Influence of Cu Surface Treatment on the Cu-Direct Via Hole Drilling Efficiency of PWBs

This report describes the quality assessment of blind via holes (BVHs) of Printed Wiring Boards (PWBs) drilled by a CO₂ laser using Cu-direct drilling. In the Cu-direct drilling method, the copper foil and the buildup layer are melted at the same time, and the surface is treated to increase the laser energy absorbed by the copper foil because an untreated copper surface reflects most of the 10.6-μm-wavelength CO₂ laser beam. We used black-oxide and V-bond treatments as surface treatment. Previously, the only black-oxide treatment was paid attention to, but the new V-bond treatment is also investigated in this report. First, a straightforward method employing infrared thermography was proposed to determine the absorbance of the CO₂ laser beam by the copper surface. Then, we used SEM to characterize the copper surfaces after surface treatment, and established the relationship between laser absorbance and surface shape. Subsequently, we observed the circumference of a point irradiated with the CO₂ laser and explained melting processes were different from surface shape. Finally, we investigated the relationship between laser absorbance and BVH quality, and showed that a high absorbance improved BVH quality.