The via treatment has to be stated in the fabrication note. If it is missing or vague, engineering review raises a query and the job is held until it is confirmed in writing.
| Treatment | Barrel condition | Typical use | Main risk |
|---|---|---|---|
| Open (untented) | Hole mouth and annular ring exposed | Test points, vias that must be soldered | Solder wicking, bridging to nearby pads |
| Tented | Mask forms a film over the mouth; barrel not required to be filled | Ordinary signal vias, preventing accidental soldering | Mask drawn into small holes |
| Solder-mask plugged | Mask ink fills and seals the barrel | Vacuum test, stopping solder at wave, support under a part | Large holes do not fill; one-sided plug leaves a blind cavity |
| Resin filled + plated cap | Resin filled, cured, planarized, copper plated over | Via-in-pad, buried vias, fine-pitch parts needing a flat surface | More process steps; must be planned in the stack-up |
1. Open vias: only where exposure is needed
An open via has no mask over its ring or mouth. It suits test points and vias that are meant to take solder. Open vias next to pads can draw solder away during reflow or bridge to the neighbouring pad. Do not leave vias open by default.
2. Tenting: small holes pull the mask in
Tenting asks the mask to form a film over the hole mouth while the barrel copper stays continuous and testable. Below roughly 10 mil, capillary action draws the mask into the barrel. Add normal mask misregistration (±0.076 mm in the SKPCB capability table) and some holes end up blocked with ink while others stay clear.
The SKPCB engineering query offers two fixes:
- Change to plugged: accept that ink enters the hole and specify plugging, giving one consistent, inspectable sealed condition.
- Open the mask about 5 mil larger per side: the mask edge sits clear of the mouth and the hole stays clear. The cost is an exposed copper ring, whose clearance to nearby conductors must be checked.
3. Solder-mask plugging: large holes do not fill
Plugging is usually requested for three reasons: the board must hold vacuum in test, solder must not come through at wave, or a part sits over the hole and needs a supported surface.
Above roughly 30 mil, the ink sinks under its own weight before cure. The hole is left with a void or a film that breaks in the middle, and none of the three goals is met. Two fixes:
- Reduce the finished hole and plug it: confirm the smaller diameter does not affect the electrical or assembly purpose.
- Change to tented: the hole size stays, but the barrel is no longer sealed, so it cannot be used for vacuum test or to stop solder at wave.
4. One-sided plugging: solder balls on HASL boards
Plugging one face and leaving the other open turns the via into a blind cavity. During HASL, molten solder enters the barrel and cannot escape, leaving a solder ball after cooling. With one end sealed, cleaning solution and air cannot circulate, and visual inspection cannot see the bottom. If the ball later breaks loose it can short adjacent pads.
The two options in the SKPCB engineering query:
- Keep the one-sided plug: accept the solder balls in writing as a permitted condition of the build.
- Open a small window of about 3 mil on the plugged face: the barrel connects at both ends, vents during HASL, and solder does not stall inside. That face stays essentially mask-covered.
With a finish other than HASL, the behaviour differs and must still be confirmed in review against the actual finish.
5. Resin filling with a plated cap: when it is required
Solder-mask plugging and resin filling are not interchangeable. Their materials, functions and acceptance criteria differ. Use resin filling for:
- Via-in-pad: the via in the pad must be filled flat and capped with copper, otherwise solder drains into the hole, giving starved joints or voids.
- Buried vias and sequential lamination: inner vias must be filled before pressing, or the next lamination leaves a dimple or void above them.
- Fine-pitch parts needing a flat surface: BGAs and similar packages are sensitive to pad flatness.
SKPCB lists screen-printed and vacuum resin plugging, with vacuum equipment below 50 Pa absolute pressure, for buried, small, thick-board and via-in-pad vias, subject to stack-up review. See resin-plugged via-in-pad process order and resin-filled via acceptance.
6. Choosing by purpose
| Purpose | Recommended treatment | Note |
|---|---|---|
| Test points, vias to be soldered | Open | Open only the holes that need exposure |
| Ordinary signal vias | Tented | If the hole is too small, plug it or enlarge the opening |
| Vacuum test, stopping solder at wave | Solder-mask plugged (both sides) | If the hole is too large, reduce it or tent it |
| Vias close to pads | Tented or plugged | Check the mask dam too; SKPCB minimum is 0.1 mm |
| Via-in-pad, buried vias, fine-pitch BGA | Resin filled + plated cap | Plan it in the stack-up |
7. Writing the fabrication note
Most queries come from three vague notes: "tent all vias" with no size split, "plug vias" without saying one side or both, and solder-mask plugging mixed up with resin filling. Instead:
- Group holes by diameter or function and state a treatment for each group, rather than one line for the whole board.
- For plugging, state one side or both, and the final mask condition on each face.
- Put resin-filled vias on their own layer or drill table and say whether a plated cap is required.
- State the acceptance requirements: vacuum test or not, allowed dimple or protrusion, microsection or not.
- Keep hole definitions and units consistent across Gerber, drill, stack-up and impedance files.
The full query triggers and options are in the engineering query reference. Via arrays under thermal pads need extra care; see thermal via array plugging.
FAQ
Q: What is the difference between via tenting and via plugging?
A: Tenting only asks the mask to film over the hole mouth; plugging asks the ink to fill and seal the barrel. A tented via cannot be used for vacuum test.
Q: How small is too small to tent?
A: Below roughly 10 mil, SKPCB raises an engineering query because the mask is drawn into the hole. Change it to plugged, or open the mask about 5 mil larger per side.
Q: How large is too large for solder-mask plugging?
A: Above roughly 30 mil the ink does not fill reliably. Reduce the finished hole and plug it, or change it to tented.
Q: Why avoid one-sided plugging on HASL boards?
A: A one-sided plug forms a blind cavity; HASL solder enters and cannot escape, leaving a solder ball. Add a vent window of about 3 mil on the plugged face, or accept the balls in writing.
Q: Can solder-mask plugging replace resin filling?
A: No. Via-in-pad, buried vias and structures needing a flat surface call for resin filling with a plated cap, and the two must be specified separately.