Transcript

Whiskers: Truth and MysteryWhiskers: Truth and Mystery

IPC/NEMI Symposium 0n Lead-Free ElectronicsIPC/NEMI Symposium 0n Lead-Free Electronics

I. BoguslavskySeptember 19, 2002Montreal, Canada

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Pure Tin Grows WhiskersPure Tin Grows Whiskers

• Bulk tin

• Vapor deposited tin films on inert substrates: paper,mica, plastics

• Metallurgically prepared polished 50%Sn/50%Al discs

• Electroplated foils delaminated from the substrate

• Electroplated tin and tin alloys

• No whiskers on high purity tin such as zone refined tinor on single crystal of tin

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Whisker TypesWhisker Types

Column Hillock

Needle Needle growing out of hillock

Flower or OSE

Needle growing out of OSE

SEM photos – courtesy of P. Bush, SUNY at Buffalo

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Whiskers on 90 Whiskers on 90 SnSn/ 10 Pb/ 10 Pb

Test conditions: 1500 thermocycles –55C to +85C

Secondary Electron Image (SEI) Backscattered Electron Image (BEI)

Substrate: C194

SEM photos – courtesy of P. Bush, SUNY at Buffalo

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Literature ReviewLiterature Review

• Most of the studies on whiskers were carried out andpublished before the end of 1970’s

• The following information is available from those papers– Theories of the whisker growth mechanism 1,2,3

– Effect of plating process parameters on whisker growth• Current density 4,5

• Temperature 6

• Mechanical and chemical preparation of the substrate 7

• Substrate material 6,8,9

• Concentration of the bath components and particularly additives1,4,5,6

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Literature Review (continue)Literature Review (continue)

• Additional information available– Deposit thickness 6,10

– Underlayers 8,11

– Post-plating annealing 2,4,11

– Crystallographic structure of whiskers 2,9,12

– Deposit structure 4, 13,14

– Effect of alloying tin 10,15,16,17

• Lead reduces propensity to whisker as low as 1%

• Even deposits with 10% Pb grow whiskers, although, less thanpure tin

• Only 60%Sn/40%Pb is immune to whiskering

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Literature Review (continue)Literature Review (continue)

• Most recent publications presented in the NASA web-site list maybe divided into two categories:– About 60%: Case studies for whisker growth and warning

against using pure tin deposits from high reliability users (J.Brusse 18 and J. Kadesch 19 dominate the list)

– About 20%: studies related to whisker testing of variouscomponents plated with tin and tin alloys

– About 20%: Publications from suppliers of plating chemicalsand industry consultants promoting their processes andexpertise

• The last type of studies created mystery and confusion

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Current Status of Plating ProcessesCurrent Status of Plating Processes

• Plating chemicals suppliers significantly improved platingprocesses by:– Testing various additives

– Utilized recommendation from various publications (improved substratepre-treatment, post-plating annealing, Ni underlayer)

• However, no reliable deposit characterization has been done tosubstantiate those improvements. As an example, there is nomethod available for deposit internal stress (IS) measurement(probably, the most important deposit property)– Standard contractometers are not sensitive enough for low value of IS in

tin (<10 Mpa for Sn vs ~100 MPa for Ni and ~300 MPa for hard Au)

– XRD method 23 has 50-100% experimental error due to low IS values,large grain structure (crystal distribution is not random) and anisotropy(different material properties in different directions) for tin deposits

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Production TrialsProduction Trials

• Another source of conflicting results are production trials.Apparent reasons for that contradiction are– No standard whisker test procedure

– No standard inspection procedure

– Different acceptance criteria. “No whiskers” as an acceptance conditionleaves uncertainty for results interpretation

• Not so obvious reasons are:– Variation in process parameters

• Pre-treatment: stamping versus etching; de-flashing, de-scaling

• Cell design: current density variation; agitation

• Contamination

• Possibly more than one mechanisms for whisker growthmay explain different effect of accelerating tests ondifferent deposits

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Problem: to resolve whiskering and create whisker-free lead-free finish, the industry rely on suppliers andcomponent manufacturers who in most of the casesdo not have enough expertise and capabilities(equipment, resources, etc.) to address this issue atan appropriate level. Need multi-disciplinary team ofexperts in the relevant fields and advancedequipment .

