Excellent point Kelly. I had forgotten that one. I too have seen this come
up when line drawn data is received from customers and the board shop
flashes pads. There is a small 1 mil outline of the features that are
filled with typically 5 mil overlapping lines. If you flash only the fill
area, you could leave a 1 mil gap between features such as lines and pads.
However, most circuits on line drawn data that we see today extend into the
pad to the pad center. This typically would eliminate this possibility on
new designs.
Ed Cosper
----- Original Message -----
From: Kelly M. Schriver <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, December 15, 1999 10:49 AM
Subject: Re: [TN] Open traces on printed circuit boards
> Lou -
>
> I ran into a similar situation with a board of similar trace widths a
couple
> of years ago. After much gnashing of teeth and wailing we found the
problem
> to be a microscopic disconnect in the CAD files at the interface of the
> trace and pad. You may want to take a real close look at this possibility
> and get a clear understanding of just how your design rules check is
> running.
>
> Regards - Kelly
>
> -----Original Message-----
> From: Lou Hart <[log in to unmask]>
> To: [log in to unmask] <[log in to unmask]>
> Date: Wednesday, December 15, 1999 8:33 AM
> Subject: [TN] Open traces on printed circuit boards
>
>
> 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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