Measurements of the median time to failure (MTTF) and deviation in the time to electromigration‐induced failure (DTTF) of Al alloy thin‐film lines are reported. As the ratio of the linewidth to the grain size decreases, MTTF decreases to a minimum and then increases exponentially. DTTF continuously increases. We show that serial and parallel failure unit models can be used to explain the grain size and linewidth dependence of the MTTF and DTTF for interconnects. We further note that extrapolation to low cumulative failures based on serial failure models must be based on knowledge of the failure statistics of individual units.