Development of New Low Melting Solder Alloys

In advanced 3D IC assembly, one of the key processes is to thin down the wafer for multiple-layer stacking vertically. Warpage is commonly seen in the IC chips due to the differences in coefficient of thermal expansion of the materials. The conventional peak temperatures for reflowing Sn-base Pb-free solders are above 230 °C. High temperature thermal processes would further induce severe warpage issues of the chips with thin wafers and cause failure and degrade the reliability. The development of new Pb-free alloys for low temperature assembly is important for the applications. In the previous years, Sn, In and Bi are the most common elements that are used for low temperature alloys. Setting the melting temperature at around 100°C for the alloy is ideal for practical applications. The two alloys reported in this study show possible applications for the advanced packaging technology. Gallium has a melting temperature of 29.8°C. Alloying Ga in Sn-Bi based solders could reduce the melting temperature to 128°C when the composition is Sn62Bi32Ga6. The alloys reflowed on Cu substrates to investigate the microstructure. The microstructure of the cross-sectioned images and the electron probe micro-analyzer (EPMA) analysis show that the initially formed intermetallic compound (IMC) is CuGa2. The IMCs became basin-type due to fast dissolution of Cu into molten solders. The basin-type IMC eventually became a continuous layer. The thermal aging tests at different temperatures yields the calculation of the activation energy, 2.82 kJ/mol. The relatively low value implies fast IMC growth and could be a possible candidate for the transient liquid phase bonding technique. Another low melting alloy, eutectic In-50wt% Bi (InBi), is developed for its melting temperature at around 95.7°C. This study compares the properties with eutectic Sn-52wt% In (SnIn), melting point of 118°C. Both alloys were reflowed on Cu substrates for cross-sectional microstructure investigation. The scanning electron microscopy (SEM) images revealed that the interfacial compounds were Cu6Sn5, Cu11In9 in eutectic Sn-In and In-Bi alloys, respectively. Shear tests were conducted on the samples with the alloys reflowed on OSP substrates. The results showed that InBi has almost twice the joint strength than the eutectic SnIn solders.