> the remnant of clay which sticks to the toner seems to react with
> the etchant and generates bubbles.

Hi John,

I had to make a couple of PCBs today and made a note to watch
closely. Everything was prepared as normal

I did see what you wrote about and then recalled a board or two
which had unexplained small unetched areas, as though a bubble
had been trapped when laying the board upside-down on the
surface. Bubbles I'm sure shouldn't and couldn't have been there

I deliberately put the first board in copper side up, on the bottom
of tray. At this point I'd usually walk away and come back in a
few minutes, but this time I watched carefully

For about a minute, with the light just right, I saw many very
small bubbles, like a slow and gentle fizzing. If that board had
been upside-down as usual they probably would have merged
to larger bubbles that blocked the etchant getting to the copper

It stopped producing bubbles after about a minute so I took it
out, rinsed and dried it, then put it back in, this time upside-down
on the surface.

It etched perfectly, as I'd hoped because this had a lot of closely-
spaced parallel lines going to an SMT memory chip

Thanks for upping my standards

Joe


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