Charles,
At a previous employer we assembled boards very similar to what you
describe. However, our panels then were 8.5" x 14.5" (slightly
larger) 6~10 layers, more signals than planes, 1/2oz ext/1oz int spec.
foil only, 1.6mm etc. We had .5mm, .4mm and in some cases .3mm QFPs
throughout the panel.
If I remember correctly, we did have hardmount stiffners along the
longest edge. This may or may not have helped, since we used them
from day one. I don't recommend you using copper as a stiffner (2oz
or more). Most copper foil is ductile and easily bends, not adding
any rigidity to the panel. While other copper foils are non-ductile
and may form cracks (open) under stress, regardless of its thickness.
This is why it is not recommended for board fabricators or assemblers
to bend boards to correct bow and twist in boards that are out of
spec.
Using thicker copper will only open another bag of process worms.
If your bareboard is not impedance controlled and the CCA receptical
can accept a .093 thk. increase the overall bareboard thickness to
offset component stresses on the PCBA. Again, cost compare the
additional laminate to adding soft/hardmount stiffners.
What I have also seen is operator handling. Some Assembly houses have
PCBA Handling Procedures in place, but regardless of who and who
doesn't, I continually see operators miss handle completed assemblies.
They either handle the PCBA by stradling their hands across the
assembly onto components or hold PCBAs from the short end allowing the
component weight to stress the laminate, in turn solder joint
failures. To name a few.
The problem may also be in your assembly process (solder paste volume,
steciling, reflow, etc). Hope this helps, I'll leave it at that.
Regards,
John Gulley
INET Inc. - A Smarter Vision
______________________________ Reply Separator _________________________________
Subject: Pins popping up
Author: [log in to unmask] (Charles J Wills) at Internet
Date: 11/12/96 9:49 PM
Technet:
I have a board about 13.5" x 4.5" (PC plug in card), .062" thick,
8 layer, FR4, 1 oz copper, 2 power planes, with 16 100 pin TQFP
packages (0.5mm lead pitch) spread across the board (2 rows of 8), as
well as many other PLCC and SOIC devices.
The problem we are having with these prototypes is that some pins
on the TQFP's pop up durring handling. The boards seem to be too
flexable, which probably causes the pins to break off the pads. Is there
a way to make the board more rigid besides using board stiffiners? In
other words can anything be done as far as changing the board stack up,
or will using thicker copper help this situation? Or are there different
types of FR4 available? Or is there something I am not looking at?
Also, what would be a good pad size for the 100 pin TQFP chip?
Currently I am using 1.2mm x .25mm, but I think the IPC recommends 1.6mm
x .35mm, but it doesn't look like it would allow any soldermask between
pads. Or does anybody even bother with soldermask between pads at .5mm
pitch? What is the middle ground?
Thanks in advance!
Charles Wills
[log in to unmask]
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