based on my knowledge, following conditions are required for whiskers to
occur:
(1) stress, could be as simple as stress due to improper plating
conditions
(2) activation energy = most likely in terms of heat or self heating
effect of the components.
(3) migration path for the atoms = sometime, the humidity plays the role
to accelerate the growth.
(4) ionics sometimes help to mobilize the atoms too by increase the
mobiility
item (1) is almost always a requirement for the whiskers...therefore,
most of the whiskers are observed at leads bending area, poor plating
(extreme case, you can see the plating cracking), or connectors and
sockets for modules...
Low stress plating is best defence to prevent the whiskers. Add few
precent of Pb helps too (min. 2% - 4% if my memory serves me
right...can't remember where is the reference come from...some good old
data from 50 or 60s).
my 2 cents...(1.28 US cents equivalent).
jk
-----Original Message-----
From: Ingemar Hernefjord (EMW)
[mailto:[log in to unmask]]
Sent: May 15, 2001 3:36 AM
To: [log in to unmask]
Subject: Re: [TN] Pure Tin, Poor Tin
Our companies have been using hundreds of tons of Tin and Lead and the
company has made many millions of boards and still few have ever seen
whiskers on a PWB. It seems to me that whiskers occur more frequently on
components, on terminals, on leads, wires, washers, contacts,
connectors and other parts with mixed geometries, bends, mixed
materials, irregularities and structures that are not like the
'singularity' of PWBs. It also seems as forces that develop in the
construction (e.g. large CTE mismatch) will enhance the creation of
whiskering. I have tried to understand what Jong and others have written
in the matter, and it all seems to be something that occurs on atom
level rather than in the understandable and visible macro world. All
that I have found seems to be based on empirical runs with different Tin
thickness (Jong means that thinner than 0.5 microns= no problem /
thicker than 10 microns= no problem)and varying Lead alloying (Jong
means adding 1-3% Lead eliminate!
s whiskering)but no deeper thermodynamic or chemical or other studies
seem to have been done. So, if we don't know the basic driving mechanism
we will still get surprises, don't we?
Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: Genny Gibbard [mailto:[log in to unmask]]
Sent: den 14 maj 2001 20:47
To: [log in to unmask]
Subject: Re: [TN] Pure Tin
Well, that's just the thing - is whiskering non-existent with lead in
the
mix? The article in the advanced packaging magazine said that lead had
nothing to do with reducing or eliminating whiskers, that tin could
whisker
even when mixed with lead and they had a couple of pictures as examples.
Their premise was more about that better processes have reduced or
eliminated the whiskering and that is the only reason why we see less
whisker problems today. They said that Tin should be considered a
simple
drop in replacement for tin-lead.
I get a little concerned when someone seriously contradicts an
organization
like NASA, not that NASA can't be wrong. But it looks like NASA has
thrown
a lot of money at the problem, so why haven't they ever come across the
problem of tin-lead having just as much propensity to whisker as
straight
tin?
As far as the boards we received, I'm pretty sure they are a white tin,
and
I'm sure that they would work fine for the uses they are designed, but
it is
a concern when a co. just substitutes one process for another without
telling you. It is questionable whether we will use that co. again.
-----Original Message-----
From: George Milad [mailto:[log in to unmask]]
Sent: May 14, 2001 12:10 PM
To: [log in to unmask]
Subject: Re: [TN] Pure Tin
Genny,
If your supplier has used immersion tin as a solderable finish; then you
should be able to assemble the parts successfully.
After assembly the immsersion tin is disolved into the solder and
becomes
part of the solder joint. Immersion tin as such would cease to be part
of
the finished product, and the probability of "Solder Whiskering" is non
existent.
I hope this is helpful.
George Milad
Shipley Ronal Inc
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