Via-in-Pad Solder Wicking: Causes and Solutions

What Via-in-Pad Means

A via-in-pad is a plated hole placed directly in the centre of a component land rather than beside it. The technique frees routing space under dense BGA and QFN packages, shortens the current path and improves thermal transfer, which is why it appears in almost every high density design. The cost of that benefit is a process risk: the same hole that carries the signal can also swallow the solder paste during reflow, leaving the joint starved. That failure is called solder wicking.

What Solder Wicking Looks Like

In a wicking defect the molten solder flows down the barrel of the via instead of staying on the pad. The joint ends up thin, voided or completely open, and solder often reappears as a ball or a bump on the opposite side of the board. Because the loss happens during reflow, the defect is invisible after paste printing and only shows up in X-ray or electrical test, which makes it expensive to catch late.

Why It Happens

Capillary action. A small open hole pulls molten solder downwards, exactly as a wick pulls up oil. Unfilled or untented vias. An open barrel is an open path for the solder to leave the pad. Excess paste volume. A stencil aperture that is too large delivers more solder than the joint needs, and the surplus finds the hole. Aggressive preheat. Long or hot preheat lowers viscosity early and gives the solder more time to drain. Poor pad geometry. A large pad with a big central hole has a short distance between the paste and the barrel. Slumping paste. Paste that flows before reflow migrates towards the via opening. Reflow vacuum or vibration. Some ovens and conveyors disturb the molten joint at the wrong moment.

What It Costs

A starved joint does not always fail immediately. It may pass functional test and then crack in the field under thermal cycling, because the remaining solder fillet is too thin to absorb the stress. Wicking also creates solder balls on the back side that can bridge to nearby features, and it can leave voids that reduce thermal performance under the very pad the design added for heat. In high reliability products, the defect is a genuine field failure risk rather than a cosmetic issue.

Fix One: Fill or Cap the Via

The most reliable solution is to remove the open barrel. Conductive or non-conductive epoxy filling followed by planarisation and copper plating produces a flat, plated surface that behaves like solid pad, with no cavity for the solder to enter. This is the standard approach for via-in-pad in high density designs. Where filling is not available, tenting, which covers the via opening with solder mask, reduces but does not eliminate wicking, and it cannot be used when the via must be tested or reworked. For vias that only carry signals and do not sit in a paste area, tenting is usually enough; for vias inside a paste deposit, fill and cap is the correct answer.

Fix Two: Stencil and Paste Volume

Reduce the amount of paste that can find the hole. Use the smallest aperture that still forms a good fillet, and consider splitting a large aperture into several smaller ones so the paste has less chance to flow towards the centre. Some designs use a paste-reducing feature such as a window pane pattern over the via. Keep the stencil thickness matched to the component pitch, because an unnecessarily thick stencil simply delivers extra volume. Aperture area ratio and release quality matter as much as total volume, since a ragged release leaves paste smeared towards the via.

Fix Three: Reflow Profile and Handling

Shorten the soak and avoid the hottest possible preheat. A profile that reaches the liquidus quickly, with a controlled ramp and a limited time above liquidus, gives the solder less opportunity to drain. Avoid mechanical disturbance, unnecessary vibration and vacuum-assisted steps on boards with unfilled via-in-pad. Handle panels gently before reflow as well, because slumping starts at room temperature if the paste is warm or the print is heavy.

Inspection and Test

Wicking is a hidden defect, so inspection has to look inside. X-ray is the standard tool: it reveals voids, thin joints and solder on the far side of the board. Cross-sectioning a sample from each lot is the reference method, because it shows the actual barrel and fillet. Electrical test catches open joints but misses partially starved ones, so combine continuity testing with X-ray and, where the product demands it, thermal cycling of a sample to expose marginal joints.

Wicking is a design and process problem at the same time, so the fix belongs in both places. Review how PCB manufacturing fills and caps vias, keep the via placement and pad geometry aligned with your PCB design and layout, and check the design and manufacturing considerations before release. A prototype PCB assembly run with X-ray inspection is the cheapest way to prove the via-in-pad process before volume.

Prevention Checklist

Fill and cap every via that sits inside a paste deposit. Tent signal-only vias that are not in a paste area. Minimise stencil aperture size and paste volume. Match stencil thickness to pitch. Shorten the soak and limit time above liquidus. Avoid vacuum and vibration during reflow. Inspect with X-ray and cross-section samples from each lot.

FAQ

Can tenting stop solder wicking? It reduces it, but only fill and cap reliably prevents wicking for vias inside a paste deposit.

Is conductive or non-conductive fill better? Both work mechanically. Conductive fill also improves thermal and electrical performance where the via carries heat or current.

Why does solder appear on the other side of the board? Because the molten solder travelled down the open barrel and emerged on the far side, which is the clearest sign of wicking.

Can X-ray see every wicking defect? No. X-ray finds most voids and thin joints, but cross-sectioning remains the reference method for a starved fillet.

Conclusion

Solder wicking happens because molten solder follows the easiest path, and an open via-in-pad is the easiest path there is. Close that path with fill and cap, keep paste volume to the minimum the joint needs, control the reflow profile so the solder solidifies quickly, and inspect with X-ray plus cross-sections. Do those four things and via-in-pad stays the layout advantage it was meant to be in 2026, not a hidden reliability risk.