Cu alloying effect in Sn(Cu) solder line has been studied. We found that the Sn0.7Cu solder line has the most serious EM damage than pure Sn and Sn3.0Cu solder lines. The dominant factor for the fast EM rate in Sn0.7Cu could be attributed to the relatively small grain size and the low critical stress, i.e., the yielding stress of Sn0.7Cu solder line. Also, we found that the shortest Sn0.7Cu solder line, 250 μm, has most serious EM damage among three solder lines of different lengths. The back-stress induced by EM might not play a significant role on the EM test of long solder lines. A new failure mode of EM test, i.e., EM under an external tensile, was observed. The external stress would be superimposed on the stress profile induced by EM. As a result, the hillock formation at the anode side was retarded and void formation at the cathode was enhanced.