This paper presents an original approach allowing determining the failure mechanism at the origin of onresistance (RDSon) drift on vertical Power N-MOSFETs, dedicated to automotive application. The studied devices failed after Temperature Cycling (TC) qualification stress. The originality of that paper concerns the necessity to use strategic Failure Analysis (FA) approaches to determine the origin of the defect, without any localization possibilities. In that perspective, an original microprobing sub-mohms resistance electrical characterization of the different conductive interfaces was performed in order to determine the failing layer. Then, physical destructive FA techniques (backside SAM, leadframe peel-off and mechanical cross-section) were combined in order to finely characterize the defect. As a result, problems in die attach layer were highlighted, confirming the electrical probing characterization. At last, root cause of this abnormal die attach will be discuss through review of assembly process parameters. These results allowed implementing corrections and improving product stress resistance.