IEC PAS 62647-2:2011

IEC/PAS 62647-2 ®
Edition 1.0 2011-06
PUBLICLY AVAILABLE
SPECIFICATION
PRE-STANDARD
Process management for avionics – Aerospace and defence electronic systems
containing lead-free solder –
Part 2: Mitigation of the deleterious effects of tin

IEC/PAS 62647-2:2011(E)
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IEC/PAS 62647-2 ®
Edition 1.0 2011-06
PUBLICLY AVAILABLE
SPECIFICATION
PRE-STANDARD
Process management for avionics – Aerospace and defence electronic systems
containing lead-free solder –
Part 2: Mitigation of the deleterious effects of tin

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
W
ICS 03.100.50; 31.020; 49.060 ISBN 978-2-88912-202-8
– 2 – PAS 62647-2  IEC:2011(E)
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 7
2 Terms and definitions . 7
3 Requirements . 9
3.1 Determination of levels . 9
3.2 Requirements for control levels . 9
3.2.1 General . 9
3.2.2 Control Level 1 requirements . 10
3.2.3 Control Level 2A requirements . 10
3.2.4 Control Level 2B requirements . 11
3.2.5 Control Level 2C requirements . 12
3.2.6 Control Level 3 requirements . 12
3.3 Implementation requirements . 13
3.3.1 Documentation of uses of Pb-free tin . 13
3.3.2 Detecting and controlling Pb-free tin finish introduction . 14
3.3.3 Methods for mitigating impact of Pb-free tin (applies to Level 2B,
Level 2C) . 14
3.3.4 Methods for analysis and evaluation of tests and mitigations for tin
whisker risk and mitigation effectiveness . 16
Annex A (informative)  Guidance on control levels, risk assessment, and mitigation
evaluation . 18
Annex B (informative) Technical guide on tin whiskers . 23
Annex C (informative) Technical guide on detection methods, mitigation methods, and

methods for limiting impact of tin . 26
Bibliography . 33

PAS 62647-2  IEC:2011(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PROCESS MANAGEMENT FOR AVIONICS –
AEROSPACE AND DEFENCE ELECTRONIC SYSTEMS
CONTAINING LEAD-FREE SOLDER –
Part 2: Mitigation of the deleterious effects of tin

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,
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
A PAS is a technical specification not fulfilling the requirements for a standard, but made
available to the public.
IEC-PAS 62647-2 has been processed by IEC technical committee 107: Process management
for avionics.
The text of this PAS is based on the This PAS was approved for
following document: publication by the P-members of the
committee concerned as indicated in
the following document
Draft PAS Report on voting
107/108/PAS 107/116A/RVD
Following publication of this PAS, which is a pre-standard publication, the technical committee
or subcommittee concerned may transform it into an International Standard.

– 4 – PAS 62647-2  IEC:2011(E)
This PAS is based on GEIA-STD-0005-2 and is published as a double logo PAS. GEIA,
Government Electronics and Information Technology Association, has been transformed into
TechAmerica Association.
This PAS shall remain valid for an initial maximum period of 3 years starting from the
publication date. The validity may be extended for a single period up to a maximum of
3 years, at the end of which it shall be published as another type of normative document, or
shall be withdrawn.
A bilingual version of this publication may be issued at a later date.

