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July 1997

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Subject:
From:
"Andrew J. Scholand" <[log in to unmask]>
Date:
Sun, 27 Jul 1997 02:35:32 -0400
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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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