IEC 61193-3:2013

Quality assessment systems - Part 3: Selection and use of sampling plans for printed board and laminate end-product and in-process auditing

IEC 61193-3:2013 establishes sampling plans for inspection by attributes, including sample plan selection criteria and implementation procedures for printed board and laminate end-product and in-process auditing. The principles established herein permit the use of different sampling plans that may be applied to an individual attribute or set of attributes, according to classification of importance with regard to form, fit and function.

Système d'assurance de la qualité - Partie 3: Choix et utilisation de plans d'échantillonnage pour cartes imprimées et produits finis stratifiés et audits en cours de fabrication

La CEI 61193-3:2013 établit des plans d'échantillonnage pour le contrôle par attributs, incluant des critères de sélection de plans d'échantillonnage et des procédures de mise en oeuvre pour les cartes imprimées et les produits finis stratifiés et des audits en cours de fabrication. Les principes établis ici permettent d'utiliser différents plans d'échantillonnage qui peuvent être appliqués à un seul attribut ou à un ensemble d'attributs, selon une classification sur l'importance portant sur la forme, le caractère adapté et la fonction.

General Information

• Status: Published
• Publication Date: 23-Jan-2013
• ICS: 31.190 - Electronic component assemblies
• Technical Committee: TC 91 - Electronics assembly technology
• Drafting Committee: WG 4 - TC 91/WG 4
• Current Stage: PPUB - Publication issued
• Start Date: 24-Jan-2013
• Completion Date: 15-Feb-2013

Overview

IEC 61193-3:2013 is an international standard developed by the International Electrotechnical Commission (IEC) focused on quality assessment systems. Specifically, it addresses the selection and use of sampling plans for printed boards and laminate products during end-product and in-process auditing. This standard outlines methodologies for inspection based on attributes, defines sampling plan selection criteria, and provides implementation procedures aiming to enhance the quality control process in the manufacturing of printed circuit boards (PCBs) and laminates.

The standard facilitates the adoption of various sampling plans tailored to individual or grouped attributes, depending on their classification related to the product's form, fit, and function. By establishing clear guidelines and best practices, IEC 61193-3:2013 supports manufacturers in delivering products that meet stringent quality requirements and regulatory compliance.

Key Topics

• Sampling Methodologies: The standard covers general sampling methods including attribute sampling plans (e.g., continuous and lot attribute sampling), variable sampling, and non-statistical sampling approaches.
• Attribute Classification: Attributes are classified according to their importance, influencing the choice and criteria of the sampling plans to ensure effective defect detection.
• Defect Classification: Detailed instructions for categorizing defects including functional, electrical, visual/mechanical, and hermetic non-conformances.
• Process Control Integration: Guidelines for incorporating sampling plans into ongoing process control and improvement practices.
• Inspection Plans: Specifies zero acceptance number (c=0) based plans and compares different inspection plans with operational characteristic (OC) curves to illustrate acceptability risks.
• Defects Per Million Opportunities (DPMO): Describes calculation and classification of DPMO metrics specific to PCB and laminate quality assessment.
• Use of Sampling Plans: Practical recommendations for grouping tests, categorizing data, applying sampling during in-process testing, and utilizing indirect measurement methods.

Applications

IEC 61193-3:2013 is essential for manufacturers, quality managers, and auditors operating in the PCB and laminate production industries. Its applications include:

• End-Product Auditing: Ensuring finished printed boards and laminates meet defined quality and functional standards before shipment.
• In-Process Auditing: Monitoring production processes in real-time to identify defects early, prevent waste, and reduce rework.
• Quality Control Systems: Enhancing existing quality management systems by integrating statistically validated sampling plans to manage inspection efforts efficiently.
• Supplier Qualification: Utilizing consensus sampling plans to assess supplier compliance with customer and industry specifications.
• Risk Management: Implementing risk-based sampling according to defect classification ensures critical attributes affecting product safety and performance are prioritized.

