Hi Jason,
It is gratyfing to find such a high retention evel from my wokshops. You
indeed recall correctly; while, of course, the 16% life reduction is specific
for the example chosen (5 reflow cycles plus design/material parameters) to
illustrate the point in the workshop, it serves to give a rough magnitude. For
manual rework the temperatures are significantly higher, some times very much
higher, and thus the reduction in lfe should be expected to be significantly
larger. The guideline of maximum 3 rework cycles transmitted by Earl is a good
one; and may by too much for real HDI boards.
On the statistical question, there really is not enough data out there
obtained under really controlled conditions with adequate sample sizes to make
definitive statements. I think, to state that "it is a virtual statistical
certainty that, after
assembly, the barrel of at least one via has cracked" can not be made in any
general sense. This may be true for marginal (for whatever reason) MLBs, but
is not generally true in my experience.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]

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