Hmm ; can see the servers reasoning :
that dialect you definitely mastered must drive the poor machine nuts in
digifiles !
Not that 'm complaining, ok (where's Sam by the way ?).
Your inclinations are quite correct in this instance ;
(whish could say same about hurricane and dry film ally):
especially between fine pitch makes huge difference ,
couple months ago I've had to iron my fab bud because he "streamlined" our
20 pitch QFP .
AFTER our tech's runned a month around fault (bridge)previously unknown ;
result was when checked the liquid photo under micro : there were just 4
blanks and that was it !
We were not amused , before we used up the guilty batch .
Also SIR values differ (still more aged in field).
Without having any indium's shares :
the nc-smq90 paste (dispensing [63/37/85%,400-500], find stenciling
ratios)proved to be phenomenally wicking;
you could (on 25, mask between) smear paste right across QFP like a
toothpaste and it came from reflow even evenly distributed, no shorts.
We trained the cutie even to creep from mask onto pads whenever we wanted to
balance wettings on thumbstonings , feed odds,
seep in holes, etc. (things you can do with needles [if you have a time for
it], on stencils you can do same within limits [flat volumes and new
stencil]).
see you paul
-----Original Message-----
From: Stephen R. Gregory [mailto:[log in to unmask]]
Sent: Tuesday, 14 September 1999 9:18
To: [log in to unmask]
Subject: [TN] Solder Mask between pads...
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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