The influence of Ni and Zn additions on microstructure and phase transformations in Sn–0.7Cu/Cu solder joints

Microalloying, in which the solidification structure is preferably and significantly modified by trace elements, is a key method for improving Pb-free interconnections in electronic devices. Microalloying Ni or Zn is expected to modify the Sn-0.7Cu alloy in different ways. This research examines the influences of minor/trace additions of Ni and Zn concurrently on the development of the microstructure, the interfacial reactions and the stability of the intermetallics in Sn-0.7Cu solder alloys and associated joints, using X-ray radiography, X-ray florescence analysis, X-ray diffraction and electron microscopy. It shows that minor Zn additions (∼0.15 wt.%) result in the formation of a CuZn intermetallic in the interdendritic region during solidification, whereas a small amount of Ni completely changes the solidification mode and a eutectic microstructure is obtained. When Ni is added, small particles of primary (Cu,Ni)Sn intermetallic forms in advance of the solidification front. Microalloying Ni and Zn concurrently refines the microstructure and leads to a more continuous, finer-grained and stable interfacial CuSn intermetallic and suppresses the growth of CuSn. The Ni and Zn are homogeneously distributed in interfacial CuSn and inhibit the polymorphic phase transformation of CuSn. This stabilizing effect minimizes the thermal expansion mismatch between interfacial CuSn and the Cu substrate. The findings have important implications for the manufacture of high-reliability lead-free microjoints.