Small　outline　transistor　(SOT)　packages,　because　of　their　low　cost　and　low　profile,　are　widely　used　in　consumer　electronics.　Ni/Fe　alloy　(A42)　is　the　most　widely　used　leadframe　material　of　SOT　packages　because　of　its　good　formability　and　coefficient　of　thermal　expansion　(CTE)　match　with　silicon　die.　The　very　small　size　of　SOT　packages　allow　a　higher　package　density　on　a　board,　but　the　small　size　and　close　proximity　of　SOT　packages　make　thermal　management　difficult.　The　low　thermal　conductivity　of　A42,　which　is　less　than　one　tenth　of　that　of　copper,　makes　the　thermal　conduction　even　more　difficult.　Thus,　the　method　of　copper　plating　on　A42　leadframe　is　used　to　enhance　the　thermal　conductivity　of　SOT　packages　while　keeping　their　low　cost　and　good　CTE　match　between　leadframe　and　silicon　die.　In　this　paper,　the　junction　temperature　T-j,　the　junction-to-ambient　thermal　resistance　theta(j-a)　and　the　junction-to-solder　joint　thermal　resistance　theta(j-sp)　are　used　to　evaluate　the　heat　dissipating　ability　of　the　package.　The　finite　element　method　is　applied　to　study　the　influence　of　copper　plating　thickness　on　the　thermal　performance　of　a　SOT　package.　A　thermal　resistance　circuit　model　and　the　corresponding　thermal　resistance　expression　on　function　of　the　copper　plating　thickness　are　then　proposed　to　quantify　the　influence　of　copper　plating　thickness.　The　proposed　expression,　which　follows　a　hyperbolic　form,　can　well　predict　thermal　resistance　of　the　model.　Finally,　die　cracking　issue　of　the　SOT　packages　with　copper　leadframe　is　examined　and　the　residual　stress　on　silicon　die　after　die　bonding　process　is　studied.　By　comparing　the　cracking　pattern　and　the　stress　state,　shear　fracture　is　proposed　to　be　the　crack　initiation　mechanism.　Results　also　show　that　copper　plating　on　A42　leadframe　has　little　influence　on　the　distribution　of　the　thermal　residual　stress　in　the　die.　The　stress　level　for　the　SOT　package　with　copper　plated　A42　leadframe　is　much　lower　than　that　with　copper　leadframe,　which　makes　the　die　cracking　risk　much　lower.