Thermo-Mechanical Reworkable Epoxy Underfill in Board-Level Package: Material Characteristics and Reliability Criteria

This work explores underfill with improved properties for rework-ability and package reliability. Reworkable underfills (Epoxy-R1 – R5) were customized by a material supplier and benchmarked with an existing non-reworkable underfill (Epoxy-E). R1 shows similar glass transition temperature (Tg), coefficient of thermal expansion 1&2 (CTE) with Epoxy-E, lower storage modulus (30% of Epoxy-E), yet still poor for rework due to major damage on printed circuit board (PCB) detected. R2 was refined with much lower modulus, 10% of Epoxy-E, but failed to meet target Tg and CTE. R3 used smaller filler size (10um) in formulation, reliability related properties were significantly improved, however, same overheat issue on adjacent component and insufficient coverage was found. For R4 and R5, both Tg reached above 130°C and low CTE-2 around 100ppm/°C, which is only 70% of Epoxy-E. For rework evaluation, R4 and R5 showed good results, no adjacent defects which are suspected due to lower adhesion, underfill is easier to remove. R5 was selected for reliability test due to its similarity in viscosity and process condition compared to Epoxy-E with minimal change in dispensing process setup. R5 test vehicle survived 1000 thermal cycling (-40°C to 85°C) meeting mechanical shock and vibration tests qualification. Lastly, it was observed that R5 achieved both rework-ability and package reliability expectations with a new defined thermo-mechanical property.