D. Hoover's response is right.  If you press 18 X 24 panels in one opening and 
12 X 18 in another, using, say 80,000 pounds RAM pressure; you will be effectively 
pressing the 18 X 24 panels at 185 psi and the 12 X 18 panels at 370 psi.  Other 
panel sizes would vary accordingly.  

If you set your pressure to the larger panel size, this is not good practice for the following reasons:

1.  Tremendous stresses are applied to your platens, which, over time, with repeated cycles
     and the typical 350F temperature could alter them to something less than parallel and flat.

2.  This practice flies in the face of SPC efforts and the desired result of having consistent, 
     repeatable processes which result in predictable physical/electrical properties.

3.  The higher pressures induced by this practice can cause glass lock at the Cu foil prepreg
     interface, evidenced by the following characteristics:
     a)  weave image transfer
     b)  increased potential for bow and twist
     c)  increased potential for measling (micro voids at the knuckles of the woven glass).
     d)  increased potential for micro voiding within the prepreg (by squeezing too much resin out).
     e)  dimensional stability problems.
     f)   thickness control.

If you set your pressure to the smaller panel size, this is not good practice for the following reasons:

1.  Slight irregularities in platen/caul/pcb laminate flatness may induce air entrapment.
2.  You may be compromising thorough flow of the resin.
3.  Reduced peel strength.
4.  Reduced resistance to the 'Pink Ring' phenomenom.
5.  Surface flatness may be compromised.
6.  Delamination.

If you 'split the difference' you run the risk of having any or all the above problems to some degree 
relative to the respective panel sizes.

D. Hoover wisely suggested you find out from your laminate supplier what the acceptable range
of pressures is for your panel size and press procedure; and stick to it.  I  would discourage
the practice under *nearly* any circumstances.

Fred J. Paul, Sr. Process Engineer
PCB Operations, FLUKE CORPORATION
direct:  206 356-5734  fax:  206 356-6070
[log in to unmask]	

On Mon, 16 Sep 1996 05:54:22 -0700 [log in to unmask] wrote:

> ______________________________ Reply Separator _________________________________
> Subject: FAB: Lamination
> Author:  [log in to unmask] at SMTPLINK-HADCO
> Date:    9/16/96 8:08 AM
> 
> 
> Does any PCB manufacturer has an experiance with lamination proccess of
> mixing different board sizes into different openings of the press in the
> same cycle (using same size of press plates and seperators)?
> 
> Shabtay Shalev
> Engineering  
> 
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> 
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> From: [log in to unmask]> Date: Mon, 16 Sep 1996 05:54:22 -0700
> Subject: Re: FAB: Lamination
> To: [log in to unmask]
> 
>      It doesn't work that way. The PCB square area receives the pressure
>      direct. (Like pounds per square inch) You can put close to the same
>      panel sizes in the press. (Like 14 X 18 in one opening and 16 X 18
>      in another) Check with your laminate supplier. They should have a
>      recommended pressure range for lamination. Work within that range to
>      achieve the pressures. Like 18 X 24 at 200 PSI and 16 X 18 at 300 PSI
>      in the same press. An important note to mention is that if you do
>      controlled impedance work, your dielectric thickness' will be all over
>      the place. (variance) Also, you will see dimensional stability 
>      differences. (growth or shrinkage) Be sure you look downstream at
>      any potential changes you may create.
>      
>      Groovy




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