Transmission Electron Microscopy of Interfaces in Joints between Pb-Free Solders and Electroless Ni-P

Abstract

Microstructures of joints between a Ni–8 mass%P UBM (under bump metallization) and three different Pb-free solders, i.e., Sn–Ag, Sn–Ag–Cu and Sn–Ag–Cu–Bi were studied by transmission electron microscopy. The phases formed near the joint interfaces as well as inside the solders during soldering were identified on the basis of electron diffraction and EDX analysis without ambiguity. In the Ni–P under bump metallization near the interface with the solders, P-denuded Ni–20 mass%P layer was observed, where a high density of columnar voids were formed. Also, in Ni(Cu) SnP intermetallic, which is adjacent to the P-denuded Ni–20 mass%P, a high density of spherical voids were observed. These voids degrade the joint strength. Change of the above microstructures during thermal cycling between 253 K and 453 K was also observed.