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April 1997

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Subject:
From:
"George Franck" <george@[11.1.1.33]>
Reply To:
Date:
Wed, 30 Apr 1997 14:09:41 -0400
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text/plain (85 lines)
John,

The LPI tenting process creates an air cavity in the Via holes. 
When you heat up the gas, it wants to expand.  Sometimes the
pressure is more than the LPI can take, and the LPI lifts.  Other
causes such as partially cured LPI and moisture in the boards can
be reduced through baking, but the basic PV=nRT problem will
remain.

Try clipping the LPI 2-3 mil larger than the hole size.  This
will allow the via hole to get solder coated, and still cover the
surface copper pads.  If you need the vias tented for SMT / Wave
solder processes, then plug the vias from the solder side AFTER
HASL.  This keeps solder off the SMT side of the boards (ie under
the smt components), avoids air cavities, and lets all your copper
to be solder coated.  

**** Please note that the assembly processing dictates
**** the best soldermask tenting design.  There is no
**** "One Size Fits All" soldermask tenting solution.

If you got some more questions,.... you have my E-mail address!
-- 
George Franck
Principal Product Engineer
Raytheon E-Systems
Falls Church Va

On Apr 30,  8:32am, LANmark Circuits,Inc. wrote:
> Subject: RE: FAB- LPISM
...We've been battling a problem with LPI covering (tenting)
larger vias (>.020).
...There is a tendency for the mask to lift around these holes,
particularly in
...ground planes, at HAL.  We've been drying the panels after
scrub in a heated
...board dryer and a quick bake in the oven.  We are using
Dynachem Epic mask.
...
...Any input would be appreciated.
...
...John Gordon
...LANmark Circuits
...LANmark Circuits,Inc.		
...400 Crown Point Circle
...Grass Valley,Ca. 95945
...(916)272-7280(voice)
...(916)272-7017(fax)
...
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>-- End of excerpt from LANmark Circuits,Inc.


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