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.

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

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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

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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

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

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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).

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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).

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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

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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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Drivers

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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

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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

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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

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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 30

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.

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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

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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

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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

SPONTANEOUS GROWTH MECHANISM OF TIN WHISKERS, B.-Z. LEE and D. N. LEE

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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

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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/) 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

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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

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Indirect (Non Stress Driven)

Mitigations

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Industry Mitigation

Documents

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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

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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

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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)

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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.

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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.

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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

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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.

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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.

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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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