Homebrew PCBs

Hi,

I'm trying to print my first ever PCBs, but I'm having some problems with
the photoresist producing small bubbles (or patches lacking in
photoresist) which are about 2 mm (0.1") wide.

I'm using copper-coated blank PCB which has been polished with emery
paper, washed in clean water, and dried in a cool gas-fired oven (~ 10
minutes at ~ 100-150 degrees centigrade), in the very hot summer sun.

I'm then spraying on positive photoresist (Electrolube PRP200) from a can,
and leaving the boards to dry overnight in a dark cupboard, before
applying a photocopied acetate template of the tracks and exposing to
direct sunlight for about 5 minutes.

I'm etching using ammonium persulphate however the problems are apparent
before etching.

If I spray on a very thin layer of the photoresist then this forms very
small bubbles ( < 0.5 mm or 1/50 ") when etched, however this seems to be
too thin a layer to withstand etching, and the bubbles form pinhole voids
in the etched copper.

If I spray on about 2-3 times as much photoresist (producing a
significantly dark layer) then the larger bubbles form when the resist
dries .. although they are sometimes obvious immediately.


Any suggestions as to what causes this problem would be appreciated.

I think that the way in which I clean the boards should eliminate dirt and
moisture as causes, but I'm open to any ideas.

Thanks in advance

Scott Gordon
Brisbane, Australia