An interposer to couple a microelectronic device package to a motherboard is formed from a PCB substrate. Multiple via holes are drilled through a copper-clad PCB substrate and then coated inside with copper. The copper surface coating is etched to form multiple traces. In one embodiment, the substrate is cut through each row of via holes and between each row of via holes to produce multiple individual beam-and-trace interposers. Two or more such interposers may be affixed together to form a beam-and-trace interposer array. Alternatively, the substrate is not cut into strips, and each via hole is filled completely with a conductive material to form an array of solid conductive columns through the substrate.