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This paper reports the experimental and theoretical exploration of the fracture mechanism active in BGA lead-free solder assemblies under high speed shear fatigue test conditions. Our investigation finds that, contrary to common assumption, the crack growth in shear fatigue is not governed by shear stress but more by crack opening stress. Our theoretical analysis indicates that fracture by crack opening mode prevails because non-uniformity in the shear deformation of solder joint creates a body rotation which results in crack opening stress rather than shear. While the crack growth in shear fatigue is found to vary sensitively with variation in the mechanical constraints on the assembly, such as solder shape and elastic modulus of the chip mold, it is also sensitive to variation in solder microstructure. This, the sensitivity to the assembly constraints and solder microstructure, makes it ideal in investigating fatigue properties of solder joints as well as identifying the structural and microstructural features responsible for reliability failure.