generally defined as their design lifetime.
Real life capability
(Confirmed at die element level)
Accelerated reliability test
(Confirmed with products)
Durable period
(In the user application)
Screening within
IC vendor’s
Early
manufacturing
failure
processes
Wear-out
Random failure
failure
Field use time
t = 1 year
ELF
IEC
Figure 1 – Bathtub curve
5.2 Early failure
5.2.1 Description
Since ICs contain very small feature sizes and are dense and complex, they are susceptible
to defects generated in manufacturing processes. For this reason, good devices which satisfy
required characteristics and functions are sorted out at the final stage of manufacturing
processes. The ratio of good devices to the total amount produced and tested in this process
is called yield. When sorting good devices, they are measured for as many items as possible
including characteristics and functions required. However, some of these sorted good devices
can include those with built-in latent defects or weaknesses which does not influence
electrical characteristics, and they operate properly during sorting. When the yield is high,
devices with these potential defects are less likely to be included. In contrast, when the yield
is relatively low, there is a high possibility of mixture of these latent defect devices with good
ones. Devices with these potential defects can eventually fail during use due to the shortened
lifetime or the intensity of the user application.
A small amount of tested good devices which contain such defects is included in the
manufacturing lot and, as such, its failure rate decreases with time. This is because non-
defective ICs which are unlikely to cause a failure remain when defective ICs are removed
after they cause a failure. In such a case, the shape parameter of the Weibull distribution: m is
less than 1 (m < 1).
Instantaneous failure rate
To be more specific, when a manufacturing lot has good devices with a potential defect as
shown in Figure 2, electronic products using such devices may cause a failure during use,
and faulty ICs are removed by application screening, repair (component replacement) or
disposition. This leaves reliable ICs.
NOTE It is much preferred to screen out these failures, latent or otherwise, at the IC manufacturer rather than
have them reach the user, where it is more expensive to correct.
Population of
Failure
Failure
manufacturing lots of
semiconductors
Defect
Failure
Immediately after manufacturing
Individual semiconductor
After a certain period of After long-term use
of semiconductors
devices
time has passed
(Semiconductor devices containing a defect cause a (Semiconductor devices containing no
failure and are removed) defect remain after time has passed)
IEC
Figure 2 – Failure process of IC manufacturing lots
during the early failure period
For this reason, reducing defects generated in manufacturing processes is the major
countermeasure against such failures. Another possible countermeasure is to change the
design to a structure not susceptible to defects if it is feasible.
There are also screening techniques such as burn-in, which operate ICs under relatively
harsh conditions of temperature and/or voltage to induce the defects to fail in advance and
remove them by sorting. This acts to consume the early failure period of ICs before shipment,
which can reduce impacts of the early failure after shipment. Screening can be optimized if
the effect of the above defect reduction is confirmed.
5.2.2 Early failure rate
5.2.2.1 Early failure rate definition
The early failure rate indicates the probability of degradation failures resulting from
manufacturing defects which occur within one year (defined early failure period) after
shipment by IC manufacturers and the operation starts in the field (within assembly
manufacturers’ processes and in end user applications).
The early failure rate is often expressed as “cumulative failure probability”, where the failure
rate which occurs during the defined early failure period is numerically expressed in
-6
percentage (%) or parts per million (10 ).
5.2.2.2 Cumulative fail probability
In general, the cumulative failure probability is expressed as follows.
When the Weibull distribution shape parameter is expressed as m, scale parameter η and time
t, the cumulative failure probability F(t): from 0 to t is defined by Formula (1).

