Phil, you are experiencing one of the less understandable, and because of
that,  more maddening phenomenon in building bare PCB's.   I have seen this
also, and it seems, that it must be related to the grain of the metal, or
some localized impurities, as it is clearly not a surface related phenomenon.
  I say this, because I have left panels in alkaline cleaner, for 15 minutes,
and then microetched off 200-400 microinches, and had them repeat the pattern
of "skips".

However, there is some good news here.  That is that the "skips", in my
experience, at least, are not truly skips, in other words, will simply not
oxide, they are merely slower to oxide, and we found that by leaving the
panels exposed to the oxide longer, you could get it all.

If you are using a light brown/red oxide, of the type designed for polyimide,
then this should not be a big problem, as most of these oxides are self
limiting, (at least at higher temperatures), and thus should not cause a
problem is left in for longer.  How much longer?  Well, we found that a
minute usually did the problem, but then we we using one serious fast oxide,
and that minute actually meant 50% longer than the normal oxide.

If this does not seem to do the job, gimme a call, maybe I can give you more
specific discussion.

Rudy Sedlak
RD Chemical Company
415-962-8004

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