Effects of UBM thickness, contact trace structure and solder joint scaling on electromigration reliability of Pb-free solder joints

Electromigration (EM) tests were performed on Pb-free solder joints having different thicknesses of Ni UBM, to examine the effect of UBM thickness on EM reliability. The UBM thickness dependency of EM lifetime was explained in terms of the current crowding effect with the help of finite element analysis (FEA). Based on the experimental results as well as FEA, the maximum current density at the UBM/solder interface was found to be a critical factor controlling EM reliability. Further analyses were conducted by FEA to evaluate the dependency of current density distribution on a contact trace structure. The results showed that an appropriate selection of a contact trace structure was as important as the UBM thickness. The effects of solder joint scaling on current crowding were also investigated. It was found that the maximum current density did not increase as much as the average current density when solder joints were scaled down.