The Effect of Toughening Agents on Capillary Underfill in the Flip Chip Package

Toughening agents were introduced in capillary underfill to prevent fillet cracking and promoting reliability in flip-chip package. The effects of toughening agents with typical structures on the performance of capillary underfill such as storage modulus, coefficient of thermal expansion (CTE), glass transition temperature (Tg), adhesion strength, rheological and curing behavior were investigated. It is found that the capillary underfill containing carboxyl terminated rubber showed the highest CTE, lowest adhesion strength, lowest moisture absorption, and highest modulus. The capillary underfill containing silicone elastomer showed low viscosity, fast flow time and proper moisture absorption, fast gel time, CTE, storage modulus and Tg, as well as good adhesion strength in five substrates. Compared with capillary underfill without toughening agent, the viscosity of capillary underfill with toughening agent increases with increasing flow time. The influence of toughening agents with different structures on capillary underfill is varios. Our work provides guidance for selecting the toughening agent for the underfill of the capillary.