Epoxy Wetting Flow and Adhesion Mechanism within a Small Gap and Small Pitch Copper Pillar Structure

The capillary underfill is the most commonly used method to fill the interconnection gap in a flip chip structure. Its main role is to give a good mechanical strength to the structure and protect the interconnections from the severe environment. With the evolution of electronic devices, and the need for higher resolutions as in the case of the display market, the interconnections are getting smaller and their pitch lower and lower. This causes the underfill operation to be more challenging to avoid voids and defects formation. Specifically, in the micro display screens, when there is lot of defects, it can create a cluster visible when the device is working. The goal of this study is to understand the wetting and adhesion mechanisms between the epoxy resin and the different materials of the flip chip structure, in order to better prevent these defects. We based our research on epoxy based underfill within a flip chip structure with a large array of copper pillar interconnections at 30μm pitch.