Ken,
Even if your vias were only in the area of selective wave soldering, the wave
process could not guarantee a fill on the .018" vias.
Steve's suggestion does work, but be careful. For a reflow/reflow board, you
only want to print solder paste in the vias on your topside print. If you print
them on the rear side (which is typically done first), during rearside reflow
they will droop out the other side (which happens to be the topside). Then when
you go to print the topside solder paste, you have these pesky solder bumps
poking out the vias, and now you've created a "standoff" and solder paste
printing suffers!!!
On your topside print, if there is enough room around the vias, print oversize
solder paste around the via to get good via fill (i.e., solder paste aperture
design can print out onto the solder resist and reflow will wick it back into
the annular ring/via hole during reflow, like pin-in-paste).
Good luck.
Jeff Hempton
United Technologies Electronic Controls
______________________________ Forward Header __________________________________
Subject: Re: [TN] How to fill Vias? ...
Author: "Stephen R. Gregory" <[log in to unmask]> at Internet
Date: 4/27/1999 7:40 PM
In a message dated 4/27/99 4:27:23 PM Pacific Daylight Time, [log in to unmask]
writes:
<< I am in dilemma. We have designed a board which require double reflow
process with selective wave operation. Here is the problem - How can I get
vias filled solder which will carry 1 amp current. Some vias are underneath
the components and many are away from the selective wave fixture opening.
Finished hole size of the via is 18 mils. Can fab house fill the vias?
re,
Ken Patel >>
Hi Ken!
They should be able to...for a price of course. But the problems with via's
that are already filled, especially if it's going to be a double-sided
reflow, is that during the first reflow you may get some solder "drooping"
out the via's on the backside which may cause some problems printing the
solder paste on the second side...especially if you have fine pitch.
I think the easiest thing to do would be to identify the via's that you need
filled, pull them into the paste mask layer of the Gerbers, get new stencils
with the vias also etched in them, and viola', filled vias!
I had to do that a lot on disk drive boards...glad I'm outta that business!
That can be as bad as building memory! hehehe...
-Steve Gregory-
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