IEC TR 60068-3-82:2024

Environmental testing - Part 3-82: Supporting documentation and guidance - Confirmation of the performance of whisker test method

IEC TR 60068-3-82:2024, which is a Technical Report, provides technical background information on the whisker test methods from IEC 60068-2-82 and guidance on test selection.

General Information

• Status: Published
• Publication Date: 14-Aug-2024
• ICS: 19.040 - Environmental testing
• Technical Committee: TC 91 - Electronics assembly technology
• Drafting Committee: WG 3 - TC 91/WG 3
• Current Stage: PPUB - Publication issued
• Start Date: 15-Aug-2024
• Completion Date: 23-Aug-2024

Overview

IEC TR 60068-3-82:2024 is a Technical Report that provides technical background and practical guidance for confirming the performance of the whisker test method referenced in IEC 60068-2-82. It focuses on tin (Sn) whisker phenomena in electronic components and surface finishes, explains underlying growth mechanisms, and gives guidance on selecting and performing appropriate whisker testing and whisker inspection methods as part of environmental testing and reliability assessment.

Key topics and technical guidance

The report covers the following key technical areas and requirements:

• Whisker growth mechanisms

  • Detailed discussion of Sn whiskers, intermetallic compound (IMC) formation, corrosion effects, and the impact of grain structure and surface finish (bright vs. matte Sn).
  • Effects of coefficient of thermal expansion (CTE) mismatch and temperature cycling on whisker initiation and growth.
  • Process- and design-related factors that influence whisker risk (plating, diffusion barriers, flux residues, mechanical stress, press-fit and bending operations).

• Whisker testing guidance

  • Preconditioning recommendations for different application scenarios (press-fit, mechanical loading, soldering/welding).
  • Environmental test types relevant to whisker formation: ambient storage, damp heat, temperature cycling, and application-specific ambient tests for press-fit connections.
  • Considerations on test severity and combining environmental stresses for realistic assessment.

• Inspection and measurement requirements

  • Comparison and guidance on detection methods including light optical inspection and scanning electron microscopy (SEM).
  • Verification of inspection methodology: detection capability, length and density measurement capability, and overall criteria for reliable whisker quantification.
  • Guidance on technological similarity when extrapolating test results between related materials or component designs.

Practical applications - who uses this standard

IEC TR 60068-3-82 is aimed at professionals responsible for electronic reliability and compliance, including:

• Reliability and test engineers assessing tin whisker risk on components and assemblies
• PCB and connector designers evaluating surface finish and press-fit interactions
• Component manufacturers and contract manufacturers validating plating and process controls
• Independent test laboratories performing environmental and whisker testing
• Procurement and quality assurance teams setting acceptance criteria for assemblies in automotive, aerospace, medical, and industrial electronics

Related standards

• IEC 60068-2-82 - Whisker test methods (main test procedures referenced by this TR)
• IEC 60068 series - Environmental testing standards and guidance for broader context

IEC TR 60068-3-82:2024 is a practical resource for understanding tin whisker mechanisms, selecting appropriate environmental tests, and implementing robust inspection methods to manage whisker-related reliability risks.

Standards Content (Sample)

IEC TR 60068-3-82 ®
Edition 1.0 2024-08
TECHNICAL
REPORT
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Environmental testing –
Part 3-82: Supporting documentation and guidance – Confirmation of the
performance of whisker test method

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IEC TR 60068-3-82 ®
Edition 1.0 2024-08
TECHNICAL
REPORT
colour
inside
Environmental testing –
Part 3-82: Supporting documentation and guidance – Confirmation of the

performance of whisker test method

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 19.040  ISBN 978-2-8322-9494-9

