Most likely the board will distort somewhat; however, the board is small - 
the amount of cup/warp/twist may be tolerable.

It would be nice if the problem of twist/warp/cupping could be 
quantified.  Attempts have been made over the past couple of decades to 
study the problem and put some numbers to it, but no success yet.  There 
are too many variables that work in concert to cause the problems.

There are simple techniques that can be used during fab that minimize the 
distortion caused by unbalanced copper.  Many high-end fabs now use 
controlled non-linear cooldown, and biased laminate layup to control twist 
and warp.

However, just to frustrate you - the care in fab can be undone in board 
assembly by reflow or hot air that bulk plasticizes the resin and warps a 
board that the fab managed to keep flat.

Bottom line after all that  - you can probably get away with the amount of 
warp you are going to see on such a small board.  Just stiffen it with 
additional mounting or other mechanical support, and you shouldn't have any 
problem keeping the stress off the components/joints.

Since the only real benefit of an external ground plane is suppression of 
radiated signal from the internal layers of the board, I assume you are 
trying to contain some interfering emission.  You also might want to 
consider some other localized shielding in lieu of the single-sided copper 
flood.


At 11:51 AM 5/14/02 -0400, you wrote:
>Hello, all:
>
>I have a double sided PCB about 4 x 4 in. square, supported by mounting
>holes, 1 in each corner.  I want to pour a polygon ground plane on the top
>side.  The plane will cover probably about 80% of the board area.  There
>will be no plane on the bottom side.  Am I going to have a problem with
>board warp with this?
SNIP

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