Study on the effect of toughening of no-flow underfill on fillet cracking

Fillet cracking of no-flow underfill in flip-chip device during reliability test such as thermal shock or thermal cycling has been a serious reliability problem. The effect of toughening agents and modification of epoxy on fillet cracking of no-flow underfill was investigated. The base epoxy formulation and the appropriate loading level of toughening agent were found regarding the anti-fillet cracking performance. In case where the epoxy was modified with polysiloxanes, the 2nd phase with fine particle size was formed and the size of the particle depended on the toughening agent. The morphology was observed by scanning electron microscopy (SEM) and confirmed by dynamic mechanical measurement (DMA) measurement. The physical properties such as the fracture toughness, flexural modulus, coefficient of thermal expansion (CTE), and adhesion were measured and the liquid-liquid thermal shock (LLTS) test under -55-125°C 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 high 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 those approaches was discussed