– 2 – IEC TR 60068-3-82:2024 © IEC 2024
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Whisker growth mechanisms . 10
4.1 General . 10
4.1.1 Sn whiskers . 10
4.1.2 Sn surface finishes . 10
4.2 Basic Sn whisker mechanisms . 12
4.2.1 General remarks . 12
4.2.2 IMC growth . 13
4.2.3 Corrosion . 21
4.2.4 Coefficient of Thermal Expansion (CTE) mismatch – Temperature
cycling test . 26
4.2.5 Influential process factors . 31
5 Whisker testing . 44
5.1 Preconditioning . 44
5.1.1 Pre-aging before testing . 44
5.1.2 Preconditioning of test specimen intended for press-fit applications . 45
5.1.3 Preconditioning of test specimen intended for mechanical loads other
than press fit . 45
5.1.4 Preconditioning of test specimen intended for soldering / welding . 45
5.2 Ambient test . 46
5.2.1 General . 46
5.2.2 Test severity . 46
5.3 Damp heat test . 47
5.3.1 General . 47
5.3.2 Test severity . 47
5.4 Temperature cycling test . 47
5.4.1 General . 47
5.4.2 Test severity . 47
5.5 Ambient test for press-fit applications . 48
5.5.1 General . 48
5.5.2 Test severity . 48
6 Whisker inspection and measurement . 48
6.1 Inspection and detection methods . 48
6.2 Comparison of the methods . 48
6.2.1 Light optical inspection . 48
6.2.2 Scanning electron microscopy (SEM) inspection . 49
6.3 Verification of inspection methodology . 49
6.3.1 General remarks . 49
6.3.2 Overall criteria . 49
6.3.3 Capability of whisker detection . 50
6.3.4 Capability of whisker length measurement . 50
6.3.5 Capability of whisker density measurement . 51
6.4 Technological similarity . 51
Bibliography . 53

Figure 1 – Cross-sectional views of component termination surface finishes . 8
Figure 2 – Grain size and whisker growth on bright Sn and matte Sn finishes [10] . 11
Figure 3 – Example comparison of IMC formation between a Sn surface deposit and
Cu based substrate . 12
Figure 4 – Whisker formation in Sn layer [14] . 13
Figure 5 – Stress gradients in Sn layer [15] . 14
Figure 6 – An example of whisker growth (length) from approximately 2,5 µm matte Sn
plated on Cu aged at ambient conditions (RT/RH) . 14
Figure 7 – Microstructures of different Sn and SnPb surface finishes [5] . 15
Figure 8 – Stress states of different Sn and Sn/Pb surface finishes [5]. 16
Figure 9 – Effect of post-bake heat treatment on microstructure and stress gradients [16] . 17
Figure 10 – Stress states of different Sn surface finishes [16] . 18
Figure 11 – 2D-XRD analysis of Sn surface finishes [22]. 19
Figure 12 – IMC formation of Sn surface finishes . 19
Figure 13 – The compressive stress levels of matte Sn finishes without and with a Ni
barrier and the corresponding whisker growth [12] . 20
Figure 14 – Whisker growth with several factors and saturation with a Ni barrier [23],
[24] 21
Figure 15 – Schematic of corrosion stress in a Sn film and its redistribution capabilities . 21
Figure 16 – Grain orientation of different Sn surface finishes [32] . 23
Figure 17 – Percentage of corroded area after contamination and damp heat aging [32] . 24
Figure 18 – Whisker density with different humidity [27] . 25
Figure 19 – Frequency and length of whiskers after a thermal cycling test . 27
Figure 20 – Comparison max. whisker length with several base materials and
combining environment [25] . 28
Figure 21 – Comparison of whisker growth in an ambient experiment and a basic
experiment combining environmental stresses [25] . 29
Figure 22 – Distribution of whisker length grown on FeNi (Alloy42) base material after
300 cycles . 29
Figure 23 – Whisker growth on FeNi (Alloy42) base material for thermal cycling with ∆t
of 65 °C, 95 °C and 125 °C . 30
Figure 24 – A relationship of ∆ϑ and number of cycles for whisker growth on FeNi
(Alloy 42) base material to reach 100 μm . 31
Figure 25 – FIB and SEM images of the imprint in the Sn film due to a probe needle
[33] 32
Figure 26 – Whisker growing between to connector pins as a result of the externally
applied stressed from the plastic overmold [24]. 32
Figure 27 – Sn plating surfaces and IMC structures after bending by Trim and Form
and without bending [23] . 34
Figure 28 – Schematic representation of a press-fit connection [35] . 35
Figure 29 – A simulation of the stress distribution and corresponding stress gradients
in a press-fit zone [34] . 36
Figure 30 – Contact grooves in the PCB hole from press-in operation and cross-section
of a Sn plated pin with Ni underlayer [35] . 37
Figure 31 – Focused-ion beam investigations of different surface finishes [36] . 38
Figure 32 – Whisker growth from an iSn with Ag additive (for whisker mitigation) plated
via after 1 000 h at 85 °C / 85 % RH aging . 39

