Underfill Affect Study on Chip Stress in Board & Set Level Drop Test

Underfill is a chemical glue whose main ingredient is epoxy resin. Capillary flow underfill has been widely used in package (PKG) surface mount technology (SMT) and flip chip package assembly process to strengthen package and board levels interconnection reliability performance. Especially, in SMT underfill improves solder joint lifetime in board level and set level drop tests significantly. Simultaneously, with the thin, small, high-density stack trend for PKG design, the PKG becomes more sensitive to the stress and we found underfill in SMT had an important affect on the thin chips inside the PKG in the drop reliability test, especially for below 50μm thin chips. Based on explicit dynamic theory, board level drop with 1500G acceleration, 0.5ms, and set level phone drop with 1m height, edge 45°tile down were modeled by ANSYS software. In the finite element model, some complex small parts without impact on the package were ignored. The whole mesh size was similar and dominated by solid hexahedral elements. Flanagan Belytschko algorithm was applied to the hourglass energy control and the hourglass energy of all computation models was less than 10% of the overall internal energy. The maximum principal stress and stress distribution in different times of the fine-pitch ball grid array (FBGA) package that contained several ultrathin chips were investigated. In the case of SMT with underfill, both board and set levels drop results showed the major principal stress of the chips was greater than in the case of SMT without underfill. Chip Corners appeared the concentration of stress and top die had higher crack risk than bottom die when underfill was used. However, when without underfill, the opposite stress distributions were observed that the stress of the bottom die and chip center were higher than others. The impact stress indicated that it was transmitted from the board to chips directly through underfill and chip corners were shocked more seriously, which raised the chip crack possibility in corners.