PAS 62647-2  IEC:2011(E) – 5 –
INTRODUCTION
This PAS is intended for use by those procuring, designing, building or repairing electronic
assemblies that will use items with Pb-free tin finishes to document processes they use to
assure performance, reliability, airworthiness, safety, and certifiability of those assemblies. It
provides a framework to communicate and agree on the processes to be used to control and
mitigate the use of Pb-free tin in these applications.
The Aerospace Industries Association (AIA), the Avionics Maintenance Conference (AMC),
and Government Electronics and Information Technology Association (GEIA) formed a joint
working group with the express purpose of generating a series of industry standards
documents for the use and handling of Pb-free solder, piece parts, and boards in aerospace
and high performance applications. This PAS – originally published as GEIA-STD-0005-2 –
was prepared by that group. It was balloted and approved by GEIA G-12 (Solid State
Subcommittee) and GEIA Avionics Management Conference (AMC Subcommittee). According
agreements between GEIA and IEC, this PAS is extended at international level.
This PAS is intended to work in concert with IEC/PAS 62647-1 (based originally on GEIA-
1 2
STD-0005-1), GEIA-HB-0005-1 , GEIA-HB-0005-2 . This PAS may be referenced in
proposals, requests for proposals, work statements, contracts, and other documents. It may
be used as a stand-alone standard or as part of compliance with IEC/PAS 62647-1.
This PAS addresses the risk of tin whiskers. However, the state of research into tin whisker
risk still does not allow accurate quantitative estimates of the risk and reliability. It defines
three baseline control levels that detail the amount of attention that should be paid to the risk
of tin whiskers: no restrictions on tin use, some restrictions on tin use, and prohibition of tin
use.
There are three informative annexes in this PAS:
– Annex A provides guidance on selecting control levels and performing risk
assessments;
– Annex B describes mechanisms of formation, properties, and potential deleterious
effects of tin whiskers;
– Annex C provides some background on various mitigation methods.
Due to a variety of real and potential health issues, many constituent materials used in the
production of electronic products have come under scrutiny. The European Union (EU) has
enacted two directives; 2002/95/EC Restriction of Hazardous Substances (RoHS) and
2002/96/EC Waste Electrical and Electronic Equipment (WEEE) that restrict or eliminate the
use of various substances in a variety of products that are produced after July 2006. One of
the key materials restricted is lead (Pb), which is widely used in electronic solder and
electronic piece part terminations, and printed wiring boards. While these regulations may
appear to only affect products for sale in the EU, due to the reduced market share of the
Aerospace and High Performance Industry in electronics, many of the lower tier suppliers are
changing their products because their primary market is consumer electronics. Additionally,
several U.S. states have enacted similar “green” laws, and many Asian electronics
manufacturers have recently announced completely “green” product lines.
The restriction of Pb use has generated a transition by many piece part and board suppliers
from tin-lead (Sn-Pb) surface finishes to pure tin or other Pb-free finishes. Lead-free tin
finishes can be susceptible to the spontaneous growth of crystal structures known as “tin
whiskers” which can cause electrical failures, ranging from parametric deviations to
catastrophic short circuits, and may interfere with sensitive optical surfaces or the movement
___________
A future IEC/PAS 62647-21, based on GEIA-HB-0005-1, is in preparation.
A future IEC/PAS 62647-22, based on GEIA-HB-0005-2, is in preparation.

– 6 – PAS 62647-2  IEC:2011(E)
of Micro-ElectroMechanical Systems (MEMS). Though studied and reported for decades, the
mechanism behind their growth is not well understood, and tin whiskers remain a potential
reliability hazard. Furthermore, the growing number of piece parts with pure tin finishes
means there are more opportunities for whiskers to grow and to produce failures.

PAS 62647-2  IEC:2011(E) – 7 –
PROCESS MANAGEMENT FOR AVIONICS –
AEROSPACE AND DEFENCE ELECTRONIC SYSTEMS
CONTAINING LEAD-FREE SOLDER –
Part 2: Mitigation of the deleterious effects of tin

1 Scope
This PAS establishes processes for documenting the mitigating steps taken to reduce the
harmful effects of tin finishes in electronic systems.
This PAS is applicable to Aerospace and High Performance electronic applications which
procure equipment that may contain Pb-free tin finishes.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply:
2.1
assemblies
electronic items that require electrical attachments, including soldering of wires or component
terminations; examples include circuit cards and wire harnesses. This may include soldered
assemblies
2.2
bright tin
tin finish with higher internal stresses and smaller grain size of 0,5 µm to 0,8 µm and carbon
content of 0,2 % to 1,0 %
2.3
critical
item or function, if defective, will result in the system’s inability to retain operational capability,
meet primary objective, or affect safety
2.4
customer
entity or organization that (a) integrates a piece part, soldered assembly, unit, or system into
a higher level system, (b) operates the higher level system, or (c) certifies the system for use.
For example, this may include end item users, integrators, regulatory agencies, operators,
original equipment manufacturers (OEMs), and subcontractors
2.5
energy dispersive (X-ray) spectroscopy
EDS
method for material composition analysis
2.6
high performance system or product
system or product which requires continued high performance or performance on-demand, or
equipment downtime cannot be tolerated, or end-use environment may be uncommonly harsh
and the equipment must function when required, such as life support or other critical systems