– 4 – IEC TR 60068-3-82:2024 © IEC 2024
Figure 33 – Pure Sn plated pin after Pb-free reflow process using solder paste under
serial production conditions . 40
Figure 34 – Sn plating after 3x reflow (40 s at 260 °C) [23] . 40
Figure 35 – Appearance of the Sn surface due to the various flux systems and their
corresponding residues after reflow (min. Profile) and 85 °C/85 % RH exposure . 41
Figure 36 – Representative whisker growth near areas where flux residue is located . 42
Figure 37 – Whisker density with no flux and several flux types . 42
Figure 38 – Sn whisker growth at the area with Al welding point . 43
Figure 39 – Feature of formation area of Sn whisker welding point . 44
Figure 40 – Effect of viewing angle on whisker detection . 49

Table 1 – Materials used for a diffusion barrier along with their typical thickness,
process parameters and quality criteria. . 16
Table 2 – Standard electrical potential for selected chemical elements . 22
Table 3 – Overview of Sn whisker results using different testing conditions . 25
Table 4 – Overview of Sn whisker results using different components . 26
Table 5 – Relationship between base material CTE, ∆CTE to Sn and the maximum
whisker length after thermal cycle testing. 27
Table 6 – Overview of situations where an external mechanical force is applied to the
Sn surface finish and their impact on whisker growth . 33
Table 7 – Overview of the tested fluxes for their impact on the whisker growth . 41
Table 8 – Examples of technological similarity . 52

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ENVIRONMENTAL TESTING –
Part 3-82: Supporting documentation and guidance –
Confirmation of the performance of whisker test method

FOREWORD
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IEC TR 60068-3-82 has been prepared by IEC technical committee 91: Electronics assembly
technology. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft Report on voting
91/1957/DTR 91/1967/RVDTR
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Report is English.

– 6 – IEC TR 60068-3-82:2024 © IEC 2024
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 60068 series, published under the general title Environmental testing,
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 webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
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ENVIRONMENTAL TESTING –
Part 3-82: Supporting documentation and guidance –
Confirmation of the performance of whisker test method

1 Scope
This part of IEC 60068, which is a Technical Report, provides technical background information
on the whisker test methods from IEC 60068-2-82 and guidance on test selection.
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 60068-2-82:2019, Environmental testing – Part 2-82: Tests – Test XW1: Whisker test
methods for components and parts used in electronic assemblies
IEC 61190-1-1, Attachment materials for electronic assembly – Part 1-1: Requirements for
soldering fluxes for high-quality interconnections in electronics assembly
IEC 62483, Environmental acceptance requirements for tin whisker susceptibility of tin and tin
alloy surface finishes on semiconductor devices
ISO 9454-2:2020, Soft soldering fluxes – Classification and requirements – Part 2: Performance
requirements
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp

– 8 – IEC TR 60068-3-82:2024 © IEC 2024
3.1
whisker
metallic protrusion that grows spontaneously during storage or use
Note 1 to entry: Whiskers typically do not require any electrical field for their growth and are not to be confused
with products of electrochemical migration. Signs of whiskers include:
– striations in growth direction;
– typically no branching;
– typically constant diameters.
Exceptions are known but are rare and can require detailed investigation.
For the purposes of this document, whiskers are considered if:
– they have an aspect ratio (length/width) greater than 2;
– they have a length of 10 μm or more.
Note 2 to entry: For the purposes of this document, whiskers have the following characteristics:
– they can be kinked, bent, or twisted; they usually have a uniform cross-sectional shape;
– they may have rings around the circumference of the column.
Note 3 to entry: Whiskers are not to be confused with dendrites, which are fern-like growths on the surface of a
material, which can be formed as a result of electro(chemical)-migration of an ionic species or produced during
solidification.
Note 4 to entry: Whiskers are not to be confused with slivers as generated by mechanical metal processing.
Whiskers are not to be confused with tubular SnO structures, which may develop under damp-heat test conditions.
These structures are hollow and are typically lacking striations occurring on Sn whiskers.
[SOURCE: IEC 60068-2-82:2019, 3.1]
3.2
termination
solderable element of a component consisting of the following elements:
– base material;
– underlayer (or underlayer system, if more than one underlayer is present), if any, located
under the final plating;
– final Sn or Sn alloy finish.
See Figure 1.
a) Gull wing termination b) Chip termination

