Designing a Power Supply PCB: Copper Pours, Thermal Reliefs and Vias

A power supply section has two layout problems that ordinary signal routing does not: it must carry significant current without overheating, and it must get rid of the heat that regulators produce. Switching regulators add a third problem, fast-changing currents that radiate noise if the layout is careless. Copper pours, vias and zone settings in KiCad are the tools that solve all three.

This article starts with the thermal arithmetic for a linear regulator, then covers copper pours, thermal relief settings, thermal vias and the hot loop of a buck converter.

Start With the Power Dissipation

A linear regulator drops the difference between input and output voltage across itself and turns it into heat. Calculate this first, because it decides whether you need a large copper area, a heatsink or a switching regulator instead.

P = (Vin - Vout) x Iload

Example: 12 V in, 5 V out, 0.5 A load
P = (12 - 5) x 0.5 = 3.5 W

Junction temperature:
Tj = Ta + P x theta_JA

theta_JA = junction-to-ambient thermal resistance (C/W),
           from the datasheet for your package and copper area

If a datasheet gave theta_JA = 50 C/W and Ta = 40 C:
Tj = 40 + 3.5 x 50 = 215 C  -> far above the usual 125 C limit

The example shows why a 12 V to 5 V linear regulator at half an amp is a bad idea without a serious heatsink. The fix is either to reduce the voltage drop, reduce current, or use a buck converter whose losses are a small fraction of output power. Notice also that theta_JA in datasheets depends on the copper area under the package, which is exactly what your layout controls.

Copper Pours as Conductors and Heatsinks

A copper pour is a filled zone tied to a net. On a power board, pours replace wide tracks for input, output and ground, and they spread heat away from components. In KiCad 8 use Place > Add Filled Zone, choose the layer and net, and set properties.

Zone setting	What it does	Power board suggestion
Clearance	Gap from the zone to other nets	At least your design rule; more for high voltage
Minimum width	Thinnest copper section allowed in the fill	0.25 mm or more to avoid fragile slivers
Pad connections	Thermal reliefs, solid, or none	Solid for high current pads, reliefs for hand soldered parts
Thermal relief gap	Gap between pad and surrounding zone	About 0.5 mm
Thermal spoke width	Width of each spoke joining pad to zone	Wide enough for the pad current
Zone priority	Which zone wins where zones overlap	Higher for smaller, local pours
Island removal	What happens to unconnected fragments	Always remove

Put the regulator's tab or exposed pad on a large pour of the net it is connected to; for many regulators this is the output or ground, so check the datasheet. Mirror the pour on the opposite layer and join them with vias to double the heat-spreading area.

Thermal Reliefs: When to Use Them

A thermal relief connects a pad to a pour through a few narrow spokes instead of solid copper. The spokes limit how fast heat flows from the pad into the plane, so a soldering iron can bring the joint up to temperature. Without reliefs, a through-hole ground pin on a large plane can be very hard to solder by hand and often produces cold joints.

Use reliefs for through-hole pins and small SMD pads on large planes, especially if the board will be hand soldered.
Use solid connections for pads that carry high current or must dump heat, such as regulator tabs, MOSFET drains and exposed thermal pads.
If reliefs are needed on a high current pad, widen the spokes so their combined cross-section can carry the current; four spokes are typical.
Set connection style per pad in footprint or pad properties when one part needs different treatment from the zone default.

Thermal Vias Under Exposed Pads

Packages such as QFN, SOT-223, DPAK and TO-263 dissipate most of their heat through the bottom pad. FR-4 is a poor conductor of heat, so a grid of vias under the pad carries heat to the copper on the other side. Follow the package datasheet for via count and size where it is given. Small drills, often around 0.3 mm, are preferred under pads that will be reflow soldered because larger holes can wick solder away from the joint; some designs tent or plug the vias for this reason.

Buck Converter Hot Loop

In a buck converter the current in the input loop switches on and off every cycle. The loop formed by the input capacitor, the high-side switch and the low-side switch or diode carries sharp current edges and must be as small as physically possible.

Place the input ceramic capacitor right at the converter's VIN and GND pins.
Place the catch diode or low-side switch so the loop from input capacitor through switch and diode back to capacitor ground is tiny.
Keep the switch node (SW) copper area small; it is a strong noise source. Make it just large enough for the current.
Place the inductor next to SW and the output capacitor right after the inductor.
Route the feedback trace from the output capacitor, away from the inductor and SW node, and place the divider resistors near the FB pin.
Keep a continuous ground plane under the whole converter.

Summary

Calculate dissipation first with P = (Vin - Vout) x I and check junction temperature with theta_JA. Use copper pours for current and heat, solid connections for high current and thermal pads, thermal reliefs for pins you will hand solder, and via arrays under exposed pads. For switching regulators, shrink the input hot loop and the switch node and route feedback away from noise. These habits make supplies that stay cool and quiet.