A New Approach to Mitigating Tin Whiskers

A New & Better Approach to Tin Whisker Mitigation
Cheryl Tulkoff ctulkoff@dfrsolutions.com SMTAI Tin Whisker Tutorial Orlando, FL 2012
1

Abstract
Tin whiskers are hair-like single crystal metallic filaments that grow from tin films. Their unpredictability is the greatest concern and the Aerospace and Defense industries consider tin whiskers the, "greatest reliability risk associated with Pb-free electronics".
The potential failure modes include:
Direct contact causing an electrical short (arcing). This requires whisker growth of sufficient length and in the correct orientation
Electromagnetic (EM) Radiation where the whisker emits or receives EM signal and noise at higher frequencies and causes deterioration of the signal for frequencies above 6 GHz. This is independent of whisker length.
Debris which results from a whisker which breaks off and shorts two leads (primarily during handling). Mitigation efforts can fail when only one source of stress is accounted for. For example, if a Ni layer is used to prevent stress from IMC formation, this will not address whisker formation due to corrosion or external pressure.
All the potential sources of stress for a particular application should be considered in the whisker mitigation process. The various sources of compressive stresses that drive whisker growth are rather well understood but to effectively mitigate tin whisker growth one needs to ensure actions are taken to address ALL sources of stress in an application. A checklist can be used as an aid in this endeavor. This approach would offer a more cost effective and better method of whisker management than the current focus on long term environmental testing.
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Outline
What are tin whiskers?
What are the potential failure modes?
Where have tin whiskers caused failure?
Root causes of whiskers
Drivers
Mitigation
Sources of compressive stress
New approach to mitigation
Proposed checklist
Discussion
Summary
3

Background
Transition to Pb-free has brought about a number of “challenges”
Higher assembly temps
Poor solderability
Hole fill, etc.
Moisture sensitivity issues
Brittle laminate materials
Pad cratering, etc.
New solders and uncertainty with reliability
Temp cycling, vibration, mechanical shock, etc.
None seems to drive more angst than tin whiskers
Pb-free
4

Tin Whiskers
Tin whiskers are hair-like single crystal metallic filaments that grow from tin films.
Their unpredictability is the greatest concern.
The Aerospace and Defense industries consider tin whiskers the, “greatest reliability risk associated with Pb-free electronics”.
Manhattan project phase 2 report
5

Tin Whiskers - Shapes
Xu, Cookson Electronics, IPC 2002
Highest aspect ratio Longest length
6

Tin Whiskers –
Filament (examples)
7

Motivation
 The response of the electronics industry
is increasingly becoming segmented
based on market demands
Consumer
Space
Everyone else
8

Response to Tin Whisker Risk
Consumer / Commercial
Very high volume / low cost / limited lifetime
Electronics dominate product cost, so any mitigations must be limited
Response: Set some rules, follow JEDEC / iNEMI, move on
Missiles / Space
Very low volume / very high cost / long lifetimes
Electronics are a very small portion of costs ($50M to $400M to launch a satellite)
Response: No tin. Period.
9

Response to Tin Whisker Risk (cont.)
Everyone Else
Biggest challenges are high reliability applications with high volumes and strong cost pressures
Examples: Enterprise servers, external defibrillators, first-responder radios, industrial process monitoring, etc.
Every mitigation must be looked at closely in regards to need and cost (led by GEIA)
Requires a clear understanding of why and when tin whiskers
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What are the potential failure modes?
Direct Contact
Causes an electrical short (arcing)
Requires growth of sufficient length and in the correct orientation
Electromagnetic (EM) Radiation
Emits or receives EM signal and noise at higher frequencies
Deterioration of signal for frequencies above 6 GHz independent of whisker length
Debris
Whisker breaks off and shorts two leads (primarily during handling)
Courtesy of P. Bush, SUNY Buffalo
Observation of tin whisker debris as reported to NASA from Sanmina-SC
DfR Solutions
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Direct Contact (Arcing)
Electrical resistance of specimen appears to be strongest indicator of whether arc will occur versus pressure & whisker geometry
Proposed arc current metric for use in design
퐴푟푐 푐푢푟푟푒푛푡 푚푒푡푟푖푐=푉푎푝푝푙푖푒푑 /푅푠푝푒푐푖푚푒푛+푅 푡푒푠푡 푐푖푟푐푢푖푡
12
S. Han et al, CALCE, Likelihood of Metal Vapor Arc by Tin Whiskers, SMTA Magazine, August 2012

