Development of 30 μm pitch Cu/Ni/SnAg micro-bump-bonded chip-on-chip (COC) interconnects

In this study, the assembly processes and reliability performance of interconnects with 30 μm pitch Cu/Ni/SnAg joints are evaluated, Plasma was treated on the both upper and lower chips before bonding, then fine gap control bonding process was applied to minimize the solder squeezing phenomenon which causes the adjacent joints bridged. Two kinds of underfills were used to fill the gap of around 20 urn between the upper and lower chips Pre-conditioning test and Temperature cycling test and -55 ~ 125 TCT for 5,000 cycles was performed on the chip-on-chip (COC) interconnects. The characteristics of micro joints with different underfills were investigated during the reliability test. The results of electrical test and microstructure analysis of the micro-bump interconnects showed that the micro joints were well-bonded in such kind of method. Fine gap filling capability of the selected underfill materials was also confirmed. The higher filler content and viscosity may cause the void issue during the encapsulation process. The major failure modes were found after the failure analysis. One is crack occurred at the center of the joint which relate to underfill's capability, another is void formation in the joint which relate to volume shrinkage of the inter-metallic-compound (IMC) transformation from the remained solder. The micro bump assembly technology was established, quality check methods were demonstrated and the failure modes for the bonded-micro-joint with different underfills were investigated at this study.