Thickness　and　roughness　evolution　of　the　intermetallic　compounds　(IMC)　layer　between　Sn3.0Ag0.5Cu　and　Cu　substrate　are　examined　under　various　isothermal　aging　times　at　150　degrees　C.　The　effect　of　the　microstructure　evolution　of　the　IMC　on　the　mechanical　behavior　of　solder　joints　is　experimentally　investigated.　The　results　show　that　the　growth　of　the　IMC　follows　the　Fick＇s　law　that　predicts　the　total　mean　thickness　increases　linearly　with　the　square　root　of　the　aging　time.　With　the　increase　of　the　isothermal　aging　time,　the　initial　scallop　morphology　of　the　solder/IMC　interface　changes　to　a　more　planar　type.　Both　the　thickness　and　roughness　of　the　IMC　layer　affect　the　tensile　strength　and　fracture　mode　of　the　solder　joints.　The　tensile　strength　decreases　with　the　increase　of　either　the　IMC　thickness　or　the　solder/IMC　interfacial　roughness.　With　the　increase　of　the　isothermal　aging　time,　the　IMC　thickness　increases　and　the　solder/IMC　interfacial　roughness　decreases,　and　the　fracture　mode　migrates　from　ductile　fracture　in　bulk　solder　to　brittle　fracture　in　the　IMC　layer.