Via in Pad: Filling, Planarization and Assembly Rules

Via in pad places the via inside the solder pad so the ball or the printed paste lands on the same feature that carries the signal to the next layer. The arrangement saves routing space on dense boards, but it moves a hole into the wettable area, and that changes what the assembly process has to do.

Why an Unfilled Via Causes Trouble

A via that is open at the pad surface acts as a reservoir. Paste printed across it flows down the barrel during reflow, so the joint that forms holds less metal than the stencil aperture was designed to deposit. The joint can pass a visual check and still fail later.

The same mechanism produces paste wicking along the barrel wall, which is rarely visible from above. Our via filling notes describe the fabrication side of the same problem.

Fill Materials and Planarization

Conductive and non conductive fills are both used. A copper filled via is plated over and can be treated as a normal pad; a resin filled via is cured, sanded back and capped with plating so that the surface is flat enough to print on.

Planarization is the step that decides whether the pad is usable. A fill that sits proud of the surface holds the stencil away from the pad, and a fill that sits below it leaves a dimple where paste collects. Our plating notes describe the cap that follows the fill.

Design Rules for Via in Pad

The pad should be at least the via diameter plus two times the annular ring, and the fill should be specified together with the planarization tolerance. A via placed on the pad edge is a different problem, because the fill has no wall to bond to on one side.

Tented and plugged vias are not the same as filled vias, and the difference matters for yield. Our hole types notes set out which construction suits which requirement.

Paste Printing and Stencil Aperture

Because a planarized via in pad presents a copper surface with a small dimple, the aperture can be sized like a normal pad. Where the cap is slightly domed, the aperture is reduced and the print is verified by volume rather than by area.

The print should be checked for paste wicking at the first article, and again whenever the fill supplier changes. Our paste volume notes describe the measurement.

Solder Voiding and Entrapment

Flux vapour and any residual solvent in the fill are the two sources of solder voiding under a via in pad joint. A fill that is not fully cured outgasses during reflow, and the gas has nowhere to go once the ball has collapsed.

The cure schedule and the reflow profile therefore belong to the same discussion. Our voiding notes describe the acceptance limits that apply.

Inspection and Acceptance

X-ray is the practical inspection for a via in pad joint, because the barrel and the cap sit under the ball. The image should be reviewed against a sample from the same fill and profile rather than a generic reference.

Where the voiding limit is tight, the fill batch should be recorded so that a change in supplier can be traced. Our X-ray notes describe the technique.

Cost and Process Selection

Filling adds process steps and it is priced per hole, so it is usually applied only where the routing gain is real. A design that can move the via out of the pad should do so, and one that cannot should specify the fill properly rather than leaving it to the shop.

The decision is best made at layout review, when the alternative is still available. Our design release notes list the points to confirm.

Verification

The verification is a filled and planarized sample measured for dimple depth, followed by a print volume check and a reflowed joint examined by X-ray. The three measurements together confirm that the fill, the stencil and the profile agree.

Records from that sequence are the evidence that a later change did or did not affect the joint. Our first pass yield notes describe how the data is used.

Process Control and Verification

On a design of this kind, paste wicking is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

Process Control and Verification

On a design of this kind, paste wicking is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

Process Control and Verification

On a design of this kind, paste wicking is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Is via in pad always filled? For a paste printed pad the via has to be filled and planarized, because an open barrel wicks paste away during reflow. The exception is a via placed beside a pad rather than on it.

Does a copper filled via cost more? Yes, because the plating and planarization steps are longer. A resin fill with a plated cap is the usual choice where the thermal path is not the deciding factor.

What does gopcb provide for via in pad work? We provide filled and planarized vias to a stated dimple tolerance, cap plating to a measured thickness, stencil apertures sized against the measured cap, print volume verification at first article, and X-ray review against a sample from the same fill batch.