You might try plugging the via holes with solder mask prior to applying the
LPI mask to the boards.To do this, you must use a reverse dot pattern stencil
on a screen (i. e. the screen is open only where there are via holes; the
rest of the screen is blocked). Next, you screen the solder mask into the
holes until they are completely plugged. Several methods are employed to do
this:

1. Use a standard green thermal cure solder mask, plug the holes, give the
mask a quick cure (say 10-15 min at  250-300 deg F). This will get rid of
most of the solvent in the mask and probably cure it a little bit too. Next,
apply the LPI mask and process as normal. The thermal cure step for the LPI
mask will also cure the thermal mask.

2. Use a clear thermal cure mask and process as in #1 above. This will
eliminate the "greem measles" effect that you will get from using green
thermal cure mask,

3. Use the same LPI mask to plug the holes as you use on the entire surface
of the board. Process as in #1 above.

4. Use the same LPI mask, but use the "wet on wet" approach. For this, you
plug the holes with the LPI mask and then directly apply the LPI mask onto
the entire surface of board. You partially dry the mask in the holes during
the normal drying step of the LPI mask and you will completely dry and cure
the mask in the holes during the normal cure step of the LPI mask.

All of these methods are being used successfully in the field. Instead of
"nests" for the solder balls to congregate in during HASL, you will actually
end up with little "bumps" where the via holes have been plugged.

Hope this helps.

Larry Fisher
Dexter Electronic Materials
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