That's exactly opposite of what is actually happening. 

Perhaps it is due to the WS process where the top side is not getting as hot as the bottom?

>>> [log in to unmask] 12/15/05 02:49PM >>>
        The side with most metal will expand more and will tend to curve the board toward the lesser metal side while is heated. The board will shrink back but not all the way to its original shape when it cools. An uneducated guess. Board fabricators will know exactly what happens.
        Regards,
        Ramón


 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Blair K. Hogg
Sent: Thursday, December 15, 2005 9:07 AM
To: [log in to unmask] 
Subject: [TN] PCB warping due to copper imbalance

We have a motherboard type PCB assembly that constantly warps during the wave soldering process. From what I've learned on this forum, I believe it is due to a copper imbalance from the top layer to the bottom layer - the PCB is a 2 layer assembly, with traces on the bottom and ground planes on the top. 

My question is this - is it typical that the warping will occur towards the side with more copper (the side with more copper will be bowed inward)? 

Thanks,

Blair


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