– 12 – IEC 60749-43:2017 © IEC 2017
m
 t 
 
F(t) = 1− exp − (1)
m
 
η
 
Figure 3 shows the concept of the early failure rate using a Weibull distribution chart.
lnln(1/(1 – F(t)))
Defined early
t : Field use time
b
failure period
t : Value converting the time between
s
screening period to shipment into
field use time.
P : Operating ratio (0 < P ≤ 1)
lnln
(1/(1 – F(t )))
b
Early failure rate
ln(t)
Shipment 1 year after Useful life
t operation starts t time
s b
t + 8 760 × P
s
IEC
Figure 3 – Weibull conceptual diagram of the early failure rate
The following sections describe how to calculate the early failure rate from the confirmation
result of the cumulative failure convergence at screening test.
5.2.2.3 Calculation of the early failure rate
Suppose that the cumulative failure probability F(t ) with the field use time t was obtained as
b b
the confirmation result of the cumulative failure convergence at screening test. The Weibull
shape parameter η is obtained from the following formula:
t
b
(2)
η =
m
[− ln(1− F(t ))]
b
Where m indicates the value obtained from the experiment result or an estimated value.
However, in the above formula, if there are zero failures, then F(t ) = 0, and the scale
b
parameter η is undefined, as the denominator goes to 0.
For this reason, the χ (chi-squared) distribution shall be used to define the cumulative failure
probability F (t ) taking account of the confidence level.
c b
However, this is based on the premise that the number of samples N is sufficiently large.
NOTE Typical confidence level used in failure rate calculations for semiconductor devices is 60 %.
The cumulative failure probability at the specified confidence level and at the field use time
F (t ) is given by Formula (3).
c b
F (t ) =χ (3)
c b d
g
g,
2×N
Where,
χ Chi-square distribution;
g Confidence level (CL in %);
d Degree of freedom = (2 × f) + 2;
f Number of failures;
N Number of samples.
Susbtituting (3) into (2) yields the scale parameter taking account of the confidence level:
t
b
(4)
η =
c
[− ln(1− F (t ))]
m
c b
When the value converting the screening period until shipment into field use time t and the
s
calculated value η are used, the early failure rate taking account of the confidence level after
c
shipment: F (t ,t ) is given by the following.
c 1 s
If failures are found during the early failure period, η is used instead of η and the early failure
c
rate takes no account of the confidence level after shipment. F(t ,t ) is given by the following:
1 s
m
 
(t − t )
1 s
F (t ,t ) = 1− exp− 
(5)
c 1 s
m
 
η
c
 
m
 
(t − t )
1 s
F(t ,t ) = 1− exp− 
(6)
1 s
m
 
η
 
For both Formulae (5) and (6), t = 365 × 24 ×P
where
P Operating ratio ranged from 0 (always off) to 1 (always on);
t Time point of 1 year after operation start measured in hours (constant on = 8 760 hours).
5.2.2.4 Calculation of a failure rate ratio
The failure rate ratio: α between the early failure rate F taking account of a confidence level
c
with g (in %) and the early failure rate F taking no account of the confidence level is
expressed by the following formulae:
F = χ (7)
c
d
g,
2×N
f
F = (8)
N
where f = Number of failures in N = Number of samples.

– 14 – IEC 60749-43:2017 © IEC 2017
χ
d
g,
2×N
F
c 2
α = = = (N/f)χ (9)
d
F
 f  g,
  2×N
N
 
α
2,2
2,0
1,8
1,6
1,4
1,2
1,0
0,8
1 10 100
Number of failure
IEC
Figure 4 – Example of a failure ratio: α (in hundreds)
and the number of failures for CL of 60 %
F is determined by the number of failures, not by the number of test samples: N.
As the number of failures increases, the ratio between the rates taking account of and without
taking account of the confidence level comes closer to 1.
From Figure 4, when the cumulative number of failures is 50, the failure rate difference
between the early failure rate taking account of the confidence level: F and the early failure
c
rate taking no account of the confidence level: F is 5 % (α = 105 for
N = 50 versus α = 100 for large N).
Therefore, if the difference in the early failure rate depending on the number of failures and
the presence of incorporation of the confidence level is allowable, the early failure rate taking
no account of the confidence level can be used.
EXAMPLE:
When a confirmation result of the cumulative failure convergence at screening test is obtained using:
m = 0,3; t = 70 298 hours; N = 2 000; and f = 69,
b
and the early failure rate F (t ,t ) using:
c 1 s
CL of 60 %; t = 70 298 hours; P = 1
s
The scale parameter with confidence level is calculated as η = 5,57 × 10 from Formula (4).
c
-6
The early failure rate with confidence level is calculated as F (t :t ) = 129 x 10 from Formula (5).
c 1 s
The scale parameter taking no account of the confidence level η = 2,15 × 10 from Formula (2).
-6
The early failure rate taking no account of the confidence level F(t :t ) = 124 × 10 from Formula (6).
1 s
Thus, in this example, the failure rate difference between the early failure rate taking account of the confidence
level F and the early failure rate without taking account of the confidence level F is (129 − 124)/124 = 4 %.
c
5.2.3 Screening
Since ICs contain very small feature sizes and are dense with complex geometry, they are
susceptible to defects generated in manufacturing processes. Therefore, some lots classified
as ‘good devices’ contain potential failures with a minor intrinsic defect which does not

