There's a thread on the PADS PowerPCB forum about the reasons for copper
thieving. Some of the posts so far:

a. "Thieving is simply a process used to balance the copper usage on a
board to
ensure that the etching is even and accurate. Usually accomplished by
adding metal squares  or rounds to open areas of a layout. They should
never be connected to any signal. They do cause some problems for
certain
types of designs, but for "everyday" stuff they shouldn't cause any
problems."

b. "Hmmm, I was taught that the purpose of thieving was more to do with
balancing the plating current over the surface of the board. (note that
theiving is not done on inner layers as they are not electro-plated)"

c. "When a board is in the etcher or plating tanks, the electrical field
needs to be balance on both sides of the board so it can be etch or
plated evenly. By adding more metal to one or both sides of the board
will change the electrical field. When the metal balance is equal, the
etch or plating will be equal."

d. "We place it on inners to Balance out the copper."

e. "These are a few other advantages of thieving -

Solectron used to request thieving on large boards to assist
with even heating across the entire board during assembly.

 From a fab standpoint, thieving can help avoid warpage issues
and separation on inner layers."

f. "If you mean thieving with regard to wave soldering, it's to aid in
breaking
surface tension of solder as it passes over a series of lands (IC's)."

g. (replying to f.) "I always confused those two also.  Now I call the
ones your referring to
Ed... scavenging pads."

h. "It also helps to keep the tanks cleaner as more copper stays on the
board and is not etched off into the tanks."

What are the real reasons for thieving and what are the possible adverse
effects of it (if any) on trace impedances, etc.? I always thought the
reason was to even out the copper distribution to get more even etching.
Something about the localized concentration of the etching agent varying
inversely with the localized copper density and therefore affecting the
localized etching rate. But some of the other reasons listed here also
seem plausible.

Thanks,

Robert

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