Thermal Relief Spoke Design For Soldering

A thermal relief is a pattern of narrow spokes that connects a pad to a surrounding copper plane. It exists because a pad that is connected to a plane by solid copper conducts heat away so effectively that the joint cannot be soldered: the iron or the wave cannot raise the pad to the melting point of the alloy before the heat has spread into the plane.

This article explains what the relief has to balance, how the spoke dimensions are chosen, and when a direct connection is the better choice.

What The Relief Has To Balance

A relief is a deliberate thermal resistance inserted between the pad and the plane. It raises the temperature the pad reaches for a given amount of heat applied, which makes soldering possible, and it also raises the electrical resistance of that connection, which is undesirable for a ground or power net. Every design of relief is a compromise between those two requirements.

The compromise matters differently for different nets. A pad on a signal net that is connected to a ground plane only for shielding can be heavily relieved with little consequence. A pad that carries several amperes into a plane needs a low resistance path, and the relief then has to be wide enough to carry the current without significant heating, which also reduces its thermal benefit. The current capacity of a given copper geometry is described under trace width and current calculation.

Spoke Dimensions

A relief is defined by the number of spokes, their width, their length and the clearance between the pad and the plane on the sides where there is no spoke. Four spokes at the cardinal directions is the most common arrangement, because it is symmetrical and easy to generate automatically, and it gives a balance between thermal resistance and electrical resistance that suits a general purpose pad.

Spoke width is the variable that is adjusted most often. A width of about half a millimetre is typical for a component pad on a hand assembled or reworked board, and narrower spokes are used where the thermal mass of the plane is small or the soldering process is more capable. The spoke length, which is the width of the clearance gap, matters almost as much: a longer spoke of the same width has a higher thermal resistance and an increased electrical resistance, so lengthening the spoke is a way of increasing the thermal benefit at a cost in current capacity.

How Many Spokes

Increasing the number of spokes reduces both the thermal and the electrical resistance, so a pad with eight spokes is thermally closer to a solid connection and electrically closer to it as well. The number is therefore chosen from the soldering requirement first: enough spokes to be manufacturable and symmetrical, and few enough that the pad can be heated. Where the pad is small, four spokes of a given width may already occupy most of its perimeter, and the relief then becomes a solid connection in practice.

For a pad that must also carry current, the arrangement that works is a small number of wide spokes rather than many narrow ones. The electrical resistance depends on the total copper area, while the thermal resistance depends on the same area, so wide spokes give a lower resistance for the same thermal resistance only in the sense that they are easier to fabricate and to keep uniform. Where the current is significant, the better solution is often a solid connection plus a separate thermal strategy such as a longer soak or a preheat. The way planes are organised in general is treated under ground routing and power trace planning.

Solderability And Rework

Solderability is the property the relief exists to protect. A joint that cannot be brought to temperature produces a cold joint, a joint with poor wetting, or a joint where the solder sits on the pad without flowing into the hole. On a board with a large ground plane the effect is dramatic: a pad connected solidly to the plane may need several times the iron contact time of an isolated pad, which stresses the component and the laminate as well as the operator’s patience.

Rework has the same requirement in reverse. Removing a component from a relieved pad needs less heat and less time than removing one from a solid connection, so a design that intends to be serviceable uses reliefs on the pads that are most likely to be replaced. Where a component will never be reworked and the current is high, the solid connection is correct and the process has to be set up to deal with it.

Where A Direct Connection Is Better

A direct connection is better for a pad that carries significant current, for a pad that is part of a thermal path intended for heat sinking away from a component, and for a pad on a board that is assembled with a process capable of heating it, such as a vapour phase or a well controlled convection reflow. In those cases the electrical and thermal performance of the connection is the priority and the soldering difficulty is handled by the process.

A direct connection is also used where a relief cannot be generated correctly. On a very small pad, or where the plane is on the opposite side of a thin board, the relief may not produce a meaningful thermal benefit and adds a feature that the fabricator has to hold to a tolerance. Where the same pad must serve as both a signal connection and a thermal path, the decision usually goes to the direct connection with a controlled process, since the two requirements pull in opposite directions and one of them has to be favoured. The choice between a mesh and a solid plane, which interacts with the same considerations, is discussed under copper flooding: mesh or solid.

Relief Geometry And The Fabrication Rules

The clearance gap of a relief is a mask and copper feature that the fabricator has to hold, and it is normally set at or above the minimum clearance the shop can produce. A relief with a gap smaller than the shop minimum is either widened or lost, and a lost relief becomes a direct connection, which changes the soldering behaviour of the board without the designer knowing. Putting the shop minimum into the design rules is what prevents that.

The spokes also have to respect the pad’s own geometry. A spoke that joins a pad at a corner creates a narrow neck where the current concentrates and the plating is thinnest, so spokes are normally placed on the flat sides of a pad. Where a pad is surrounded on all sides by a plane and the relief would leave the pad connected by very little copper, the pad is better connected directly and the plane relieved around it instead of the other way round. The interaction of a pad arrangement with the plane it sits in is part of power plane splitting rules.

FAQ

How wide should a thermal relief spoke be? It depends on the current the pad must carry and on the thermal mass of the plane. Half a millimetre is a common starting point for a hand soldered pad, and a pad carrying significant current needs more, or a direct connection.

Does a relief affect the electrical performance of a ground connection? It adds a small resistance and a small inductance, which matters at high frequency and at high current. On a ground plane for a high speed circuit, a relieved via can be the difference between a quiet return path and a radiating one.

Can a relief be added after the layout? It is generated automatically by most layout tools from a rule, so it can be added or removed at any time. The decision should still be made deliberately, because the effect on the soldering process is significant and it is not visible in the schematic.