A high throughput and reliable thermal compression bonding process for advanced interconnections

In this study, a liquid phase contact thermal compression bonding (LPC TCB) process was investigated for fine-pitch copper pillar with solder cap flip chip packages. It offers a high throughput and reliable interconnection with a controllable solder height. A series of LPC TCB experiments using both Chip-on-substrate (CoS) and Chip-on-chip (CoC) packages were performed. The bonding profiles clearly indicate that a high throughput, UPH (units per hour) of 1.2k, could be achieved compared with UPH of ~600 for a conventional TCB-flux process. The results on cross section examination, interfacial microstructure, shear strength and failure mode demonstrated that excellent wetting and robust solder joint could be achieved using LPC TCB process. With a bond head (BH) cooling step, a precise solder height could be obtained based on a pre-determined bonding level, while without a BH cooling step, the solder height remained at a constant value regardless the bonding level. The restoring force from the surface tension was calculated to explain this phenomenon. Finite element analysis (FEA) was also carried out to predict the joint fatigue life for the packages bonded with different solder thickness values.