Studies on CO2 laser drilling: formation mechanism of residual thin materials at the bottom of laser via

Formation mechanism of residual thin materials at the bottom of a microvia processed with CO₂ laser is studied with a bromine (Br)-containing epoxy film designed for sequential build-up printed wiring boards. By observing Br content distribution in a cross section of the epoxy film after laser processing, it is estimated that the film around the via is heated up to 1600 K and that the residual thin film at the bottom of the via is heated up to 2000 K upon laser irradiation. Based on heat transfer simulations for epoxy and copper layer structures, the epoxy film within a 0.1-μm distance from the copper surface is unable to be removed by laser irradiation because the temperature of this region cannot be heated above the decomposition temperature of ≈2500 K due to a large heat flow from the epoxy layer to the copper