Laser-assisted via hole metallization in PCB materials

A novel via hole metallization method is presented, where the vias are dril led in polyimide/copper (PI/Cu) flexible printed circuit boards (PCBs) usin g KrF excimer pulses, and then pre-metallized using a scanned Ar+ laser. In the premetallization step, a thin (20-50 nm) and narrow (2-10 mum) palladi um layer is deposited on the polyimide-covered side of the PCB and on the w all of the vias using the laser-induced chemical liquid-phase deposition me thod. After the pretreatment, the Pd covered holes are immersed into a Cu e lectroless plating bath. Plated copper vertical and horizontal interconnect s are analyzed by optical microscopy, focused ion beam, profilometry and re sistivity measurements. The results show that the copper deposits formed on the pre-metallized surface of PCBs have high chemical purity, excellent ad hesion and almost bulk conductivity, but, so far, due to unclear reasons, h igh through hole resistance.