– 8 – PAS 62647-2  IEC:2011(E)
2.7
lead-free
defined as less than 0,1 % by weight of lead in accordance with Waste Electrical and
Electronic Equipment (WEEE) guidelines
2.8
matte tin
tin finish with lower internal stresses and larger grain sizes typically of 1 µm or greater and
carbon content less than 0,050 %
2.9
Pb-free tin
tin defined to be pure tin or any tin alloy with <3 % lead (Pb) content by weight. This means
that some Pb-free finishes other than pure tin, such as tin-bismuth and tin-copper, are
considered to be “tin” for the purposes of this PAS. Many of these alloys have not been
assessed for whiskering behavior
2.10
Pb-free tin finish
Pb-free tin final finishes or underplates either external or internal to a device, board or other
hardware. This includes all leads and surfaces, even those coated, encapsulated, or
otherwise not exposed. It may include finishes on electrical piece parts, mechanical piece
parts, and boards. It does not include Pb-free bulk solders, assembly materials, solder balls,
or those devices where the Pb-free tin finish has been completely replaced
2.11
piece part
electronic component that is not normally disassembled without destruction and is normally
attached to a printed wiring board to perform an electrical function
2.12
rework
act of reprocessing non-complying articles, through the use of original or equivalent
processing in a manner that assures full compliance of the article with applicable drawings or
specifications
2.13
repair
act of restoring the functional capability of a defective article in a manner that precludes
compliance of the article with applicable drawings or specifications
2.14
sub-contractor
organization, within the given high-reliability industry, that supplies, maintains, repairs, or
supports electronic systems, and is not the direct supplier to the customer or user of those
systems
2.15
supplier
entity or organization that designs, manufactures, repairs, or maintains a piece part, unit, or
system. For example, this includes original equipment manufacturers (OEMs), repair facilities,
subcontractors, and piece part manufacturers. In some cases, a single organization may be
both a customer and a supplier. They should follow the requirements for suppliers when
addressing their customer’s contracts and should follow the requirements for customers when
flowing down requirements to their lower tier suppliers
2.16
system
one or more units that perform electrical function(s)

PAS 62647-2  IEC:2011(E) – 9 –
2.17
tin whisker
spontaneous crystal growth that emanates from a tin surface. They may be cylindrical, kinked,
or twisted. Typically they have an aspect ratio (length/width) greater than two, with shorter
growths referred to as nodules or odd-shaped eruptions (OSEs). See Annex B for further
description of tin whiskers and their physical attributes
2.18
unit
one or more assemblies within a chassis to perform electrical function(s)
2.19
X-ray fluorescence
XRF
method for material composition analysis
3 Requirements
3.1 Determination of levels
The customer is responsible for determining the control level they are seeking and identify it
in their request for proposal when this PAS is imposed. They should also determine the level
of oversight and review the program will require. For some programs, different control levels
may be required for different products. In these cases, the customer is responsible for
defining these different levels and their applications or define a process by which they and the
supplier will determine the levels.
The customer and supplier shall agree on the control levels and shall document this
agreement in appropriate control documents.
There will be cases where errors will be made in the finish determination or in the application
of mitigation methods. Customers and suppliers should have processes in place to document
and assess the impact of these errors. Already existing deviation or waiver processes may be
acceptable if technical experts on tin whiskers are consulted.
3.2 Requirements for control levels
3.2.1 General
Each program or system has the responsibility of determining the appropriate control level for
their product. This document is not intended to imply that any category of aerospace or high
performance application is more or less reliable or critical than any other category; nor is it
intended to imply that any aerospace or high performance system will be more or less
reliable, depending on the control level that is selected from the above list. Reliability is
assured by a wide range of design, production, use, and support decisions and activities, of
which tin whisker mitigation is only one. It is expected that, whatever level of mitigation
category is used, the system reliability will be assured by the totality of all the methods
available to the producer and user of the system.
There are many aspects to tin whisker control. For the purposes of this PAS, the activities
have been grouped into four categories:
– documentation of uses of Pb-free tin;
– detecting and controlling Pb-free tin introduction;
– tin whisker risk mitigation;
– tests and analyses of tin whisker risk and mitigation effectiveness.

