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Date: | Mon, 19 Feb 96 11:34:29 EST |
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Mark,
I'm curious on this subject. I've never used those "odd
ball" glass fabric styles you've listed. I've stayed away from
them for another reason. I know that cost is driving more single
ply usage and that they had to bring in more selections to cover
thickness' that we weren't able to achieve in the past. The
possibility of inherent mechanical stress from single ply glass
styles is highly possible. Two ply construction could provide
an interface that allows more shear movement between the ply's
providing a less stress cured substrate. Think about it.
<One of Dave's <stupid> simple analogies> When ever I go
backpacking I have to wear two pairs of socks. First I put on a
thin one, then a thick one. All the walking I do puts all the
friction wear between the two socks. (and not on my feet) The two
ply issue may be similar. When we (Fabricators) process innerlayers
and laminate into MLB PCB's, we experience layer/material movement.
(Most of the time shrinkage caused by the fiberglass trying to
stress relieve everytime the material sees Tg)
The two ply issue may be similar. Single Ply usage is still in it's
infancy. ~3 - 4 yrs old for production.
<I dunno...makes sense to me>
Keep me posted. If there's anything I can help you with, feel free
to call me or email direct.
Dave Hoover
(408) 728-6677
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______________________________ Reply Separator _________________________________
Subject: flatness affected by build-up?
Author: [log in to unmask] at SMTPLINK-HADCO
Date: 2/15/96 11:00 PM
Hello,
We assemble a six-layer two-sided SMT board which has been troublesome
lately because of increased bow/twist problems. The increased bow/
twist appears to coincide with our recent changing of suppliers for this
PWB. Unfortunately, I cannot say this definitively because I don't have
historical bow/twist data - only qualitative perceptions from our SMT
line operators.
Another difference we noticed between the old supplier and the new
supplier is the old supplier's boards tended to be on the high end of the
thickness spec (.065"-.067") whereas the new supplier's boards tend to be
on the low end of the thickness spec (.057"-.059"). This led us to
speculate whether there might be some correlation between the material
chosen (core laminate and pre-preg) and the bow/twist properties. Our
documentation doesn't specify what to use for core laminate or pre-preg so
it's quite possible the two suppliers chose different core-laminate and
pre-preg material.
I'm looking for some feedback: What build-up would you use to fabricate a
six-layer .062" board with 1 oz copper inner layers and 2 oz copper outer
layers (after plating)? Do you think the build-up could have any
influence on bow/twist? (Also, the copper distribution on all layers appears
fairly balanced.)
We're going to cross section some of the old supplier's boards to determine
their build-up. The new supplier has the following build-up:
1 oz copper foil
7629 B-stage
.015 copper-clad core
7629 B-stage
.015 copper-clad core
7629 B-stage
1 oz copper foil
I believe the 7629 B-stage is .008" thick. Would there be any advantages
to using two sheets of 1652 B-stage which is .005" thick per sheet? Since
the new supplier's boards are on the low end of the thickness spec we
could add the additional .006" per board, I think, without a exceeding our
board thickness specification.
Please e-mail replies to [log in to unmask]
Thanks!
Mark Lettang
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