Solder joint shape formation under constrained boundaries in wafer level underfill

Standard underfill processing occurs at the package level and is slow since it relies on the capillary flow of the resin. The dispensing operation requires from 30 to 120 seconds depending on the die geometry, bump pattern, standoff, and material characteristics. Even with the “no-flow” materials introduced recently in the market, processing is still carried out at the unit level. Upon completion of the underfill operation, batch curing is done for an additional amount of time. Thus, when multiplied by the number of dies on a wafer, the total process time can be quite substantial. Wafer level deposition of underfill is the next paradigm shift in flip chip packaging, offering both cost savings and higher throughput than with the conventional process. The resin is applied to the wafer and soft-cured prior to singulation. Final cure of the film is achieved during assembly reflow. A number of challenges do exist in this new process relating to material formulation and processing. This paper discusses the issue of solder wettability during reflow, while the solder balls are being constrained by a polymeric underfill layer