Oops
I answered the specific question that was asked, which was how to
determine the grain direction. I didn't mean to bring up the cross-ply
versus no cross-ply rathole. Assuming no cross-ply made the analogy
simpler to convey. Also I said board shops specify grain orientations
for their materials, which Fred also confirmed.
I didn't say that end-users should be specifying grain orientations
because IMHO it's an unnecessary detail that the end use spec (i.e.
warpage, etc.) will take care of anyway.
As for tailoring CTE to match parts, which was the other question that I
didn't address, using X and Y laminate CTE values would be a futile
exercise for the reasons Fred and Werner have already stated.
Sorry for the confusion
Gerry
-----Original Message-----
From: Fred Johnson [mailto:[log in to unmask]]
Sent: Wednesday, July 15, 1998 9:52 AM
To: [log in to unmask]
Subject: Re: [TN] CTE of PCB Laminates -Reply
Sorry, folks, but I must disagree a little with what's said here.
Although I
might tell you the grain direction in your parts, I most likely would
not be
too thrilled about having the grain direction specified, since that
might
affect how I lay out the parts on the panel - for best utilization (and
thereby lowest cost). In some cases, parts might be oriented with the
longest dimension in both X and Y, if that would be the way to fit the
most parts on the panel. Also, I have built many multilayer panels
with
the grains crossed between the cores and the prepreg, since that was
what was necessary to reduce the warpage on the parts (10-12 layers,
.053" thick, large size boards).
In my opinion (which doesn't count for much), if you need to match the
CTE of the parts that closely, you should be adding internal or external
stiffeners (heatsinks, etc.). Go back to the drawing board.....
Fred J.
>>> "Gerald G. Gagnon" <[log in to unmask]> 07:18 15
July 1998 >>>
Hello KK
Although I'm not 100% sure, I believe the Laminate data convention is
the X is the warp (i.e. grain or machine direction of the treated
fabric) and the Y is the fill (i.e. weft or cross grain direction of the
treated fabric).
If you know the lay-up construction detail, you will know which is
which. Most board shops specify grain orientation for their laminates
and prepregs. Most board shops would be happy to tell you.
If it is not known, the only way I know to determine the fabric warp
direction, assuming the lay-up does not cross-ply prepregs, is to burn
the resin & copper layers off in a vacuum furnace. This should leave the
glass fabric layers only. Using a "linen tester" you would then count
the number of threads per inch in both sample directions. You would
then
compare the numbers to a fabric chart (either IPC spec or glass fabric
vendors can provide) which will tell you the thread count in warp and
fill directions.
Hope this helps.
Gerry
-----Original Message-----
From: KK Chin [mailto:[log in to unmask]]
Sent: Tuesday, July 14, 1998 7:29 PM
To: [log in to unmask]
Subject: [TN] CTE of PCB Laminates
Specs from laminate manufacturers show a substantial difference in
coefficient of thermal expansion between X-axis and Y-axis for FR4,
FR2,
CEM3, CEM1, etc. Is there any way I can tell from a finished PCB which
is X
and which is Y? How can I specify in my PCB drawing such that a fab.
house
always orientates my PCB in a particular axis? I consider this as a mean
to
match with the CTE of certain components on that PCB.
Thanks!
K.K.Chin
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