The　effects　of　the　intermetallic　compound　(IMC)　microstructure　on　the　tensile　strength　and　the　fracture　behavior　of　Sn3.5Ag0.5Cu/Cu　solder　joints　are　investigated.　The　soldered　samples　are　isothermally　aged　at　150　degrees　C　for　0,　72,　288,　500　hours,　respectively,　and　then　are　tested　at　the　strain　rate　of　2x10(-4)　s(-1)　at　room　temperature　in　air.　The　experimental　results　show　that　with　the　increase　of　the　isothermal　aging　time,　the　IMC　layer　thickness　increases,　and　the　roughness　of　the　solder/IMC　interface　decreases.　The　tensile　strength　of　the　solder　joint　decreases　with　either　increasing　IMC　thickness　or　increasing　solder/IMC　interfacial　roughness,　and　the　fracture　mode　migrates　from　ductile　fracture　in　bulk　solder　to　brittle　fracture　in　the　IMC　layer　with　increasing　aging　time.