Robert,
According to our experience whiskers occur both at electroless and
electrolytic tin plating. Only difference is the length that a whisker can
growth. We are making TCP film carrier for LCD drivers. The lead pitch is 75
um. At this kind of fine circuitry even minimum growth of tin whisker will
cause short. Therefore many method were used to prevent whiskering. If you
use it for normal SMT pad I believe there should not have such problem. But
be aware of the oxidition of the tin layer which in term lower the
solderibility of the tin plating. Nitrogen may be necessary for packaging if
you need to guarantee certain shelf life of your product.

Chris Wu
Wus Printed Circuit
ENG Dept
[log in to unmask]


>At 02:03 AM 1997/1/17 +0800, you wrote:
>>
>>Hello to all,
>>
>>I thought " tin whiskers" only occurred with electrolytic 
>>plated tin.   Is it not the stresses induced during plating that 
>>provide the energy to produce the whiskers?  In any case I
>>have used immersion tins on pcbs  for many years, both
>>as a short term finish in captive shops and as a rework  for
>>outdated HASL boards.  The new 1 part liquids make the 
>>process much easier.  You need only  about 20 mins at 
>>100 F to put down about 20 millionths.  thats millionths with
>>six zeroes, certainly not enough to sustain whisker growth.
>>
>>As with all plating processes the base metal must be 
>>"meticuously"? well very clean.  Copper  will require a 
>>microetch , tin/lead may require a solder brightener step
>>if highly oxidized.   If the copper is not clean immersion 
>>tin will not take thus making inspection easy.  As always
>>the solderability time limit will depend storage conditions. 
>>I've seen them last for 6-8 weeks on the floor during a brutal 
>>South Carolina summer.  
>>
>>     Remember, all the tin is doing is protecting that perfectly
>>clean  copper surface from the elements with something that
>>will help your solderpot ratios. Contact your supplier for the
>>processing details.  But try it you'll like it.  And if the project
>>gets hung up for some reason don't fret, just wrap them up like
>>Cristmas leftovers and keepem in the fridge.  
>>Robert E. Welch
>>Process Specialist
>>Email: [log in to unmask]
>>Modem: 804-239-9120
>>Fax:   804-237-3048
>>
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>

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