1. There are some people that use microetch only prior to application of LPI
solder mask with good success. Normally you want to microetch about 30-49
microinches to obtian a good surface. The important thing to remember with
microetch only is to apply the solder mask after the microetch/rinse/dry as
soon as possible (within 15 minutes is recommended and no longer than 1
hour). Microetched copper surfaces oxidize much faster than mechanically
scrubbed copper and these oxides can interfere with adhesion.

2. Pumice is actually ground up lava from volcanos. It is a highly abrasive
material and does produce an excellent surface for the adhesion of LPI solder
masks. Tests have shown that it provides the overall best surface; however,
pumice machines are expensive and require a lot of maintenance because of the
abrasive nature of the pumice.

Hope this helps.

Larry Fisher
Dexter Electronic Materials
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