The mass transport of Cu during electromigration (EM) testing is typically dominated by interface diffusion. If a mechanism other than interface diffusion begins to affect the overall transport process, then the effective diffusivity, D, of the EM process would deviate from that of interface diffusion only. This would have fundamental implications. We have preliminary evidence that this might be the case, and we report its implications for EM lifetime assessment in this manuscript. (11 References).