With standard sodium or potassium carbonate developing solutions (generally
0.9 to 1.1%), feed and bleed systems for LPI solder masks are maintained at a
pH level of 10.7 to 10.8. Since the initial makeup of the developing solution
in the holding tank with DI water is in the range of 11.0-11.2 pH, the system
is fairly easy to control by feeding directly into the developing sump. A
number of companies make feed and bleed systems for controlling developing
solutions. The feed rate is usually controlled by pH of the solution in the
sump.

These days, a number of companies (Surfact-Tek, Morton, Dexter, etc.) offer
"doped" developing solution concentrates that are designed for the liquid,
photoimageable (LPI) solder masks. These concentrates contain amounts of
sodium or potassium hydroxide that increase the pH of the operating solution
in the sump (up to 12.0 pH),  increase the loading capacity of the developing
solution and allow for better development of the solder mask out of the holes
(important if screening is used to apply the mask).

Whichever way you go, I'm sure you will have success with using the feed and
bleed method to control your developing with your LPI solder mask.

Hope this helps.

Larry Fisher
Dexter Electronic Materials
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