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NEMI Fundamental Group ApproachNEMI Fundamental Group Approach

• Short-term practical solution: establish whisker performancefor “best in class” pure tin plating processes (supplied bycomponent manufacturers) utilizing three test methods andcombination of those– Thermal cycling

– Ambient temperature and humidity

– Elevated temperature and humidity

• Use that information as a basis for specification for pure tinproduction processes

• Long-term study: Fundamental understanding of whiskergrowth mechanism and quantitative characterization of thefactors affecting whiskering. Predict incubation period, growthrate and maximum length of whiskers based on measurabledeposit properties and modeling

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Work Done by NEMI Modeling GroupWork Done by NEMI Modeling Group

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Use of synchrotron radiation to determine the state ofstrain in tin coatings on integrated circuit packages.

Dr. Peter W. StephensDepartment of Physics & AstronomyStony Brook University

Verification of XRD MethodVerification of XRD Method

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•Advances in x-ray technology, especially use of synchrotron radiation, permitmicroscopic studies of materials that were previously impossible/difficult.

•The premise of powder diffraction is to look at a statistical sample of grains.Complements measurements of individual grains/whiskers such as recentlydiscussed by K.N. Tu.

•Preliminary work was done on sample N2, QFP package (14mm x 14mm, 100leads), ~10 _m matte tin over alloy A42 (fcc, a=3.588). Temp. and humiditycycled 1,500 times.

•Microscopy (Peter Bush) shows grains 5 – 50 microns.

•Films are evidently one grain thick.

XRD StudyXRD Study

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10 12 14 16Sample angle theta (degrees)

0

20000

40000

60000

X-r

ay c

ount

s pe

r se c

ond

_

NO! Large fluctuationsimply that sample hasrelatively large grains. Alsoknown from microscopy

Sn (220) reflection_ = 0.83 Angstrom

typ. 5-50 _m

Rock the sample at fixed diffraction angle.

Is the sample a good powder(observe many small grains)?

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20 30 40 502theta (degrees)

0

4000

8000

12000

16000

X-ra

y Int

ensit

y (a r

b un

i ts)

10 mm Sn film on Cu IC leadl = 1.15 Å.

(200

) I=

100

(101

) I=

8 8

(22 0

) I =

32(2

11)

I =5 9

(30 1

) I =

1 4

( 11 2

) I =

1 8(4

00)

I =8

(32 1

) I =

1 6

S ub s

t rat e

(22 0

)

Subs

tr ate

(11 1

) not

ob s

erv e

d

FWHM = 0.016º

Diffraction Pattern (low penetration)Diffraction Pattern (low penetration)

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15 20 25 302theta (degrees)

0

500

1000

1500

2000

X-ra

y In

tens

ity ( a

r b u

n its

)

10 mm Sn film on Cu IC leadl = 0.65 Å.

(200

) I=

100

(101

) I=

8 8

( 22 0

) I=

32(2

1 1)

I =59

(30 1

) I =

1 4

( 11 2

) I =

18( 4

00)

I =8

(32 1

) I =

16

Puzzle: why did (220) getso weak? The rest ofthe pattern is about thesame as 1.15A.

Diffraction Pattern (mediumDiffraction Pattern (mediumpenetration)penetration)

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10 12 14 16 18 20 22 242theta (degrees)

0

20000

40000

X-ra

y in

tens

ity (a

rb u

n it s

)

10 mm Sn film on Cu IC leadl = 0.500Å.

(200

) I=

100

(101

) I =

88

( 22 0

) I=

32(2

11)

I =59

( 30 1

) I=

14

(112

) I =

18( 4

0 0)

I=8

( 321

) I =

1 6

Subs

tr at e

(220

)

Subs

trate

(111

)

Diffraction Pattern (high penetration)Diffraction Pattern (high penetration)

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Diffraction patterns have been collected at three differentwavelengths.

Used this data to measure strain at different depths of thesample.

Re-plotted three diffraction patterns as a function of sin(_)/_ =1/2d, which should be the same for each diffraction line.

Larger angles => shorter d-spacing

Stress/Strain MeasurementsStress/Strain Measurements

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0.17 0.171 0.172 0.173sin(theta) / lambda = 1/2d

0

1000

2000

3000

X-r

ay in

tens

it y (a

r b u

nit s

)

1.15 Å

0.5 Å

0.65 Å

Tin (200)Motorola 14mm square pack

Nominal position, d = 2.9155

dd / d = 0.0012

Deeper penetration ->Smaller d, negative strain.

