Thermal relief

Should a ground pin tie into the plane through a thermal relief?

Yes, if it gets soldered. On a 4-layer board with the fab’s default copper, a 402 °C iron melts the solder on the far side through KiCad’s default spokes in 3 s. Tied in solid, it needs 536 °C. A Hakko FX-888D tops out at 480 °C.

The same iron, the same pin

From 0.5 to 5 s

Past 217 °C
Every 50 °C

A section through a 2.54 mm header pin, between the spokes. Lead-free solder melts past 217 °C. The far side is the pad on the bottom layer.

What a thermal relief is

KiCad’s default, on a header pin

A through-hole ground pin’s pad sits in the ground copper on every layer it passes through. Tied in solid, that copper pulls the iron’s heat out sideways before the far side gets hot enough to melt the solder. A thermal relief cuts a gap round the pad and bridges it with a few thin spokes. KiCad’s default is four spokes 0.5 mm wide across a 0.5 mm gap.

From above

The top layer round the pin’s 1.7 mm pad. Every ground layer gets the same spokes.

Through the board

Half the section through the pin, between the spokes, on 4 layers with the fab’s default copper. Every ground layer has the gap.

How hot the iron has to be

Relief
Solid

The iron behind 7 K/W, taken for a 1.6 mm chisel tip with a solder bridge.

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Thermal relief

Solid needs a 536 °C iron

Field marks. With KiCad’s default relief, 402 °C melts the far side in 3 s on a 4-layer board with the fab’s default copper.

Look-alike. A 2-layer board is easier, 370 °C against 475. Its pin only touches copper on the two outer layers.

Range. More copper makes it harder. With 1 oz inner planes it’s 434 °C against 596. A big chisel tip lowers every number by 8 to 15%, and a fine cone raises them by 14 to 25%. The relief always wins.

Habitat. A 2.54 mm header pin on JLCPCB’s 1.6 mm boards, with ground poured on the outer layers and a ground plane on layer 2.

169

Set the iron yourself

3 boards, 300 to 500 °C

Board

Iron

400 °C

Thermal relief

4 layers, fab default, iron at 400 °C

Relief
Solid
Solder melts, 217 °C

Relief melts in: 3.3 s
Solid melts in: Over 15 s

The far side of the same header pin, with the iron behind 7 K/W. The 4-layer board has the fab’s default copper.

What the spokes cost later

Relief
Solid

Once the joint is made: the temperature rise per watt from the pad to the air, with a 50 mm square board cooling off both faces.

170

Thermal relief

The spokes add 7 to 10 K/W to a pin

Field marks. If a part dumps 1 W through the pin, it runs 7 to 10 °C hotter with the relief.

Look-alike. Current barely notices. The spokes add 54 to 81 µΩ to the board’s edge. At 10 A that’s 5 to 8 mW.

Range. A via with a relief pays 5 to 7.5 K/W and 34 to 52 µΩ. Nobody solders a via, so it gains nothing.

171

Where each belongs

Pins and vias

A pin and a via

A pin you solder, and a via you don’t, at one scale.

172

Key to ground connections

Should this pad get a thermal relief?

1aNobody solders itSolid, like a via, 7e
1bIt gets solderedGo to 2
2aIt has to shed heatSolid, and a bigger iron
2bIt only needs a ground connectionA relief, 7e

KiCad puts reliefs on pads in a zone by default and ties vias in solid unless the zone is hatched.

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Conditions

Board: 50 by 50 mm, JLCPCB’s 1.6 mm stackups. On 4 layers, 0.2 mm of 7628 prepreg under each outer layer and a 1.065 mm core.
Copper: Ground poured on the outer layers, a ground plane on layer 2 and a power plane on layer 3. 1 oz outside, 0.5 oz inside as the fab’s default or 1 oz.
Pin: KiCad’s 2.54 mm header pad, 1.7 mm on a 1.0 mm hole, with a 0.64 mm brass pin and the rest of the hole full of SAC305. 20 µm of plating.
Via: 0.6 mm on 0.3 mm, KiCad’s default, empty
Relief: KiCad’s zone default on every ground layer: 4 spokes 0.5 mm wide across a 0.5 mm gap
Iron: A fixed tip temperature behind 7 K/W, on the pad and the pin. 4 and 12 K/W move every number but never the order. A big chisel tip is about 4 K/W and a fine cone about 12.
Far side: The average over the bottom pad. Melted is 217 °C, where SAC305 starts to melt.
Solver: A 3D finite-volume heat model over a quarter of the board, 0.05 mm cells near the hole, stepped in time
Check: Halving the cells near the hole moves the steady K/W by 0.2%

Simulated only. Nothing on this page has been measured on a board yet.

Limits

Material properties don’t change with temperature, and the solder’s latent heat isn’t modeled. That keeps the model linear, so other iron settings are scaled from 350 °C runs.
The iron holds its tip temperature. A real tip cools as it pours heat in.
The pad is round and the spokes run along x and y.

Files

tsim/model.pyThe heat model
tsim/run.pyThe soldering runs, K/W and µΩ
tsim/iron.pyHow hot the iron has to be

Sources

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Rev 1 · 2 Nov 2026

Rev 12 Nov 2026First published

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