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Reworkable underfills for flip chip, BGA, and CSP applications

Underfill is a polymeric material used in the flip-chip devices that fills the gap between an IC chip and an organic board, and encapsulates the solder interconnects. This underfill material can dramatically enhance the reliability of the flip-chip devices as compared to the non-underfilled devices. Current underfills are mainly epoxy-based materials that are not reworkable after curing, which places an obstacle in Flip-Chip on Board (FCOB) technology developments. Reworkable underfill is not only the key material to address the non-reworkability of the FCOB packages, but it can also be used to enhance the board-level reliability of BGA, CSP devices without losing their good reworkability feature. The objectives of this study are to determine the process viability, material performance, and reliability of reworkable underfills for advanced area array packaging and assembly technologies. Several reworkable underfills along with standard non-reworkable underfills (baseline materials) are evaluated in the study. Rework processes for BGA, CSP and flip chip devices are developed. Reworkable underfills allow flip chip, BGA, and CSP packages to be reworked using developed rework processes, while standard underfills show no reworkability. Reliability assessment of these reworkable underfills is ongoing.