Where have tin whiskers caused failures?
Bright Tin on the case of a pacemaker crystal component (in 1986).
13

Where have tin whiskers caused failures?
Satellites
Whisker growth and subsequent short results in a low pressure arc in vacuum (vaporized tin creates a plasma).
Don’t use Sn plating in satellite applications (risk may be low but the cost of failure is very high).
14

Where have tin whiskers caused failure?
Unintended Acceleration – Automobiles?
Study by CALCE found tin whiskers on the contacts to the electronic throttle control system.
Shorting of some of these could cause acceleration.
No direct shorts were found – but possibility exists that this was the cause of the issue.
Ref: B. Sood, M. Ostermann, M. Pecht, Tin Whisker Analysis of Toyota’s Electronic Throttle Controls”, Circuit World, 3, Aug, 2011.
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Training Required to Find Tin Whiskers

Theory of Tin Whisker Formation
The presence of compressive stresses drive the preferential diffusion of tin atoms
To capture the causes for tin whiskering, we need to understand
Tin (Sn)
How Tin responds to stress
How Tin diffuses
What elements can change or introduce stresses or modify diffusion behavior
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Tin whiskers have been heavily studied.
The primary driving mechanism is a compressive stress (or stress gradient) in the tin.
This compressive stress drives the preferential diffusion of tin atoms (to lower stress regions).
There are additional factors that contribute to the propensity of whisker formation, such as grain structure, oxide thickness, tin thickness, base metal, etc.
However, without compressive stress the whiskers will not form.
Root Cause of Whiskers
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Tin Electroplating
Electroplating is the process of depositing metal (tin) from a solution/bath on to an electrically conductive surface
For electronics, copper / Alloy42 / steel
Tin has been electroplated since the 1850’s
Most common application is over steel to prevent rust
20

Tin Plating
Tin electroplating baths can be alkaline or acid
Alkaline baths (stannate) are somewhat easier to operate from a process standpoint, and provide a matte plated surface.
Acid baths are can produce matte deposits or when combined with brightening agents can produce bright plated surfaces.
Acid baths also provide a higher deposition rate but require a lower operating temperature.
The common requirement for all baths is that they must be capable of maintaining the correct amount of the material being deposited in the solution
21

Tin Plating Process
Initial tin plating baths used sulfate based electrolytes (acidic stannous sulfate); plating chemistry of recent has been methane sulfonic acid (MSA)
MSA baths consist of water, tin concentrate/salt, methane-sulfonic acid (MSA) concentrate, organic brighteners, and antioxidants
22

