Metallurgical reaction of the Sn-3.5Ag solder and Sn-37Pb solder with Ni/Cu under-bump metallization in a flip-chip package

Several international legislations recently banned the use of Pb because of environmental concerns. The eutectic Sn-Ag solder is one of the promising candidates to replace the conventional Sn-Pb solder primarily because of its excellent mechanical properties. In this study, interfacial reaction of the eutectic Sn-Ag and Sn-Pb solders with Ni/Cu under-bump metallization (UBM) was investigated with a joint assembly of solder/Ni/Cu/Ti/Si₃N₄/Si multilayer structures. After reflows, only one (Ni,Cu)₃Sn₄ intermetallic compound (IMC) with faceted and particlelike grain feature was found between the solder and Ni. The thickness and grain size of the IMC increased with reflow times. Another (Cu,Ni)₆Sn₅ IMC with a rod-type grain formed on (Ni,Cu)₃Sn₄ in the interface between the Sn-Pb solder and the Ni/Cu UBM after more than three reflow times. The thickness of the (Ni,Cu)₃Sn₄ layer formed in the Sn-Pb system remained almost identical despite the numbers of reflow; however, the amounts of (Cu,Ni)₆Sn₅ IMC increased with reflow times. Correlations between the IMC morphologies, Cu diffusion behavior, and IMC transformation in these two solder systems will be investigated with respect to the microstructural evolution between the solders and the Ni/Cu UBM. The morphologies and grain-size distributions of the (Ni,Cu)₃Sn₄ IMC formed in the initial stage of reflow are crucial for the subsequent phase transformation of the other IMC.