In　this　paper,　the　failure　mechanism　of　VDMOS　in　DC/DC　converter　was　analyzed　in　detail.　The　result　was　gained　by　the　reliability　project　that　included　reliability　experiment　and　reliability　analysis.　The　VDMOS　devices　were　used　in　the　circuit　of　DC/DC　converter　which　can　achieve　the　function　of　28V　into　15V.　The　accelerated　life　test　was　imposed　on　the　VDMOS　of　the　circuit,　which　has　the　temperature　stress　5C/day　and　the　electrical　stress.　The　test　was　began　at　75C,　and　the　VDMOS　devices　failed　at　215　°C.　Then　the　failure　analysis　which　recurred　to　the　failure　analysis　tools　such　as　the　photon　emission　analysis　(PEM)　and　the　physical　failure　analysis　(PFA)　to　the　failed　device　showed　the　failure　mechanism　which　is　junction-break-down　leakage　inside　source　contact,　under　source　wire　bond.　And　the　possible　root　cause　of　failure　probably　is　AI　spiking/　diffusion　into　source　contact　causing　junction　leakage　under　reliability　test　condition　of　bias　and　temp.　So　in　this　way,　the　reliability　of　VDMOS　in　DC/DC　converter　can　be　tested　and　evaluated　effectively.　©2009　IEEE.