Key
a base material;
b underlayer (or underlayer system, if more than one underlayer is present), if any, located under the final plating;
c final tin or tin alloy finish.
Figure 1 – Cross-sectional views of component termination surface finishes

3.3
∆CTE
CTE mismatch
coefficient of thermal expansion mismatch
coefficient calculated by taking the absolute after subtracting the CTE of the base material from
the CTE of the surface finish layer:
∆CTE = | C – C |
f b
where
C is the coefficient of thermal expansion of the surface finish layer;
f
is the coefficient of thermal expansion of the base material
C
b
Note 1 to entry: No underlayer system (e.g. Ni, Cu) has any influence on the CTE mismatch.
3.4
mechanical load
load related to the intended mounting/assembly condition of a particular specimen (e.g. press-
fit application: stress exerted by the plated through-hole on the press-fit pin), or as a transitional
load related to a mechanical process in a trim and form operation to adapt the shape of the
specimen to the intended use condition (e.g. bending of a connector pin)
Note 1 to entry: Mechanical load in the context of these test methods is not related to external factors, e.g. thermo-
mechanical loads arising from the mismatch of the coefficients of thermal expansion of the various constituents of a
particular test specimen upon temperature change.
3.5
classification
3.5.1 Level A
consumer products, some computer and computer peripherals,
and hardware suitable for applications where the major requirement is function of the completed
assembly
3.5.2 Level B
communications equipment, sophisticated business
machines, and instruments where high performance and extended life is required, and for which
uninterrupted service is desired but not mandatory
Note 1 to entry: Typically, the end-use environment would not cause failures.
3.5.3 Level C
equipment where continued performance or
performance-on-demand is mandatory; equipment downtime cannot be tolerated, end-use
environment can be uncommonly harsh, and the equipment shall function when required, such
as life support systems and other critical systems
Note 1 to entry: The classification of levels A, B and C is based on IEC 61191-1 [1] .
___________
Numbers in square brackets refer to the Bibliography.

– 10 – IEC TR 60068-3-82:2024 © IEC 2024
4 Whisker growth mechanisms
4.1 General
4.1.1 Sn whiskers
4.1.1.1 Features of Sn whiskers
Sn whiskers are metallic protrusions, which can grow spontaneously during storage or use from
Sn or Sn alloys. For information on whisker characteristics and their various forms, refer to 3.1
and in IEC 60068-2-82:2019. They can grow up to millimeters, even centimeters long, which is
long enough to branch over to a neighboring electrical contact. Being metal growths, this has
often led to short circuit failures and system malfunctions. They are not produced during plating
(e.g. electrochemical effects during galvanic deposition or dendrite growth), but instead grow
afterwards, during storage or use.
4.1.1.2 Sn whiskers growth rate and mechanism
Depending on the conditions, Sn whisker have been observed growing within hours after
deposition or first found creating a system failure after 10 years in service, which is why they
are often referred to as spontaneous growths. Sn whiskers were already reported as early as
1951 by Compton [2] including the investigation of other metals. Over the last 70 years many
contradictory findings have been reported in the literature as well as several whisker theories
formed, but still no universal model exists today [3]. However, it is commonly agreed that
compressive stress in the Sn film is the fundamental driving force behind whisker growth [4],
[5], making whisker growth a stress relief mechanism for the surface finish. Compressive stress
sources within the film or applied on the film lead to stress gradients within the deposit,
generating atomic migration, which promotes the transportation of atoms to a whisker
nucleation site. Migration takes place over long-range diffusion [6] throughout the film,
predominately along the grain boundaries, however, also along the surface and interface. As
atoms continue to build-up at a nucleation site, a whisker, in return, can grow out of the film,
reducing the stress state. In general, all factors that create stress gradients within the film or
promote diffusion increase the whiskering tendency.
4.1.1.3 The role of this document on Sn whiskers
Although Sn is not the only metal known to whisker, it is the surface finish of focus for this
technical report, since Sn whiskers have been the culprit for many system malfunctions
throughout the past and Sn is commonly used in industry due to its beneficial characteristics.
Its low melting temperature (231,9 °C) makes it very attractive for soldering applications and its
contact resistance, corrosion resistance and low cost has made it, in general, one of the more
favorable platings of choice for electronic finishes. The incorporation of lead (Pb) as an alloying
element in Sn is the most universally successful prevention method against Sn whisker induced
failures. However, due to mandated regulations restricting the use of Pb in electronics [7], [8],
the risk of electrical shorts due to Sn whisker growth remains. Still to this day whisker growth
characteristics, such as propensity, length and rate are unpredictable, and vary from plating to
plating as well as with substrate material. Therefore, it is important to define a set of tests as
guidelines to be able to compare and assess the whiskering propensity of different surface
finish/substrate combinations in a structured manner. To effectively do this one must first have
an understanding for the basic Sn whisker growth mechanisms.
4.1.2 Sn surface finishes
4.1.2.1 General
For all practical industry purposes, Sn is plated through galvanic deposition or hot-dip tinning.
Due to the ease using these plating options for Sn and its low material cost, it generally does
not deem appropriate to use other more expensive plating techniques such as chemical or
physical vapor deposition. Sn alloys are also used depending on application. For information
regarding Sn alloys, see also [9].

