In　this　work,　stress　relaxation　behavior　of　Sn3.5Ag0.5Cu　solder　joint　was　investigated　under　398　K.　Three　strain　ranges　(4,　6,　and　8　%)　and　two　strain　rates　(0.002　and　0.01/s)　were　utilized.　The　results　show　that　the　effect　of　strain　range　on　stress　relaxation　is　not　significant　under　high　strain　rate.　However,　it　firstly　increases　with　strain　range　and　then　decreases　after　approaching　the　maximum　value　under　low　strain　rate.　Moreover,　Stress　relaxes　more　under　high　strain　rate　than　that　under　low　strain　rate.　Feltham　equation　can　describe　the　stress　relaxation　behavior　well　under　high　strain　rate.　After　a　certain　relaxation　time,　crack　appears　at　the　corner　of　the　solder　joint　and　the　stress　appears　to　increase　a　little.