There's no real reason for burning stencil just because you switch to AC voltage. A couple of questions, what voltages are you using for DC and for AC? How long are you holding pad on stencil at each phase? From what I've read (and happened to me) the quickest way to burn a stencil is to hold pad direct on stencil too long at a time. There should be VERY little (if any) heat build up during etching process. Put pad on stencil, count 1 Alabama, 2 Alabama, Roll Tide 3, lift pad from stencil for a couple of seconds. Repeat about 10 times or so, switch to AC and do the same thing. The trip is NEVER keep the pad on stencil longer than 2 to 3 seconds at a time to prevent heat built up. Lifting pad also allows gas to escape to make a sharper etch.
Ken H>