A microelectronic contactor assembly can include a probe head having microelectronic contactors for contacting terminals of semiconductor devices to test the semiconductor devices. A stiffener assembly can provide mechanical support to microelectronic contactors and for connecting a probe card assembly to a prober machine. A stiffener assembly may include a main body and a plurality of mounting points, wherein at least one of the mounting points is flexibly connected to the main body by one or more laterally extending beams that has a section modulus normal to the lateral direction significantly greater than in the lateral direction. The stiffener assembly allows for differential thermal expansion of various components of the microelectronic contactor assembly while minimizing accompanying dimensional distortion that could interfere with contacting the terminals of semiconductor devices.