Mixed Mode Tension Test of Underfills

In flip-chip packages (FCPKG), underfill (UF) is applied to reinforce solder bumps and enhance reliability of the package. Loading mode around cracks in UF is divided into tension (Mode I) and shear (Mode II). These two modes occur depending on the external load and the component structures. As for real components, these two modes are usually mixed. For higher precision of analyses and material design, it is essential to evaluate fracture toughness for each mixture rate of Mode I/ Mode II.Fracture toughness and fracture mode are influenced by mode mixture rate of the load, resin layer thickness as well as resin ingredients. We investigated the relationship between these factors. Mixed mode tension tests have been conducted for UF samples with different filler content and compound. Measurements were conducted at several different angles. Maximum load values were measured and fracture modes were observed.Most of specimens showed interfacial delamination, but lower angle (higher rate of mode I) induces bulk cohesive fracture and gives lower load toughness. High resin content increases max load value, and elastomer additive reinforces interfacial adhesion. Besides, resin layer thickness has relation with the fracture mode.Through this study, we obtained relationship between loading mode and fracture mode, and toughness enhancement effect of elastomer additive.