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Date: | Tue, 15 Aug 95 21:15:01 EST |
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Kelly:
I assume that you are using a 1/2 oz. cu. foil on which
to build the outer layers. Since 1/2 oz. cu./sq. ft. equates
to .0007", and the etch occurs on both sides of the pad, a
maximum etch loss of .0015" would be expected. This is also
assuming that the etch is in control, and functioning at a
ratio of 1:1;i.e., as much is etched inward as it is down.
There may be a slight growth of the pad width due to
possible "mushrooming", thus a span of .0025" should be
quite acceptable to board fabricators. We like to use
+.001", -.002" as a maximum with 1/2 oz. cu. I would guess
that the pads would be blown up by .001" overall to try to
more center the tolerance around the finished requirement.
I don't believe that the finish of either HASL or OSP would
affect this much, because the typical thickness of solder
levelled solder is .0001" or less. Bottom line: if you are
looking for .011" (+/-.002"), the pads should probably be
bumped up to .012" on the fabricator's artwork to avoid the
phenomenon of running at or below the minimum acceptable
finish.
Tom Coyle
Field Services Engineer
HADCO Corporation
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