Via

A via is a plated hole that carries an electrical connection from one copper layer of a printed circuit board to another. The hole is drilled through the board and its barrel is electroplated with copper, so a signal or power net can drop from the top layer down to an inner plane or the bottom layer. On the dense multilayer boards used in ASIC hashboards, vias number in the thousands and are central to routing the high-current and high-frequency nets between domains.

Via types

A through-hole via spans the full board thickness. A blind via connects an outer layer to one or more inner layers without passing all the way through, and a buried via connects only inner layers. A micro-via is a small laser-drilled via used in high-density designs. A via-in-pad places the via directly inside a component land, which saves space but must be filled and capped to keep solder from wicking down the barrel during reflow.

Why vias fail

Repeated heating and cooling expands the board in the z-axis far more than the copper barrel can tolerate. Over many cycles this can crack the plated barrel, producing an intermittent or open connection that is hard to diagnose because it may conduct when cold and open when hot. Vias near high-power components and along board edges see the worst stress. On hashboards, a failed power-rail via can starve a whole group of chips while leaving the rest working.

Identifying a broken via often means tracing a net under magnification and comparing it to a known-good board. Related concepts include the annular ring that surrounds the via pad, the PCB stackup that determines which layers a via can reach, and the copper pour planes that vias stitch together.