The effect of toughening of no-flow underfill on fillet cracking of flip-chip device

The effect of toughening agents and modification of epoxy on fillet cracking was investigated. Nylon, polycarbonate (PC) and polysiloxanes with amine and epoxy functional groups were employed as the toughening agents. Nylon and PC were miscible and reacted with epoxy resin, and a homogenous phase was found. However, Nylon and PC were ineffective in enhancing the anti-fillet cracking properties of the no-flow underfill. In the case where the epoxy was modified with polysiloxanes, a second phase with fine particle size was formed and the particle size depended on the toughening agent. The morphology was observed by scanning electron microscopy (SEM) and confirmed by dynamic mechanical analysis (DMA) measurement. The physical properties such as the fracture toughness, flexural modulus, coefficient of thermal expansion (CTE), and adhesion were measured and liquid-liquid thermal shock (LLTS) tests in the -55~125°C range were performed with different formulations. One of the formulations toughened by amine/epoxy terminated polysiloxane, which has higher die shear strength, lower modulus, and higher toughness, passed 1000 cycles of the LLTS test. As such, in order to obtain highly reliable no-flow underfill, the physical properties of the no-flow underfill should be well controlled and balanced. Finally, correlation between physical properties of no-flow underfill and anti-fillet cracking capability for these approaches was discussed