The requirement for closely coupled, highly integrated circuits in the semiconductor industry has spawned alternative packaging innovations such as 2.5-D/3-D integration. The incredible potential of this alternative comes with great challenges, not the least of which is the unprecedented reduction in package interconnection pitch. Market acceptance of new fine-pitch microelectronic products is strongly dependent on the development of flawless assembly processes that align with the traditional Moore-like expectation of higher performance without cost penalty. One such process is the application of flux to the interconnect surfaces to achieve effective joining. Insufficient flux quantity or flux activity can impede the formation of solid, reliable joints, whereas excessive quantities or activity can cause solder bridging or difficulties with downstream operations such as residue cleaning or underfill reinforcement. This delicate balance, already complex for traditional chip joining, is further challenged by the geometrical and spatial reductions imposed by pitch miniaturization, especially where large die, with over 100,000 interconnects, are concerned. This article presents an overall development protocol to evolving a flux dipping operation to production-level thermocompression assembly of large die (8 × 11 × 0.780 mm) with 11,343 ultrafine pitch (62 μm) copper pillar interconnections. After reviewing the state of the art for fluxing technology and detailing the specific technical issues, we present and defend the chosen flux application approach with its corresponding parameters of interest. Physical and chemical characterization results for selected flux material candidates are reported in conjunction with an analysis of how their properties correlate to the flux dip application parameters. As part of this fundamental understanding, we investigate and report on flux dip coating behavior and how it compares to other industrial dip coating applications. Finally, the results of process assembly experiments in a production-type environment are reviewed and discussed with respect to the previous characterizations. These experiments span downstream assembly process compatibility (i.e., cleaning and underfill) as well as product reliability.
