To use a TechNet quote
It Depends

The usual quoted heat rise is 6 - 8 degC per minute between 60 and 140 degC.
(11-14 degF per minute between 140 and 284 degF) but this depends on what
your prepreg supplier recommends.

The applied pressure depends on the panel size being pressed. With a vacuum,
lower pressures can be used. Some maximum quoted figures are:-
Panel size 12"x12" - 250 psi.  18" x 16" - 300 psi. As before it depends on
what your pre-preg supplier recommends.

If you undercure in the press I guess that means the Tg will be lower which
will lead to more movement. Don't forget we're dealing with a plastic
material which has a transition or softening point, usually about 130 - 140
degC for standard FR4.
Below the Tg the CTE in the X & Y direction is around 11 ppm/degC. Above the
Tg the CTE is ........ much higher (over 100 ppm/degC but I can't remember
figure.

Geoff Layhe

-----Original Message-----
From: Jonathan C Whitcomb [mailto:[log in to unmask]]
Sent: 29 October 2002 14:20
To: [log in to unmask]
Subject: Re: [TN] Dimensional stability


      Would you care to comment on some specifics as to what you consider
too high a pressure, and too fast a heat up rate?
      Also what would be the dimstab consequences (assume other properties
are OK) of slightly undercuring the resin system in the press (with no post
bake)?  I know that such a panel can shrink a few hundred ppm if it sees
heat later on above Tg, which can be a nuisance.  I've never really known
if  the dimstab coming out of the press is actually worse, but it doesn't
seem to be.

Jonathan Whitcomb  IBM Endicott   607-757-1326   [log in to unmask]
Dept T49G bldg 257-2  1701 North St.  Endicott NY 13760   aisle H office
X013         pager 2930  fax 607-757-1156


Geoff Layhe <[log in to unmask]>@IPC.ORG> on 10/29/2002 08:05:59 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
       to Geoff Layhe <[log in to unmask]>

Sent by:    TechNet <[log in to unmask]>


To:    [log in to unmask]
cc:
Subject:    Re: [TN] Dimensional stability


If you want to improve registration between layers you need to start way
sooner than the press.

Here are a few quick pointers to improving registration/stability.

Choose cores and pre-pregs carefully. Cores with a high glass to resin
ratio
will be better for stability.
Some pre-pregs give better results than others. I've always found 2116
better than 2125 for instance.
If you use compensation factors on inner layer artwork make sure they
reflect what the laminate is doing. (Signal layers will need more
compensation than ground layers).
Brushing can stretch inners. Chemical clean is better.
Always have copper around tooling holes in inner cores.
Make sure the heat up rate in the press is not too fast. If it's too fast
it
will give too much flow and "stretch" the inner cores.
Make sure the pressure is not too high use a vacuum press if possible.
Make sure the cure time is long enough to fully cure the resin.
Cool down rate should be the same as the heat up rate. (This affects bow &
twist more than stability.

Hope this helps

Geoff Layhe
Lamar UK Ltd

-----Original Message-----
From: [log in to unmask]
[mailto:[log in to unmask]]
Sent: 29 October 2002 09:34
To: [log in to unmask]
Subject: [TN] Dimensional stability


Fellow Technetters,

Pls suggest a way out of this problem

      1. If I want to reduce the dimensional instability after Multilayer
pressing, what all the variables I should consider

      2. By reducing the rate of cool down, will I be able to achieve the
required stability


Thanx in advance.

Warm regards,

      Madhu




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