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November 1998

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Sun, 15 Nov 1998 08:45:38 EST
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Yves/Paul:
- The basis for the limitation on repairs was (like many other former
requirements) based on a requirement from the U. S. Military.  The rationale
behind the limitation on the number of rework/repair actions stems from the
concept that additional heating cycles increase the thickness of the
intermetallic, thereby decreasing the long term reliability and limiting the
number of field repairs possible before the intermetallic increases.
- Given that a surface starts clean/bare, is subjected to tinning or HASL (1),
is pretinned in house prior to assembly (2), soldered on a wave solder machine
(3) [or dual wave machine (4)], then subjected to rework or repair (4 or 5),
you can have as many as 4 or 5 heat applications to the materials before they
are ever deployed to field service.  Given that DOD equipment was (before
Commercial Off The Shelf) COTS) designed with a 20-year life expectancy and a
certain number of field repairs were anticipated, it was necessary to minimize
the intermetallic to the greatest degree possible before the equipment was
deployed for use.  The DOD folks also did a lot of agonizing about the older
solder-cut-solder systems (which engender an additional heat cycle) before
biting the bullet and agreeing that users of a solder-cut-solder system would
not be penalized by losing one rework opportunity.
- In today's world the U. S. DOD is mandated to use COTS when available and
avoid referencing the former (canceled) military specifications and standards.
Since most COTS has a design life of substantially less than 20-years, and
since field repairs are minimal (usually throw away and replace) the
importance of minimizing the number of rework/repair actions is reduced.
- Absent any requirement to the contrary you can pretty much rework/repair as
many times as you want, unless of course your conscience causes you some
problem.
Regards, Jim Moffitt, Moffitt Enterprises Consulting

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