influence measured electrical characteristics. This allows the device to operate to
specification during the sorting process. Removing potential failures before shipment and thus
reducing or eliminating the early field failure rate is called screening.
General screening methods to remove devices containing such minor intrinsic defects include
the application of stricter stresses such as voltage and temperature than those under actual
use conditions, combining a product test and burn-in, methods to remove initial defects in
packages using X-ray inspections and visual inspections and combining mounting stress and
stresses under a temperature cycle test, etc. Screening conditions need to be adjusted when
necessary depending on the target early failure rate. They should be examined in a manner
that the screening itself will not influence the useful life significantly, i.e. reduce the time to wear out.
By stabilizing the product manufacturing lines and manufacturing the product carefully to reduce
intrinsic manufacturing defects, the product early failure rate can also be reduced.
Burn-in has generally been conducted after packaging, but recently, it is common to conduct it
to die on wafers. Both methods will produce the same effect in terms of the purpose of
removing manufacturing defects in device die. Figure 5 shows the relationship between the
screening and the early failure rate which can be statistically estimated.
Early failure distribution
(e.g. m ≈ 0,3)
percentage
fraction of
estimated early
Screening failure failures
probability
Time taking account of the Converted into actual
acceleration factor by burn-in use: Approx. 1 year
t = 1 year
Before burn-in After burn-in
ELF ln (t)
IEC
Figure 5 – Screening and estimated early
fail rate in Weibull diagram
Figure 5 shows the verification method for the early failure rate after screening which can be
estimated statistically. In some cases, burn-in is repeated until no actual burn-in failure occurs
to confirm the convergence of the screening but, as shown in Figure 5, the early failure
always occurs even at a slight rate when it is statistically estimated in the Weibull distribution.
For this reason, the sampling becomes meaningless unless data is statistically analysed
taking account of the experimental parameter, acceleration factor, and failure mode even
when the burn-in is unreasonably repeated until no burn-in failure occurs.
The m value calculated with the failure probability of the burn-in process does not reflect the
failures which occurred and were removed in the test processes before the burn-in process. If
the m value is calculated without considering failures in the test processes before burn-in, it
becomes larger. Therefore, the early failure rate estimated from the m value tends to become
a poorer estimate than the actual quality. In general, it is necessary to calculate the m -value
taking account of the failure probability before and after burn-in as shown in Figure 5 to
estimate the early failure.
Cumulative failure probability

– 16 – IEC 60749-43:2017 © IEC 2017
Verification of the early failure distribution and rate based on this concept should be
incorporated into the verification of the product reliability, which is also the most effective
means to improve the actual delivery quality.
Since the early failure depends on the intrinsic manufacturing defect rate of the manufacturing
line, the early failure rate of other products in the same manufacturing line and with the same
design standard can also be estimated easily as long as the manufacturing defect rate and
the convergent characteristics (m -value of the Weibull distribution) of the early failure are
understood.
EXAMPLE:
Calculation method for the early failure probability in the field:
When t is the time until shipment taking account of the screening (converted into the time in the use environment
per Figure 6), then the cumulative failure probability after shipment is:
m m
 
(t + t ) − t
1 1
( ) = −
F t : t 1 exp  (10)
m
 η 
 
and the failure rate after shipment (see Figure 7) is:
(m−1)
m × (t + t )
λ(t : t ) = (11)
m
η
Note that the cumulative failure probability for one year after shipment is F(8 760 h: t ) where 1 year = 8 760 h.
When m and η are obtained from the screening data, quantitative calculations are enabled.
Failure rate
t
after production
Removed by
screening
Failure rate: λ(t)
Immediately Shipment t
after
production
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Figure 6 – Bathtub curve setting the point immediately
after production as the origin

Failure rate: λ(t:t )
Shipment
t
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Figure 7 – Bathtub curve setting the point
after screening as the origin
5.3 Random failure
5.3.1 Description
Random failure generally indicates a failure which occurs between the end of the defined
early failure period (in this case, 1 year) and the end of the useful life or onset of the wearout
period. It is appropriate to estimate that failures whic
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