Laser Drilling of Epoxy-Glass Printed Circuit Boards

The drilling of microvias for epoxy-glass printed wiring boards (PWBs) by CO 2 laser radiation was studied experimentally and theoretically. In the experimental approach, the effects of laser pulse duration and frequency on the thickness of the carbonized layer surrounding the drilled holes were investigated for pulse durations of 50-120 s and frequencies of 20-80 Hz. It was found that carbonization takes place at frequencies higher than a certain value and that the frequency becomes lower as the pulse duration increased. Heat accumulation was investigated theoretically in the region near the drilled holes during the removal process. It was reasonably assumed that sudden carbonization takes place when the temperature of the drilled hole surface exceeds the carbonizing temperature of epoxy resin (573 K) by the heat accumulation. The analytical study revealed there exists a practically acceptable region of pulse duration and frequency where the temperature of the drilled surface does not reach the carbonizing temperature and no carbonization takes place due to the heat accumulation. Based on the above results, a CO 2 laser drilling system was developed to make microvias in epoxy-glass PWBs.