XRD StudyXRD Study

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0.24 0.242 0.244sin(theta) / lambda = 1/2d

0

4000

8000

12000

16000

20000

X-ra

y in

tens

it y (a

rb u

n its

)

1.15 Å

0.5 Å

0.65 Å

Tin (220), d = 2.0615Motorola 14mm square pack

Ni40Fe60 (111)

dd / d = 0.0007

XRD StudyXRD Study

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1. Sn films are severely textured. (220) or (321) appears to be the main growthdirection.

2. Films are made of rather large grains, which makes accurate powderdiffraction measurements difficult for characterization of both crystal orientationand deposit strain/stress.

3. It appears that the deeper material is compressed, perpendicular to thesurface. Cannot tell if there is such compression along the surface direction.Estimated deep strain is equivalent to a stress of -0.001 x 40 GPa = -40 MPa(compression)

4. More work requires more manpower.

Preliminary Conclusion of XRD StudyPreliminary Conclusion of XRD Study

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Deposit Orientation:Deposit Orientation:effect of current densityeffect of current density

5 ASD 10 ASD 20 ASD321

321

321112

112112

431

431

431211

111

111

211

211

111 200

101332332

Slide is courtesy of Shipley Co

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XRD Method for Crystal OrientationXRD Method for Crystal Orientation

• In addition to internal stress, deposit crystalorientation is a critical parameter that may influencewhisker growth

• There are two reasons that complicate the use ofXRD orientation data for deposit characterization andprediction of whisker behavior:– Large experimental error when powder diffraction method is

used (similar to the interpretation of XRD data for strainstress measurements)

– Influence of plating process parameters on crystal orientation

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XRD Method for Crystal OrientationXRD Method for Crystal Orientation

• To obtain reliable XRD data for deposit orientation bypowder diffraction method, statistical approach shouldbe utilized (multiple measurements of the samesample)

• XRD micro-probe can be used to determineorientation of individual grains

• The effect of plating process parameters on crystalorientation needs to be studied to define the operatingwindow of a plating process which would provide thebest whisker behavior

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Auger Analysis of Tin Oxide FilmAuger Analysis of Tin Oxide FilmThickness: Effect of Aging ConditionsThickness: Effect of Aging Conditions

Peter J. Bush

SUNY at Buffalo

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Auger Study of Oxide FilmAuger Study of Oxide Film(sample after ambient conditions)(sample after ambient conditions)

Sn

C

O

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Auger Study of Oxide FilmAuger Study of Oxide Film(sample after 50C/85% RH)(sample after 50C/85% RH)

Sn

C

O

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Whisker Growth TheoriesWhisker Growth Theories

• Surface oxidation develops stress that causes screwdislocations to move up. Counter-argument: whiskersgrow in vacuum

• Stress caused by hydrogen incorporated in the depositduring electroplating drives whiskers. Counter-argument:whiskers grow on bulk tin and deposits produced fromvapor phase.

• IMC formation, particularly, SnCu intermetallics, inducestress and increase propensity to whisker. Counter-argument: whiskers grow if tin has no interface withcopper

• Most of the theories emphasize only one factor influencingwhiskers

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Diffusion theoryDiffusion theory

• Diffusion theory provides the most comprehensiveapproach

• Electrodeposits have higher amount of crystal defects(vacancies and dislocations) than metallurgical material.Those defects in combination with high mobility of tinatoms explain fast diffusion in tin deposits

• Diffusion mechanism: for each tin atom leaving thedeposit, the vacancy is formed below the root of thewhisker which is then absorbed by long-range diffusion oftin atoms

• Macro- and micro-stress, surface energy, stored strainenergy and crystal orientation may influence vacancydiffusion

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Simulation of thevacancy mechanism

Simulation of the direct interstitial mechanism

Atomic Diffusion MechanismAtomic Diffusion Mechanism

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Diffusion theoryDiffusion theory

• Two-stage model:– 1st stage – dislocation loop formation and expansion within the

deposit (gliding). Vacancies are formed and absorbed by lattice andgrain boundary diffusion and through dislocation pipesSEE 34

– 2nd stage – dislocation loop climbing (upwards) by motion of graininterior atoms, grain boundary atoms and grain boundary sliding.The latter mechanism is dominant for deposits with columnar grainsand flat grain boundaries like tin SEE 35

• If the 1st stage is the rate-limiting process, the whiskergrowth rate maybe described by the equation:

kTR

vDh

w

azs

⋅⋅=

s2WhereDs is self-diffusion coefficientsz is stress normal to the surfaceVa is atomic volumeRw is whisker radius

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Dislocation Loop GlidingDislocation Loop Gliding

