Abstract: In this paper, the interfacial reactions and IMC evolutions of Cu/Sn-3.0Ag-0.5Cu/Cu butt solder joints were investigated during electromigration test, meanwhile, the difference were analyzed between solid-solid and solid-liquid diffusion under current stressing. The results indicated during solid-liquid diffusion. The IMC thickness is increasing at both anode and cathode with the extension of stressing time, the thickness of anode interface IMC layer is thicker than cathode after the same stressing time. With the increase of stressing time, the grain size increases in radial direction, but the grain size first increases then decreases in axial at the cathode. And the grain size increases not only in radial direction but also in axial at the anode. The average grain size of IMC at the anode is bigger than cathode in the stressing process. Compared to solid-solid diffusion, the morphologies of IMC were from irregular becoming irregular and the surface smoothness was improved at the cathode, the microscopic appearance of IMC was polygon ball in solid-solid diffusion, and it was polygon columnar in the solid-liquid diffusion.