Modeling of Cu Direct Laser Drilling Process

Drilling of printed wiring boards (PWBs) by CO2 laser radiation was studied both experimentally and theoretically. Two types of processes are generally used for drilling: resin direct drilling (RDD) and Cu direct drilling (CDD). RDD is a single-layer removal process and is explained using the evaporative removal model under the condition that the laser pulse width exceeds a nanosecond. In this model, the processed material evaporates upon heating by a laser beam. In the CDD process, Cu and resin layers are simultaneously removed. However, Cu and resin have different physical characteristics. Therefore, the evaporative removal model cannot be applied to the CDD process, and a new model is required. An experimental study of CDD shows that via holes open at a certain irradiation energy. Thermal simulation revealed that the thermal decomposition of the resin interface is complete before copper is evaporated. These results show that as a result of thermal decomposition, resin's internal pressure increases rapidly and the pressure pushes upward the liquid Cu foil in the upper layer. Then, the vaporized resin scatters liquid copper. Furthermore, the via hole shape can be calculated using this model.