Slide courtesy of Ohio State University web-site

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Dislocation Loop ClimbingDislocation Loop Climbing

Slide courtesy of Ohio State University web-site

b d

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Diffusion theoryDiffusion theory

• For the 2nd stage (climbing), the main obstacle is intersectionwith forest dislocations that creates opposing pressure - si.The whisker growth rate maybe described by the equation

• For low stress, the 2nd stage is rate-determining and whiskergrowth strongly depends on internal and external stress andsomewhat on temperature

• For high stress, the 1st stage is dominant and the whiskergrowth rate has reverse dependence on temperature (higherrate at RT)

nikh )( ss -=

Wherek and n depend on the temperature but not stresss is the stress driving whisker growthsi depends on deposit structure

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Features Grown on the Scratched AreaFeatures Grown on the Scratched Area

SEM photos – courtesy of P. Bush, SUNY at Buffalo

Scratching is plastic deformation process that creates numerous dislocations..Step-wise growth visible on the side correlates with diffusion theory

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Step Growth and Voids AroundStep Growth and Voids AroundWhisker BaseWhisker Base

Whiskertestconditions:55C dry heat, 4 weeks

Substrate: A 42

Note steps atthe whiskerbase

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Calculation of Stress along GrainCalculation of Stress along GrainBoundaries (Modeling)Boundaries (Modeling)

Balasubramaniam Radhakrishan

Oak Ridge National Lab

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Calculation of Stress along GrainCalculation of Stress along GrainBoundaries (Modeling)Boundaries (Modeling)

•Estimation of normal force was done for the existingdeposits with various degree of whiskering based on theirXRD spectra (crystal orientation)

•The normal strain, e33´, was calculated for each grainorientation

•The difference in normal strain between two grainorientations is proportional to the stress acting to promotewhiskers

•Larger the difference in normal strain, greater is theprobability for initiating whisker growth (Lee and Lee,1998)

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Calculated ResultsCalculated Results

Orien- tation e'_33 Intensity Normal force --------------------------------------------- Good practice ------------ 101 0.014080 800 0.0 211 0.013813 200 0.000267 112 0.008393 400 0.005687 111 0.008720 140 0.00872 103 0.007674 130 0.006406 40 whiskers per lead -------------------- 220 0.007860 2300 0.0215 211 0.013813 2000 0.015547 321 0.010886 800 0.018474 101 0.014080 500 0.01528 200 0.029360 400 0.0 No whiskers ----------- 321 0.010886 1220 0.018474 111 0.008720 1180 0.02064 431 0.009578 600 0.019782 211 0.013813 440 0.015547 200 0.029360 200 0.0

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Discussion of the ResultsDiscussion of the Results

• For the “good practice” texture, set 1, the magnitude of normal straindifference is significantly smaller than for the set 2 (40 whiskers perlead) and set 3 (no whiskers). Sets 2 and 3 represent the same platingprocess

• The presence of <200> component (which has the highest normalstrain) in set 2 and set 3 results in a larger magnitude of strain differencethat in set 1 and, thus, higher probability of whiskering

• Volume fraction of <200> component in set 3 is lower than in set 2;therefore the probability of finding grains with high stress is lower

• Based on the components that have the highest volume fractions(probability of finding such pairs of grains in the samples is high), thestress value for set 1 is significantly lower than in sets 2 and 3.

• Good correlation of the modeling results with practice confirms thefeasibility of the approach

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Future WorkFuture Work

• Determine deposit characteristics that affect whiskergrowth and techniques for their measurements. Develop amodel for whisker growth mechanism.

• Based on the modeling, predict long-term whiskerperformance:– Incubation period, growth rate and maximum length (whisker

seizure mechanism)

• Use the knowledge of whisker growth mechanism toidentify accelerated test conditions and determineaccelerating factor

• To coordinate various types of activities and thoroughlyconduct the experiments, long-term planning andappropriate funding should be done

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Capabilities of NEMI Modeling GroupCapabilities of NEMI Modeling Group

• NEMI Fundamental Group has a unique set of expertiseand capabilities and unbiased attitude necessary toresolve such a complex problem as whisker growth. Itincludes experts in the following areas:– Electroplating chemistries and processes

– SEM characterization of whiskers (quantitative approach)

– XRD for macro- and micro-stress measurements and crystalorientation

– Crystal mechanics, modeling and computation

– Metallurgy

– Material science

– Engineering/manufacturing

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ReferencesReferences

1. R. Kawanaka, K. Fujiwara, S. Nango and T. Hasegawa, “Influence ofImpurities on the Growth of Tin Whiskers,” Japanese Journal of AppliedPhysics, vol. 22, pp. 917-921, March 19, 1983.

