Lou..
              We have had problems with fine traces (<.006) during different
  stages of bare board  and assembly test ( more with robotic than manual
  probing) with the test probe accurately causing the open. Might want to
  checking that possibility. Just something that I've experienced. Good
  luck and have a great day.
                                                         Regards,
  Bob Vanech                                                   Bob
  BVAN Enterprise




>Technetters, I'd like to have a clearer, more precise question to ask, but
maybe you can help with any comments from experience.
>
>Our fairly small assembly shop has been troubled with open traces, found at
final test, on the circuit boards of some pretty expensive assemblies.   The
bare boards come from our sister division of the same company.  They are
sizeable multilayers with traces 4 or 5 mils in width.  The opens appear
most commonly near the surface mount pads of a cluster of SSOPs (0.65 mm
pitch).
>
>Test here had believed that the bare board test might not be catching the
opens.  The PC shop suggested the opens were coming from overstress during
assembly testing.  But the appearance of these opens did not resemble that
of traces that had been overstressed in test.
>
>This week one of the test techs found an assembly that worked OK, then an
open appeared.  This behavior seemed to support the thesis, proposed by the
PC shop, that the traces could have been greatly reduced during fabrication,
but were conductive enough to pass bare board test.  (Bare board test
delivers 20 ma for the continuity test.)  In the operating assembly, the
trace could carry as much as 200 ma, and for a longer time than that
consumed by the bare board test which could blow the tiny trace.
>
>Many thanks for any comments or ideas.  Lou Hart   Compunetix
412-858-6184
>
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