Interfacial Reaction Between Molten Sn-Bi Based Solders and Electroless Ni-P Coatings for Liquid Solder Interconnects

This paper reports on the interfacial reactions and lifetime of electroless Ni-P coatings in contact with molten Sn-Bi based solders. A layer of approximately 4 mum thick electroless Ni-P in contact with the molten Sn-58Bi solder began to fail at 48 h at temperatures between 200degC and 240degC . Elemental additions to modify the solder, included 1-2wt.% of Al, Cr, Si, Zn, Ag, Au, Ru, Ti, Pt, Nb, and Cu. Of these, only Cu modified the interfacial intermetallic compound growth from Ni₃Sn₄ to (Cu,Ni)₆Sn₅ , resulting in significantly decreased consumption rates of the Ni-P substrate in contact with the molten solder and increasing the lifetime of the Ni-P layer to between 430 and 716 h. Micro cracks were observed in all but the thinnest Ni-P layers, allowing the solder to penetrate.