Related Standards

To fully leverage IEC 61193-3:2013, consideration of related standards and guides within the IEC and ISO frameworks is recommended:

• IEC 62326-4: Guides multilayer printed board requirements, closely linked for conformity with consensus sampling plans.
• ISO 2859 (Sampling Procedures for Inspection by Attributes): Provides methodologies that align with IEC sampling principles for broader quality control applications.
• ISO 9001 (Quality Management Systems): Complements with general requirements for quality assurance applicable in conjunction with IEC sampling plans.
• IEC 61193-2 (Quality assessment systems – Part 2): Focuses on sampling plan application for electronic components, a beneficial companion for end-to-end quality assessment.

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Keywords: IEC 61193-3, sampling plans, quality assessment systems, printed boards, laminate products, in-process auditing, end-product inspection, attribute sampling, defect classification, DPMO, process control, PCB quality standards, international standards IEC, quality control sampling, manufacturing auditing.

Standards Content (Sample)

IEC 61193-3 ®
Edition 1.0 2013-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Quality assessment systems –
Part 3: Selection and use of sampling plans for printed board and laminate end-
product and in-process auditing

Système d'assurance de la qualité –
Partie 3: Choix et utilisation de plans d'échantillonnage pour cartes imprimées et
produits finis stratifiés et audits en cours de fabrication

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IEC 61193-3 ®
Edition 1.0 2013-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Quality assessment systems –
Part 3: Selection and use of sampling plans for printed board and laminate end-

product and in-process auditing

Système d'assurance de la qualité –

Partie 3: Choix et utilisation de plans d'échantillonnage pour cartes imprimées et

produits finis stratifiés et audits en cours de fabrication

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XB
ICS 31.190 ISBN 978-2-83220-585-3

– 2 – 61193-3 © IEC:2013
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references. 7
3 Terms and definitions . 7
4 Sampling methodologies . 9
4.1 General . 9
4.2 Attribute sampling plans. 10
4.2.1 General . 10
4.2.2 Continuous sampling. 10
4.2.3 Production lot attributes . 10
4.2.4 Production lot variables . 10
4.3 Non-statistical sampling plans . 11
4.4 Defining c = 0 plans . 11
5 Classification of attributes . 16
5.1 General . 16
5.2 Classification assignment . 17
5.3 Classification and adjustment of sampling plan criteria . 18
5.4 Process control . 18
6 Defects and process deviation indicator (PDI) evaluation . 19
6.1 General . 19
6.2 Process control and process improvement requirements . 19
7 Inspection plans . 19
7.1 General . 19
7.2 Zero acceptance number-based sampling plans . 20
7.3 Responsible authority . 20
7.4 Application. 20
7.5 Sampling plan specification . 20
7.6 Submission of product . 21
8 Classification of defects . 23
8.1 General . 23
8.2 Customers detail specification (CDS) data . 23
9 Percent defectives per million opportunities . 23
9.1 General . 23
9.2 Classes of DPMO . 24
9.2.1 General . 24
9.2.2 DPMO-1 – Functional non-conformances only . 24
9.2.3 DPMO-2 – Electrical non-conformances . 24
9.2.4 DPMO-3 – Visual/mechanical non-conformances . 24
9.2.5 DPMO-4 – hermetic non-conformances . 24
9.2.6 DPMO-5 – all non-conformances . 24
9.3 Estimation of DPMO . 24
9.3.1 General . 24
9.3.2 DPMO reporting . 24
9.4 DPMO calculations . 25

61193-3 © IEC:2013 – 3 –
9.4.1 General . 25
9.4.2 Sampling requirements . 25
10 Use of sampling plans . 25
10.1 General . 25
10.2 Grouping of tests . 25
10.3 Categorization . 26
10.4 In-process testing and control . 26
10.5 Indirect measuring methods . 27
Annex A (informative) Example of consensus sampling plan for three levels of
conformance to requirements of IEC 62326-4 multilayer printed boards . 28
Annex B (informative) Example of consensus sampling plan . 49
Annex C (informative) Operating characteristics curves and values . 52
Bibliography . 60

