In-situ study of the effect of electromigration on strain evolution and mechanical property change in lead-free solder joints

We report a localized mechanical property study by using the nano-indentation continuous stiffness measurement (CSM) technology on Pb-free solder joints with and without electromigration. The tensile test structure of a solder ball connected by two Cu wires of 300mum diameters was prepared and tested. At 100 degC, the applied current density was from 0 A/cm² to 5times10³ A/cm² and the time from 3 to 144 hrs. An array of 500 nm indentations was created by the nano-indenter from the cathode area, across the bulk of the solder, to the anode area. The change of Young´s modulus and the hardness at the cathode and the anode was calculated from the CSM. The results show that, the Young´s modulus and the hardness of the anode were higher than the original values. And it increases with increasing electromigration time or higher current density. On the other hand, the Young´s modulus and hardness at the cathode were lower than the original values and they decrease with increasing electromigration time or higher current density. Therefore, there is a change of mechanical properties at the cathode and the anode of the lead-free solder joints due to electromigration. Furthermore, an in-situ micro-DISC technology, digital image speckle correlation technology, was used to study the strain evolution at the lead-free solder joints during electromigration. It is found that, during electromigration, with the atoms moving from the cathode to the anode, a tensile strain was created at the cathode region; while a compressive strain was observed at the anode region. And the strain difference between the cathode and anode kept increasing with the time of electromigration. The localized mechanical property change in the lead-free solder joints due to electromigration will be discussed