An evaluation method of interface crack between solder ball and copper based on the non-destructive monitoring of direct current potential difference was proposed. The validity of the method was examined based on the results of electric field analysis by the finite element method. In the analysis, it was assumed that a solder ball was simply joined on the surface of a copper plate and the interface crack was evaluated by the potential difference between the top of the wire inserted into the solder ball and the copper plate. It was found from the analysis that the relationship between the crack area ratio and the increase in the potential difference was given by a single formula for all the conditions assumed in this study. As an experimental verification, a cyclic tensile test was carried out for the interface between Sn-3Ag-0.5Cu lead-free solder balls and a plate specimen of pure copper. Similar relationship between the crack area ratio and the increase in the potential difference held for the test. This suggests that the proposed method is applicable to the evaluation of actual interface cracks initiated in the specimens or in various electric packages.