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October 2001

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Subject:
From:
"<Peter George Duncan>" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 9 Oct 2001 13:16:00 +0800
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If my understanding is correct, IMC's continue to grow throughout the life
of a board. If we're discussing what is too thick and what is too thin, we
need to look at the stage in the board's life at which the IMC thickness is
being considered and the effect that it's thickness has at that point.

Military boards are intended to last for 15+ years, operating (in theory)
between -40C and +125C. This degree of thermal stressing is going to be a
fertile breeding ground for IMC growth, especially when coupled with
stresses due to vibration. Many "older technology" boards last the distance
still in working condition, so the IMC layer growth can't be a major issue
there.

I know SMT is a lot more fussy and fragile, but are we being more concerned
about IMC's that we need be, apart from ensuring products get through the
warranty period before we make money on repairs or replacements? When is
max IMC thickness achieved (how many 'degree.hours' does it take)? Is IMC
thickness self-limiting in any way, or does the whole solder joint
eventually become a homogenous IMC? How MUCH weaker than the newly-born
solder joint is the senior citizen solder joint? What can we do, if
anything to ensure that the old, fully-matured joints are still strong
enough to do their job? i.e. what homgenous IMC alloy is strong enough as a
solder joint?

Just a (cynical) thought to stoke the fire with.

Regards
Peter Duncan




                    Earl Moon
                    <[log in to unmask]        To:     [log in to unmask]
                    M.COM>               cc:     (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
                    Sent by:             Subject:     Re: [TN] IMC's
                    TechNet
                    <[log in to unmask]
                    ORG>


                    10/09/01
                    02:58 AM
                    Please
                    respond to
                    "TechNet
                    E-Mail
                    Forum."






Phil,

So the following is of concern:

:The thickness of the intermetallic formed by thermal aging had little
effect on the peel strength until the intermetallic thickness reached above
25 microns (0.001 inch). Then the lead rich layer appeared to be a major
the
contributor, it formed large lead crystals the fractured easily along grain
boundaries."

Is this then a thickness specification not to be exceeded, and how easily
under what conditions did the fracture occur?

Earl

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