The　evolutions　of　intermetallic　compounds　(IMCs)　around　the　reinforcing　particles　in　composite　solder　matrix　during　isothermal　aging　markedly　influenced　the　mechanical　properties　of　composite　solder　joints.　In　this　research,　solder　joints　fabricated　using　the　eutectic　Sn-3.5Ag　composite　solder　with　micro-sized　Cu　or　Ni　reinforcing　particles　as　matrix　were　subjected　aging　at　different　temperatures　(125,　150　and　175　degrees　C).　The　growth　kinetics　and　activation　energies　of　interfacial　layers　around　the　reinforcing　particles　were　characterized.　The　effects　of　the　growth　of　interfacial　layers　on　the　mechanical　properties　of　composite　solder　joints　were　also　investigated.　Experiment　results　indicate　the　growth　rate　of　interfacial　layer　around　the　Cu　reinforcing　particles　in　composite　solder　matrix　was　faster　than　that　around　Ni　reinforcing　particles　during　isothermal　aging.　The　activation　energy　of　Ni　composite　solder　is　higher　than　that　of　Cu　composite　solder.　In　addition,　the　mechanical　properties　of　Cu　composite　solder　joints　decreased　quickly　during　isothermal　aging.　Besides,　finite　element　method　was　used　to　analyze　the　stress　distribution　of　IMCs　around　the　reinforcing　particles.　It　suggests　that　IMCs　around　the　reinforcing　particles　give　birth　to　stress　concentration　and　have　tendency　to　crack　with　the　growth　of　IMCs　during　isothermal　aging.