The Impact of Hydrogen Gas Evolution on Blister Formation in Electroless Cu Films

Abstract

Electroless Cu plating is a key process to provide electrically conductive layers on insulating substrates for the subsequent Cu electroplating of printed-circuit boards (PCBs). Recently introduced substrate materials are prone to spontaneous delamination failure (blistering) of the electroless layer during deposition. A higher Ni content in electroless Cu baths prevents delamination failure by increasing the internal tensile stress in the Cu layer. This effect is achieved by suppressing Cu self-diffusion through the incorporation of small amounts of Ni in the grain boundaries and the grain boundary junctions. Whether the hydrogen gas evolved inevitably during electroless Cu deposition and potentially trapped in the deposit may cause blistering is still under debate. Here we test this hypothesis by quantifying the generated hydrogen gas volume during Cu deposition in baths with Ni concentrations between 0 and 1000 ppm, and observing the rate of blister formation in the deposit. We find a constant molar ratio of 0.65 ± 0.05 mol H2/mol Cu independent of the Ni content in the bath, in contrast to a systematic and substantial change of the blister probability. From this lack of a correlation between the blister rate and hydrogen evolution, we rule out hydrogen evolution as a determining factor for blister formation.