4.1.2.2 Galvanic Sn plating
Galvanic Sn plating is often the method of choice since it offers a larger spectrum of plating
possibilities, especially selective plating for other surface finish options on the mating side. The
plating can be applied for different geometries as individual piece parts through barrel or rack
plating, or in a strip format on a plating line. The electrolytes and therefore, their finishes are
mainly divided into two different types: bright or matte. Other than just appearance, the main
differences between the two deposit types are smaller grain size and higher carbon (C) content
for the bright Sn, typically < 1 µm and > 1 000 ppm C respectively, whereas matte Sn generally
has an average grain size of a few microns and < 150 ppm C in the finish, as stated in Table 1
of IEC 60068-2-82:2019. Though bright Sn has an excellent cosmetic appearance, the higher
amount of co-deposited C, as a result of the organic brighteners used in the electrolyte, end up
creating a higher internal stress in the Sn finish. Carbon in a Sn deposit can be measured using
different methods, such as Glow Discharge Optical Emission Spectroscopy [11] or Auger-
electron spectroscopy in combination with sputter-depth profiling.
Furthermore, the smaller grain sizes in bright Sn do not help against whisker growth. Smaller
grains in general lead to more grains on the surface and the probability for more low index
grains, which make for great whisker nucleation sites. Smaller grains also mean additional grain
boundaries, which promotes diffusion and easier acccess to a whisker root. For example, a
layer with an average grain size of 1 µm has a 1 000x higher diffusion rate than a layer with
approximately 10 µm grains. An example of grain size and whisker growth from a bright Sn
("SnA" sulfuric acid base) vs. matte Sn finish ("SnC" MSA base) is given in Figure 2. Here the
maximum whisker lengths are compared after 5 years of incubation at room temperature /
relative humidity (RT/RH) conditions, where the impact on whisker growth from a bright Sn finish
is clear.
Bright Sn Matte Sn
a) grain size/orientation
b) whisker growth between a bright Sn deposit and a matte Sn both approximately 2 µm to 3 µm thick

Figure 2 – Grain size and whisker growth on bright Sn and matte Sn finishes [10]