2. B.-Z. Lee, D. N. Lee, “Spontaneous Growth Mechanism of Tin Whiskers”,Acta Met., vol. 46, pp. 3701-3714, 1998.

3. U. Lindborg, “A Model for the Spontaneous Growth of Zn, Cd, and SnWhiskers”, Acta Met., vol. 24, p. 181, 1976

4. . A. Selcuker, and M. Johnson, “Microstructural Characterization ofElectrodeposited Tin Layer in Relation to Whisker Growth”. Capacitor andResistor Technology Symposium: CARTS, pp 19-22, October, 1990.

5. L. Zakraysek, et. “Whisker Growth from a Bright Acid Tin Electrodeposits”,Plating and Surface Finishing, vol. 64, pp. 38-43, 1977.

6. V.K. Glazunova and N.T. Kudryavtsev, “An Investigation of the Conditions ofSpontaneous Growth of Filiform Crystals on Electrolytic Coatings”Translated form – Zhurnal Prikladnoi Khimii, vol 36, no. 3, pp 543-550, March1963.

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ReferencesReferences

7. D. Endicott, K.T. Kisner, “A Proposed Mechanism for Metallic WhiskerGrowth”, Proceedings of AESF SURFIN Conference, July, 1984.

8. M. Endo, S. Higuchi, Y. Tokuda and Y. Sakabe. “Elimination of WhiskerGrowth on Tin Plated Electrodes”, Proceedings of the 23rd InternationalSymposium for Testing and Failure Analysis, pp 305-311, October 27-31,1997.

9. P. Harris, “The Growth of Tin Whiskers”, ITRI, pp. 1-19, 1994

10. N. A. J. Sabbagh, H.J. McQueen, “Tin Whiskers: Causes and Remedies”,Metal Finishing, March 1975.

11. S.C. Britton, “Spontaneous Growth of Whiskers on Tin Coatings: 20 Yearsof Observation,” Transactions of the Institute of Metal Finishing, vol. 52, pp95-102, April 3, 1974.

12. . N. Furuta and K. Hamamura, “Growth Mechanism of Proper Tin-Whisker,”Journal of Applied Physics, vol. 8, no. 12, pp 1404-1410, December 1, 1969.

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ReferencesReferences

13. K. Fujiwara, M. Ohtani and T. Isu, “Interfacial Reaction in BimetallicSn/Cu Thin Films”, Thin Solid Films, vol. 70, pp. 153-161, 1980.

14. T.Kakeshita, R. Kawanaka and T. Hasegawa, “Grain Size Effect ofElectro-Plated Tin Coatings on Whisker Growth”, Journal of MaterialsScience, vol. 17, pp. 2560-2566, 1982.

15. P.L. Key, “Surface Morphology of Whisker Crystals of Tin, Zinc andCadmium,” IEEE Electronic Components Conference, pp. 155-157,May 1970 or 1977.

16. S. M. Arnold, “Repressing the Growth of Tin Whiskers”, Plating, vol.53, pp. 96-99, 1966.

17. . K.M. Cunningham and M.P. Donahue, “Tin Whiskers: Mechanism ofGrowth and Prevention.” 4th International SAMPE ElectronicsConference, p.569, June, 1990.

18. J. Brusse, et, “Tin Whiskers: Attributes and Mitigation”, CARTS, March2002, pp. 68-80

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ReferencesReferences

19. J. S. Kadesch, J. Brusse, “The Continuing Dangers of Tin Whiskersand Attempts to Control them with Conformal Coat”, NASA’s EEE LinkNewsletter, July 2001.

20. B. Hom, S. Winkler, “Back to the Future: A Look at the Past Reveals aLead-Free Drop-In Replacement”, MEPTEC Report, March-April 2001

21. Y. Zhang, et., “An Alternative Surface Finish for Tin/Lead Solders –Pure Tin”, SMI’96 Proceedings, Sept. 1996, p. 641

22. R. Schetty, “Tin Whisker Study – Experimentation and MechanisticUnderstanding”, Proceedings of AESF SURFIN Conference, June2002, pp. 7-17.

23. Y. Zhang, et., “Understanding Whisker Phenomenon: Whisker Indexand Tin/Copper, Tin /Nickel Interface”, Proceedings of IPC SMEMACounsel APEX, pp. S061-1 through 11, January 2002


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