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Novel thermally reworkable underfill encapsulants for flip-chip applications

Underfill encapsulant is critical to the reliability of the flip-chip solder interconnections. Current underfill encapsulants are mainly filled epoxy-based materials that are not reworkable after curing, which place an obstacle in Flip-Chip On Board (FCOB) and Multi-Chip Module (MCM) technology developments, where unknown bad dies (UBD) is still a concern. Accordingly, approaches have been taken to develop the thermally reworkable underfill materials in order to address the reworkability problem of the commercial underfill encapsulants. These include introducing the thermally cleavable block to thermoset resins, and adding additives to thermoset resins. For the first approach, two diepoxides with thermally cleavable blocks were synthesized and characterized. These materials were shown to undergo thermosetting reactions with cyclic anhydride in a very similar fashion to a commercial cycloaliphatic epoxide, and they meet the underfill encapsulant requirements in regard to their glass transition temperatures (Tgs), storage moduli (G's), coefficients of thermal expansion (CTEs), and viscosities. However, these cured samples degrade at much lower temperatures than the commercial epoxies. For the latter approach, two additives were shown that after being added to typical cycloaliphatic epoxy formulation, do not interfere with epoxy curing, and do not affect the typical properties of cured epoxy system, yet provide reworkability to the epoxy. In this paper, we present the characterization results of this new class of materials-thermally reworkable underfill encapsulants-by DSC, TMA, DMA, and rheometer. Furthermore, we will present the preliminary reworkability test results of this class of materials with flip-chip devices on PWB.