Lou:
Look at Figures 6.6 through 6.8 ( pp 147-149) in Failure Modes & Mechanisms in Electronic Packages, 1998 , Chapman & Hall. Your answer is there. Yes bare board can have many defects that pass ET and opens or intermittents show up in later stages of assembly operations. But each failure has to be verified to accept that assumption.

Lou Hart wrote:

> 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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Pratap Singh

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