a new & better approach to tin whisker mitigation
DESCRIPTION
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.TRANSCRIPT
A New & Better Approach to
Tin Whisker Mitigation
Cheryl Tulkoff
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.
2
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
10
11
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
11
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 𝐴𝑟𝑐 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑚𝑒𝑡𝑟𝑖𝑐 = 𝑉𝑎𝑝𝑝𝑙𝑖𝑒𝑑
/ 𝑅𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛 + 𝑅 𝑡𝑒𝑠𝑡 𝑐𝑖𝑟𝑐𝑢𝑖𝑡
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. 15
16
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
17
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
18
Drivers
19
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
Decre
asin
g p
H
23
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
24
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
25
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
26
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
27
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)
29
Grain Size (Bright Tin)
Tin Whisker EBSD Bright and Matte Finish Preliminary Investigation
Chris Meyer / Rex Smith, 06/26/08 30
Grain Size (Matte Tin)
Tin Whisker EBSD Bright and Matte Finish Preliminary Investigation
Chris Meyer / Rex Smith, 06/26/08
31
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
33
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
36
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 38
Texture
SPONTANEOUS GROWTH MECHANISM OF TIN WHISKERS, B.-Z. LEE and D. N. LEE
39
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
40
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
41
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
SPONTANEOUS GROWTH MECHANISM OF TIN WHISKERS, B.-Z. LEE and D. N. LEE
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.
44
Intermetallics (Cu6Sn5) –
Irregular Growth
Pascal Oberndorff, Philips CFT, Eindhoven, The Netherlands, EFSOT Europe (http://www.efsot-europe.info/servlet/is/837/) 45
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
Intermetallics (cont.)
47
Intermetallics
(Nickel)
48
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
49
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
50
51
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
61
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
Delphi Example (cont)
65
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
86
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.
87
Contact Information
Questions?
Contact Cheryl Tulkoff,
512-913-8624
www.dfrsolutions.com
Connect with me in LinkedIn as well!
88