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Whiskers: Truth and Mystery - · PDF fileWhiskers: Truth and Mystery IPC/NEMI Symposium 0n Lead-Free Electronics I. Boguslavsky September 19, 2002 Montreal, Canada. 2 ... •The last

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  • 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

  • 2Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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

  • 3Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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

  • 4Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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

  • 5Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    Literature ReviewLiterature Review

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

    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

  • 6Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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

  • 7Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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

  • 8Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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 (

  • 9Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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

  • 10Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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 .

  • 11Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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

  • 12Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    Work Done by NEMI Modeling GroupWork Done by NEMI Modeling Group

  • 13Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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

  • 14Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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

  • 15Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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)?

  • 16Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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 .

    (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 = 0.016

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

  • 17Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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 .

    (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)

  • 18Connect with and Strengthen Your Supply ChainConnect with and Strengthen Your Supply Chain

    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 = 0.500.

    (200

    ) I=

    100

    (101

    ) I =

    88

    ( 22 0

    )

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