– 10 – PAS 62647-2  IEC:2011(E)
If only Level 2, with no sub-level, is identified in a control document, the default level shall be
assumed to be Level 2A.
3.2.2 Control Level 1 requirements
3.2.2.1 Requirements for documentation of uses of Pb-free tin
There are no requirements. The supplier should provide general information regarding types
of platings, finishes, and solder used and plans for process controls on those processes in
accordance with 3.3.1. If the supplier is unable to determine some materials, this shall be
stated.
3.2.2.2 Requirements for detecting and controlling Pb-free tin finish introduction
No requirements.
3.2.2.3 Requirements for tin whisker risk mitigation
No requirements.
3.2.2.4 Requirements for tests and analyses of tin whisker risk and mitigation
effectiveness
No requirements.
3.2.3 Control Level 2A requirements
3.2.3.1 Requirements for documentation of uses of Pb-free tin
There are no supplier requirements. The supplier should provide general information
regarding types of platings, finishes, and solder used and plans for process controls on those
processes in accordance with 3.3.1. If the supplier is unable to determine some materials,
this shall be stated.
The customer is responsible for listing any applications where Pb-free tin is not allowed.
3.2.3.2 Requirements for detecting and controlling Pb-free tin finish introduction
No requirements.
3.2.3.3 Requirements for tin whisker risk mitigation
The supplier shall provide descriptions of any mitigation methods assumed to be in use for the
tests and analyses in 3.2.3.4. The supplier shall provide descriptions of any mitigation
measures taken for hardware.
The customer is responsible for defining any mitigation measures that are required or
disallowed.
3.2.3.4 Requirements for tests and analyses of tin whisker risk and mitigation
effectiveness
The supplier shall provide an analysis addressing the risk of tin whiskers in accordance with
3.3.4. This analysis is expected regardless of whether mitigations are applied. If no
mitigations are applied, the analysis should demonstrate why they are not needed. If
mitigations are applied, the analysis should demonstrate that they are effective.
For Level 2A, these analyses may be performed at the process level. For example, the
analysis might address all devices with a particular mitigation technique employed.

PAS 62647-2  IEC:2011(E) – 11 –
3.2.3.5 Exceptions
Specific piece parts, soldered assemblies, units, or applications may be required to meet a
higher level of control. These requirements shall be specified in contractual documents.
3.2.4 Control Level 2B requirements
3.2.4.1 General
For Level 2B hardware, these control plans may cover families of piece part types or
applications. Separate assessments and control plans for each individual item are not
required. For example, one assessment might allow use of all tin-plated capacitors in a variety
of applications.
3.2.4.2 Requirements for documentation of uses of Pb-free tin
The supplier shall provide lists of families of tin-finished piece parts and/or location and
material information for categories of applications where they would like to use Pb-free tin in
accordance with 3.3.1.2. If there are other uses of tin, the supplier shall provide a list of
additional specific applications of Pb-free tin that fall outside these families in accordance
with 3.3.1.3. If the supplier is unable to determine some materials, this shall be stated.
The customer is responsible for listing any applications where Pb-free tin is not allowed.
3.2.4.3 Requirements for detecting and controlling Pb-free tin finish introduction
The supplier should provide a plan for monitoring materials on a sample basis, including
method of test and sampling scheme, in their product in accordance with 3.3.2.1.
3.2.4.4 Requirements for tin whisker risk mitigation
The customer is responsible for defining any mitigation measures that are required or
disallowed.
The supplier shall implement the mitigating measures contractually required by the customer.
It is recommended that at least two mitigation measures in accordance with 3.3.3 be required
and performed.
3.2.4.5 Requirements for tests and analyses of tin whisker risk and mitigation
effectiveness
If a specific risk algorithm or other method for evaluation measure is required, the customer is
responsible for describing them in the request for proposal. The customer is also responsible
for communicating any documentation review or oversight requirements to the supplier.
The supplier shall have documentation covering the following elements:
– the mitigation measures taken for each family of piece parts or applications of Pb-free tin
finish in the product;
– the tests or analyses performed for each family of piece parts or applications using Pb-
free tin finishes, to determine risk of whisker growth in accordance with 3.3.4;
– if there are other uses of Pb-free tin outside the families, the mitigation measures taken
for each piece part or application of Pb-free tin finish in the product outside the families;
– if there are other uses of Pb-free tin outside the families, the tests and analyses performed
for each of these piece parts or applications to determine risk of whisker growth in
accordance with 3.3.4;
– 12 – PAS 62647-2  IEC:2011(E)
– provide the risk assessment and mitigation measures to the customer for their review, as
requested or required by customer.
3.2.5 Control Level 2C requirements
3.2.5.1 General
For Level 2C hardware, separate assessments and mitigation plans are required for each
instance of Pb-free tin finish use. For example, instead of one assessment and mitigation plan
covering all tin-plated capacitors, each capacitor type and application must be reviewed and
approved, even if the same strategy is applied to each situation.
3.2.5.2 Requirements for documentation of uses of Pb-free tin
The supplier shall avoid use of Pb-free tin whenever possible. The supplier shall not allow use
of Pb-free tin finishes without prior written permission of the customer and shall provide the
customer with a list of all uses of Pb-free tin. Each individual use shall be reviewed and
approved or disapproved by the customer.
The supplier shall provide a plan for passing the requirement to lower level suppliers in
accordance with 3.3.1.4.
3.2.5.3 Requirements for detecting and controlling Pb-free tin finish introduction
The supplier shall provide a plan for monitoring materials in their product in accordance with
3.3.2.1. The supplier and customer shall reach an agreement regarding this plan.
For critical piece parts, assemblies or systems, the plan should include sampling at least one
part per lot of all piece parts not approved for tin.
3.2.5.4 Requirements for tin whisker risk mitigation
The customer is responsible for defining any mitigation measures that are required or
disallowed in their request for proposals and contractual documents.
The supplier shall implement at least two mitigation measures in accordance with 3.3.3.
3.2.5.5 Requirements for tests and analyses of tin whisker risk and mitigation
effectiveness
The customer is responsible for describing the risk algorithm or other methods for evaluating
mitigation measures in the request for proposal, if applicable. The customer is also
responsible for communicating any documentation review or oversight requirements to the
supplier.
The supplier shall have documentation covering the following elements:
– the mitigation measures taken for each piece part or application of Pb-free tin finish in the
product;
– the tests and analyses performed for each piece part or application using Pb-free tin
finishes, to determine risk of whisker growth in accordance with 3.3.4;
– provide the risk assessment and mitigation measures to the customer for their review, as
requested or required by customer.
3.2.6 Control Level 3 requirements
3.2.6.1 Requirements for documentation of uses of Pb-free tin
The supplier shall not allow use of Pb-free tin finish.

