Laser drilling of epoxy-glass printed circuit boards

Abstract

The drilling of microvias for epoxy-glass printed wiring boards (PWBs) by CO2 laser radiation (λ = 10.6μm) was studied experimentally and theoretically. Laser drilling is a thermal removal process and the biggest problem to be solved is carbonization of the epoxy resin.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 μs 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 (⋍ 300 °C) 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 CO2 laser drilling system was developed to make microvias in epoxy-glass PWBs.The drilling of microvias for epoxy-glass printed wiring boards (PWBs) by CO2 laser radiation (λ = 10.6μm) was studied experimentally and theoretically. Laser drilling is a thermal removal process and the biggest problem to be solved is carbonization of the epoxy resin.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 μs 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 (⋍ 300 °C) by the heat accumulation. The analytical study revealed there exists a pra...