Electroless Ni-W-P Alloys as Barrier Coatings for Liquid Solder Interconnects

The control of the interfacial reaction rate is of great importance for liquid solder interconnects for high temperature electronic assemblies. Conventional electroless Ni-P barrier metallizations have been found to be inadequate for providing long term protection of the underlying metallization from the attack of molten solders. In this paper, binary Ni-P was modified with the co-deposition of a refractory alloying element, tungsten (W), from its soluble metal salt added to the plating bath. Critical parameters for the deposition of ternary Ni-W-P were identified. The long term reaction between Ni-W-P and molten Sn-Bi solder at 200 degC was studied. The results indicated that Ni-W-P barrier coatings with higher W contents have much longer service lifetime as a barrier than normal Ni-P coatings. A mechanism for the reaction between Ni-W-P and molten Sn-Bi solder, and a failure mechanism for the Ni-W-P layer, are also proposed