Effects of Sb and Bi addition on IMC morphology and reliability of Pb-free solder/Cu-OSP

Abstract

In this study, the interfacial reaction, shear strength, Cu pad consumption after thermal aging, drop test, and temperature cycling performance of five solder ball alloys (SnAgCuNi-1% Bi, SnAgCuNi-1.5% Bi, SnAgCuNi-3% Bi, SnAgCuNi-4% Bi, and SnAgNi-3% Bi-0.05% Sb) on organic solderability preservative (OSP) were investigated. The interfacial intermetallic compound (IMC) of Sn-Ag-Cu-Ni-Bi is identified as (Cu, Ni$)_{6}\text{S}\text{n}_{5}$ by energy-dispersive X-ray spectrometer (EDX) after soldering. On the other hand, Sb signals are detected at the interface of Sn-Ag-Ni-Bi-Sb/Cu-OSP that finer grain structure compared to Sn-Ag-Cu-Ni-Bi was observed by the top view of IMC surface morphology, which implies Sb atoms are involved in the interfacial Cu-Sn IMC reaction. The fracture mode of shear test of all solder alloys after 2 times reflow presents 100% solder fracture mode by using 300um/sec shear speed, and shear strengths of 3% and 4% Bi-doped solders increase 25% and 37% as compared to 1% Bi-doped solder, respectively. Furthermore, Cu pad consumption is identified as a critical index of solder alloy performance since the thickness of substrate and Cu trace are getting thinner in electronic packaging industry. The results of Cu pad consumption at 150 and 175°C for 1000h showed no significant difference among Sn-Ag-Cu-Ni-Bi alloys. However, Sn-Ag-Ni-Bi-Sb alloy without Cu additions showed 1um higher Cu pad consumption as compared to Sn-Ag-Cu-Ni-Bi alloys. In the end, temperature cycling and drop test on board level reliability are tested to evaluate the performance of five solder alloys. Lower Bi additions (1% and 1.5%) alloys showed significantly worse TC than that of higher Bi additions (3% and 4%) alloys. In addition, SnAgNi-3% Bi-0.05% Sb gives the best temperature cycling performance among these five solder alloys and typical solder fatigue crack was found from failure analysis, which implies that Sb addition can enhance temperature cycling performance. On the other hand, a typical IMC break at package corner was found on SnAgNi-3% Bi-0.05% Sb/Cu-OSP, which might result from finer and brittle IMC grain structure.