PCB Design
What is a thermal relief on a PCB?
A thermal relief connects a pad to a copper plane through a few thin spokes across a ring of clearance, instead of solid copper. A plane joined solid pulls the soldering iron's heat away faster than the iron can supply it, and the solder won't flow. The spokes slow the heat enough for the pad to pass solder's melting point, while still carrying the pin's current to the plane.
Why a solid pad won't solder
Treat heat like current: watts through °C/W give a temperature rise. An iron pushes about 18 W into the pad. Some escapes to the air, about 30 °C/W, and solid copper into the plane is only about 2 °C/W, with the plane so big it barely warms. The two paths are in parallel:
Lead-free solder melts at about 217 °C. The pad never gets near it.
Spokes slow the heat
Each spoke is a narrow neck of copper, about 80 °C/W. Four side by side make 20 °C/W:
Now the iron wins: 241 °C, past solder's 217, and the pin is still grounded through the spokes.
Live experiment
Spokes, not solid
An iron pushes 18 W into a pad, the air takes some through 30 °C/W, and the connection to the ground plane takes the rest. Join the pad solid and it sits at 59 °C; with eight spokes it reaches 160 °C, with four 241 °C, past solder's 217, and with two 333 °C.
“Spokes, not solid” is lesson 2 of 6 in Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 15 before it first.
How many spokes?
More spokes let more heat escape: six make about 13 °C/W and the pad stalls near 191 °C; eight make 10 °C/W and it reaches only about 160 °C. Fewer spokes solder more easily, but the spokes are the pad's electrical connection too, and one thin spoke can't carry much current or survive a knock. Four spokes of about 0.3 mm suit most pins.
Where you'll meet it
PCB tools such as KiCad and Altium draw thermal reliefs on pads in a copper pour by default, with settings for spoke width, spoke count and clearance. Wave soldering needs them as much as hand soldering does. Pads that carry tens of amps are the exception: they're connected solid and soldered with a hotter iron or a preheated board. Reflow ovens heat the whole board, so reliefs matter less there, but they still help small parts heat evenly.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Spokes, not solid” is lesson 2 of 6 in Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 15 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: The plane pulls the heat away faster than the iron can supply it. A plane is a huge, low-θ heatsink. The iron's watts flow straight into it and the pad never gets hot.
- Why not “Solder doesn't stick to ground”? Solder wets ground copper like any other copper. The trouble is temperature, not which net it is.
- Why not “The plane is too cold to touch”? The plane starts at room temperature like everything else. It's its size that drains the heat.
- Why not “The pad has too little copper”? The pad's size is fine. The solid connection to the huge plane is what drains the heat.
Show the answer
Answer: Four thin spokes across a clearance ring. The spokes still connect the pad to the plane for current, but each is a narrow neck that slows the heat.
- Why not “A thicker pad”? A thicker pad would hold more heat and be even harder to bring up to temperature.
- Why not “A wider plane”? A wider plane would drain the heat even faster.
- Why not “No connection at all”? The pad still needs to connect to the plane, for current and for its ground.
Show the answer
Answer: One thin spoke can't carry the pad's current or hold its ground well. The spokes are the pad's electrical connection too. Two to four share the current and survive a knock.
- Why not “The plane would melt”? A spoke carries the pin's current, far too little to melt a copper plane.
- Why not “It would get too cold”? One spoke drains the least heat, so it would let the pad get hotter, not colder.
- Why not “Solder can't wet a spoke”? Solder wets a spoke's copper as well as any other copper.
“Spokes, not solid” is lesson 2 of 6 in Gerber Gulf, PCB Design World 3. Free with an account. Lessons unlock in order, so you’ll do the 15 before it first.
Questions people ask
What is a thermal relief on a PCB?
A pad joined to a surrounding copper area through a few narrow spokes across a ring of clearance, instead of solid copper. It slows the flow of heat from the pad into the plane, so the pad can be soldered, while keeping the electrical connection.
Do thermal reliefs reduce current capacity?
Yes. The spokes are the only copper between the pad and the plane, so they set how much current the connection can carry. For high-current pads use wider or more spokes, or connect them solid and solder with more heat.
Should SMD pads have thermal reliefs?
Usually, for small parts on a plane: if one end of a tiny resistor heats faster than the other during reflow, it can stand up on end. Large power pads under regulators or MOSFETs are normally connected solid, for the sake of heat and current.
Related topics
- What is a ground plane on a PCB?A whole layer of copper for ground: milliohms for steady current, about a nanohenry for fast current.
- How to calculate junction temperatureHeat flows like current: watts through °C/W give a temperature rise, so size the copper before it cooks.
- PCB trace width for currentCount the squares: how a trace's width sets its ohms, its voltage drop and its heat, and how wide to draw it.
- What is a decoupling capacitor?Why every chip has a capacitor beside its power pins, how big, and why it must be close.