DC-DC｜Design

Placement of Thermal Vias

2021.04.07

Points of this article

・When PCB mounting alone results in inadequate heat dissipation, thermal vias are provided to conduct heat to the opposite side of the board and reduce thermal resistance.

・In order to improve thermal conductivity, it is recommended that thermal vias have a small inner diameter of approx. 0.3 mm, enabling filling with plating.

・If the hole diameter is too large, solder suction problems occur in the reflow soldering process.

・The thermal via interval should be about 1.2 mm, with the vias positioned directly below the heat sinks on the bottom surfaces of IC packages.

・If thermal vias below IC bottom-surface heat sinks are by themselves inadequate for heat dissipation, thermal vias should also be provided on the periphery of the ICs.

・When an IC bottom-surface heat sink is at ground potential, a large-area copper foil trace can be used without adverse EMI effects.

Up to this point, we have explained placement of input capacitors, output capacitors and freewheel diodes, and inductors. In this article we address placement of thermal vias, which play an important role in heat dissipation.

Placement of Thermal Vias

The copper foil area of a printed circuit board contributes to heat dissipation, but in general the foil thickness is insufficient, and so for boards larger than a certain area, the heat dissipation is inadequate for the board area. In such cases, thermal vias are used to effectively cause heat to be conducted to the opposite side of the board, reducing the thermal resistance.

As thermal vias, small-diameter vias with an inner diameter of about 0.3 mm, which can be filled with plating to enhance thermal conductivity, are recommended. If the hole diameter is too great, problems with solder suction occur in the reflow soldering process. Intervals between thermal vias should be about 1.2 mm, and they should be positioned immediately below the heat sinks on the bottom surfaces of IC packages.

When adequate heat dissipation cannot be achieved by positioning thermal vias only directly below the bottom heat sinks, thermal vias are also provided on the periphery of the ICs. When a bottom heat sink is at ground potential, there are no adverse EMI effects even when a broad copper foil trace is used.

Below, an example of simulations of the heat dissipation effect of thermal vias is shown. The simulation indicates that by providing thermal vias directly below ICs, a temperature reduction of about 15°C can be expected.

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