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August 1997

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Wed, 13 Aug 1997 15:48:16 -0400
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Yuan:  As I understand it the key word here is "rework".  Both John Guy and
Gunter Grossman are correct, as far as they go.  John is correct when he says
that the smaller grain size will yield higher reliability via increased
elasticity.  As John also notes, the connection metalurgy begins to change as
soon as the reflow process ends.  A solder connection has always been a
dynamic medium of attachment and characteristics will change throughout it's
useful life.  However, the finer the grain density at the beginning of that
useful life, the longer the useful life will be.  Gunter correctly notes that
stress in a connection will be self-relieved within a brief period of time.
 Gunter also correctly notes that the coarse lead phases are initially noted
adjacent to the pads/components, where thermal energy is extracted slowly by
limited conductive cooling attributable to the huge thermal mass of the
pad/component interface.  Those interfaces are the points where you see
connection failures, attributable in part to lead phase concentration and the
combined effect of intermetalic formation.  In the repair of equipment you
have two things to contend with, the presence of an intermetalic before you
start and the application of only localized heating.  The intermetalic works
against you because you need to minimize it in-so-far-as possible, while the
localized heating works for you because it will allow the pwa to
accept/dissipate thermal energy more quickly than a pwa which was preheated
for machine soldering.
  During several years of running a Reliability Test laboratory for the Navy
(in a former life) it was demonstrated to my own satisfaction that a
connection subjected to accelerated cooling will be more reliable than one
allowed to cool slowly.  My advice would be to train your repair technicians
well, minimize the thermal energy used during the repair, and use accelerated
cooling to the maximum extent allowable by the components you are using.
 Regards, Jim Moffitt


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