Comparison of properties of thermo-compression bonded 3D stacks using a liquid and a dry-film wafer level underfills

This papers reports on the study of two wafer level underfills (WLUF) for 3D stacked IC applications. WLUF either as a liquid (material A) or a dry-film (material B) were applied at wafer level before singulation and stacking in a die to die configuration using a thermo-compression bonding (TCB) process. The materials were applied on the top die on Cu/Sn microbumps 8.5μm thick to reach a film thickness about 15μm. The bottom die had Cu microbumps 5μm thick. The total number of microbumps for the test vehicle used is 8250 with a bump pitch of 50μm. The material performances were evaluated in terms of deposition technique, dicing performances, electrical results and reliability testing. It was observed that the deposition of the liquid material was not trivial due to the high viscosity. Minimum peeling and delamination was observed on the b-staged underfill material after dicing. In the case of the dry-film underfill, the lamination process was straightforward but severe delamination of the material was observed after dicing. The best stacking performances were obtained with a temperature of 270°C and a total time of 125s: material A showed good electrical performances (>80%) with no voids and the yield of material B was 100% with no voids. Samples were also subjected to thermal cycling tests (-40/+120°C) up to 500 cycles, showing a yield loss down to 35% for both materials.