Electromigration (EM) is one of the major indices to evaluate the reliability performance in IC devices. From the point of view of permutation and combination, there are many combination types for temperature and current density that used to determine the model parameters (Ea and n) in EM. Fourteen types among these combinations are chosen for this study and confidence intervals for model parameters are used as theoretical criteria. The study shows the most popular stress combination type is not the best one both from theory and experiment. Also, it is found that the tight confidence interval is a trade off for either the equipment capacity or the test cycle time in choosing the stress condition combination. Further, we recommend the appropriate stress combination types to be implemented in the experiment in order to save capacity and/or shorten test cycle time with still acceptable confidence interval.