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From [log in to unmask] Fri Dec 6 15: |
22:46 1996 |
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Good day!
I may have missed the first email on this thread seeing how I just
subscribed today, but in reference to the following:
>>>If you do not wave solder, I do not know of a reason to tent vias...
> Our boards are done in some kind of convection oven rather than wave
> soldered, and we tent our vias because the distance from pad to via is
> sometimes too small to get a reliable mask dam: e.g. a via is 8 mils
> from the pad it connects to and the board house leaves a 3 mil gap
> around vias and pads, and the LPI SM we use won't stick if it's only
> 2mils thick. So to prevent solder from getting sucked across the trace
> and into the vias we tent them.
> Andy B
Andy does have a point, however, in my experience the amount of solder that
wicks and flows down the via is pretty minimal and normally doesn't reduce the
volume of solder to the point that causes a problem with the fillet at that
joint...unless of course the via's are HUGE!
But the comment before that about not tenting them if you don't wave solder
may not be correct all the time. There's assemblies out there that are all
surface mount on both sides (double sided reflow) except for maybe a connector
or two, and they will be installed using a "Airvac" solder pot or selective
wave soldering only the connector...the rest of the PCB is sheilded by the
fixture, so the via's won't be filled. If the assembly is ICT'ed, you need to
be able to maintain vacuum on the ICT fixture, thus tenting is needed.
There have been cases where I've had my stencil made with the via's imaged
along with the surface mount pads to fill them because the fab wouldn't be
revised to tent the via's until the 10,000 fabs that are already made, were
built.
One last note about tenting, and this is only because I've seen it happen.
When specifying the tenting, make sure the bottomside is tented (the side
thats going to be hitting the wave), not the top! What happens is that you can
get outgassing from the flux and have what look like "little blobs" of solder
hanging off the via's, plus if there are any via's that are open beneath a QFP
it makes it tough to clean the flux residues that wick-up through the via
during wave. This may seem pretty obvious, but I still see it happen now and
again.
__\/__
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