It is based on square inches of copper on the outside layers of the 
PWB. If one side has alot less square inches of copper than the other 
side, they will add nonfunctional copper to balance the design. The 
purpose is to reduce bow and twist. Inside layers being non balanced 
also plays a significant role as well and that it is harder to 
balance and control. There are other factors that also contribute to 
bow and twist and that is the prepreg and grain structure of the prepreg.

Many OEM's or contract manufactuers place blame on the PWB 
manufacturer when it is a design issue. In many cases the PWB 
manufacturer cannot control this. The control of bow and twist begins 
at the designing stage of the circuit board. The problem is also 
compounded because more and more product is required to be in array 
format instead of individual boards fabbed out. This reduces 
stability in the array as well and creates problems for contract 
manufacturers performing their assembly processes due to excess boaw and twist.

Gary




At 09:10 AM 02/23/2007, you wrote:
>Dear colleagues,
>
>very often, the PCB layout designers would draw the circuitry and 
>write a note on the fab drawing asking that the copper be balanced 
>by the PCB house, by adding non-functional copper.
>
>The question I have is: how does the PCB house know how much copper 
>to add and where to add it? Are there any scientific bases, some 
>calculations, or is it based on experience? Otherwise said, if the 
>PCB house will balance the copper, this means the warpage worries 
>are gone, or just that there are chances it will warp less?
>
>And to make it tougher: if we leave the latitude of choosing the 
>right substrate and doing the balancing to the board shop, can they 
>guarantee stable boards?
>
>Thanks,
>
>Ioan
>
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