Hi Dennis,
We have required tenting on some types of boards for over a decade, and
we've never had a problem. We've used both LPI and dry film. We have also
used reinforced tenting in applications where the board is subjected to a
pressure differential. Reinforced tenting involves mask applying a thin
resin (LPI type of material) pad over the via, then tenting over the pad.
There are some real concerns - as long as you are aware of them, you
shouldn't have a problem. For example, if you tent the non-solder side of
via-in-pad for BGA's it forms a gas pocket when soldering that can result
in blown out joints under the BGA devices. Such tenting would be a rather
unusual arrangement anyway. You also don't want to tent over vias under
soldered thermal pads - rework becomes impossible.
You will also see an argument against tenting that suggests chemicals used
in processing the board will be trapped in the tented pocket, thus
shortening the life of the board. While this could happen, it also
requires that the board has been subjected to rather harsh chemical
treatment. Such problems have been ameliorated by modern processes using
more benign chemicals. We've never seen the problem on any of the hundreds
of tented board designs we have done in the past.
With the exception of gas driven solder BGA blowouts and specific rework
cases, I would say that tenting is a non-problem. In those instances where
tenting can't be used for fear of blowouts, there is the option of silver
or copper fill before tenting. Fill adds cost to the board, but you do
what you have to do.
As an aside, looking at the specs from eprotos, it appears they use some
rather old fab equipment. The internal clearance capabilities of 15-18mils
is not consistent with the 4-5mils we see from fabs we have used. Likewise
their minimum finished pads sizes. It looks like they are geared toward
simpler designs. Their advice regarding LPI tenting may be more a function
of their experience and equipment rather than the process itself.
Regards - Harry
At 08:24 AM 10/22/2005, you wrote:
the following snip is from the e-protos web site:
(http://www.eprotos.com/Design_help.pdf)
i never had a fab object or had any troubles w/ tenting, what do you all
think?
Suggestions for Tenting Via holes: If Liquid Photo Imageable (LPI) solder
mask is required, we advise that the via holes not be tented. Tenting the
via holes with LPI solder mask will result in the following problems:
incomplete encapsulation of the via hole, exposed copper/metal on via
holes Solder, on via hole pads and in holes, uncured solder mask in the
holes, which results in peeling of LPI and strapping contamination in side
the holes. The solder mask pads for via holes can be made about 5 mils
bigger than the via drill size. This will result in deposition of solder
in the via holes, reduce the possibility of bridging problems during wave
soldering operation. If the via plugging is used to fill open holes with
the solder mask to block air leakage during In Circuit Test, via plugged
one side is the best. We hope the above mentioned is of some help to you.
If you need additional information or have any questions, suggestions,
please feel free to contact us at (847) 439-5447.
ds
_______________________________________________________________________
Integrated Controls, Inc. Tel: 415-647-0480 EXT 107
2851 21st Street Fax: 415-647-3003
San Francisco, CA 94110 www.integratedcontrolsinc.com
snip
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