Simulation and Experimental study on IMS (Injection Molded Solder) bumping with expanded resist patterning for reinforcement of fine-pitch capability

Abstract

Injection Molded Solder (IMS) process has been developed as one of the advanced micro-bumps forming techniques. This paper presents a novel IMS process utilizing a double-layer resist patterning with expanded opening for the reduction of required injection pressure. It consists of upper resist layer with larger opening diameter than lower resist opening. With this configuration, necessary injection pressure is adjusted to upper resist size, while lower layer determines the position and the size of fabricated bump. Furthermore, two different types of resist opening shape, an isolated opening and connected opening are proposed and evaluated in this study from both simulation and experimental perspective. For simulation, CFD analysis is performed to demonstrate the behavior of liquid molten solder flow and its bump forming process. It is revealed that solder bump was successfully formed in accordance with theoretical bump size in both designs. However, the result indicates that insufficient non-wetting property of resist material could lead solder bridging between adjacent bumps in connected design. Experiment is also conducted using the novel resist structure with the dimension correlated to the simulation model which has lower resist diameter of 10 μm, upper resist diameter of 15 μm, and the pitch of 20 μm. IMS is operated and it is confirmed that the uniform bumps can be favorably formed without any defects.