The　Sn3.0Ag0.5Cu/Cu　lead-free　solder　and　Cu　joints　were　isothermally　aged　at　125,　150　and　175°C　for　36,　72,　216,　360　and　720　h.　The　microstructure　and　mechanical　properties　of　the　solder　joints　were　studied.　The　results　showed,　after　soldering　the　interfacial　intermetallic　compounds　(IMC)　of　solder　joints　were　small　serrate.　The　size　of　interfacial　IMC　was　found　to　increase　with　the　aging　temperature　and　aging　time.　The　IMC　growth　rate　increased　with　aging　temperature　but　decreased　with　aging　time.　With　the　increase　of　aging　time,　the　amount　of　IMC　serration　decreased,　and　its　diameter,　height　and　width　became　bigger,　higher　and　wider　respectively.　Through　analysis　of　the　interfacial　IMC,　a　scientific　method　to　characterify　IMC　size　was　found.　When　the　IMC　equivalent　thickness　was　calculated,　the　quantitative　relationship　between　IMC　equivalent　thickness　and　aging　time/temperature　was　established.　The　IMC　activation　energy　is　88　kJ/mol.　In　addition,　the　relationship　between　isothermal　aging　and　tensile　strength　was　developed.　The　tensile　strength　firstly　increased　and　then　decreased　with　the　aging　time,　which　could　be　attributed　to　the　welding　residual　stress　relaxation　and　the　IMC　evolution.