Abstract: An apparatus for supporting a substrate such as a printed circuit board (PCB) having various electronic components installed on one side with that side down so that other components can be installed on the opposite side. An assembly of three perforated plates has deformable, typically rubber-like, pins projecting through the perforations. Springs below the assembly bias the pins to a position extending above the plate. When a PCB is laid on the pins with mounted electronic components downward, the components with pres the pins down distances corresponding to component thickness. The pins provide uniform support while working on the opposite side. The center perforated plate of the three is moved to the side, clamping the pins in position, so that the PCB can be removed and replaced with another identical PCB. Different devices for causing the plate clamping action and devices for indexing PCB position and for maintaining the PCB flat and level are also provided.