The disclosure relates to assemblies and methods for preheating objects in the process of underfill. Specifically, the disclosure relates to assemblies and methods for treating printed circuit boards (PCBs), integrated circuits (ICs) wafers and the like, in the preheating stage of the underfill process using an assembly operable to create a customizable hot-air bath with adjustable depth to account for surface topology and necessary clearances, for soaking the PCBs, ICs, wafers and the like in hot air, bringing these to operating temperature.