Evaluation of electromigration resistance of lead-free solders using unpassivated stripe-shaped samples

Abstract

To improve the reliability of electronic devices, electromigration (EM) in lead-free solders is actively being studied. In particular, EM testing methods that use 1-D stripe samples have significant merit. However, some aspects of the reasonability of the EM resistance evaluation remain to be discussed. In this study, the validity of the evaluation method utilising stripe-shaped samples without passivation was verified. Sn-3.0Ag-0.5Cu (SAC) and Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge (SACNG) solders (in wt.%) with a reference of four nine Sn were used as testing materials for the verification. EM resistance was evaluated by measuring the total volume of hillocks observed in the designated region per unit time of current supply. First, the effect of sample length on EM was confirmed by observing the hillock volume, which increased based on the sample length. In addition, EM resistance was ranked Sn < SAC < SACNG under a combination of experimental conditions as well as under another combination of the conditions. Therefore, the consistency of the EM resistance ranking regardless of the experimental conditions was shown through the experiment. Furthermore, an evaluation formula including the effects of sample length on the EM was derived. Finally, it was theoretically shown that the present method of EM resistance evaluation was consistent regardless of the experimental conditions. These experimental results and theoretical discussion demonstrated the validity of the presented evaluation method.