Figure 1 – Typical OC curve for c ≥ 0 plan . 13
Figure 2 – OC curve comparisons between c ≥ 0 and c = 0 plans . 14
Figure 3 – Systematic path for implementing process control . 19
Figure 4 – Non-conforming attributes with specification requirements . 22
Figure C.1 – Lot size 2 to 8 . 53
Figure C.2 – Lot size 9 to 15 . 53
Figure C.3 – Lot size 16 to 25 . 54
Figure C.4 – Lot size 26 to 50 . 54
Figure C.5 – Lot size 51 to 90 . 55
Figure C.6 – Lot size 91 to 150 . 55
Figure C.7 – Lot size 151 to 280 . 56
Figure C.8 – Lot size 281 to 500 . 56
Figure C.9 – Lot size 501 to 1 200 . 57
Figure C.10 – Lot size 1 201 to 3 200 . 57
Figure C.11 – Lot size 3 201 to 10 000 . 58
Figure C.12 – Lot size 10 001 to 35 000 . 58
Figure C.13 – Lot size 35 000 to 150 000. 59
Figure C.14 – Lot size 150 001 to 500 000 . 59

Table 1 – Inspection plan comparison . 14
Table 2 – Risk management index values (Associated AQ Limits) . 15
Table 3 – Sample size selection guideline . 16
Table 4 – Worst-case use environments . 17
Table 5 – General sample plan criteria per industry markets/technology sectors . 21
Table 6 – Process control . 27
Table A.1 – Performance requirements . 28
Table B.1 – Guideline for qualification and conformance inspection . 50
Table C.1 – Lot sizes . 52
Table C.2 – Small lot characteristics . 52

– 4 – 61193-3 © IEC:2013
INTERNATIONAL ELECTROTECHNICAL COMMISSION
______________
QUALITY ASSESSMENT SYSTEMS –
Part 3: Selection and use of sampling plans for printed board
and laminate end-product and in-process auditing

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
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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 61193-3 has been prepared by IEC technical committee 91:
Electronics assembly technology.
The text of this standard is based on the following documents:
FDIS Report on voting
91/1061/FDIS 91/1080/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.

61193-3 © IEC:2013 – 5 –
A list of all parts of the IEC 61193 series, under the general title Quality assessment systems,
can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until the
stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to
the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
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 – 61193-3 © IEC:2013
INTRODUCTION
A clear description in IEC standards and specifications and their reference to sampling plans in
order to insure adherence to customer requirements is essential. All the details should be clear
as to their implementation or adjustment for evaluation of the product to be shipped, the use of
process control and SPC, or the applicability for using these principles in controlled
experimentation. The general characteristics of these principles relate to a gradual reduction
that might be needed in examining the product being manufactured. As such, they are
sometimes referred to as the logical steps to process improvement. These steps are as follows.
a) STATISTICAL SAMPLING: where, when, and why
­ To determine a proper amount of examples from a given lot of product and using
statistics to evaluate the occurrence of anomalies.
b) ZERO DEFECT STANDARDS: role of specifications
­ To adopt the role of attempting to achieve no defects in a production lot through the
recommendations identified in standards or specifications that define the product
requirements.
c) ECONOMICS: AQL versus cost of defects
­ To establishing the highest level of non-conforming product characteristics, determining
the cost that is incurred when these are discovered or delivered accidentally to the
customer (cost of quality) and establishing an acceptable quality assessment
methodology in order to reduce these occurrences.
d) SPC REDUCED INSPECTION: rules for use and control
­ To create a process control program that is based on reject criteria, followed by
controlled experimentation to improve the process and then using statistical analysis in
order to determine that the process improvement has reduced the occurrences of these
reject criteria.
The explosion of the electronics industry has led to a situation where the design of the printed
board mounting structure or the material used to produce the product is so complex, that the
quality level of these items delivered with known failures are no longer acceptable. The
acceptable number of non-conforming products should be directed toward approaching zero in
producer-customer contracts.
This has led to the development of new methods of quality assurance like the application of
Statistical Process Control (SPC). The low number of permitted non-conforming product
according to the AQL tables caused many to resort to 100 % testing or inspection.
At the same time the quality thinking has developed so that the idea to accept failures has
become impossible, and the use of the AQL tables in the traditional way has been diminishing
very rapidly.
61193-3 © IEC:2013 – 7 –
QUALITY ASSESSMENT SYSTEMS –
Part 3: Selection and use of sampling plans for printed board
and laminate end-product and in-process auditing