– 12 – IEC TR 60068-3-82:2024 © IEC 2024
Consequently, matte Sn is usually used for electronic finishes to reduce the internal film
stresses and thus, the whisker risk. All galvanic surface finishes discussed throughout the
remainder of this chapter will therefore, be based on matte Sn deposits.
There exist numerous different matte Sn electrolytes available on the market today with the
majority of them using a methanesulfonic acid (MSA) base. In order to achieve a homogeneous
layer thickness throughout the surface and to reduce the redox reaction, different additive
systems and an antioxidant are utilized. The design of the electrolytes can be optimized for
certain plating "purposes" such as complicated geometry, high plating speed, low plating speed,
appearance, plating distribution, etc. To achieve different demanding targets the concentration
of the electrolyte components (e.g.: Sn, MSA, and additives) as well as the plating parameters
can also be adjusted (e.g.: temperature, current density, and agitation). The whisker propensity
for galvanic Sn platings strongly depend on the electrolyte chemistry, process parameters and
plating method. Therefore, every component-geometry, plating method, electrolyte and set-up
of plating parameters requires a specific assessment and qualification.
4.1.2.3 Hot-dip tinning
Due to the low melting point of Sn (231,9 °C), it can be hot-dip plated. Hot-dip plating is the
immersion of the base material into the molten Sn after the surface has been first appropriately
prepared (e.g.: cleaning, fluxing), which means that selective plating is not an option for tinning.
The molten bath can be easily alloyed, often with Ag and/or Cu and the temperature typically
varies between 250 °C and 290 °C depending on alloying elements and process set-up. It is
broadly used for mechanical components as pre-plated material, but also for various electronic
components. The resultant surface tends to be smooth and shiny.
A significant difference between a Sn finish which is hot-dip plated compared to galvanic plated,
especially when regarding whisker growth, is the intermetallic compound (IMC) formation
between Sn and Cu based substrate materials, due to the high temperatures needed for tinning,
see Figure 3. IMC formation is automatically present in hot-dip Sn deposits, in the as plated
state, but not in galvanic plated finishes, which are plated at much lower temperatures typically
between approximately 20 °C to 40 °C depending on the electrolyte. The various IMCs between
Sn and Cu at different temperatures and their effects on whisker growth are explained below,
in 4.2.1.
a) a hot-dip plated Sn finish b) an electroplated Sn finish
Figure 3 – Example comparison of IMC formation between
a Sn surface deposit and Cu based substrate
4.2 Basic Sn whisker mechanisms
4.2.1 General remarks
As stated in 4.1.1, whiskers grow as a result of compressive stress gradients within the film,
which promote Sn atom migration. In the past, the focus tended to lie mainly on the macroscopic
stress of the Sn finish. However, macroscopic stress is not the only relevant driving force for
whisker formation. In fact, it is the microscopic compressive stress sources, creating
microscopic stress gradients throughout the film, which play a significant role in whisker growth
[12]. If these factors are present within a certain range, whisker growth can be expected.

The basic stress sources known for generating Sn whisker growth, as well as any combination
of them together, include (in no particular order):
a) irregular IMC growth between Cu and Sn, which can vary between Cu-based substrates;
b) corrosion at the Sn surface and/or at the grain boundaries;
c) coefficient of thermal expansion (CTE) mismatch between base material and Sn surface
finish;
d) compressive external mechanical forces on the Sn finish.
Throughout the following clauses these stresses will be based around galvanic matte Sn
finishes. The individual factors will be discussed in greater detail throughout the following sub-
clauses. It is evident that the Sn grain orientation as well as shape/size directly influence the
whiskering properties of the deposit. Depending on the system as well as its working and
environmental conditions, any of these factors can take precedence over the other when it
comes to whisker growth.
4.2.2 IMC growth
Sn and Cu react with one another at ambient room temperature (RT) conditions, forming an
irregular IMC, Cu Sn , through Cu diffusion into the Sn grain boundary. This IMC formation is
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accompanied by a specific volume increase, resulting in a residual compressive stress in the
depth of the Sn coating where the IMC forms due to the constraint imposed by the substrate
[13]. Such localized stress sources create stress gradients (compressive stress by the IMC and
tensile by the surface), resulting in Sn-atom diffusion away from the area where the IMC is
formed. The stress gradients exist vertically in the Sn layer towards the surface, as well as
horizontally in the surface. An illustration of the situation is shown in the following Figure 4 a)
compared to a FIB cut from an actual matte Sn film deposited on Cu based substrate in b),
where a whisker is growing out of the surface directly in the area of one of the high IMC stressed
locations. The columnar grains seen here are typical for matte Sn finishes.

