Abstract: The present disclosure relates to a heat exchanger for thermal control of heat producing devices. In accordance with aspects and embodiments, this is achieved in the present design by placing a parallel flow path near the heat sink in the test head with a variable restriction in each flow path. The restrictions may be operated in a complementary manner to keep the fluid inertia in the main supply and return lines substantially constant. This arrangement results in a test head with higher cooling capacity and quicker response time than achievable with prior art semiconductor testing systems. The test head includes a pair of nested pneumatic actuators having non-circular pistons designed to provide Z axis motion and roll and tilt compliance. The pneumatic actuator assembly may be mounted on segmented flexures to provide X axis, Y axis and yaw compliance.