Any problems I've had with board warpage in the past have been due to either

Bad design with respect to copper density across and through the board, or un-even 
density of drill holes.
Bad processing, the board having stresses frozen into it during manufacturing.
Bad build up, using asymmetric build up, either by design or one time due to the 
manufacturer not controlling their pre-preg thickness correctly through each sub-step 
of build.
I'm sure that there are others, that I've not come across yet.....

Some say that an odd number of layer pairs can cause problems, indeed we had problems 
with 6 layers at first, though we now use 6 layers to great success.  Problems with 
that were caused by bad process control, not the design (though the manufacturer tried 
to blame design at the time!!)

If forces are frozen into the board during manufacture, they'll re-appear as bowing, 
twisting, bumping etc, during reflow.

I've also seen the opposite, where a board, bent from the fab straightens during 
reflow, I'd be less worried about mechanical stress in this than the other way round.

I recall one board (not mine I hasten to add) of about 100mm along each edge that had 
a 5mm hump in the middle, and the customer wanted the assembly house to put BGA's on 
it.

If the boards are that important, re-make them.   Try a different vendor or a 
different build up, sometimes vendors choose the build up for low cost/fast 
turnaround, not for the best structure.


Regards,
Jason.

-----Original Message-----
From: Robison Michael R CNIN [mailto:[EMAIL PROTECTED]
Sent: 20 July 2004 16:40
To: 'Protel EDA Forum'
Subject: [PEDA] Board warpage?


This email is not a Protel-specific question but instead concerns a 
generic problem with some boards I'm getting, so I apologize for being
off-topic, but this forum has more PCB experience than any other I
know of, so I would like to get your advice.  

A partner in a manufacturing effort is sending us full populated
boards (ICs and passive components) that are badly warped.  We sent
them back and rather than get the boards remade and repopulate them,
they want to heat the boards up (fully populated) for three days at
150 degrees Fahrenheit in order to straighten them out.  These boards
perform an extremely important job and I'm worried about the stress
on both the electrical components and the boards themselves.  Should
I allow them to do this or should I demand the boards be remade?

Thank you, Michael Robison 





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