Challenges in the Assembly of Large Die, High Bump Density Pb-Free Flip Chip Packages

It is well established that Sn/Pb flip chip bumps tend to solder more readily than the lead free Sn/Ag bumps. Underfill process is also well understood. For a large die, typically greater than 20 mm per side, with Pb-free bumps, the wetting and spreading appears to become less effective. Due to the higher reflow temperature requirement, a Pb-free compatible no clean flux is needed. Poor compatibility between the flux residue and the underfill can lead to the formation of voids, a potential reliability concern. These new fluxes are formulated to work well with most underfill chemistries. While this is generally the case for small die size applications, these new fluxes do not always work well with large dies and high bump count. Balancing the flux effectiveness in soldering and in achieving compatibility with the underfill is one of the most difficult challenges in getting a manufacturable Pb-free process on large dies. As next generation flip chip products move into tighter bump pitch and higher bump count, the process margin challenge will remain, if not increase. The process window for the rosin and the synthetic-based fluxes typically used in Sn/Pb application is much wider than the Pb-free fluxes. For Pb-free, soldering performance is sensitive to flux thickness, reflow profile, substrate pad surface finish/topography and substrate shrinkage. Underfill flow performance varies considerably with bump layout, density and aspect ratio. Because of the increased ratio of flux residue to underfill volume on high density bump designs, the underfill cure step is also becoming a major factor in the system. This paper gives a detailed description of the challenges encountered during assembly, such as the need for flux and underfill compatibility, flux selection process, underfill flow studies, flux height study, effect of dispense pattern, and the effect of underfill curing.