HI Jeffry  but you can manufacture much better line resoultion then 15 mils
in 3 oz of plated copper , using the method I stated even 3/3 is possible,
through my research we found thick dry film up to 20 mils, produced almost
as good a line resolution as thin 1.5 mil dry film did, in fact I made many
boards for IBM with 20 oz of plated copper at 10/10 technology. we started
with thin base copper and in fact did not use a tin protective  layer but
just a quick etch as the 1/8 oz base copper etched very quickly with no
damage to the plated copper. The exposure and developing is not tricky but
does require a  bit of the "out of the box" additude.
robert Tarzwell  megadawn.com

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