A Study on the Joint Properties and Reliability of 25μm Cu/Ni/Sn-3.5Ag Bonding Process with Chip on Chip Thermal Compression Bonding

With the further shrinking of chip feature size, Thermocompression bonding (TCB) is widely used in ultra-fine pitch flip welding products. Therefore, research and master important process parameters such as temperature, time and force are crucial to improve the throughput and robustness of TCB processes. In this study, we chose Cu/Sn–3.5Ag 50 μm pitch chip-on-chip (CoC) interconnection with Ni barrier microbumps to evaluate the bonding structure and the bonding strength. The scanning electron microscopy(SEM) was used to investigate the inhibition of Ni on the IMC formation and observe the gap between the upper and lower pillar varied during the TCB processes. We also carried out shear tests on samples fabricated to probe into their effects on the mechanical properties at various TCB processes. Experimental results showed that Cu diffusion to Sn–3.5Ag solder was restricted by the Ni barrier. In addition, the shear strength and gap height of samples exhibited a sensitive dependence of the TCB processes (time, force and temperature).