Tin Plating (cont.)
Metal deposition occurs when an electrical potential is established between the anodes and the cathode.
Electrical field initiates electrophoretic migration of tin ions to the cathode
At the anode, sufficient tin erodes into the electrolyte to replace deposited material, maintaining a constant concentration of dissolved tin.
Tendency of electrical charges to build up on the nearest high spot, creating higher electrical potential, which attracts metal ions, which makes the high spot higher…
Potential for a runaway reaction
Prevented by organic brighteners
Decreasing pH
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The Role of Brighteners
Brightener is attracted to points of high electro- potential, temporarily packing the area and forcing metal ions to deposit elsewhere
When deposit levels, high potential disappears and the brightener drifts away
By continuously moving with the highest potential, the brightener prevent the formation of large clumps of tin whiskers, giving the smooth, bright deposition that results from a properly maintained and operated acid tin plating bath.
http://www.thinktink.com/stack/volumes/volvi/tinplate.htm
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Role of Antioxidants
In acid baths, divalent Tin (Stannous) is the plating species
Readily oxidized to Tetravalent Tin (Stannate) by free oxygen in the bath
Stannate Tin is sparingly soluble and readily precipitates.
To prevent this, vendors add antioxidants, which scavenge any free oxygen in the bath
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Matte vs. Bright Tin Plating
Bright Tin
Matte Tin
Grain Size
Tyco
< 1 mm
~ 3 mm
iNEMI
0.5 – 0.8 mm
1 – 5 mm
Carbon Content
Tyco
< 0.3%
< 0.05%
iNEMI
0.2 – 1.0%
0.005 – 0.05%
Samtec
< 0.15%
< 0.015%
Bright tin can be “bad” plating and matte tin can be “better” plating, not necessarily “good” plating
Problem: Quantitative definitions of bright tin and matte tin can vary
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Why is Bright Tin Plating so Bad?
Smaller grains
More grain boundary area; faster diffusion rates
Not as columnar as matte tin
Increased likelihood of grain tilted with respect to orientation of stress state
Higher amounts of carbon content
Increases internal stress
Some indications of greater degree of texture
May induce higher local stress states
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Tin Plating (cont.)
Electroplating process is actually quite complex
Additional reactions taking place at the anode and cathode
The solution chemistry can be complicated
Nature of the current used is very important
Surface preparation can be critical
28

How Does Plating Change Tin?
Smaller grain size
Texture
Incorporation of elements from plating bath
Carbon, hydrogen, etc.
Residual stress
Reaction to base metal (Intermetallic)
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Grain Size (Bright Tin)
Tin Whisker EBSD Bright and Matte Finish Preliminary Investigation
Chris Meyer / Rex Smith, 06/26/08
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Grain Size (Matte Tin)
Tin Whisker EBSD Bright and Matte Finish Preliminary Investigation
Chris Meyer / Rex Smith, 06/26/08
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Comparison to Cast Tin
(Melted / Solidified)
 Bulk tin and tin alloys
tend to have grain sizes
(50-500 um) orders of
magnitude larger than
plated tin
 Very dependent on
cooling rate and geometry
 Plated tin grain size
driven by plating rate
(current density) and
organic content 32

Study on Stresses due
to Plating
 Plating over
nickel under layer
(2um)
 Captured grain
size, carbon /
hydrogen /
oxygen content,
stress, and
texture
 Thermal shock
followed by T/H
IEEE TRANSACTIONS ON ELECTRONICS PACKING MANUFACTURING, VOL. 28, NO. 1, JANUARY 2005,
Role of Intrinsic Stresses in the Phenomena of Tin Whiskers in Electrical Connectors, Sudarshan Lal and
Thomas D. Moyer
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Findings: Texture
Most of the platings had a preferred orientation of <220>
Platings with compressive stress (high organic brightener content), showed <321> orientation
Madra employed tensor analysis to correlate crystallographic grain orientation (h, k, l) with residual stresses and whisker formation.
Preferred orientations of <110>, <210>, <220>, <320>, and <420> were considered whisker resistant, whereas grain orientations 211 and 321 were considered whisker prone.
Similar claims have been made by Schetty et al. that 220 is a whisker-resistant preferred orientation.
34

Texture (Other Findings)
 Gaylon suggested that
texturing influenced whisker
behavior through the lower
modulus in certain
crystallographic planes
 Driven through the process
of recrystallization, in which
an individual grain realigns
its crystallographic
orientation such that the
elastic moduli are minimized
in the x-y plane
The Integrated Theory of Whisker Formation- A Stress Analysis
G. T. Galyon 35

Findings: Plating Elements
No correlation to whisker behavior and hydrogen / oxygen content
Much stronger correlation with carbon content
Inline with previous studies
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Whiskering Results
Whiskered
37