1 Scope
This part of IEC 61193 establishes sampling plans for inspection by attributes, including
sample plan selection criteria and implementation procedures for printed board and laminate
end-product and in-process auditing. The principles established herein permit the use of
different sampling plans that may be applied to an individual attribute or set of attributes,
according to classification of importance with regard to form, fit and function.
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 60194:2006, Printed board design, manufacture and assembly – Terms and definitions
IEC 62326-4:1996, Printed boards – Part 4: Rigid multilayer printed boards with interlayer
connections – Sectional specification
ISO 9000:2005, Quality management systems – Fundamentals and vocabulary
ISO 14560:2004, Acceptance sampling procedures by attributes – Specified quality levels in
non-conforming items per million
3 Terms and definitions
For purposes of this document, the terms and definitions given in IEC 60194:2006,
ISO 9000:2005 and the following apply.
3.1
attribute
aspect or characteristic of a unit of a defined product in terms of actual requirement and
allowable deviation
Note 1 to entry: An actual requirement signifies the following:
• a requirement that is stated as a measurement with an allowable more and/or less deviation;
• a requirement stated as an absolute desired condition with allowable anomalies;
• a requirement stated as an absolute without exception (go/ no-go).
3.1.1
critical attribute
attribute where a defect, that judgment and experience indicate, is likely to result in hazardous
or unsafe conditions for individuals using, maintaining, or depending upon the product; or
where a defect is likely to prevent performance or function of a major end item such as a ship,
aircraft, computer, medical equipment, or telecommunication satellite

– 8 – 61193-3 © IEC:2013
3.1.2
major attribute
attribute where a defect, other than critical, is likely to result in failure, or where a defect
reduces the usability of the unit of a product for its intended purpose
3.1.3
minor attribute
attribute where a defect is not likely to reduce materially the usability of the unit of product for
its intended purpose, or where a defect is a deviation from established standards having little
bearing on the effective use or operation of the unit
3.2
acceptable quality level
DEPRECATED: AQL
maximum percent of defects that can be tolerated as a risk, stated for the purposes of
sampling inspection
Note 1 to entry: Sample inspection with associated risk tolerance is employed only where all units of a product
within an inspection lot is expected to completely conform to the specification requirements.
Note 2 to entry: See 3.3.
3.3
acceptance quality limit
lower than perfect quality level
Note 1 to entry: Revised term for AQL.
Note 2 to entry: The term is used to indicate a certain degree of risk in that some products may have non-
conforming characteristics. However, they do not impact the final performance. These decisions are based on
customer/supplier agreements.
Note 3 to entry: The use of the abbreviation AQL to mean “acceptable quality level” (refer to 3.2) is no longer
recommended.
3.4
defective
unit of product that contains one or more defects
3.4.1
critical defective
unit of product that contains one or more defects of critical attributes, and that may also
contain defects of major or minor attributes
3.4.2
major defective
unit of product that contains one or more defects of major attributes, and may also contain
defects of minor attributes, but contains no defects of critical attributes
3.4.3
minor defective
unit of product that contains one or more defects of minor attributes, but contains no defects of
major or critical attributes
3.5
inspection
process of measuring, examining, testing, or otherwise comparing the unit of product with the
specified requirements
61193-3 © IEC:2013 – 9 –
3.5.1
inspection by attributes
inspection of individual attributes (aspects or characteristics) of the unit of product per
specified requirements, procedures, and/or instructions
3.5.2
inspection lot
collection of product units that are identified and treated as a unique entity from which a
sample is drawn and inspected in order to determine conformance with acceptability criteria
3.5.3
inspection rate
number of features per unit of time that can be evaluated at specified false-alarm and escape-
rate settings
3.6
risk management factor
RMF
maximum tolerable percentage of possible defects within a lot (group) of units, based on
approximately 95 % confidence level
3.7
shipment-ready product
product shipped to the customer without having to meet any further acceptance criteria
3.8
unit of product
item(s) being inspected in order to determine conformance to specific requirements
Note 1 to entry: These requirements consist of the following:
• a single article, a pair, a set, a length, an area, an operation, a volume, a component of an end product, or the
end product itself;
• may or may not be the same as the unit of purchase, supply, production or shipment.
4 Sampling methodologies
4.1 General
There is a considerable number of ISO standards on acceptance sampling (see Annex D for
details). However, most of these standards contain plans that allow a lot to be accepted even
when the sample from the lot contains one or more non-conforming items, although there are
some exceptions (ISO 18414 and ISO 21247).
The zero acceptance number plans (c = 0) were originally designed and used to provide equal
or greater consumer protection with less inspection than that required by corresponding
sampling plans. The c = 0 plans are simple to use and administer since there is greater
emphasis on zero defects and product liability prevention. The concepts stated herein provide a
set of attribute plans for product lot inspection. The acceptance number in all cases is zero.
This means that for some level of protection, a sample size is selected and if one or more non-
conforming attributes are present, the lot will be withheld.
The terminology "withhold the lot" does not necessarily mean rejection. A lot is not
automatically accepted or rejected if one or more non-conformances are found. It is only
accepted if zero non-conformances are found in the sample.
Withholding the lot obliges engineering/management personnel to review the results and to
withdraw the lot depending on the seriousness of the case. This relates to whether the attribute

