PCBs (Printed circuit boards) have been widely used in electronic products such as wearables, smartphones, and table computers. Recent trends of miniaturizing electric components require improvement of component density and electronic functionality by decreasing the size of micro via holes (50-110 μm), which interconnect electric signals between adjacent layers in high density interconnection (HDI) PCBs. To generate micro via holes, we studied CO₂ laser drilling with the help of pulse shape controlling using acousto-optic modulator (AOM). Pulse shape controlling is one of the key factors to reduce heat effect during the laser drilling process. To increase laser absorption, the substrate was subjected to black oxidation prior to CO₂ laser drilling. We designed a diffractive optical system using a circular aperture. Micro via holes were obtained by optimizing the optical distance. The laser drilled via hole was studied both experimentally and theoretically.