Wettability of electroless Ni in the under bump metallurgy with lead free solder

Abstract

This study investigates the wettability of several lead-free solders, including Sn, Sn-Ag, and Sn-Bi, on electroless Ni (EN) with various phosphorus contents. The role of phosphorus on solder wettability is studied. Microstructure evolution in the lead-free solder/EN joint is investigated with the aid of electron probe microanalyzer (EPMA) to relate metallurgical reactions between the solder and the EN. The Sn solder exhibits better wettability on EN, while the Sn-Bi solder has a larger contact angle. Wettability degrades as the phosphorus content in EN decreases. The dependence of wetting angle on the phosphorous content can be attributed to the surface roughness and density of EN, along with the interfacial reaction between the solders and EN. An EPMA analysis reveals the presence of a Sn-Bi-Ni-P solid solution at the interface of solder/EN joints due to the interdiffusion of major constituent Ni and Sn. The interaction zone of the solid solution increases with increasing temperature. Wettability of Pb-free solders on EN degrades with the presence of NiO due to oxidation or the existence of Ni3P due to precipitation after annealing. For an adequate wetting behavior in the Sn (Sn-Bi, Sn-Ag)/EN joint, EN deposited with phosphorus contents in the range of 9 to 12 wt.% is suggested.