Direct Laser Drilling in Copper Clad Printed Wiring Boards (Effect of Surface Treatment on Drilled Blind Via-Hole Quality)

Micro-via drilling technology using a laser has become the dominant method of drilling smaller blind via-holes in copper clad multi-layer printed wiring boards (PWBs). Cu-direct laser drilling has attracted attention as a new method. However, the defect of overhang occurs when copper and resin, having different processing thresholds, are drilled at the same time. The absorptance is an important factor for Cu-direct laser drilling. Therefore, we propose a new method using thermography to measure the absorptance of a PWB's surface for a CO2laser. Moreover, we investigate how surface treatment of the outer copper foil influences the quality of a laser-drilled hole. Two kinds of surface treatment, roughening and application of black-oxide, were attempted as ways to improve the absorptance of a surface for the laser. As a result, it was found that not only the surface absorptance but also the method of surface treatment affects the quality of a drilled hole.