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
        [log in to unmask]
______________________________ 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