PCB Thermal Via vs Filled Via: Key Differences

Two Ways to Use a Via

Vias are usually thought of as signal paths, but in a power or high brightness design they are also thermal hardware. Two terms come up constantly: the thermal via and the filled via. They are not opposites, and they are not the same thing. A thermal via describes what the via is for, moving heat away from a hot component. A filled via describes how the via is made, with its barrel plugged by conductive or non-conductive material and plated over. A design can use one, the other, or both at once, and confusing them leads to layouts that do not remove the heat they were supposed to.

What a Thermal Via Is

A thermal via is an ordinary plated hole placed under or around a heat generating component to conduct heat down to an internal or bottom copper plane, where the copper spreads it and the ambient air or a heatsink removes it. Thermal vias are most effective in an array, because heat flow is roughly proportional to the total copper cross section. A single via carries little heat; a grid of twenty small vias under a power pad can carry a great deal. The copper planes they connect to matter as much as the vias, because a via can only deliver heat to a plane that is large enough to spread it.

What a Filled Via Is

A filled via is a via whose barrel has been filled with conductive epoxy, non-conductive epoxy or plated copper, then planarised and usually plated over so the surface is flat. Filling is done for three reasons: to allow a via to sit inside a solder pad without wicking paste, to improve thermal and electrical conduction relative to an empty barrel, and to provide a flat surface for a component to sit on. Conductive fill adds a real thermal and electrical path; non-conductive fill is mainly structural, closing the barrel so solder cannot drain through it.

How They Differ

Purpose. Thermal vias move heat; filled vias close or reinforce a barrel, usually for assembly or conduction reasons. Fabrication. Thermal vias are simply plated holes. Filled vias require an extra filling, planarisation and plating process. Electrical. An empty thermal via carries current on its plated wall only; a conductive filled via carries current through the fill as well. Thermal. A filled via with conductive material transfers heat better than an empty barrel, because the cross section is solid rather than a thin wall. Assembly. An open barrel in a solder pad causes wicking; a filled and capped via does not. Cost. Thermal vias are essentially free beyond the drill and plating cost. Filled vias add a dedicated process step and a clear premium.

Thermal Performance in Practice

The thermal resistance of a via depends on its wall thickness, its length and the number of vias in the array. Making vias larger and fewer is not automatically better: many small vias often fit under a component pad, which is where the heat source actually is, so the array geometry matters more than the individual diameter. What limits performance in practice is usually the receiving end. If the heat is dumped into a small internal plane with no path to the outside, the junction temperature stays high no matter how many vias are used. A thermal design therefore budgets copper area on the far side, not only via count.

Via-in-Pad and Solderability

When a thermal via grid sits directly inside a component pad, the design becomes a via-in-pad problem as well as a thermal one. An open barrel under a paste deposit will wick solder during reflow, starving the joint. The usual answer is to fill and cap the vias, which keeps the thermal path and removes the wicking risk at the same time. For components where solderability is not a concern, such as a pad that is only glued or clamped, open thermal vias remain perfectly acceptable.

Design Rules

Place vias directly under the heat source. Heat spreads laterally through copper, so distance costs performance. Use an array, not a single via. Total cross section is what matters. Keep the receiving plane large. A via is only as good as the copper it feeds. Tie the vias to a plane that can reach the outside. Heat has to leave the board somewhere. Fill and cap vias that sit in solder pads. This prevents wicking and improves conduction. Respect the drill to pad rule. Vias under a pad still need adequate annular ring or a filled design. Check for solder mask over the vias. Tenting over a thermal array traps air and wastes the path.

Thermal vias and filled vias are both manufacturing decisions, so they belong in the conversation with the fabricator rather than in the layout alone. Review how PCB manufacturing fills and plates vias for thermal designs, apply the via rules in your PCB design and layout, and check the design and manufacturing considerations before release. A prototype PCB assembly run with thermal measurement confirms that the junction temperature actually meets the budget.

Cost

Thermal vias add only drill and plating cost, so they are cheap insurance on any board with a power device. Filled vias add a separate process, so they are specified where they are needed: inside solder pads, under fine pitch components and where the thermal or electrical gain justifies the step. A common and economical compromise is to fill only the vias that sit in paste areas and leave the remaining thermal vias open.

FAQ

Is a thermal via always filled? No. Thermal vias can be open barrels. They are only filled when solderability or extra conduction requires it.

Does filling a via improve heat transfer? Yes, if the fill is conductive, because the cross section becomes solid instead of a thin plated wall.

How many thermal vias do I need? As many as fit under the heat source. Total copper cross section and the receiving plane area matter more than the count alone.

Can open thermal vias cause assembly problems? Only if they sit inside a solder pad, where they can wick paste. Elsewhere they are harmless.

Conclusion

A thermal via is about where the heat goes, and a filled via is about how the barrel is built. Use thermal via arrays under every power device, give them a copper plane that can actually spread and release the heat, and fill the vias that sit inside solder pads so the joint is not starved. Design them together and the board will run cooler and assemble cleanly in 2026.