Effects of Zn addition on undercooling of Pb-free solder alloys and their interfacial reactions with Cu and Ni-P UBMs

Abstract

Solder is the most common material to interconnect various components, such as chip package and board, in electronic packaging. Eutectic Sn-Pb solders have been used the most notably to attach discrete components to printed circuit boards (PCBs). However, due to an increased awareness of the potential health hazards associated with the toxicity of lead (Pb) in humans and several demands, the use of Pb in the solders have been restricted or banned. The extensive searches for Pb-free solder alloys in last several years were conducted. A few promising Pb-free candidates have been identified to produce reliable Pb-free solder joints, which include Sn-0.7Cu, Sn-3.5Ag and Sn-3.8Ag-0.7Cu (in weight percent unless specified otherwise). Recently among the Pb-free candidates, the near-ternary Sn-Ag-Cu solder alloys are recognized as the most promising candidate for surface-mounted card assembly such as BGA (ball grid array) solder joints. However, many problems in using the Sn-Ag-Cu solder alloys have been reported, such as the formation of large primary intermetallic compounds (IMCs) inside solders, large undercooling in solidification, large IMCs growth and voids formation at the interface of a Cu under bump metallurgy (UBM) and fast Cu UBM consumption. In addition, these concerns are related to the lifetime of the solder joint during various reliability tests, such as drop test, thermo-cycling test and die shear test. Recently, many studies have been conducted to solve the problems as well as accomplish more stable and reliable solder joints by adding fourth alloying elements to SAC solder alloys, such as Ni, Ti, Mn, Co, Bi, Sb, Zn, Fe and In-Ni. The beneficial effect of adding a small amount of Zn is outstanding among various minor alloying elements. Although the beneficial effects of the Zn addition to Pb-free solders on the undercooling and the interfacial reactions with Cu UBMs are well documented, the mechanisms of most beneficial effects of Zn addit...