In this thesis, I systematically investigated and discussed the mechanical reliability challenges of Cu-Sn IMC-based microbump from the as-reflowed condition, after multiple reflows, to solid state aging. I designed three types of experiments of tensile, nanoindentation, and complex mixed mode fracture mechanics testing to quantitatively and qualitatively study how brittle Cu-Sn IMC-based microbump is. For tensile properties of microbump, I prepared more than hundreds of wire-type specimens for tensile testing in order to evaluate microbump brittleness in terms of effective modulus, fracture strength, and elongation to fracture of the joints. In addition, by preparing a series of wire-type joint specimens with decreasing solder joint thickness from 500 um to 5 um, I found ductile to brittle transition occurs with joint thickness downward scaling. Next, from nanoindentation study of polished sub-20 um micro-joint specimens, I used continuous stiffness measurement with controlled cracking technique to measure Young’s modulus, hardness, as well as fracture toughness of micro-joints. Furthermore, I prepared complex mixed mode specimens to evaluate critical strain energy release rate (toughness) to investigate the micro-joints mechanical reliability not only in mode I, mode II loading, but also under mixture mode of loading to mimic the mechanical integrity of microbump under high strain rate impact or dropping. Last but not the least, I further studied microbump brittleness from chemical bonding and first principles perspectives to explain the fundamental reasons that why Cu-Sn microbump is brittle yet electrical conducting. In my best knowledge, this thesis is probably the first systematic study regarding the mechanical reliability challenges, mechanical properties, and fundamental reasons of the brittleness of Cu-Sn IMC-based microbump.