AVF · Auto Via Filling Plating Line
Any-layer HDI is built on filled vias. A via that is 90% filled is a scrap board, because the next lamination has nothing to land on. Filling is a chemistry problem wearing a machine.
Filling a 100 µm via looks like the easy version of plating a 4 mm hole. It is not. The failure modes are different, the flow has to be gentler, and the additive package does most of the work.
A laser via of 95 to 100 µm diameter and a similar depth — an aspect ratio near 1:1 — has to end up solid copper, planarised, so that the next copper layer can land on it and the via can be stacked. Three numbers define acceptance:
Bottom-up fill is an adsorption effect. The suppressor coats the field and the via walls; the accelerator displaces it at the via bottom, where surface area is increasing as the bottom rises. Copper then deposits faster at the bottom than at the mouth — that is the whole mechanism, and it is why a fill bath has three additives and not one.
Which means the flow in a fill cell must be uniform and slow enough to deliver the additives without shearing the adsorbed layer off the wall. A side-spray manifold tuned for 40:1 through holes will strip a 1:1 via clean and fill nothing. This is the single most common mistake we are asked to correct on a converted line.
Bottom spray plus gentle side flow, targeted at 0.3 to 0.8 m/s at the panel face and mapped to ±10% across the panel. Enough to refresh the via, not enough to shear off the suppressor.
Nozzle spacing and pump curves are set at commissioning against a flow map, and the VFD recipe stores the fill setting separately from any through-hole recipe on the same machine.
Our standard fill cell runs DC at 500 A / 12 V, one unit per copper cell. On a nine-cell line that is nine rectifiers, each with its own current loop, so a drifting cell is visible in the trend rather than hidden in a shared bus.
DC also keeps the contact simple: no reverse edge, no arcing at the V-seat, no extra cooling. On a bath that runs 24/7 at high current density, fewer things to fail is a specification, not a compromise.
A fill bath draws high current for hours at a time. Soluble anodes under that load change area, shed sludge and need bag changes; the fill profile drifts with them.
IrO2 mesh plus copper oxide powder keeps both anode area and copper concentration flat, so the fill profile at the edge of the panel matches the centre without a shield change.
Three knobs: current ramp at entry so the mouth does not pinch shut, additive dosing by amp-hour, and flow held within ±10% across the panel.
The line logs current, amp-hours and thickness per lot. When a bath ages, the trend shows two days before the section does.
| Parameter | Range / standard |
|---|---|
| Blind via | 3.75 mil (95 µm) minimum, AR 1:1 |
| Fill result | void-free; fill rate >90% with matched chemistry |
| Panel size | up to 24.5 × 28.5 in (622 × 724 mm); minimum 12 × 18 in |
| Panel thickness | 0.1 – 3.2 mm on the standard transport, no frame required |
| Plating thickness | 25 µm fill typical |
| Thickness uniformity | full-panel R value ≤0.3 mil (7.6 µm) |
| Current density | 0 – 30 ASF (0 – 3 ASD) machine range; fill recipes run 5 – 15 ASF |
| Plating time | 150 – 180 min for a 25 µm fill |
| Copper cells | 6 to 12, set by the required m²/hour |
| Rectification | one DC rectifier per copper cell, sized to the plated area |
| Anode | insoluble IrO2/Ti mesh + CuO dissolution |
| Filtration | filter-bag type, shared with the spray pump |
| Control | PLC + HMI + PC, recipe management, alarm and trend logging |
| Panel handling | 6-axis robot load and unload, with a manual load station |
Fill rate and void results depend on the chemistry you run. We commission the line with your supplier's process and hold the guaranteed numbers from a pilot run, not from this table.
The panel range runs from 0.1 mm to 3.2 mm and does not need a frame on the standard transport. Six-axis robots load and unload at both ends, so no operator lifts a loaded frame by hand.
A fill line is time-bound, not speed-bound. The dwell in the copper section is set by how long 25 µm takes (150 to 180 minutes), so the speed is whatever pushes panels through that length in that time:
line speed = copper section length ÷ plating time
m²/hour = panel width (m) × line speed (m/min) × 60 × number of strips
A nine-cell copper section is about 13.5 m long. At a 180 minute fill that is 0.075 m/min, and one 620 mm strip yields 2.8 m²/hour. Ten times the dwell of a pattern plating line gives roughly a tenth of the throughput — which is why fill capacity is bought in cells and strips, and why the number surprises people who compare it with a VCP quotation.
The same platform, with a different flow package and chemistry, plates through holes — that is the VCP plating line, which holds 40:1 with matched chemistry. Where the constraint is dwell time rather than agitation — backplanes thicker than 5 mm, or a heavy copper target — the gantry line is the correct machine.
DC, on a dedicated fill line. Reverse pulses are specified for high-aspect-ratio through holes, where the reverse cycle re-dissolves the hole mouth and improves throw. At 1:1 there is nothing to throw into: the via fills by bottom-up additive action, and a reverse cycle only removes the accelerator you just paid for, while adding arcing at the contact and time to the cycle.
Physically yes — the tank, the anode and the rectifier are the same. Practically, the flow settings are opposite: a through-hole cell wants high jet velocity at the hole mouth, a fill cell wants uniform low velocity so the suppressor stays adsorbed. If more than about 70% of the output is filling, we quote a dedicated fill line and keep the through-hole work on a VCP.
Cross-section nine points per panel plus X-ray on every lot at first article, then a sectioned coupon every 500 panels and after every bath change. We supply the sectioning fixture with the line and train the lab on the measurement, because a fill measurement taken with the wrong preparation is a false pass.
Related lines: VCP plating lines · gantry lines for 40:1 HDI · ENEPIG lines
Send us the board: layer count, panel size, hole diameter and depth, copper target and required m²/hour. We will return a cell count, a rectifier schedule and a pilot-run plan.