Thermal Reliefs in Pads and Planes

What a Thermal Relief Does

A thermal relief is a pad connection that is deliberately narrowed, usually to four spokes, so that a pad connected to a large copper plane does not conduct heat away too quickly. The plane is an excellent heat spreader, which is exactly what a soldered joint does not want: a pad sitting on a plane pulls heat out of the iron, the wave or the selective nozzle, and the joint does not reach soldering temperature in the time available. The relief reduces the cross section of the connection and slows the loss, which makes the joint solderable.

Spokes and the Soldering Benefit

The relief is defined by the number of spokes, their width and the clearance between the pad and the plane. Four spokes at 45 degrees is the common pattern, and the important parameter is the total cross section of copper connecting the pad to the plane. A wide spoke carries heat away quickly, which defeats the purpose, and a narrow spoke is difficult to etch consistently. The second parameter is the clearance, which has to be wide enough that the etchant can remove the copper between the pad and the plane, since a clearance that etches incompletely turns the relief back into a solid connection.

When Reliefs Do Harm

A relief is a thermal resistor, and where the pad exists to move heat rather than to be soldered, that resistor is a defect. A power device pad, a thermal via array and a grounding pad under a radio frequency shield all want the shortest, widest connection to the plane, not the narrowest. Applying a relief by habit to every pad on a board is a common error, because the layout tool applies the rule globally unless it is told otherwise, and the result is a power design that runs hotter than the thermal model predicted and a ground connection with higher impedance than the design assumed.

Spoke Width and Count

The spoke should be wide enough to be manufacturable and to carry the current the connection has to pass, and narrow enough to be effective. In practice spokes are often specified at or slightly above the minimum trace width, with four of them, which usually gives a total cross section comparable to a normal trace. Where the pad carries significant current, the spokes have to be sized for that current rather than for the soldering benefit, and at that point the relief is either widened or the connection is made solid and the soldering difficulty is solved another way.

Reliefs on Through-Hole and Surface Mount

Through-hole pads on a plane benefit most from a relief, because the barrel and the plane together form a very effective heat sink and the joint is made by an iron or a wave that has limited thermal energy to give. Surface mount pads on a plane also benefit, particularly on a board with a heavy plane, because the paste and the pad have to reach reflow temperature and the plane will pull heat away during the ramp. The parts that should not have a relief are those whose function is thermal or electrical rather than solderability: power pads, thermal via lands, and the ground connections of a shielded area.

Reliefs, Planes and Impedance

A relief changes the electrical path as well as the thermal one. The inductance of a narrow spoke connection is higher than that of a solid connection, and the return path impedance of a signal referenced to that plane is affected. On a high frequency board, a ground pad connected by four narrow spokes is not the same as a ground pad connected solid, and the difference appears as a small discontinuity. Where the ground connection is part of a controlled impedance or a shielding path, the connection should be solid or the relief should be sized for the current and the frequency rather than for the iron.

Design Rules

Apply reliefs by exception rather than globally: default to a solid connection and add reliefs where a specific pad is hard to solder. Size the spokes from the current the connection carries, not from the soldering benefit alone, and check the clearance against the etch process. Remove the relief on thermal and shielding pads. Where a pad carries both a soldering problem and a thermal function, decide which matters more, and if the answer is the thermal function, change the soldering process rather than the pad.

Reliefs and Rework

The value of a relief is most obvious during rework. Removing a component from a pad that is connected solid to a plane requires a great deal of heat, and the heat spreads into the plane and into neighbouring parts rather than into the joint, so the operation either takes a long time or damages the laminate. A relief makes the rework practical within a reasonable temperature and time, which matters on a board with a high value or a repair contract. This is one of the few arguments for applying reliefs more widely than the soldering process alone would justify: where a design is expected to be reworked, a relief on the pads most likely to be repaired is a deliberate trade of a small thermal penalty for a large improvement in serviceability. The trade should be recorded on the drawing, since a later reviewer who sees only the thermal numbers would remove the reliefs as an optimisation.

FAQ

What is a thermal relief? A narrowed connection between a pad and a copper plane, usually four spokes, that slows heat loss so that the joint can be soldered.

Should every pad have one? No. It should be applied where a pad is difficult to solder, and removed where the pad exists to transfer heat or to provide a low impedance ground.

How wide should the spokes be? Wide enough for the current and the process, narrow enough to be effective. A common starting point is the minimum trace width with four spokes.

Do reliefs affect impedance? Yes. A relief increases the inductance of the connection compared with a solid link, which matters on high frequency and shielding paths.

Why does a relief sometimes not form? Usually because the clearance around the pad was too narrow to etch cleanly, leaving the pad effectively solid.

Conclusion

A thermal relief is a tool for making a joint solderable, not a default for every connection, and applying it indiscriminately creates thermal and electrical problems elsewhere. Use reliefs where the soldering needs them, size the spokes for the current, remove them from thermal and shielding pads, and check the clearance the etch process can hold. Plane design and pad connections are covered under PCB design and layout, the thermal side by PCB manufacturing capability limits, and the assembly consequences by SMT PCB assembly. Boards with heavy planes are normally tuned during prototype PCB assembly in 2026.