Hi Andrew,

When you laminate copper and prepreg together you are, by the laws of Physics,
going to get internal stresses built in due to the difference in the CTE's of
the copper and the Epoxy or whatever polymer system. If you cross ply the two
or however many peices of prepreg such that the warp and fill are in different
planes, this will cause the boards to warp. 

How's that for an oversimplified answer?

Regards,

[log in to unmask]
   
>  From: "Andrew J. Scholand" <[log in to unmask]>, on 7/27/97 2:35 AM:
>  Well, no-one has offered any suggestions on this, so I'll try putting 
>  up a trial balloon.  Please remember I'm only a student however-
>  so take this with a grain of salt!
>  
>  During the cooling phase of lamination, residual stresses can be 
>  introduced if thermally anisotropic plies are oriented in different 
>  directions and/or if the cool-down path is poorly managed.  
>  [Karalekas et al., 1987]
>  
>  I have also heard of PCB fab shops meeting warpage specs by 
>  placing weights on top of warped boards and baking them.  
>  This produces flat boards, but at the cost of building in residual 
>  stress.
>  
>  Assuming that your assessment of the low ambient
>  humidity of your storage area is correct, 
>  my guess is that some combination of factors lead to high 
>  residual stresses in your PCBs, and over time, aging and 
>  creep led to delamination of the bare boards.  The bad 
>  news is, any assemblies built on those boards may have 
>  the same stresses.  The good news is, any thermal processing 
>  steps you may have taken during assembly may have 
>  altered the residual stress (i.e. the boards may have popped 
>  back to their warped, but stress-free shape).  Also, if you 
>  had through holes on the board that were wave soldered, 
>  they exert a compressive stress on the board.  
>  Mirman (1991) has shown that compressive stress in 
>  bimaterial assemblies can increase the shear stress 
>  before delamination substantially.
>  
>  Any one with more experience want to comment on this?
>  
>  Cheers,
>       Andy
>  
>  
>  REFS:
>  Mirman, Ilya B. "Effects of Peeling Stresses in Bimaterial Assembly," 
>  Journal of Electronic Packaging, December 1991, Vol. 113, pp. 431-433.
>  
>  Zewi, I.G., Daniel, I.M., and Gotro, J.T., "Residual Stresses and Warpage in 
>  Woven-Glass/Epoxy Laminates," Experimental Mechanics, March 1987, pp. 44-50.
>  
>  Karalekas, D., Daniel, I.M., and Gotro, J.T., "The Influence of Lamination 
>  Parameters on Warpage of Woven-Glass/Epoxy Laminates", ANTEC 1987, pp. 
>  339-341.
>  
>  ----------
>  From:  Orna and Yehuda[SMTP:[log in to unmask]]
>  Sent:  Wednesday, July 23, 1997 11:05 PM
>  To:  'IPC TechNet'
>  Subject:  Re: "Delayed dalamination"
>  
>  Bill,
>  I absolutelly agree with you about the hygroscopic nature of the polyimide. 
>  If boards are not baked prior to soldering we get quite ugly delaminations. 
>  That is very right, and that's why the phenomena described in my original 
>  message was so puzzling:
>  
>  Two years ago the boards were received in the assembly shop with reflowed 
>  Pb/Sn finishing. There was NO sign of measling, delam, etc. The lot was 100% 
>  visually examined.
>  
>  Last month some boards from the ORIGINAL LOT were taken out of storage and 
>  showed signs of measling/delam. This was WITHOUT subjecting the boards to 
>  any thermal or mechanical shock. As if the delam was developed during 
>  storage.
>  
>  
>  Even with the highly hygroscopic nature of polyimide this seems very 
>  inprobable to me (they were not stored in an aquarium !!). I haven't heard 
>  of such a failure till now and I can't find a mechanism that describes it.
>  
>  Can you or any of the technetters offer an explanation ???
>  
>  Yehuda
>  
>      ************************************* 
>      *          Yehuda E. Weisz              
>      *  e-mail: [log in to unmask]
>      *  Tel: 972-3-5240362                    
>      ************************************
>  
>  
>  
>  
>  
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