Use of Wafer Applied Underfill for 3D Stacking

A key element for improving 3D IC stacking and packaging reliability is the choice of the right underfill (UF) material. The underfill is a specialized adhesive whose main purpose is locking together the top and bottom dice; it must fill the gap between bumps and between dice, while reducing the differential movement that would occur during thermal cycling. Traditional underfill processes are based on local dispensing after solder bump reflow (i.e., capillary dispensing), or before flip chip operation with no need of reflow (no flow underfill, NUF). In case of 3D stacking, such processes present some limitations: the need for a dispensing area (die size increase); material flow (spacing between dice), and cost (low throughput). After an introduction on typical underfill applications such as die-to-package and die-to-die assembly, we report the work done to assess the properties of several wafer applied underfill (WAUF) materials and their integration in 3D stacking. These materials were initially applied on silicon wafers in order to assess the minimum achievable thickness and material uniformity. The wafers were coated with different methods: spin coating and film lamination. After this initial assessment, the most promising materials were used for 3D stacking. The test vehicle used has Cu/Sn microbumps with a pitch of 40 μm. The quality of the materials is judged by electrical test, SAM (surface acoustic microscope), and X-SEM (scanning electron microscope).