On Tue, 8 Oct 2002 14:54:55 -0700, JaMi wrote:

>> Nothing you can do will eliminate the problem, just hope to reduce the
>> effects. A lot of copper in one area of the PCB will take longer to etch
>> resulting in over etching (undercutting) in areas with less copper. The
>> board house can't afford to over etch because some tracks will undercut
>> unacceptably (or even completely and fall off the board), under etching
>> risks leaving shorts in the dense areas. You are giving them process
>> control and yield problems seems reasonable they want you to pay extra.

>I can accept this as applies to a board with different weights of copper on
>each side, but not for the same thickness of copper (specifically not to the
>extent that you appear to be stating here), where the only issue is "area",
>and there is proper replensihment and removal of etchant.

The trouble is your idea of 'proper' isn't possible. If each molecule of
etchant came into contact with its own molecule sized area of the board and
was magically whisked away then copper density would make no difference but
that doesn't happen. 

It hits the board surface then moves around a bit. It reacts with the first
copper atom it meets and for the rest of its journey across the board
surface it is just getting in the way of the remaining active etchant
molecules. 

The more copper area on the board surface the higher the proportion of
spent etchant in contact with the board. You can not avoid this effect just
try to reduce it by reducing the time each etchant molecule stays on the
board surface (by spraying or agitation or bubbling or whatever).
Cheers, Terry.

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