Thermal Relief Design For Plane Connections

A pad that is connected directly to a copper plane is thermally part of that plane, and that is exactly the problem during soldering. The plane conducts heat away from the joint as fast as the iron or the wave can supply it, so the solder does not reach temperature and the joint does not form. The thermal relief exists to limit that conduction while still making the connection.

This article explains what the relief does, how its geometry is chosen, and the cases where the trade should go the other way and the pad should be connected solidly.

Why A Plane Is Hard To Solder

Copper conducts heat very effectively, and a plane is a large mass of it. A joint connected to the plane has a thermal path in all directions, and the heat that the soldering process delivers is drawn away into the plane faster than the joint can be brought to the melting temperature. The result is a joint that wets partially, looks dull, or does not form at all.

In a wave soldering process the same effect produces a different symptom. The plane acts as a heat sink and the joint stays below temperature, so the solder does not fill the barrel. In a reflow process the plane draws heat from the paste and the deposit does not fully melt, which produces a joint with a grainy appearance and a poor fillet.

What A Thermal Relief Does

The relief replaces the solid connection with a small number of narrow spokes that bridge the gap between the pad and the plane. The spokes carry the electrical connection and the heat, and because they are narrow their thermal conductance is much lower than that of a solid connection. The joint therefore reaches temperature during soldering, and the plane is isolated from the pad by the mask gap.

The relief is normally combined with a mask opening around the pad, so that the plane is separated from the pad by a ring of bare laminate. The width of that ring and the width of the spokes are the two parameters that set the thermal and the electrical behaviour of the connection.

Spoke Width And Number

Four spokes at right angles is the common arrangement, and the width of each spoke is typically a fraction of the pad diameter. Wider spokes conduct more heat into the plane and make the joint harder to solder, while narrower spokes make soldering easier and raise the resistance and the inductance of the connection. The choice is a compromise between the process and the electrical requirement.

The number of spokes matters for the same reason. Four spokes give a symmetric connection with a defined thermal resistance, and reducing to two makes soldering significantly easier at the cost of a higher resistance. For a small via connected to a plane, a single spoke is sometimes used, and for a pad that carries a signal rather than a supply the electrical consequence is negligible.

Where A Solid Connection Is Correct

A solid connection is the right choice where the thermal path is the function. A pad on a power device that has to conduct heat into the plane should be connected solidly, because the whole purpose of the connection is to carry heat away as efficiently as possible. The same applies to a pad that carries a large current, where the resistance of the spokes would produce a local hot spot.

The soldering problem is then solved in another way. The process can be adjusted with a longer preheat or a higher soldering temperature, or the joint can be made with a selective method that delivers more energy. The layout can also help by reducing the thermal mass under the pad, for example by removing copper from the plane immediately beneath a through hole pad while keeping the connection on the top side. That adjustment is part of the layout decisions that affect the production process.

The Cost At High Current And High Frequency

At direct current the cost of a relief is the resistance of the spokes, which can be calculated from their width and length. A relief with narrow spokes on a pad that carries several amperes produces a measurable voltage drop and a local temperature rise, and in a current sense path it also produces an error that depends on the temperature.

At high frequency the cost is inductance, which matters even for a ground connection. A spoke is a short length of narrow conductor, and its inductance adds to the return path of the signal above it. Where a via is required to connect a decoupling capacitor to a plane, a relief on the via adds inductance that partly defeats the purpose of the capacitor, and the solid connection is preferable there. The way the plane itself is arranged is described under copper flooding and in the rules for splitting a power plane.

Specifying It In The Layout

The relief is applied automatically by most layout tools for pads connected to a plane, but the default values are not always appropriate. The spoke width and the mask gap should be stated in the design rules rather than left to the tool, and the exceptions should be listed explicitly. A power pad that must be solidly connected, a via under a decoupling capacitor, and a pad that carries a current sense signal are the usual exceptions.

The choice also interacts with the return path planning for the board as a whole, which is described under ground and power routing. A design that leaves the relief settings at the default values and then struggles with a cold joint on a heavy plane has created a process problem that is easier to prevent in the layout than to solve on the line.

Additional Considerations for This Build

Practical attention to solderability pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating solderability explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Deliberate attention to plane connection pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating plane connection explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Careful attention to heat sinking pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating heat sinking explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, solderability is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

FAQ

Why does a pad on a ground plane fail to solder? Because the plane conducts the heat away faster than the process can supply it, so the joint never reaches the melting temperature of the solder.

Should a power device pad be connected with a thermal relief? No. Its purpose is to conduct heat, so it should be connected solidly and the soldering process adjusted to suit the additional heat load.

Do reliefs matter for a via connected to a plane? They matter because a spoke adds inductance to the connection. For a via under a decoupling capacitor the added inductance reduces the effectiveness of the capacitor.