The　intermetallic　compounds　(IMC)　growth　between　lead-free　solder　and　Cu　pad　has　an　important　influence　on　the　primary　device　reliability.　Cu　butt　joints　were　welded　by　Sn3.8Ag　0.7Cu　lead-free　solder,　and　the　joints　were　aged　at　125,　150　and　175°C,　aging　time　is　0,　24,　72,　144,　256　and　400　h.　The　growth　and　morphological　characteristics　of　the　IMCs　were　investigated　by　means　of　optical　microscope,　SEM　and　EDX.　The　diffusion　constant　and　active　energy　of　the　IMCs　were　fitted　by　experimental　data.　The　relationship　between　aging　and　the　tensile　strength　of　butt　joints　was　also　researched.　During　the　process　of　aging　the　tensile　strength　enhances　at　first　and　then　degrades.　Aging　could　influence　the　form　of　fracture.