A PALLADIUM SULFIDE CATALYST FOR ELECTROLYTIC PLATING

During the manufacture of circuit boards, a palladium-tin colloid is a dsorbed onto an epoxy substrate and serves as a catalyst for the subse quent electroless deposition of copper in nonconductive areas, viz., t he drilled holes. This is normally followed by a thicker layer of elec trolytically deposited copper. A simpler process has been developed wh ich replaces the electroless step with a sulfide-containing solution w hich alters the adsorbed palladium-tin colloid and renders it resistan t to acidic solutions and very effective for the subsequent electrolyt ic deposition of copper. Use of x-ray absorption spectroscopy techniqu es (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 pla ting characteristics are determined for this novel palladium sulfide a ctivator and compared to those of a conventional palladium-tin catalys t. A ''stepwise propagation with continuous conduction'' model is prop osed, based in part on the results of scanning tunneling microscopy wh ich reveals PdS particles in intimate contact with each other.