Intermetallic　compounds　(IMC)　play　a　key　role　in　the　mechanical　reliability　of　solder　joints.　The　present　work　investigates　the　diffusion-induced　stress　developed　in　the　Cu　pad/IMC/solder　sandwich　structure　during　a　solid-state　isothermal　aging　process.　An　analytical　model　and　a　numerical　approach　are　proposed　to　predict　the　stress.　The　model　consists　of　a　Cu6Sn5　layer　sandwiched　between　a　Cu　pad　and　a　solder　layer,　and　it　is　assumed　that　the　diffusivity　of　the　Cu　atoms　is　much　greater　than　that　of　the　Sn　atoms.　We　use　the　Laplace　transformation　method　to　obtain　the　distribution　of　the　Cu　atoms　concentration.　The　diffusion-induced　stress　is　determined　analytically　by　the　volumetric　strain　resulted　from　the　effect　of　the　atomic　diffusion.　It　is　found　that　the　Cu6Sn5　layer　is　subjected　to　compressive　stress　due　to　the　Cu　atoms　diffusion.　As　the　diffusion　time　is　long　enough,　the　diffusion-induced　stress　shows　a　linear　relationship　with　the　thickness　of　the　Cu6Sn5　layer.　A　finite　element　approach　to　calculate　the　diffusion-induced　stress　is　proposed,　and　it　is　compared　and　validated　by　the　analytical　solution.　The　results　show　that　the　proposed　approach　can　give　a　well　estimation　of　the　diffusion-induced　stress　in　the　Cu6Sn5　layer,　and　is　also　efficient　in　predicting　the　diffusion-induced　stress　in　the　structures　with　more　complex　geometry.　The　distribution　of　the　Cu　atoms　concentration　and　the　diffusion-induced　stress　in　the　model　with　a　scallop-like　or　flat-like　Cu6Sn5/solder　interface　are　calculated　by　the　numerical　approach.　The　results　show　that　the　interfacial　morphology　of　the　Cu6Sn5/solder　has　great　influence　on　the　evolution　of　the　Cu　atoms　concentration,　and　the　diffusion-induced　stress　in　the　Cu6Sn5　layer　with　the　scallop　edge　is　less　than　that　with　the　flat　edge.