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: they stitch ground planes together, feed the high-current power domains, and route the daisy-chained UART and clock nets that snake through every chip on the board.

Via types

A through-hole via spans the full board thickness and is the workhorse of most designs. 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 — both cost more to fabricate but free up routing space. 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 under fine-pitch BGA parts but must be filled and capped at the fab, otherwise solder wicks down the barrel during reflow and leaves the joint starved. Under hot-running parts, designers also place thermal via arrays — grids of vias whose only job is to conduct heat from a component's pad into an inner or bottom copper plane.

Why vias fail on hashboards

The failure mechanism is thermomechanical. FR-4 laminate expands along the z-axis far more than copper does when heated, so every thermal cycle stretches the plated barrel like a piston pulling on a sleeve. A hashboard is close to a worst-case environment: chips run hot, workloads and curtailment cycle the board between ambient and operating temperature over and over, and immersion or aggressive overclocking setups widen the swing further. Over enough cycles the barrel cracks — typically near the middle of the board or at the junction with a pad — producing the classic intermittent fault: the net conducts when cold and opens as the board warms and the crack gapes. Vias near high-power components and along board edges see the worst stress. Because power on a hashboard flows serially through the hash domains, a single failed power-path via can starve an entire group of chips while everything upstream measures fine.

Finding a broken via on the bench

Cracked vias are among the most frustrating hashboard faults precisely because they heal when the board cools on your bench. Useful techniques: measure continuity while the board is hot (or warm it locally with gentle hot air) rather than cold; gently flex the suspect area while watching a continuity beeper; compare net-to-net resistance against a known-good board; and inspect suspect vias under magnification for the telltale ring crack at the pad. Domain voltage measurements help localize the region first — find the domain whose rail is missing or sagging, then trace its feed. A repaired via is usually bridged with a short wire jumper soldered between the two layers' pads, since the barrel itself cannot be replated in the field.

Repairing a failed via

Field repair is a bypass, not a restoration. Scrape a little solder mask to expose copper on both layers the via should join, tin the exposed pads with flux and a fine iron, and bridge them with a short wire jumper — through the original hole when it is accessible, alongside it when it is not. On power nets, size the jumper generously; a hash domain can pull serious current, and a thin strand becomes a fuse. Verify with domain-voltage measurements against a known-good board and, ideally, a burn-in at operating temperature, since the whole point is catching the fault that only appears hot.

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 via arrays stitch together. For chip-level rework above the vias, see BGA rework.