Pretreatment of Blind Via Holes before Ni/Au and Cu Plating Applied with Atmospheric Pressure Plasma Jet

The effectiveness of atmospheric pressure (AP) plasma preprocessing before Ni/Au or Cu plating has been examined by applying it to a build-up printed circuit board (FR-4 grade) and polyimide-based flexible circuit film, both with blind via-holes (BVHs). The AP plasma applied with a dielectric barrier discharge is generated inside a 56 mm wide quartz vessel by an RF power generator using Ar-O2 gas mixture. One side of the vessel is open and the plasma jet is blown on the sample substrate transported 5 mm downward from the outlet of the vessel. The deposit failure rate of Ni/Au electroless deposit to 50 μm-diameter BVHs formed on a photo resist on the printed circuit board is 12.5% without preprocessing but is decreased to 0% after applying the AP plasma processing. As for 50 μm-diameter BVHs formed with a YAG laser on a polyimide-based flexible circuit film, the bump formation using electrolytic copper plating fails without preprocessing, but a 100% bump formation rate is achieved after applying AP plasma processing. It is presumed that the AP plasma processing improves the wetting property of the BVH walls and allows the plating solution to uniformly cover the entire wall surfaces without generating bubbles. The removal of organic substances attached to the BVH bottom surface also helps to improve the adherence of metal plating.