Improved Electroless Copper Coverage at Low Catalyst Concentrations and Reduced Plating Temperatures enabled by Low Frequency Ultrasound

Electroless Copper plating is a critical process in printed circuit board (PCB) manufacture where it is employed to make the through holes and vias of a PCB conductive for subsequent electroplating. The most important pre-treatment step before Electroless Copper plating is the catalyst solution where palladium is deposited on the surface of the substrate to initiate the plating reaction. However palladium is a critical raw material and the most expensive part of the Electroless Copper process. Therefore it would be beneficial if methods to reduce the amount of palladium used in the catalyst could be found without affecting the subsequent quality of the Electroless Copper deposit, particularly with respect to plating coverage. In this paper it is shown that the use of low frequency (40 kHz) ultrasound can enable the employment of a catalyst solution containing half the concentration of palladium typically used in PCB manufacture. This was achieved without any loss in coverage (as determined by backlight and SEM) even when lower temperature Electroless Copper plating was utilised. It is proposed that ultrasound can improve Electroless Copper coverage due to modification of the deposit grain structure rather than increased plating rates