Hi, Mike,
How are you masking the stencil? From the top or the bottom? Too thick a
mask, especially on the bottom, will cause the stencil to deform and you'll
lose the gasketing of the stencil to the board. For low volume, the
simplest form of masking that works is Kapton tape on the top side. It's
thin and strong enough, and doesn't cause additional wear to the stencil.
If you paste a very fine pitch location but don't place a component, there
are no leads to "soak up" the solder when it melts. Particularly if the
pads are over-pasted (width), there is a greater tendency for bridging to
occur. If you have excessive stencil wear, then you're probably looking at
replacing the stencil soon anyway, so either consider omitting the QFP
pattern next time or reducing the aperture size.
Peter Duncan
Michael Bell
<Michael.Bell@ To: [log in to unmask]
GPC.CO.NZ> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
Sent by: Aero/ST Group)
TechNet Subject: [TN] QFP Solder Pad Shorts
<[log in to unmask]
RG>
11/07/01 07:13
PM
Please respond
to "TechNet
E-Mail Forum."
Hi all,
I have just come across a problem with a reflowed PCB that has a footprint
for a QFP on it, but no QFP placed. It seems the footprint has paste
stenciled onto it, various other components placed, and then reflowed.
However, after reflow, it is notorious for having a number of solder
shorts.
We have tried masking off the apetures in the stencil to avoid depositing
paste on the pads, but I am noticing wear on the stencil. Has anyone got
any ideas of what is causing the solder shorts between the pads, or how to
solve this???
Cheers
Mike
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