The　changes　of　electrical　conductivity　(resistance)　between　Sn-3.0Ag-0.5Cu　solder　joints　and　printed　circuit　board　(PCB)　assembly　during　aging　at　125　degrees　C　were　investigated　by　the　four-point　probe　technique.　The　microstructural　characterizations　of　interfacial　layers　between　the　solder　matrix　and　the　substrate　were　examined　by　optical　microscopy　and　scanning　electronic　microscopy.　Different　types　of　specimens　were　designed　to　consider　several　factors.　The　experimental　results　indicate　that　electrical　conductivities　(resistances)　and　residual　shear　strengths　of　the　solder　joint　specimens　significantly　decrease　after　1000　h　during　isothermal　aging.　Microcracks　generate　in　the　solder　matrix　at　the　first　250　h.　Besides,　the　evolutions　of　microstructural　characterizations　at　the　interface　and　the　matrix　of　solder　joints　were　noted　in　this　research.