Plating Stresses
 Residual stresses of tin
electrodeposits were measured as a
function of storage time
 7um thick tin electrodeposited on
70um phosphor bronze from acid
stannous sulfate bath at room temp
 Range of current densities
 Residual stress defined by modulus
(E), thickness (t), Poisson’s Ratio (v),
and radius of curvature (R)
SPONTANEOUS GROWTH MECHANISM OF TIN WHISKERS, B.-Z. LEE and D. N. LEE
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Texture
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Texture
Johns Hopkins demonstrated ability to modify grain size, shape & texture with pulse plate deposition + additive
Nanocopper underlayers may prevent whiskers in polycrystalline tin while polycrystalline copper underlayers do not
Conflicting results from previous study
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Effects of Tin & Copper Nanotexturization on Tin Whisker Formation, Lee & Pinol, Johns Hopkins, SMT Magazine August 2012

Texture
Move from oblong grains to coalesced appearance
Drop in surface roughness
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Effects of Tin & Copper Nanotexturization on Tin Whisker Formation, Lee & Pinol, Johns Hopkins, SMT Magazine August 2012

Plating Stress
 Initial plating stress is
tensile
 Transitions to
compressive stress in
a few days
 8 MPa is close to the
yield strength of tin (11
MPa)
 Important to note that
this a ‘macro’
measurement
 Provides an overall
stress measurement
 No indication of
multiple layers of
stress or stress within
individual grains
42

Plating Stress Theory
Lee hypothesizes that the growth of intermetallic into the tin grain boundaries is the driver for the change in stress state over time from tensile to compressive
Anneal changes the morphology of the intermetallic layer
43

Intermetallics
Tin reaches with copper to form Cu6Sn5 intermetallics
Irregular growth of this intermetallic can introduce 58% additional volume in the plating layer
This Cu6Sn5 is irregular after storage at lower temperatures, because grain boundary diffusion predominates over bulk diffusion.
At higher temperatures (anneal), at higher temperatures (> ~75oC), the predominating bulk diffusion forms a homogeneous Cu3Sn/Cu6Sn5 layer, resulting in less stress
Starts to limit maximum tin whisker test temperatures!
Further beneficial effects of high temperature treatment are recrystallization of the Sn and annealing of already present stress.
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Intermetallics (Cu6Sn5) – Irregular Growth
Pascal Oberndorff, Philips CFT, Eindhoven, The Netherlands, EFSOT Europe (http://www.efsot-europe.info/servlet/is/837/)
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Intermetallic
Pascal Oberndorff, Philips CFT, Eindhoven, The Netherlands, EFSOT Europe (http://www.efsot-europe.info/servlet/is/837/)
CuSn intermetallic after storage at room temperature for 6 months and subsequent selective etching of pure Sn
CuSn intermetallic after 1 hour at 150 °C and subsequent selective etching of pure Sn
46

Oxidation similar to
Intermetallic
 Oxidation preferential to
grain boundaries
 Expansion induces
compressive stress
states (just like
intermetallic
formation)
 Starts to explain ability
of elevated temperature
/ humidity to accelerate
whisker behavior
 Overcomes self-limiting
behavior of tin
oxide
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Much of the industry’s focus has been on indirect causes of whisker formation.
Typical test methods attempt to reproduce the compressive stress through thermal cycling, heat aging, elevated humidity, bending and the like.
These tests can be expensive and time consuming.
This is fine, however, ideally it is the stress itself that would be modified, measured, and tracked over time to capture whisker behavior.
Mitigation
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JESD201A Testing
Strengths
Industry standard
Consistent comparisons
Tests main environmental factors
Weaknesses
No audit or enforcement provisions
Testing can be performed on coupons or dummy parts
Cannot be used to determine field reliability
Does not replicate use conditions

Testing / Measuring Standards
IEC 60068-2-82, Ed.1 “Environmental Testing – Part 2- 82: Tests – Test XW1: Whisker Test Methods for Electronic and Electric Components”, May 2007
IEC/PAS 62483 ed1.0 TC/SC 47 “Test method for measuring whisker growth on tin and tin alloy surface finishes”, Sept 2006
IEC 60512-16-21 ed1.0 TC/SC 48B “Connectors for electronic equipment - Tests and measurements - Part 16-21: Mechanical tests on contacts and terminations - Test 16u: Whisker test via the application of external mechanical stresses”, May 2012
JEDEC Standard JESD22-A121A, “Test Method for Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes
52

