A microscopic nucleation model for electromigration is proposed. The solution to the master equations for site occupancy produces a variable current exponent with 1 < n < 2. These results disprove previous claims that n = 2 in nucleation models. We also address the long-standing speculation regarding an additional temperature dependence in Black's equation. Under the premise that the activation energy in Black's equation, E a , should be equal to the energy barrier for atomic hopping, we provide evidence for an additional T 2 dependence of the failure time. Microscopic modeling provides an explicit connection between the macroscopic model behavior and the microscopic parameters of the model. A continuum treatment of our model is derived and compared with standard continuum treatments.