Spot welds have two separation modes: interfacial and button pullout. Most of existing publications [8,9,10,11,12] focused on button pullout. This is because for the same sheet metal and gage combination, button pullout leads to higher separation load than interfacial separation. With the push for lighter vehicles, high strength and ultra high strength steels are used. To further reduce mass, welding flanges are getting narrower. The welding tips are getting smaller. The weld nugget diameters are smaller as a result. The separation mode for certain load cases is no longer nugget pullout, but interfacial instead. This lowers the weld's maximum load capacity. In order for CAE simulated prediction to correlate to physical behaviors of vehicle structures, it is important to define and reconfirm separation criteria. New tests and analyses are necessary. In this paper, an in-depth investigation is presented based on the recent GM tests on spot welds of hot stamped boron (HSB), martensitic (MS), dual phase (DP) and mild steels. The results and discussions of the separation modes give unique insight into spot weld behaviors of ultra high strength steels such as martensitic and boron steels. These results are compared to the results of lower grade steels.