Novel thermally reworkable underfill encapsulants for flip-chip applications

The flip-chip technique of integrated circuit (IC) chip interconnection is the emerging technology for high performance; high input/output (I/O) IC de vices. Due to the coefficient of, thermal expansion-mismatch between the si licon IC (CTE = 2.5 ppm/degrees C) and the low cost organic substrate such as FR-4 printed wiring board (CTE =:18-22 ppm/degrees C), the hip-chip sold er joints experience high shear stresses during temperature cycling. Underf ill encapsulant is used to couple the bilayer structure and is critical to the reliability:of the flip-chip solder interconnects. Current underfill en capsulants are filled epoxy-based materials that are normally not reworkabl e after curing. I This places an obstacle in flip-chip on board (FCOB) tech nology development, where unknown bad dies (UBD) are still a concern. Appro aches have been taken to develop the thermally reworkable underfill materia ls in order to address the nonreworkability problem of the commercial under fill encapsulants. These approaches include introduction of thermally cleav able blocks into epoxides and addition of additives to the epoxies, In the first approach, five diepoxides containing thermally cleavable blocks were synthesized and characterized. These diepoxides were mixed removal capabili ty to the epoxy formulation without interfering with, the epoxy cure or pro perties of the cured epoxy system. Furthermore, the combination of the two approaches showed positive results. with hardener and catalyst,Then the mix ture properties of Tg, onset decomposition temperature, :storage modulus, C TE, and viscosity were studied and compared with those of the standard form ulation based on the commercial epoxy resin ERL-4221E, These mixtures all d ecomposed at lower temperature than the standard formulation. Moreover, one mixture, Epoxy5, showed acceptable Tg, low viscosity, and fairly good adhe sion. In the second approach, two additives were discovered that provide di e removal capability to the epoxy formulation without interfering with, the epoxy cure or properties of the cured epoxy system, Furthermore, the combi nation of the two approaches showed positive results.