This paper investigated the effect of heat spreading on the boiling of the Novec 649™ for two-phase immersion cooling of electronics. Reference pool boiling tests were performed by attaching a 25.4 mm by 25.4 mm square copper plate to a same-sized heater, thus minimizing lateral heat spreading. Experimental measurements showed that the critical heat flux (CHF) happened at a heat flux of 17.4±0.8 W/cm2. Then, lateral heat spreading through the heat spreader was studied by attaching larger (47 mm by 47 mm) spreaders with four different thicknesses to the copper plate. With an increase in the integrated heat spreader (IHS) thickness from 1 mm to 6 mm, the CHF increased by more than 60% at the saturation condition. One plate was a 1 mm-thick IHS removed from a commercial microprocessor. In this case, the CHF happens at 8.6 W/cm2 (50% lower compared to the reference case) in the saturation condition. At CHF, the boiling can be observed on the whole surface, with columns and slugs regime at the center and the fully developed nucleate boiling regime at the edges. This nonuniform boiling was more pronounced in subcooled conditions, in which the CHF occurred at the center while there were regions at the edges that had no boiling. Finally, the performance of a microporous-coated IHS (with 3.15 mm thickness) was compared to the 6 mm thick IHS. The thermal resistance was almost equal for powers above 200 W. This indicates that lateral heat spreading is a critical parameter for the thermal design of immersion cooling along with microporous coating.