An enlarged top view of via pattern 520 is shown in FIG. 5B. Each of the via patterns 520, 522, 524, 526 may have the same configuration. In the example of FIGS. 5A and 5B, each via pattern 520, 522, 524, 526 of attachment layer 460 includes a first signal via 530 and a second signal via 532, which form a differential signal pair. The signal vias 530 and 532 extend vertically through the attachment layers and may have diameters in the attachment layers that are selected to accept the contact tails 156 of backplane connector 150. In forming the board, a ground plane 540 is partially removed, such as by patterning a copper layer on a laminate, to form an antipad 542, forming a ground clearance between ground plane 540 and signal vias 530 and 532, so that the dielectric sheet of the attachment layer 460 is exposed. The antipad 542 has a size and shape to preclude shorting of ground plane 540 to signal vias 530 and 532, even if there is some imprecision in forming the vias relative to ground plane 540, and to establish a desired impedance of the signal path formed by signal vias 530 and 532. In the embodiment of FIGS. 5A and 5B, antipad 542 is rectangular in shape, and the signal vias 530 and 532 are centrally located in antipad 522. However, the antipad 522 may have any suitable shape and may have rounded corners.