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 [log in to unmask] *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To subscribe/unsubscribe send a message <to: [log in to unmask]> * * with <subject: subscribe/unsubscribe> and no text in the body. * *************************************************************************** * If you are having a problem with the IPC TechNet forum please contact * * Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] * ***************************************************************************