A palladium sulfide catalyst for electrolytic plating

During the manufacture of circuit boards, a palladium-tin colloid is adsorbed onto an epoxy substrate and serves as a catalyst for the subsequent electroless deposition of copper in nonconductive areas, viz., the drilled holes. This is normally followed by a thicker layer of electrolytically deposited copper. A simpler process has been developed which replaces the electroless step with a sulfide-containing solution which alters the adsorbed palladium-tin colloid and renders it resistant to acidic solutions and very effective for the subsequent electrolytic deposition of copper. Use of x-ray absorption spectroscopy techniques (x-ray absorption fine structures and x-ray absorption of near-edge structures) was conclusive in establishing that the altered catalytic film was a mixture of palladium sulfide, which remains stable for at least several months, and mixed valence sulfidized tin species, which begin to oxidize to stannic oxide within 24 h. Electrolytic copper plating characteristics are determined for this novel palladium sulfide activator and compared to those of a conventional palladium-tin catalyst. A ''stepwise propagation with continuous conduction'' model is proposed, based in part on the results of scanning tunneling microscopy which reveals PdS particles in intimate contact with each other.