Learn how thermal vias move heat through a PCB, how many to use, how to size and space them, when to use via-in-pad, and how to avoid solder wicking.
Short answer: Thermal vias are plated holes placed under or next to a hot component to carry heat from the top copper down to inner planes, the bottom layer or a heat sink. A single via moves only a little heat, so designers use arrays of them: typically 0.25 to 0.35 mm drills on a 1.0 to 1.2 mm grid under a component’s exposed thermal pad, connected to large copper areas on other layers. Good PCB thermal management combines thermal vias with enough copper area, the right copper thickness, solid plane connections and a clear path to the heat sink or enclosure. Use via-in-pad with filled and capped vias when solder wicking is a risk. AI design tools help by flagging hot parts that lack a thermal path and checking via counts, spacing and plane connections against datasheet guidance before fabrication.
A thermal via is an ordinary plated through hole used for heat instead of signals. Standard FR-4 laminate is a poor conductor of heat, at roughly 0.3 W/m·K, while copper is around 400 W/m·K, more than a thousand times better. A via puts a copper barrel straight through the laminate, so heat can travel vertically to copper on other layers, which then spreads it sideways.
Thermal vias matter most for parts that dump heat through their bottom: QFN and DFN packages with an exposed pad, power MOSFETs in D-PAK or PowerPAK packages, LED packages, voltage regulators and motor drivers. Their datasheets usually say the junction-to-ambient numbers assume a specific copper area and a specific number of vias. If you skip them, the part runs hotter than the datasheet suggests.
Heat leaves the die, flows into the exposed pad, then into the top copper. From there it has two routes: sideways through top copper, or down through the vias. Each via acts like a small thermal resistor. Its value depends on the plating thickness, the drill size and the board thickness. A single standard via in a 1.6 mm board has a high thermal resistance, on the order of 100 °C/W, so one via is almost useless on its own.
Vias in parallel work like resistors in parallel: ten vias have roughly one tenth of the resistance of one. That’s why arrays work. The returns shrink after a point, though, because the copper planes the vias connect to also have to spread the heat. A big via array feeding a tiny copper island won’t help much.
These rules cover most power, LED and mixed-signal boards:
Via-in-pad means placing vias inside the component’s solder pad rather than beside it. It gives the shortest thermal path, which is why it’s common under exposed pads. The catch is solder wicking: open vias in a pad pull solder down the barrel during reflow, leaving voids under the part and solder blobs on the other side.
You have three common options:
Tell your fabricator exactly which via treatment you want on the drawing, and check it against your DFM rules.
Copper does most of the heat spreading, so copper thickness and area matter as much as vias. Standard boards use 1 oz copper (about 35 µm) on outer layers. Moving to 2 oz doubles the cross-section for sideways heat flow and current, and is common on power boards. Heavier copper needs wider spacing and costs more, so use it where the heat actually is.
Area matters too. A larger copper pour around a hot part lowers its temperature noticeably, up to a point. Inner planes are excellent heat spreaders because they cover the whole board, which is another reason to plan thermal paths as part of the stackup.
Copper and vias have limits. When a part dissipates more than a few watts, or the board sits in a sealed enclosure, you’ll often need a PCB heat sink. Common approaches are a clip-on or adhesive heat sink on top of the package, a heat sink or metal chassis pressed against the bottom copper through a thermal pad, or a metal-core PCB for high-power LEDs. In each case, the thermal vias carry heat to the surface where the heat sink sits, so the via array and the exposed bottom copper must line up with it.
Thermal problems are easy to miss because they don’t show up in a DRC. An AI design assistant can read the thermal guidance in each component’s datasheet, list the parts that need a thermal path, and check that every exposed pad has the recommended via count, drill size and plane connections. It can flag thermal relief connections on heat-spreading pads, estimate copper area around hot parts, and suggest via-in-pad treatments that match your fabricator’s capabilities. During component selection, it can also compare packages by thermal resistance, so you pick a part that is easier to cool in the first place.
Start with the number in the component datasheet. Without guidance, fill the exposed pad with an array of 0.3 mm vias on a 1.0 to 1.2 mm pitch, then confirm with a thermal estimate or measurement. More vias help, but only if they connect to enough copper.
Most designs use finished holes of 0.25 to 0.35 mm. Smaller vias fit more densely and wick less solder, while larger vias carry more heat each. Check your fabricator’s minimum drill and aspect ratio.
Fill and cap them when they sit inside a solder pad under fine-pitch or high-power parts, to prevent solder wicking and voids. Vias beside the pad can usually stay open.
No. Thermal vias should connect solidly to planes so heat can flow. Thermal reliefs are for pads that need easier soldering, not for heat paths.
AI can find hot components from their datasheets, check via arrays, copper area and plane connections around each one, flag thermal relief on heat paths, and suggest via-in-pad options before fabrication.
Want every hot component checked for a real thermal path while you design? Talk to the IntelCAD team about AI-assisted PCB design and component selection for your next board.
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