Enhancement of antifillet cracking performance for no‐flow underfill by toughening method

Abstract

Fillet cracking of no-flow underfill in a flipchip device during a 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 best epoxy formulation and the appropriate loading level of toughening agent regarding the antifillet cracking performance were found. In the case where the epoxy was modified with polysiloxanes, the second-phase particle with a submicron particle size was formed and the size of the particle depended on the kind of toughening agent. The morphology was observed by a scanning electron microscopy and confirmed by a dynamic mechanical analyzer measurement. The physical properties such as the fracture toughness, flexual modulus, coefficient of thermal expansion, and adhesion were measured, and the liquid-liquid thermal shock (LLTS) test under Ϫ55 to 125°C was 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. In order to obtain a high reliable no-flow underfill, the physical properties of the no-flow underfill should be well controlled and balanced. Finally, a correlation between physical properties of the no-flow underfill and the fillet cracking capability for those approaches was discussed.