Solder joint shape formation under constrained boundaries in waferlevel underfill

摘要

Standard underfill processing occurs at the package level and isslow since it relies on the capillary flow of the resin. The dispensingoperation 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 ofthe underfill operation, batch curing is done for an additional amountof time. Thus, when multiplied by the number of dies on a wafer, thetotal process time can be quite substantial. Wafer level deposition ofunderfill is the next paradigm shift in flip chip packaging, offeringboth cost savings and higher throughput than with the conventionalprocess. The resin is applied to the wafer and soft-cured prior tosingulation. Final cure of the film is achieved during assembly reflow.A number of challenges do exist in this new process relating to materialformulation and processing. This paper discusses the issue of solderwettability during reflow, while the solder balls are being constrainedby a polymeric underfill layer