PAS 62647-2  IEC:2011(E) – 13 –
The supplier shall provide a plan for passing the requirement to lower level suppliers per
3.3.1.4.
The supplier shall maintain records of their monitoring of materials in their product.
3.2.6.2 Requirements for detecting and controlling Pb-free tin finish introduction
The supplier shall monitor the material in their product per 3.3.2.2.
3.2.6.3 Requirements for mitigation of tin whisker risk and mitigation effectiveness
Not applicable, as Pb-free tin finish is not allowed.
3.2.6.4 Requirements for tests and analyses of tin whisker risk and mitigation
effectiveness
Not applicable, as Pb-free tin finish is not allowed.
3.3 Implementation requirements
3.3.1 Documentation of uses of Pb-free tin
3.3.1.1 Documenting general materials and processes (applies to Level 1, Level 2A)
Documentation of general materials and processes should describe the types of finishes and
solders used in the products and the processes corresponding to the manufacturing or
installation of products with Pb-free tin finishes. If different materials and processes are used
for different applications, the application and the corresponding materials and processes
should be listed. Although Levels 1 and 2A do not impose any Pb-free tin finish controls, the
supplier may have processes that limit but do not prohibit the use of Pb-free tin finishes in
some applications.
3.3.1.2 Documenting uses of Pb-free tin finishes by family (applies to Level 2B)
Documentation of specific uses of Pb-free tin finishes shall include a list of each family of
piece parts with a tin-finished surface, which surfaces are tin, and a description of the finish
composition. A family of piece parts might include multiple part numbers of a particular type of
piece part, for example, capacitors or it might include all piece parts from a particular
supplier. The supplier shall also provide a list of what applications will include piece parts
from the family.
3.3.1.3 Documenting specific uses of Pb-free tin finishes (applies to Level 2B and
Level 2C)
Documentation of specific uses of Pb-free tin finishes shall include a list of each individual
piece part with a Pb-free tin-finished surface and a description of the finish composition, and a
list of what applications will include that piece part.
3.3.1.4 Flowing requirements to lower level suppliers (applies to Level 2B, Level 2C,
and Level 3)
Requirements for tin whisker control, analysis, and mitigation are applicable to all purchased
and subcontracted elements and materials for the program. This may require flowing down
these requirements to lower level supplier or performing extensive analysis of purchased
material. The supplier should be prepared to document how they addressed the risk from
purchased equipment if requested or required by the customer.