Indirect (Non Stress Driven) Mitigations
53

Industry Mitigation Documents
54

Alloying Elements: Lead (Pb)
How does Pb prevent tin whiskers?
Uniaxial grain structure (no one grain has a lower stress state)
Lowers stress levels (remember Pb behavior)
Changes grain boundary behavior
Pb is insoluble in tin
Pb in grain boundaries slows diffusion rates
Any effective mitigation (such as Bi) should demonstrate a propensity to mitigate BOTH stress and diffusion
55

Conformal Coating
How does conformal coating prevent whiskers?
In only one way: moisture barrier
Allows tin oxide formation to be self limiting
Compressive state does not grow over time
Constant temperature/humidity testing overcomes this behavior
Does not necessarily replicate of actual field environment
56

The stresses that drive whiskering primarily derive from five sources.
Base metal (intermetallic formation)
Base metal (differences in coefficient of thermal expansion)
Bulk plating conditions
Oxidation/Corrosion
External pressure
The magnitude of these stresses can be fixed at the time of production or can evolve over time in the application.
Sources of Compressive Stress
57

The formation of Cu6Sn5 creates a volume expansion of 58% compared to Cu and Sn alone.
Cu diffusion occurs faster along the Sn grain boundaries, so this is where the most IMC forms.
This is the most common cause of compressive stress that produces tin whiskers.
Cu Base Metal (Intermetallic Formation)
58

Use of Ni as an underplate is a common method to prevent interdiffusion of the tin and copper and thus formation of Cu6Sn5.
The resulting Sn3Ni4 is relatively thin and uniform due to the low dissolution rate of Ni in Sn compared to Cu.
A slight tensile stress is created with this IMC.
Mitigation – Ni Barrier
Ni Underplate (> 1.27 micrometers)
59

When Ni underplate is not practical (a formed leadframe for instance) then annealing is often used as a whisker mitigation.
Heating the tin coating to 150-170 C immediately after plating forces the formation of Cu3Sn intermetallic (thermodynamically stable at temperatures over 60C).
This IMC does not induce compressive stress and provides a uniform layer that reduces the rate of additional copper diffusion into the tin.
Similar, but not as effective, as the Ni underlayer.
Mitigation - Annealing
60

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What Annealing is Supposed to Do

Compressive stress can also arise when the base metal has a lower coefficient of thermal expansion (CTE) than tin plating and the component is subjected to repeated changes in temperature.
Best mitigation is to avoid components with tin plating on alloy 42, steel, or bronze, where whiskers can grow very long.
Thermal Expansion Stress
62

Thermal Expansion Stress
“Low Stress” study varying bias voltage, contamination, and part lead finish
Copper lead whiskers were short with very few observed
Contaminated Alloy 42 leads grew the longest whiskers.
Difference in whisker growth between Alloy 42 and copper indicates that the source of the whisker formation stress is due to the thermal expansion mismatch between the Alloy 42 and the SAC305 solder.
63
Ref: S. Meschter et. al, “Tin Whisker Testing at Low Stress Conditions”, BAE & Celestica, ICSR conference, 2012.

