Intermetallic　compound　(IMC)　layers　formed　during　soldering　processes　significantly　affect　the　reliability　of　solder　joints.　The　atomic　diffusion　effect　during　reflow　and　isothermal　aging　leads　to　growth　and　morphological　evolution　of　IMC　layers,　and　stress　is　developed　in　the　IMC　layers.　The　changed　microstructure　and　the　stress　in　the　IMC　layer　result　in　degradation　of　mechanical　performance　of　solder　joints.　Based　on　the　mechanism　of　atomic　diffusion-reaction,　the　diffusion-induced　stress　during　the　growth　of　the　IMC　layer　is　investigated.　An　analytic　model　with　two　interfaces　(Cu/Cu6Sn5/Solder)　at　the　early　stages　of　IMC　formation　is　proposed,　and　then　the　copper　concentration　distribution　in　the　IMC　layer　is　calculated　using　the　Laplace　transformation　method.　Diffusion-induced　stresses　are　obtained　analytically　by　transforming　atomic　diffusion　effects　into　bulk　strain.　The　results　show　that　the　diffusion-induced　stress　is　compressive,　and　it　reaches　its　peak　at　the　Cu/Cu6Sn5　interface.　The　diffusion-induced　stress　increases　with　the　increase　of　the　isothermal　aging　time,　and　finally　becomes　stable　and　changes　linearly　along　the　thickness　of　the　IMC　layer.