Shirley you will have some difficulty getting 7 mil line and space on 3 oz
Copper, however it is feasible, and is done commonly.

Cupric Chloride etch is the preferred etch chemistry for this aspect ratio,
if only because it is so controlled.  Ammoniacal is usually only controlled
by density (Baume') and for really perfect etching, you need to control pH,
and air throughput as well.

With a proper control system (Density, acidity, and ORP) and a well designed
etcher, which is in good shape (clean, nozzles in good shape, etc.) you may
well be able to do this sort of etch configuration.  This sort of etching is
done commonly in the chemical milling industry, but they do all sorts of
interesting tricks to the art work/image to assist, such as making the spaces
smaller, and deliberately undercutting.

I have written some things about the chemistry of etching, which you can find
at www.pcbfab.com.  One of the sharper people I have met on equipment/etcher
design is Don Ball of Atotech USA.

Good luck.

Rudy Sedlak
RD Chemical Company

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