– 14 – PAS 62647-2  IEC:2011(E)
3.3.2 Detecting and controlling Pb-free tin finish introduction
3.3.2.1 Sample monitoring plans (applies to Level 2B and Level 2C)
A monitoring plan, including method of test and sampling scheme, should be agreed to by the
customer and the supplier for Level 2B and shall be agreed for Level 2C.
For Level 2B, the monitoring plan and sampling scheme should focus on critical hardware
where Pb-free tin usage approvals have not been obtained.
For critical hardware following Level 2C where Pb-free tin usage approvals have not been
obtained, the monitoring plan described in 3.3.2.2 should be used.
Some testing methods and their limitations are described in the Annex C.
3.3.2.2 Lot monitoring requirements (applies to Level 3)
A lot screening program is required for all items with metallic finishes. Items containing
metallic finishes shall be tested by a method agreed to in writing by the customer, at least one
sample per lot or batch received, unless otherwise specified by the customer. A minimum of
3 % Pb by weight is required. Some testing methods and their limitations are described in
Clause C.1.
Customer may allow an exception for monitoring if material specification defines all finishes
as gold with no Pb-free tin finishes and a visual inspection of the device shows it to be gold-
colored. This exception should be stated in contractual paperwork if used.
3.3.3 Methods for mitigating impact of Pb-free tin (applies to Level 2B, Level 2C)
3.3.3.1 General
Since the failure mechanisms for whisker growth are not fully understood, no single method is
an assurance against whiskers in all applications, environments, and lifetimes. The method or
methods used shall be documented in the Pb-free tin finish mitigation plan.
Many mitigations methods have been proposed in the literature and are summarized in Clause
C.2. Additional information can be found in JP002. Recommended mitigations, and some
information on circumstances where they might be most useful, are presented below. The
categories in this section represent four independent approaches; multiple actions taken in
one category are not considered separate mitigations. Other mitigations methods may be
applied but documentation regarding their effectiveness shall be provided.
The selection of the appropriate mitigation techniques will depend on the assessment of the
application and data regarding the mitigation strategies. The discussion of risk assessments
provided in Clause A.3, may help in selecting appropriate mitigation combinations. Some
combinations of mitigations may be more effective and make more sense than others. The
combination for a particular application should be carefully evaluated to make sure that the
mitigations will successfully work together to reduce the risk of whiskers.
3.3.3.2 Design to reduce whisker impact
Although design for reduced whisker impact could include several areas, such as piece part
selection and process changes, for the purposes of this PAS it means board design to reduce
the chance of whiskers causing shorts even if the whisker has grown. Design mitigations
could include the spacing of surfaces on which a whisker could bridge and cause shorts and
the use of physical barriers.
Empirical studies over the past several decades have shown the distribution of lengths and
density of tin whisker population can span wide ranges. For example, many tin finishes

