In this study, fatigue crack propagation testing on underfill resins with different filler content and finite element method (FEM) analysis were carried out to investigate the effects of filler content on fatigue crack propagation rate. The fatigue crack propagation rate of the underfill resin decreased with increasing filler content. FEM analysis clearly showed that fillers constrain the deformation of resin around them, which leads to inhibition of the amount of crack opening and a decrease in the energy release rate surrounding a crack tip. Since the amount of crack opening decreased with increasing filler content, it is conceivable that the decreased fatigue crack propagation rate can be attributed to a decrease in the amount of crack opening. The constraint on deformation of the base resin implies an increase in macroscopic static strength, and thus the resistance to fatigue crack propagation correlates with static strength.
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