Jeff,
Having dealt with the manufacturing end of double sided tented vias, I can assure you it is a nightmare to build. The problem is that with both sides tented, escaping gasses and moisture from the holes will leave a volcano effect dimple on one or both sides during the mask curing process. This will create problems when screening on the solder paste. The raised " dimples" will not allow the stencil to lay flush to the board. You can end up with solder paste shorts between fine pitched SMD pads. I have found that the no clean fluxes available today and a specified bare board cleanliness has solved most of our customers concerns. Another , although more costly, alternative is to design a test fixture that utilizes mechanical compression against the test pins rather that vacuum. Just my thoughts. Hope it provides some insight.
Sincerely,
Ed Cosper
Director Quality Assurance and Engineering
Graphic Electronics Inc.
----------
From: Jeff Seeger[SMTP:[log in to unmask]]
Sent: Tuesday, February 25, 1997 2:02 PM
To: [log in to unmask]
Cc: [log in to unmask]
Subject: Fab/Assy: Class3, 1/2 tented vias
Good day, TechNet -
Can't help needing to post in this time of too much traffic.
Perhaps you'll soon see why:
I've been asked to make a soldermask change on a very dense,
double-active, sensor type design. The assembly is subject
to In-Circuit-Test, and the non-test-side clearances are such
that the contract manufacturer is requesting that we tent the
testpoint vias on the non-test side. There are troubles with
solder shorts.
The board is HASL, 2x reflow, with hand solder, and an aqueous
cleaning process. The soldermask is LPI. Volumes are modest.
The via geometry is small, such that the current soldermask
appears to give a reliable covering.
The application is life support.
My fear is that one-sided tenting will allow entrapment of
processing chemistry and will not allow adequate cleaning.
It would seem to me that switching to OSP or shrinking the
via clearance to just above the finished hole size would be
a safer alternative for long term reliability.
Can a sometimes-too-conservative ol' design guy draw out any
support or denial opinions?
Thanks in advance, and regards,
--
Jeff Seeger Applied CAD Knowledge Inc
Chief Technical Officer Tyngsboro, MA 01879
[log in to unmask] 508 649 9800
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