Hi all,

We seem to have noticed that when etching copper foil in the through
hole areas on our boards, the etch rates for 1 ounce foil differ from
those seen on 1/2oz foil.  We are looking at reasons for the different
etch rates, but so far all we can come up with is some theoretical
reasons about lateral etching inward rates vs. horizontal copper
removal.  In short on adjacent layers in through holes, 1 oz. and 1.2
oz. foil, the 1/2 oz foil exhibits a greater negative etch back (more
copper loss).  Any thoughts on why this is, or better how to create a
system that etches evenly all foil weights?
We use a Sodium Persulfate microetch at 12.5oz/gal and a touch of the
obligatory Sulfuric acid.  Its a room temp dip for 1-2 minutes.

Thanks,

Sean Clinton
(no relation to that philanderer)