good comments Harry, thanks
i also have had no troubles w/ tenting, that's why i was surprised to
see these admonitions
e-protos appears to be part of the chicago area pcb belt
some of these shops (in the old days) used to supply the automotive
market w/ high volume single and double bds
there are quite a number of fabs in that general area
sometimes i selectively expose bits of vias where i know i might want to
either probe or kluge, something like a negative clearance that still
leaves the hole exposed but covers the edges of the pad (about splitting
the annular difference between hole and pad)
this seems to help quite a bit with avoiding bridging issues and still
allows rework
of course it is not as effective as fully tenting but i have found it to
be a useful compromise
ds
_______________________________________________________________________
Integrated Controls, Inc. Tel: 415-647-0480 EXT 107
2851 21st Street Fax: 415-647-3003
San Francisco, CA 94110 www.integratedcontrolsinc.com
Harry Selfridge wrote:
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
____________________________________________________________
You are subscribed to the PEDA discussion forum
To Post messages:
mailto:[email protected]
Unsubscribe and Other Options:
http://techservinc.com/mailman/listinfo/peda_techservinc.com
Browse or Search Old Archives (2001-2004):
http://www.mail-archive.com/[email protected]
Browse or Search Current Archives (2004-Current):
http://www.mail-archive.com/[email protected]
