Joint reliability study of solder capped metal pillar bump interconnections on an organic substrate

Flip chip technology is widely used on the electronic packaging, and the market forces drive toward finer pitch interconnection. Cu pillar bump structure is a currently trend of the fine pitch flip chip package with less than 100μm bump pitch. But there is a solder volume limitation on solder capped Cu pillar bump structure. In addition, Cu may easily react with Sn-based solder into intermetallic compounds (IMCs). So it may be difficult for much finer pitch application with the structure. In this paper, we studied the fine pitch (50μm) flip chip interconnections with various bump structures and the die thickness. The 3-types of solder capped metal (Cu, Cu-Ni and Ni) pillar bumps and the 3-different die thickness (100μm, 300μm and 725μm) were evaluated. Finite Element Method (FEM) simulation was performed to analyze the thermal mechanical stress on the solder joint and on the root of metal pillar first. The result showed that the stress on the root of pillar on Ni pillar bump was higher than that of Cu pillar bump, and the stress on solder joint and the root of pillar was reduced by thinning the die thickness. To verify these results, the initial solder joints were observed using an optical microscope and a Scanning Electron Microscope (SEM). The IMC phases, the IMC growth and the interconnect microstructures were compared. After these observations, a thermal cycling test (Condition : -55°C/+125°C) was performed, then the lifetime was compared between Cu pillar bump and Ni pillar bump. In the study, we obtained good reliability data on both pillars, but the lifetime of Cu pillar bumps was 1.5 times better than that of Ni pillar bump with the same structure. In addition, the electro-migration (EM) test was performed to investigate the EM behavior of microjoints on Cu pillar bump and Ni pillar bump. In the study, there is no significant difference on the electrical resistance variation until 2,000hrs.