Delphi Example
S. Platt, “Management and Mitigation of Sn whiskers for Lead-free electronics”, IPC Midwest, Chicago, 2010.
64

The plating conditions used to deposit the tin can play a significant factor in the tendency for whiskers to form.
This relates to the unusual lattice structure of tin (body centered tetragonal) – grain orientation has large impact on modulus, stress & strain (few slip planes).
Tin Microstructure and Plating Conditions
66

Rapid growth at small grain size.
Growth Rate vs. Grain Size (at 25C)
Ref: John Osenbach (LSI)
67

Matte tin is dull looking because it consists of large grains and an uneven surface.
For improved aesthetics carbon brighteners are added to the plating bath; these essentially provide nucleation sites for new grains to grow.
The result is a tin plating with small grain size and a smoother brighter surface.
Brighteners
68

D-Sub Connectors with bright tin shells have been known to grow whiskers that can short our pins (if connector is unmated).
Bright Tin Whisker Examples
Whiskers also found to grow in screw holes.
Ref: L. Flasche & T. Munsun, Foresite, Inc. 9/09.
Ref: Emerson
69

High angle grain boundaries provide faster GB diffusion rates.
Impact of Grain Orientation
Ref: John Osenbach (LSI)
70

Impact of Grain Orientation
The tilted grain can also slip and move (grain boundary sliding)
71

Plating conditions can create internal stress.
Bright tin (small grain size) greatly increases whisker growth risk (should be avoided).
Grain orientation influences whisker growth risk.
A well controlled plating process is required.
Plating Summary
72

Just as with IMC formation, the process of tin oxidation/corrosion can also induce compressive stress.
Oxygen diffusion will occur fastest along the grain boundaries.
The volumetric expansion can result in large compressive stresses within the plating.
A similar situation occurs with various corrosion products.
Oxidation/Corrosion
73

Various tin plated components soldered to the board were exposed to different levels of contamination/corrosion.
SAC305 assemblies cleaned – showed no whisker growth
As-received assembles (noncleaned) – showed some small whiskers (hillocks).
Assemblies intentionally contaminated with NaCl or Na2SO4 – showed more components with long whiskers.
Corrosion Testing
QFP44, contaminated with NaCl
Ref: P. Snugovsky et. al, “Influence of Board and Component cleanliness on Whisker Formation”, BAE & Celestica, IPC & SMTA conference, 2010.
74

85C/85% RH Test Results
Note: The authors found that only as-received boards that exceeded IPC cleanliness standards showed whisker growth.
Ref: P. Snugovsky et. al, “Influence of Board and Component cleanliness on Whisker Formation”, BAE & Celestica, IPC & SMTA conference, 2010.
75

Corrosion Testing
“Low Stress” study varying bias voltage, contamination, and part lead finish
Contaminated Alloy 42 leads grew the longest whiskers.
76
Ref: S. Meschter et. al, “Tin Whisker Testing at Low Stress Conditions”, BAE & Celestica, ICSR conference, 2012.

Extrinsic forces can also introduce compressive stress in the tin plating.
One of the first studies of tin whiskers was triggered by the finding that tin plated steel ring clamps grew long whiskers that depended on the clamp pressure.
The larger the stress the longer the whiskers must grow to relieve it.
Common pressure points in electronics include connectors, standoffs, card guides, washers/terminals, shielding, etc.
Of particular concern is the contact pressure on flexible circuit cables.
External Pressure
77

Tin Whisker Case Study: Compressive Stress +
Long-Term Storage
 Inspection of a tin-plated solder terminals
subjected to storage environments for 20
years
 The maximum whisker length was 18
mils (450 microns)
 Within the inner ring, at the location of
maximum compressive stress
 This length is inline with survey values
 Length is only slightly higher than
a 12 mil (300 microns) whisker
observed on the bottom of the
solder terminal.
 Compressive stresses may drive
an acceleration of growth rates,
as opposed to a definitive
increase in maximum length.
78

Contact Pressure on Flex Cables
Flex Circuits with Connector Mating
Pressure from contacts with the soft polymer substrate creates force over a large area of tin.
Don’t use Sn plating in mated flex with a spacing less than 200 micrometers.
Use gold plating under such conditions.
79

Mitigation efforts can fail when only one source of stress is accounted for.
For example if a Ni layer is used to prevent stress from IMC formation, this will not address whisker formation due to corrosion or external pressure.
All the potential sources of stress for a particular application should be considered in the mitigation process.
A New Approach to Mitigation
80