a) Schematic illustration of the presence of the b) a FIB-cut of Sn on Cu-alloy
stress gradients in the Sn layer due to the
formation of irregular IMC (Cu Sn )
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Figure 4 – Whisker formation in Sn layer [14]
A modulation of the stress distribution will be established because of the certain grain size,
leading to hot spots (intensive source of vacancies) near the surface, creating tensile stress.
Figure 5 shows a FEM simulation of the stress formed due to the irregular Cu Sn IMC growth
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in a 5 µm thick a) and 10 µm thick b) Sn finish [15]. Here the stress flow in a Sn finish can be
clearly seen – from the compressive stress due to the IMC formation to the tensile stress on
the surface – explaining the stress gradients and Sn atom migration to a whisker root near the
surface. From these simulations, it is clear that thicker Sn layers result in smaller stress
gradients and less localized tensile stressed, hot spots at the surface. Instead, the size of the
hot spots are larger and wider, meaning a larger volume where the Sn can be redistributed,
leading to a reduction in whisker lengths and therefore, risk of whisker induced failure as well.

– 14 – IEC TR 60068-3-82:2024 © IEC 2024

a) 5 µm thick b) 10 µm thick
Figure 5 – Stress gradients in Sn layer [15]
As a result of this irregular RT Cu Sn IMC growth and its effect on whisker growth, where
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these occur, it is crucial to carry-out ambient whisker tests, as depicted in 5.1, Figure 2 and
details described in 6.1 of IEC 60068-2-82:2019. Since whiskers grow as a stress relief
mechanism for the Sn finish, the growth tends to eventually saturate at some point over time.
Different deposits have different saturation tendencies depending on their grain size/orientation,
grain growth/recrystallization, IMC formation, etc. For practical comparison of the whiskering
propensity between Sn finishes, 4 000 h was chosen for ambient tests as described in
IEC 60068-2-82:2019, 6.1. An example is given in Figure 6. However, this does not guarantee
that all Sn whisker growth is completed, for example, refer to Figure 14.

Figure 6 – An example of whisker growth (length) from approximately 2,5 µm matte Sn
plated on Cu aged at ambient conditions (RT/RH)
One method to effectively counteract whisker growth due to irregular IMC formation is to
implement a way to redistribute the stress gradients in the Sn layer. The average columnar
grains deposited from typical matte Sn electrolytes cannot redistribute the stress effectively in
the Sn matrix, since diffusion takes place along the grain boundaries. Here equi-axed or
globular grains would be very useful. In fact, before the restrictions on Pb, when SnPb galvanic
platings were utilized, the grains in the SnPb deposits were globular, which helps explain why
SnPb platings did not result in Sn whisker induced failures. However, this is not the natural
tendency for pure matte Sn deposits, but such a structure could be produced by manipulating
the plating parameters and/or adjusting the electrolyte. Figure 7 c) shows just such a case
where the Sn deposit was manipulated in lab (beaker glass) into a more globular structure and
compares it to the typical columnar matte Sn a) as well as SnPb b) grain structures [5]. Here
the columnar Sn grains a) and globular Sn grains c) where both deposited from the same
electrolyte. Only the plating parameters were changed.

c) manipulated globular Sn on
a) normal columnar Sn b) SnPb
Cu at different time periods
after plating to compare grain
growth and IMC formation over
time
Figure 7 – Microstructures of different Sn and SnPb surface finishes [5]
Ambient whisker tests were carried out on all 3 variations. After 1 year of incubation at RT/RH
conditions no whiskers could be found from the SnPb or globular Sn samples. However, the
columnar Sn finish produced whiskers up to 500 µm long. This demonstrates why it is crucial
that even if just a plating parameter is changed, the resultant Sn deposit needs to be tested
and re-qualified.
In Figure 7 it is clear, that the globular grains grow over time, whereas the columnar grains do
not. It is also noticeable that the IMC formation for both pure Sn films is comparable in intensity.
From the IMC formation point of view, one could expect a similar response for the whisker
growth, but this is not the case. The imposed compressive stress due to irregular IMC formation
could be redistributed not only vertically, but horizontally as well, by the system with a more
globular grain structure. The energy from the irregular IMC induced compressive stress is used
for internal grain growth instead of whisker growth, as Sn atoms are no longer limited to only
vertical diffusion to the surface. This was not the case for the columnar gr
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