Robert,
I understand that once soldered, the solder pads on an IWT board will be
alloyed.  What about any traces or ground planes which are not covered with
solder mask?  These locations would not get solder coated, therefore would
remain IWT.  Wouldn't there be a concern of tin whisker growth over time in
the field?

Jim Marsico
Senior Engineer
Production Engineering
AIL/Electronics Systems Group
An EDO Company
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        -----Original Message-----
        From:   [log in to unmask] [SMTP:[log in to unmask]]
        Sent:   Monday, February 12, 2001 9:20 AM
        To:     [log in to unmask]; [log in to unmask]
        Subject:        Re: [TN] more about white tin


        In a message dated 2/12/01 8:07:09 AM, [log in to unmask]
writes:

        JAMES: If immersion tin does whisker, wouldn't exposed traces (not
solder
        pads) be

        a concern even after assembly?
        ROBERT: In my experiences tin "whiskering" of nonporous immersion
white tin
        (IWT) ceases to be a theoretical possibility after assembly, as the
greater
        mass of the solder alloy provided at assembly displaces the IWT. You
can't
        find the IWT after proper assembly -- including pad edges -- as the
matte
        nature of the IWT promotes whole-pad wetting (unlike OSP).

        JAMES: Is whisker growth a function of the environment or just time?
        ROBERT: In the case of IWT (versus electroplated tin), "whiskering"
exists
        only as a theoretical possibility. Search the TechNet archives (and
anywhere
        else); I assure you that you'll find not a single complaint about
nonporous
        immersion white tin whiskering, irrespective of time or environment.

        JAMES: Could the tin in the immersion tin process be alloyed (with a
small
        percent of something else) to prevent whisker growth?
        ROBERT: Whether you assemble with tin/lead, tin/silver, tin/bismuth
or tin
        kryptonite, the IWT is alloyed by a large percentage of something
else upon
        assembly. You've got a greater likelihood that the tin from your
assembly
        solder will "whisker", but I'll leave that aspect to the abundant
solder
        experts.

        - Robert Lazzara

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