We propose both a Checklist and Process Control be used in the mitigation effort.
Critical to fail industries such airlines and medical are used to checklists to eliminate failures.
A tin whisker checklist would confirm that all sources of stress that can induce whiskers in an application are accounted for and adequately controlled.
A New Approach to Mitigation
81

The proposed checklist would require at least one “Yes” per question.
Are stresses due to intermetallic formation adequately controlled?
Yes, through annealing (150°C for an hour within 24 hours of plating)
Yes, through use of an appropriate underplate (nickel, silver, etc.)
Yes, the base metal is treated to limit anisotropic intermetallic growth (i.e., surface roughening)
No
Are stresses due to differences in coefficient of thermal expansion adequately controlled?
Yes, the base metal is copper
Yes, the coefficient of thermal expansion is greater than or equal to nickel (13 ppm)
No
Proposed Checklist
82

Are stresses in the bulk plating adequately controlled?
Requirement: The supplier measures in-plane stresses on a monthly basis and ensures the stresses are tensile or mildly compressive. Grain orientation is understood and measured with diffraction.
Yes, the supplier only uses low carbon/organic content tin plating
Yes, the plating is subjected to reflow temperatures that melt the tin
No
Proposed Checklist
83

Are stresses due to oxidation or corrosion adequately controlled?
Yes, the device will not directly exposed to corrosive conditions (residual aqueous flux residues, corrosive gases, salt spray, etc.)
Yes, the device will be used in a vacuum
Yes, the application has sufficient power dissipation to drop the humidity below 40%RH and the application is always on
Yes, the device is covered with conformal coating or potting material
No
Are stresses due to external loads adequately controlled?
Yes, the tin plating does not have separable mechanical load being applied
No
Proposed Checklist
84

High volume, high reliability OEMs, and the industry organizations they belong to, should require component manufacturers to report on tin plating stress measurements and grain orientation (pole figures).
These measurements are common in other industries and there are a number of methodologies available, including spiral contractometer, bent strip, and the I.S. meter.
Prior work has clearly shown that plated tin with stresses close to the yield strength (7 to 15 MPa) tend to drive severe whisker behavior.
The electronics industry should determine if only platings with tensile stress are acceptable, or if some minimal level of compressive stress still provides sufficient risk mitigation.
Discussion
85

The various sources of compressive stresses that drive whisker growth are rather well understood.
To effectively mitigate tin whisker growth one needs to ensure actions are taken to address all sources of stress in an application.
A checklist can be used as an aid in this endeavor.
Additionally, more process control should be done to ensure internal stress in the tin deposit is sufficiently low.
This approach would offer a more cost effective and better method of whisker management than the current focus on long term environmental testing.
Summary
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Instructor Biography
Cheryl Tulkoff has over 22 years of experience in electronics manufacturing with an emphasis on failure analysis and reliability. She has worked throughout the electronics manufacturing life cycle beginning with semiconductor fabrication processes, into printed circuit board fabrication and assembly, through functional and reliability testing, and culminating in the analysis and evaluation of field returns. She has also managed no clean and RoHS-compliant conversion programs and has developed and managed comprehensive reliability programs.
Cheryl earned her Bachelor of Mechanical Engineering degree from Georgia Tech. She is a published author, experienced public speaker and trainer and a Senior member of both ASQ and IEEE. She holds leadership positions in the IEEE Central Texas Chapter, IEEE WIE (Women In Engineering), and IEEE ASTR (Accelerated Stress Testing and Reliability) sections. She chaired the annual IEEE ASTR workshop for four years and is also an ASQ Certified Reliability Engineer.
She has a strong passion for pre-college STEM (Science, Technology, Engineering, and Math) outreach and volunteers with several organizations that specialize in encouraging pre- college students to pursue careers in these fields.
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Contact Information
Questions?
Contact Cheryl Tulkoff, ctulkoff@dfrsolutions.com, 512-913-8624
askdfr@dfrsolutions.com
www.dfrsolutions.com
Connect with me in LinkedIn as well!
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A New Approach to Mitigating Tin Whiskers

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