Simultaneous molding and under-filling for void free process to encapsulate fine pitch micro bump interconnections of chip-to-wafer (C2W) bonding in wafer level packaging

Abstract

The encapsulation of chips with fine pitch micro bump interconnections in chip-to-wafer (C2W) bonding has a known two steps process in wafer level packaging. First step is underfilling process that fills the gap between bumps underneath the chips. Under-filling process can either be using liquid dispensing which allow it to flow underneath the bumped chips by capillary force or using a non-flow under-fill material. The second step is molding process that encapsulates the entire C2W with a molding compound. Through simultaneous molding and under-filling process to encapsulate, the two steps will become a single step process. Although, this method has been widely used for flip-chip Ball Grid Array (BGA) packaging using Moldable Under-fill (MUF) material in transfer molding, it is not yet fully utilized for wafer level C2W packaging using MUF material in wafer level compression molding. One major challenge during under-filling is voids formation underneath the bumped chips as shown in Fig. 1. This study aims to implement simultaneous molding and under-filling to achieve a void free process in wafer level C2W packaging. Mold flow simulation using ANSYS FLUENT 14.5 commercial software is being used to predetermine the major factors affecting the voids formation. Based on the simulation result, we have identified several factors that can significantly affect the voids size. The identified factors are mold temperature, mold compound dispensing pattern and mold filling speed. The mold flow simulation results are being validated using test chips with 90um micro bump pitch in the actual molding experiment. We validated the actual void formation by comparing the result of low and high molding temperature, comparing round and straight line dispensing pattern and also validated the use of slower filling speed during molding process. The experimental result confirms the total elimination of voids formation during simultaneous molding and under-filling process. The experimental result indicates that the actual molding perfectly match with the mold flow simulation result.