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Effect of Joule heating on the determination of electromigration parameters

The use of accelerated electromigration (EM) wafer level reliability (WLR) tests aims at performing tests on a large sample size faster than at package level (PLR). Meanwhile, the correlation of times to failure determined from package level tests and wafer level tests is sometimes not obvious. Therefore, little trust is put in extrapolated lifetime determined from WLR tests. To elucidate this apparent inconsistency, we developed a lifetime model, based on the flux divergence under high thermal gradient at the end of EM line. We found an apparent evolution of the current density exponent n from 1.5 to 2.1 due to the coupling between current density and temperature in WLR test. Besides, no important change of the activation energy Ea is found.