A stiffener molded to a semiconductor substrate, such as a lead frame, and methods of molding the stiffener to the substrate are provided. The stiffener is molded to the substrate to provide rigidity and support to the substrate. The stiffener material can comprise a polymeric material molded to the substrate by a molding technique such as transfer molding, injection molding, and spray molding, or using an encapsulating material. One or more dies, chips, or other semiconductor or microelectronic devices can be disposed on the substrate to form a die assembly. The stiffener can be molded to a substrate comprising one or more dies, over which an encapsulating material can be applied to produce a semiconductor die package.