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    SEM tin whisker.jpg

    SEM image of tin whiskers taken by ERA Technology Ltd

     

    What are tin whiskers?

    • Whiskers are thin fibres of tin that grow apparently  spontaneously from electroplated tin surfaces
    • Tin whiskers can cause short circuits and have caused several satellites, missiles, heart pacemakers and a nuclear power station to fail
    • Electroplated  tin coatings  are used  on most component terminations  to aid soldering and to provide corrosion resistance
    • Whiskers of several mm are possible  although ~100µm  is more common.
    • Only long whiskers cause failures.


    Causes of Tin Whiskers

     

    • Whiskers are caused by compressive stresses in tin coatings
    • Stresses are induced  by:
      • Irregular intermetallic crystals that grow at copper / tin interfaces
      • Due to stress induced  by thermal expansion mis-match between layers of coatings
      • Due to formation of bulky oxides between tin grains in humid environments


    Prevention of Tin Whiskers

     

    • A lot of research has been  carried out into the causes and prevention
    • iNEMI has published  guidance - http://thor.inemi.org/webdownload/projects/ese/tinhttp://thor.inemi.org/webdownload/projects/ese/tin_ whiskers/Pb-Free_Finishes_v4.pdf
    • Risk should be small if this is followed but:
      • Equipment manufacturers buy pre-plated components from suppliers and have no influence over production process
      • Test methods are available but
        • These take at least 3 months  – no use for QA
    • Equipment manufacturers need  a whisker mitigation strategy:
      • Approved supplier list for COTS components
      • Document  coating specification for custom made  parts
      • Design requirements
      • Conformal coatings


    Measures to avoid  Tin Whiskers

     

    • Whisker risk very low if the following are used:

     

    1. Use Ni/Pd/Au termination coatings  (no risk but an uncommon coating)
    2. Use tin/lead terminations  (extremely low risk but infrequently available)
    3. Thin matte  tin on copper with a non-porous nickel barrier layer (very low risk)
    4. Bake matte  tin on copper at 150°C for 1 hour within 24 hours of plating (no good  later than this). Very low risk but only a few component manufacturers do this
    5. Melt electroplated tin – this usually prevents whiskers but the high temperature may cause heat damage. Hot dipped  terminations  are OK


    How to avoid  Tin Whiskers

     

    • Do not use SnCu plating but SnAg is OK
    • Avoid alloy 42 lead-frame  components or other low TCE materials if there is a choice
    • Whisker resistant  matte tin plating processes are new, not all electroplaters use them, check  that they do and that operating  procedures are followed
    • Matte tin is usually less susceptible to whiskers although whisker resistant  bright tin is available
    • SnBi termination coatings  are OK but ensure  <6% Bi to avoid reliability problems  with PbBiSb phases

     

     

    Long tin whiskers.jpg

    Unusually long tin whiskers photographed by ERA Technology Ltd

    Design to avoid  Tin Whiskers

     

    • Follow iNEMI guidelines as far as possible
    • Avoid components from sources you do not trust
    • Avoid fine pitch terminations  (if possible) – most whiskers are up to ~150µm  in length
    • Consider  conformal coatings  – even if whiskers form and break-through, they are unlikely to re-penetrate the coating on an adjacent termination
    • Choose components that have been  tested by the manufacturer
      • JEDEC test results should be freely available
    • Remember that silver and zinc plating also produce whiskers
    • If custom parts  are to be plated with tin for corrosion  resistance – use nickel barrier layer & matte  tin


    Conclusions

     

    • Tin whiskers are a real threat
    • However their causes are understood
    • Measures to avoid whisker failures have been  developed

    With thanks to ERA Technology

     

     

     

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