– 10 – 61193-3 © IEC:2013
was critical, major, or minor, or whether identifying the non-conformance to the requirements
was defined as a critical, major, or minor defect.
The word "defective" is commonly used in quality control to describe a part, component, item,
or any other unit of product that contains one or more defects. The word "defect" is commonly
used to describe a particular non-conforming characteristic on a unit of product.
4.2 Attribute sampling plans
4.2.1 General
The following subclauses provide an overview of lot size description attribute plans while
relating them to other plans. Two broad categories of sampling exist and these are
a) continuous;
b) production lot.
4.2.2 Continuous sampling
Continuous sampling is often used when product units are submitted one at a time. This can
apply to production processes where a product moves through various steps. Product moving
along a conveyor can also be thought of as being a candidate for continuous sampling. Industry
has moved away from inspecting quality at the end of the line; thus, in-process inspection or
sampling is a way in which many companies maintain statistical process controls.
The continuous sampling plan may call for frequency checks, i.e. one unit out of five. Even if
the products are good, this frequency check is maintained. If, however, a unit is non-
conforming, 100 % inspection is reverted to until the specified number of consecutive
conforming products result. At that point, the process returns to frequency inspection.
As an example, a quality decision for continuous sampling would be to examine five samples,
within a particular hour, out of a total of thirty products passing through a process. Based on
the characteristics being inspected (i.e., solder bridging on a particular part) nothing is
observed in a certain number of hours, the time can be increased without changing the sample
size. At this point, the sample taken represents a larger portion of an amount of products being
processed. The samples are then monitored for a longer period of time before reducing to
fewer samples again, or to increase the allotted time in which the samples are randomly
selected.
4.2.3 Production lot attributes
Production lot size descriptions involve units of products that are presented in a group, batch,
or lot for inspection, as opposed to being presented one at a time. In these cases, a sample of
a specified quantity is drawn and compared with some acceptance criteria. In the past,
sampling plans allowed a certain quantity of defectives in the sample; the c = 0 plan does not.
In c = 0 plan, the attributes evaluated either conform or do not conform. Go/no go type gauges
are often used in attribute plans.
4.2.4 Production lot variables
Another production lot sampling procedure involves the analysis of measured characteristics
where the attributes vary with respect to their requirements. Variable sampling compared with
attribute sampling essentially involves the inspection of a smaller sample size to obtain the
same protection afforded by an attribute plan. The economics of these smaller sample sizes,
however, are quite often offset by the calculation involved and the need to obtain and record
measurements. In addition, the essential difference between variables and attributes sampling
is not the relative sample sizes, but that variables sampling is based on measurements
whereas attributes sampling is based on classifications.

