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Propensity of Immersion Tin surface finish fo whiskers

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Amol Kane

#38301

Propensity of Immersion Tin surface finish fo whiskers | 8 December, 2005

during my researches for LF transition, a question being asked is -- why even consider immersion tin if it whiskers.

does it really matter?, because during reflow, the Sn dissolves in the SAC solder alloy and does not exist in a "Sn whisker" formable form for a lack of suitable term. the real issue should be the component surface finish.....am i correct on this one?

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Amol Kane

#38347

Propensity of Immersion Tin surface finish for whiskers | 9 December, 2005

looks like everyone is in dark on this one......and i thought it would be pretty intuitive

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#38362

Propensity of Immersion Tin surface finish fo whiskers | 10 December, 2005

We believe imm tin suppliers say that their product does not whiskey, oops whisker, because they use 'white tin' not the 'gray tin' that causes whiskers. But as soon as you alloy their tin with SAC or any other high tin content solder, all bets are off. Your are then open to all the vararies of tin whisker formation.

NASA maintains a web page on the whiskering issue http://nepp.nasa.gov/whisker/

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Sergei

#38366

Propensity of Immersion Tin surface finish fo whiskers | 10 December, 2005

Amol Kane

#38381

Propensity of Immersion Tin surface finish fo whiskers | 12 December, 2005

thank you devef....My point is this....If you solder all the areas that have Imm Sn on them, there wont be any whisker formation DUE TO THE IMM TIN COATING.....whiskers could still potentially form as a result of COMPONENT FINISHES and effective mitigation would then lie in the realm of component finish mitigation techniques (like eg. Ni barrier), but thats a different ball game......I am trying to validate the choice of Imm Sn as a potential coating for use.....is this argument correct??

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#38383

Propensity of Immersion Tin surface finish fo whiskers | 12 December, 2005

OK, I feel the need to jump in here. Bright Tin has the tendency to easily whisker, and is almost never used, except on some through-hole connectors, I believe. Matte Tin is much less susceptible to whiskering, although under the right stress conditions, it is not immune to it. Immersion Tin finish is a form of Matte Tin, I believe. Matte tin is a VERY common finish used on lots of components right now. Some chip caps have been matte tin since 1998!!! Stresses that are believed to cause whiskers are temperature cycles, humidity, and bends, scratches, etc. As far as white tin and grey tin - that is a whole 'nother ballgame, and has nothing to do with whiskers. White Tin and grey tin are "allotropes". different forms or states of the same chemical element. White tin has metallic properties, grey tin does not, and is otherwise known as tin pest, tin plague, tin disease. It is the phase change that happens when white tin is held below 13degC for prolonged periods of time. I believe the phase change may be accelerated the colder it is held at. It still can take months and years, to totally convert all white tin to grey tin. But it cannot be converted back, so once it is grey tin it is useless.

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Kris

#38388

Propensity of Immersion Tin surface finish fo whiskers | 12 December, 2005

do you intend to use Immersion tin as a PCB surface finish ? Then Whisker concern is as below

if you do not past your test pads, the pads will be stressed when probed on a ICT or a flying probe tester. This is a POTENTIAL for whiskers

There is a lot of general whisker debate published and discussed on the forum as one post pointed out earlier

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