As　the　failure　of　solder　joints　under　thermal　cycling　is　as　a　result　of　creep-fatigue　damage　evolution,　the　failure　mechanism　of　SnAgCu　solder　was　studied　by　using　the　theory　of　continuum　damage　mechanics　(CDM)　and　a　new　damage　model　was　proposed　here.　A　special　bimetallic　load　frame　with　single　joint-shear　sample　was　designed　to　simulate　actual　joints　in　electronic　packages.　Thermo-mechanical　cycling　and　thermal　cycling　tests　were　conducted　to　determine　material　parameters　in　the　creep-fatigue　interaction　damage　model,　in　which　the　damage　variable　D　showed　a　power-dependence　upon　thermal　cycles.　The　damage　variable　D　=　1-R-0/R　was　selected　and　measured　every　dozens　of　cycles　during　thermal　cycling　tests　to　verify　the　model.　The　results　showed　that　the　experimental　damage　data　can　be　fitted　reasonably　well　by　the　relationship　of　the　damage　model　proposed　here　for　SnAgCu　solder.　And　the　evolution　of　solder　microstructure　during　thermal　cycling　was　observed　by　using　metallographic　sectioning　and　optical　microscopy　analysis,　which　gave　the　microscopic　explanation　for　creep-fatigue　damage　evolution　law　of　SnAgCu　solder.