61193-3 © IEC:2013 – 11 –
Where variables' data is required from an inspection operation, variables' plans shall definitely
be considered. The use of variable plans is necessary when the distribution of the variable data
can significantly improve the process. It may also be important to establish an upper and lower
characteristic so that the customer is aware of the changes that might be necessary to bring
the two limits closer together in a manner that meets the customers’ requirement (target). By
the manufacturer retaining the records regarding meeting the target value of a particular
requirement, the data can indicate when the process is starting to become out-of-control due to
the distribution of measurements within the specified upper and lower acceptance limit. In
variables' production lot sampling, the information is collected primarily to help assure the
manufacturing of acceptable products by indicating the distance from the target that the lot
inspection provides.
4.3 Non-statistical sampling plans
There are cases where zero defects can visually be assured, although the sample size cannot
logically be defined in terms of statistical risks. Such sample sizes are generally exceptionally
low for the more important attributes and, therefore, knowledge of the process and the control
factors is essential. The drilling of printed circuit boards might use first article inspection as a
methodology to determine that the automated tools creating the number of holes in the board
meet the criteria of the requirements. No further inspection of the product is carried out.
However, to ensure that the production process is still under control, a sampling may be made
regarding the number of uses of a drill, any changes in speed or feed characteristics, or other
features of the automated process that might impact the quality that was approved by the first
article.
In order to avoid any confusion in justifying such sample sizes on inspection plans, specific
notations should be used to avoid any tie-in with statistical risks. The reason for such a
selection should be noted, either directly in the plan or in the quality engineering standards.
An example might be a sampling operation where just the first and last item from a lot, are
inspected dimensionally. This is also accomplished where the first and last time a drill bit is
used, it is drilled into an inspection coupon. This permits the first and last characteristics of the
drilled hole to be examined and determined that all holes drilled in between are of a good
quality. Another example might be evaluating a number of products during a particular time
sequence. If the products are different, the technique can be normalized by evaluating the
amount of unit area being processed along a conveyor over a particular time. In this case, a
variety of products can be measured and evaluated. The system then would be judged in or out
of control, depending on non-conformance per unit area over specific time sequences.
The higher index values in the c = 0 plans are also used where favourable process control has
been demonstrated and just an audit is required. Although the statistical risks seem high, the
risks from a practical standpoint would be exceptionally low.
4.4 Defining c = 0 plans
There are many plans that have used the c ≥ 0 concepts. These plans are acceptable quality
level (AQL) oriented. Essentially, the AQL is a specified percent that is considered to be good
quality. In any sampling plan, an operating characteristic curve can be generated to define the
risk of accepting lots with varying degrees of percent non-conforming or defective. These plans
went out of favour in the late 1980's, due to the misunderstanding that it was good practice to
release shipment-ready products with known, non-conforming attributes.
When the AQL concept is used, a high probability of acceptance associated with the AQL
percentage exists. Normally, this is in the order of a 0,90 to 0,98 probability of acceptance level.
The risk of rejecting this AQL percentage is in the order of 0,10 to 0,02 probability level. This
rejection risk is called the "producer's risk."
The assumptions in employing the AQL concept, is that some agreement has been reached
between the producer and the consumer. Although the term ‘quality’ is implied by the initials
AQL, selecting this method is the worst tolerable quality level, since non-conforming products

