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.

By patterning finely with a laser with a thickness of 100 μm or less such as ABS and forming an electronic circuit through plating, a high-density flexible PCB applicable to wearable and mobile devices can be realized. ABS films with a thickness of 60, 90, and 120 μm were prepared, and a crater measuring 100 μm or less was formed by irradiating a fiber laser with a wavelength of 1064 nm with a single pulse. The size of the craters is affected by the intensity of laser irradiation and the thickness of the film, and the heat dissipation layer reduces the change in size caused by the difference in the thickness of the film. For films with a thickness of 60 μm, it has been found that small craters of more than 10% can be obtained due to the heat dissipation layer. Thermal analysis showed in the ABS film without the heat dissipation layer, the maximum temperature increased to 373oC, but decreased to 261℃ in the ABS film with the heat dissipation layer. With a decrease in the thickness of the film, the heat dissipation layer further reduces the pattern by laser irradiation.

Silicon nitride/cobalt tungsten boride (SiN/CoWB) passivation layer improves mass transport rate at copper thin film layers of semiconductor wafers after chemical mechanical polishing process. This study evaluates mass transport at the interface between copper and passivation layers by stress relaxation method, followed by deduction of interface diffusivity via a kinetic model. For comparison, SiN/CoWB, SiN, silicon carbon nitride (SiCN) and silicon carbide (SiC) passivation layers are introduced. A thin layer of SiN/CoWB demonstrates an outstanding performance as diffusion retarding material, especially at high temperature. The order of stress relaxation in terms of passivation layers is SiN/CoWB < SiN < SiCN < SiC, implying the order of mass transport at the interface. Using the kinetic model, the diffusivities and activation energies regarding passivation layers are calculated and reveal a good agreement with experimental results.