This　paper　investigates　the　formation　and　the　growth　of　the　intermetallic　compound　(IMC)　layer　at　the　interface　between　the　Sn3.0Ag0.5Cu　Pb-free　solder　and　the　Cu　sub-strate　during　isothermal　aging　at　150　degrees　C.　We　measure　the　thickness　of　the　IMC　layer　and　the　roughness　of　the　solder/IMC　interface,　and　these　two　factors　are　assumed　to　control　the　tensile　behavior　of　the　solder　joints.　First,　it　utilizes　the　tensile　tests　of　the　aged　solder　joints　for　analyzing　the　effect　of　the　IMC　growth　on　the　tensile　behavior　of　the　solder　joints.　Then,　the　microcracking　behavior　of　the　IMC　layer　is　investigated　by　finite　element　method　(FEM).　In　addition,　qualitative　numerical　simulations　are　applied　to　study　the　effect　of　the　IMC　layer　thickness　and　the　solder/IMC　interfacial　roughness　on　the　overall　response　and　the　failure　mode　of　solder　joints.　The　experimental　results　indicate　that　when　the　aging　time　increases,　both　the　thickness　and　the　roughness　of　the　IMC　layer　have　a　strong　influence　on　the　strength　and　the　failure　mode　of　solder　joints.　The　numerical　simulation　results　suggest　that　the　overall　strength　of　solder　joints　is　reduced　when　the　IMC　layer　is　thick　and　the　solder/IMC　interface　is　rough,　and　the　dominant　failure　mode　migrates　to　the　microcracks　within　the　IMC　layer　when　the　IMC　layer　is　thick　and　the　solder/IMC　interface　is　flat.