A stiffener assembly for use with testing devices is provided herein. In some embodiments, a stiffener assembly for use with testing devices can be part of a probe card assembly that can include a stiffener assembly comprising an upper stiffener coupled to a plurality of lower stiffeners; and a substrate constrained between the upper stiffener and the plurality of lower stiffeners, the stiffener assembly restricting non-planar flex of the substrate while facilitating radial movement of the substrate with respect to the stiffener assembly.