– 12 – 61193-3 © IEC:2013
may be found in the sample size and yet the production lot is still delivered to the customer,
see Table 1. Since sampling is used, the producer assumes a risk of having a lot rejected,
although the actual percentage defective in the lot is equal to or less than specified in the AQL.
It is also important that a clear distinction be made by either the customer or the requirements
of the specification regarding the characteristics of the non-conforming attribute. Many printed
board or laminate standards identify some characteristics as a process indicator and allow
these to be delivered since they do not impact the performance of the product. The sampling
plan, therefore, allows a lower inspection number and uses the occurrences of the process
indicators as something that needs to be improved. Scratches on copper conductors are an
example of such an indicator. Other attributes are defined as defects since they do impact
performance and, therefore, impact the entire production line.
If no prior AQL agreement exists, and sampling is to be performed simply because 100 %
inspection is impractical, then over-inspection is usually the result. Also, when 100 % sampling
is impractical, the producer is encouraged to inspect a small number of units of product on less
critical attributes. To illustrate the concept, if the c ≥ 0 plan were used, a 1,0 % AQL might be
used for critical attributes and a 4,0 % AQL might be used for major attributes. The technique
for sample selection under an Acceptable Quality Limit would correlate to a c = 0 plan which
would allow no non-conforming product in the sample size.
It is a statistical fact that zero accept number (c = 0) plans provide equivalent statistical
assurance than do plans associated with defect acceptance (c ≥ 0). This can be verified by
examining the operating characteristics (OC) curves, which should normally be provided with
sampling plans. Figure 1 shows a typical OC curve from a c ≥ 0 plan. There is a probability
scale on the Y-axis and an incoming defective possibility scale on the X-axis. The curve is
generated through probability calculations based on a sample size of 125 with an acceptable
number of 10. Also shown is the producer's risk, which is a risk of rejecting a good lot of
product and the associated consumer's risk, which is the risk of accepting a bad lot of product.

61193-3 © IEC:2013 – 13 –
Typical OC curve
Defect # > 0 acceptance plans
1,0 Producers risk
α = 1 – Pa; α = 0,014
0,9
0,8
n = sample size = 125
0,7
c = accept No. = 10
0,6
0,5
0,4
0,3
0,2
Consumer’s risk
0,1
β = Pa; β = 0,1
1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0 11,0 12,0 13,0
Incoming percent defective  (P′) IEC  161/13

NOTE 1 Values come from Poisson distribution analysis. (α is 0,013 7 by Poisson distribution and is 0,011 9 by
binomial distribution; β is 0,101 3 by Poisson distribution and is 0,086 8 by binomial distribution.)
NOTE 2 For typical OC curve refer to ISO 2859-1.
Figure 1 – Typical OC curve for c ≥ 0 plan
In addition to the AQL and producer's risk, there is a parameter called the lot tolerance percent
defectives (LTPD). This LTPD is considered poor quality, and is sometimes identified as
consumer’s risk quality. Several sampling plans can have OC curves pass through the same
AQL/producer's risk point. For each of these plans, however, there will be a different LTPD at
some constant probability of acceptance level. This probability of acceptance level
corresponding to the LTPD is usually low with a 0,10 being widely accepted. This probability
level is called the "consumer's risk".
The user of sampling plans shall select the plan that will provide reasonably good protection
against accepting lots with percent defectives not a lot greater than the AQL. With the
AQL/producer's risk point fixed, the closer the LTPD gets to the AQL, the larger the sample
size and the acceptance number becomes. Figure 2 is a comparison of the c ≥ 0 OC curve and
an equivalent OC curve from the zero defect c = 0 plan. This example illustrates that the c = 0
curve with a small sample of 18 and an accept number of 0 is equivalent or better than the c =
0 plan with a relatively large sample of 125 and an acceptance number of 10. The producer's
risk probability may be greater at certain levels with the c = 0 plan.

Probability of acceptance  (Pa)
AQL (4,0)
LTPD = 12,3
– 14 – 61193-3 © IEC:2013
OC curve comparison
Zero defect and defect > 0 acceptance plans
1,0
Sampling plans
0,9
0,8
0,7
n = 125
c = 10
0,6
0,5 n = 18
c = 0
0,4
0,3
0,2
0,1
1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0 11,0 12,0 13,0
∆X
∆X
IEC  162/13
Figure 2 – OC curve comparisons between c ≥ 0 and c = 0 plans
What industry has tended to do, is to measure output, determine yields, and then resign to an
acceptable level of defectives based on the information. These systems usually AQL-based,
remove incentives to review the validity of specifications, investigate defect causes, or to
improve overall product quality.
Table 1 shows a comparison of a set of c = 0 plans with previous plans of c ≥ 0.
Table 1 – Inspection plan comparison
AQL Sample size Accept No.
c ≥ 0 Plan 1,0 % 125 3
4,0 % 125 10
Associated AQL Sample size Accept No.
c = 0 Plan
1,0 % 42 0
4,0 % 18 0
The c = 0 plan provides equal to or greater LTPD p
...