Direct correlation between mechanical failure and metallurgical reaction in flip chip solder joints

We tested flip chip solder bonded Si samples under tensile and shear loading as a function of annealing time at 200 degrees C. The solder bump was eutectic SnPb and the underbump thin film metallization was Cu/Cr deposited on oxidized Si. We found that the failure mode is interfacial fracture and the fracture strength decreases rapidly with annealing time. From scanning electric microscope observations, the fracture occurs at the Cu-Sn/Cr interface. We conclude that it is the metallurgical reaction that has brought the solder into direct contact with the Cr surface. The weak joint is due to the spalling of Cu-Sn compound grains from the Cr surface, especially near the edges and corners of the joint. (C) 1999 American Institute of Physics. [S0021-8979(99)05006-9].