A study on the rheological characterization and flow modeling of molded underfill (MUF) for optimized void elimination design

The molded underfill (MUF) process has a lot of advantages compared with capillary underfill process in view of reduction of process, cycle time of process, material and equipment cost since this application utilizes the conventional mold materials and equipment systems. But the extremely narrow gap at the underfill area for flip-chip bonding make it difficult to get stable MUF material set and appropriate processing conditions because of the severe void trapping problem. In this paper, the MUF process of flip-chip package on package (POP) is investigated. The rheological and cure- kinetic parameters of commercial MUF compound are acquired using parallel plate rheometer and dynamic DSC (differential scanning calorimeter) analysis. The experimental results showed severe void at the underfill area for both top- left and bottom-right pin type gate locations. The modeling results by full 3D mold filling simulation show good agreement with the actual void occurring area investigated by the SAT pictures. Applying the benchmarked constitutive relations and finite elements model, seven kinds of different gate types and locations are tried, but the void trapping is not completely removed. To resolve void trapping phenomena of current flip-chip structure, we devised a proprietary mold design. From the modeling results, the void trapping zone under the large die are effectively removed through the optimized design. From this study, we can conclude that the optimized mold design will be very effective on the elimination of void in the underfill area for flip-chip MUF processing.