Kelly,
We also assemble PWA's that have fine pitch parts(.5mm), that are
12+ layers, aprox. 350X400mm, and 20mm thick. Our experience is that
there is inherently some dimensional instability in boards this big
when fabricated with FR4 and FR5. We generally see some GROWTH in at
least one of the axis on these boards. I haven't experienced a problem
with the boards actually being smaller or shrinking.
You're right! This problem is most challenging at our solder printing
operation.
Although we have not actually measured the growth, it's in the area
of .127mm (.005")max. in either the X or Y dimension. Our stencils are
stainless steel and are produced with no compensation for shrink or
growth in the PWB. We have been dealing with this by compromising the
solderpaste print registration, and splitting the difference across the
board. As you indicated the problem gets worse at the extreme outside
edges of the board. At component placement, we are using local etched
fiducials to optimize component lead to pad registration. We do see
some solder bridging as a result of the solder paste deposits being
slightly off the pads.
I am interested in hearing from the Board Fabricators as to what they
feel is reasonable and possible at their end in helping us deal with this
problem.
Dan Botts
Hughes Training Inc.
Binghamton, NY
______________________________ Reply Separator _________________________________
Subject: PCB vs. Paste Stencil Comp
Author: [log in to unmask] at CCGATE
Date: 10/18/95 4:08 PM
From: Kelly Kovalovsky, PCB Quality Engineering
~ EMail:[log in to unmask]
Subject: PCB vs. Paste Stencil Comp
We have a fairly large circuit board that we are assembling. The card
has fine pitch SMT at extreme ends. We have noticed a mismatch between
the solder paste stencil and the printed circuit board. The circuit
board features are actually closer together than the stencil.
My question to any card assembly site is whether it is common practice
to compensate a solder paste stencil for shrinkage of a PCB?
IBM Microelectronics Division
6800 IBM Drive MG12/251
Charlotte, NC 28262-8563
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