PAS 62647-2  IEC:2011(E) – 15 –
seemingly may never grow whiskers while others may grow large numbers of whiskers in
excess of several millimetres in length. In extremely rare cases maximum lengths on the order
of 10 mm have been documented. Intuitively, the risk of tin whisker induced shorting
decreases as the minimum shorting distances in the circuit design increase. More information
on whisker growth rates and lengths is provided in the informative Annex B.
The spacing requirements are left to the discretion of the customer and supplier, as different
applications may be tolerant of different levels of risk.
Placement of a physical barrier can prevent whiskers from growing from one conductive
surface to another. Because whiskers can grow through oils, greases and softer lacquers,
care must be taken in selecting the material of the barrier; the harder materials are much
more effective, provided they remain intact. While a barrier may prevent a whisker growing
from one conductive surface to another, it cannot mitigate the risks associated with free-
floating whiskers unless a combination of barriers fully encase the tin-finished area.
3.3.3.3 Select lower risk Pb-free tin finishes
Although all tin finishes (including those with >3 % Pb content) have some risk of whiskers,
some finishes have greater risk than others. Selection of a matte tin, preferably with a nickel
underplate, or a Pb-free tin alloy with nickel underplate are generally considered lower risk
finishes than bright tin or tin finishes over copper. Impact of underplate thicknesses and other
lower risk finishes is under review in the literature. Groups such as international Electronics
Manufacturing Initiative (iNEMI) may have additional guidelines on finish selection.
There are also an increasing number of suppliers who provide Pb-free tin finishes that are
warranted against whiskers. Use of these finishes is considered mitigating if the application
environment and product life length are compatible with the warranty.
If this mitigation method is applied, references or documentation supporting selected finish
performance shall be provided.
For the purposes of this PAS, heat treatments are considered to be in the category of lower
risk finishes. Heat treatments could include reflow, fusing, or annealing at the piece part or
board levels. More information on the impact of these processes is provided in C.2.2.4 and
C.2.2.5.
3.3.3.4 Tin finish replacement
As stated in the definition, if all Pb-free tin finishes on the device have been replaced through
replating or solder-dipping then the device is no longer considered to be tin-finished.
However, if only some tin-finished surfaces have been reworked, then the actions are
considered to be mitigations. For example, if tin-finished leads are solder dipped in Sn-Pb, but
the dipping does not reach the piece part body, the dipping is only a mitigation and not a full
replacement. If there are tin-finished surfaces that cannot be reworked because of
encapsulation, then the replacement of exposed Pb-free tin finishes is only a mitigation.
Replacement of finish may lead to suppliers no longer guaranteeing the performance of piece
parts. Therefore, some analysis and possibly qualification of the re-processed piece parts
may be needed to verify that they will function as intended. It is recommended that customers
require objective evidence that the functionality of the piece part has not been compromised
by the aftermarket processing. This is an area of active research, and several groups are
working on best practices to avoid damaging the piece parts.
3.3.3.5 Conformal coat
The use of conformal coating does not eliminate the potential for whisker failure entirely;
however, it is a mitigating measure. For most coatings, tin whiskers have been shown to
eventually grow through thin coatings. Although there is believed to be low risk of a whisker

– 16 – PAS 62647-2  IEC:2011(E)
penetrating the coating of an adjacent surface, whiskers could still short to other uncoated
surfaces in the area. For some piece part types, it is difficult to entirely encapsulate the
individual interconnects so there remains a direct path for shorting due to whisker growth.
Other drawbacks include bubbles between tightly spaced leads and connector mating pin
overspray contamination. Coating materials and processes should also be carefully reviewed
to ensure their compatibility with the hardware design and application. For example,
excessively thick coatings and/or mismatches of coefficients of thermal expansion may reduce
life expectancy of solder joints or crack piece parts.
It is recommended that an assessment of the conformal coating process be performed to
assure that coverage and thickness are adequate and consistent. It is also recommended that
the conformal coat process be regularly evaluated and that the evaluation results be available
for customer review.
3.3.4 Methods for analysis and evaluation of tests and mitigations for tin whisker
risk and mitigation effectiveness
3.3.4.1 General
The customer and supplier should evaluate their products, applications, and environments
and evaluate how test data and mitigation strategy applies to those conditions. A number of
different analyses might be appropriate for this requirement. Determination of the appropriate
analysis should be made by the supplier and customer.
3.3.4.2 Analysis of application tolerance
Analyses of application tolerance might include descriptions of spacing distances versus
distribution of whisker sizes and whisker density, examination of barrier locations or
conformal coat coverage, or Failure Modes and Effects Analyses (FMEA) of the impact of
shorts, plasma events, and possible micromechanical dysfunction.
Tolerance analyses might determine that tin whiskers have no impact or that there is some
level of impact. If there is possible impact, the supplier and customer should evaluate those
risks and impacts and determine if they are acceptable.
In some cases, the analysis may involve a discussion of the overall reliability model or unit
design. For example, it may be that the application has adequate margin, a redundancy, or a
maintenance schedule to allow for a probability of whiskers without impacting mission
performance. It may also be the case that field data on particular tin-finished piece parts in
similar environments exists, and can be used to support a low risk of whisker impact.
3.3.4.3 Analyses of whisker propensity tests
Although the mechanisms for tin whisker growth are still unknown, there are several test
methods being used by suppliers in the industry. Industry standard tests do provide a
common method and comparable data.
Many piece part suppliers are involved with developing tin whisker test methods and finish
qualification methods. Some examples include JESD22-A121 “Test Method for Measuring
Whisker Growth on Tin and Tin Alloy Surface Finishes” and JESD201 “Environmental
Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface
Finishes.” Tests typically include aged samples being put through thermal cycling, humidity,
high temperature, and ambient testing. Acceptable test results alone should be
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