Characterization and property correlation of no-flow underfill for handheld WLP

To increase solder joint survival rate, particularly against drop impact failure, no-flow underfills have been applied to a WLP (wafer-level package) in this work. This application of no-flow underfill for WLP does not require re-design and re-tooling since the standard SMT assembly process can be integrated. The WLP test vehicle, with potential for handheld applications, was fabricated based on a 12 mm×12 mm die size with 480 I/Os at a bump pitch of 0.4 mm. This paper discusses the findings of some no-flow underfills, the use of these in a WLP and the eventual reliability performance. Results of board-level temperature cycling (1000 cycles), viscosity changes during reflow and other characteristics are discussed. Reliability performance of the WLP with and without the use of underfill is compared. A shift of failure location away from the underfill/interconnect region to that between the die and nitride passivation is observed for some underfill which may be related to their physical properties, as indicated by some preliminary shear tests.