Hi ya'll!
I know this issue has been addressed before in the past (I tried to do an
archive search, but the server isn't letting me for some reason?), but since
I've been here at my new position, I've had to deal with a lot of boards that
have dry film solder masks.
As we all know, most of the time with dry film, the artwork is "windowed"
around all the leads in each quadrant on a QFP. I also know a lot of other
things come into play (pad geometry, stencil thickness, placement accuracy,
etc.) that determines whether you get solder bridges on QFP's or not, but as
a general rule, from your experiences, does solder mask between the pads on
fine pitch really make a difference? I'm inclined to feel that it does, but
then again, I've been able to build some boards without the dams between the
pads and get good first pass yields.
This particular board has a 304-pin QFP (with a flat metal heatsink in the
top), 10-mil wide pads (leads are the same width), .020" pitch, "windowed"
dry film, and a friggen' HASL finish on the PCB. There's been a few come out
of the oven without any bridges at all...but most have at least a few. I even
tried something an ol' mentor of mine showed me (Mike Briskey), which was to
increase the heat on the bottom of the board in the profile to try and keep
the solder down on the pads instead of bridging and wicking up the leads (the
bridge usually has formed about halfway up the leg) ...but not much changed.
So my real question is; does it really make a difference to have mask between
the pads or not? If I remember correctly the last time this was talked about
on the list, it was split about 50% for dams, and 50% that it didn't make a
difference...
Thanks!
-Steve Gregory-
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