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 first and second stiffener bodies that are connected together at their central portions with adjustment mechanisms such as three differential screw mechanisms. A probe head may be attached to a first stiffener body at locations outside its central portion, while a prober machine may be attached to a second stiffener body at locations outside its central portion. The first and second stiffener bodies may have different coefficients of thermal expansion. 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. The adjustment mechanisms allow for quick, sensitive adjustment of the positions of microelectronic contactors relative to semiconductor devices to be tested.