--- title: "Should a ground pin tie into the plane through a thermal relief?" topic: "Thermal relief" answer: "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." url: "https://nearfieldnotes.com/notes/thermal-relief/" site: "Near Field Notes" author: "Michael Walker" note: 7 family: "Board and fab" published: "2026-11-02" revised: "2026-11-02" revision: 1 video: "https://nearfieldnotes.com/videos/thermal-relief/" --- Note 7 · [Fab](https://nearfieldnotes.com/families/fab/) · Rev 1 · [Watch the video · 0:14](https://nearfieldnotes.com/videos/thermal-relief/) Thermal relief # Should a ground pin tie into the plane through a thermal relief? **Short answer:** 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. **Note 7’s opening figure: The same iron, the same pin.** From 0.5 to 5 s ![Note 7’s opening figure: The same iron, the same pin](https://nearfieldnotes.com/notes/thermal-relief/fig-opening.png) *What the figure shows:* Two sections through a header pin on a 4-layer board, an iron at 400 °C on its pad. In the first the pad is tied into the ground copper solid, in the second through a thermal relief, which leaves a gap round the pad on each ground layer. The region past 217 °C runs down the hole from the iron. At 0.5 s the far side is 138 °C solid and 194 °C with the relief. At 1.0 s the far side is 149 °C solid and 204 °C with the relief. At 2.0 s the far side is 159 °C solid and 212 °C with the relief. At 3.0 s the far side is 166 °C solid and 216 °C with the relief. At 3.3 s the far side is 168 °C solid and 217 °C with the relief. At 5.0 s the far side is 175 °C solid and 221 °C with the relief. With the relief it melts at 3.3 s. Solid never melts in 5 s. **Key** - 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. **Fig. 7: From above.** ![Fig. 7: From above](https://nearfieldnotes.com/notes/thermal-relief/fig-7.png) *What the figure shows:* Figure 7: a header pin's 1.7 mm pad from above, in the ground pour. On the left it's tied in solid, copper all the way round. On the right a thermal relief: a 0.5 mm gap round the pad, bridged by four spokes 0.5 mm wide. The top layer round the pin’s 1.7 mm pad. Every ground layer gets the same spokes. **Fig. 7b: Through the board.** ![Fig. 7b: Through the board](https://nearfieldnotes.com/notes/thermal-relief/fig-7b.png) *What the figure shows:* Figure 7b: half of the 4-layer board in section, from the hole's center out, with the relief. Ground poured on the top and bottom layers in 1 oz copper, a 0.5 oz ground plane on layer 2 and a 0.5 oz power plane on layer 3. Each ground layer stops 0.5 mm short of the pad, except where the spokes cross. The power plane clears the pad by the same 0.5 mm. The hole is full of the pin and solder. Half the section through the pin, between the spokes, on 4 layers with the fab’s default copper. Every ground layer has the gap. **Fig. 7c: How hot the iron has to be.** ![Fig. 7c: How hot the iron has to be](https://nearfieldnotes.com/notes/thermal-relief/fig-7c.png) *What the figure shows:* Figure 7c: the iron temperature that melts lead-free solder on the far side of the pin in 3 s, solid and with the relief. 2 layers: 475 °C solid, 370 °C with the relief. 4 layers, fab default: 536 °C solid, 402 °C with the relief. 4 layers, 1 oz inside: 596 °C solid, 434 °C with the relief. A Hakko FX-888D's top setting, 480 °C, is marked. The iron behind 7 K/W, taken for a 1.6 mm chisel tip with a solder bridge. 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. ## Set the iron yourself *3 boards, 300 to 500 °C* **Note 7’s explorer: Thermal relief.** ![Note 7’s explorer: Thermal relief](https://nearfieldnotes.com/notes/thermal-relief/fig-explore.png) *What the figure shows:* The far side's temperature against time with the iron on, solid and with the relief, for the chosen board and iron setting, with the 217 °C where lead-free solder melts. The relief always gets there first. 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. **Fig. 7d: What the spokes cost later.** ![Fig. 7d: What the spokes cost later](https://nearfieldnotes.com/notes/thermal-relief/fig-7d.png) *What the figure shows:* Figure 7d: the thermal resistance from the pad to the air once the joint is made, solid and with the relief. Pin, 2 layers: 37 K/W solid, 47 with the relief. Pin, 4 layers, fab default: 32 K/W solid, 41 with the relief. Pin, 4 layers, 1 oz inside: 30 K/W solid, 37 with the relief. Via, 2 layers: 53 K/W solid, 60 with the relief. Via, 4 layers, fab default: 46 K/W solid, 52 with the relief. Via, 4 layers, 1 oz inside: 43 K/W solid, 48 with the relief. 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. 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. ## Where each belongs *Pins and vias* **Fig. 7e: A pin and a via.** ![Fig. 7e: A pin and a via](https://nearfieldnotes.com/notes/thermal-relief/fig-7e.png) *What the figure shows:* Figure 7e: at one scale, a header pin's 1.7 mm pad tied in through a thermal relief, and a 0.6 mm via tied in solid. A pin you solder, and a via you don’t, at one scale. Key to ground connections ### Should this pad get a thermal relief? 1. 1a Nobody solders it Solid, like a via, 7e 2. 1b It gets soldered Go to 2 3. 2a It has to shed heat Solid, and a bigger iron 4. 2b It only needs a ground connection A relief, 7e KiCad puts reliefs on pads in a zone by default and ties vias in solid unless the zone is hatched. #### 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.py The heat model - tsim/run.py The soldering runs, K/W and µΩ - tsim/iron.py How hot the iron has to be #### Sources - [KiCad’s zone defaults, pcbnew/zones.h](https://gitlab.com/kicad/code/kicad/-/blob/master/pcbnew/zones.h) - [JLCPCB’s stackups](https://jlcpcb.com/impedance) - [Hakko FX-888D, 50 to 480 °C](https://au.element14.com/hakko/fx888d/soldering-station-digital-70w/dp/2280753) What should the next note be? [Ask for one](https://nearfieldnotes.com/#ask-for-a-note) --- Source: https://nearfieldnotes.com/notes/thermal-relief/ Cite as: Michael Walker, “Thermal relief”, Near Field Notes, Note 7, Rev 1, 2 Nov 2026. https://nearfieldnotes.com/notes/thermal-relief/ Figures: free to reuse under CC BY 4.0 (https://nearfieldnotes.com/license/). Each one's image is linked under its name. Credit Michael Walker, Near Field Notes, and link the figure: this address plus #fig- and its number, like https://nearfieldnotes.com/notes/thermal-relief/#fig-7.