Thermal Relief Design and Solder Wicking

A pad that connects to a ground plane is connected to a great deal of copper, and that copper carries heat away from the joint during soldering. The result is a pad that does not reach the soldering temperature while its neighbours do, and a joint that is incomplete even though the process is working correctly.

Thermal relief is the geometry that solves this. It connects the pad to the plane with a limited number of narrow spokes instead of a solid connection, which reduces the heat that can escape while keeping the electrical and thermal path that the circuit needs.

Why Solid Connections Are a Problem

During wave soldering the pad and the plane are both at the temperature of the solder, so the plane acts as a heat sink and pulls energy out of the joint. During reflow the same effect appears as a slower ramp and a lower peak on the affected pads.

The visible symptom is a joint that looks cold, with a fillet that does not rise up the lead, or a joint that is only partly wetted. On a ground connected pad the process window narrows sharply, and a profile that works for the rest of the board may be inadequate for that one location.

How the Spokes Work

The spokes reduce the thermal conductance between the pad and the plane by restricting the cross section through which heat can flow. The number and the width of the spokes set that conductance, and both are a compromise between soldering and electrical performance.

Fewer, narrower spokes solder more easily and increase the impedance of the connection. Where the pad is a ground connection for a circuit, the spokes are usually acceptable; where it carries a return current for a fast signal, the added inductance matters and the geometry should be reconsidered.

Thermal Relief in the Two Processes

In wave soldering the relief is essential for a pad connected to a plane, because the board is heated from below and the plane competes directly with the wave. Without relief, the joints on the plane connected pads are the ones that fail.

In reflow the requirement is less absolute, because the whole assembly is heated together and the plane is at the same temperature as everything else by the end of the profile. The relief is still useful for the pads on a large plane, but it is the wave process that makes it mandatory.

Solder Wicking and Its Causes

Solder wicking is the movement of solder away from the joint and along a lead or into a barrel. It leaves the joint with insufficient material and produces a weak connection that may still pass a continuity check.

The cause is usually a thermal imbalance: the solder flows towards the hottest part of the assembly. A pad that is slower to heat than the lead encourages the solder to travel up the lead rather than to form the fillet, and a thermal relief that delays the pad is one of the contributing factors.

Heat Sink Pads and Their Geometry

A heat sink pad is intended to conduct heat, so the design wants a solid connection rather than a relief. When the same pad also has to be soldered, the two requirements conflict, and the resolution is usually to solder it with a profile that suits it rather than to compromise the thermal path.

Where the pad is connected to an internal plane, the relief spokes can be made wider than usual, since the copper there is thicker and the thermal demand higher. The geometry should be decided with the thermal requirement and the soldering process in view, not by applying one rule set everywhere.

The Role of the Solder Mask

The solder mask defines the soldering area and prevents solder from spreading onto the plane. Where the mask is missing near a relief, solder can bridge across the spokes and effectively create the solid connection that the relief was designed to avoid.

Mask slivers and mask slivers beside reliefs are therefore related problems. A mask dam that lifts during reflow can expose the plane and change the thermal behaviour of the pad at the same time, which is why the mask geometry is checked as part of the same review.

Design Rules in Practice

The typical rule specifies a spoke width and a number of spokes, together with a mask defined boundary around the pad. The values are a starting point, and they should be adjusted for the copper weight, the plane thickness and the soldering process.

Where the board carries both a heavy plane and fine pitch devices, the relief geometry for the plane connected pads is one of the items that decides whether the process has a single window or two. Splitting the profile for different regions of the board is possible but expensive, so the layout should aim to keep the thermal demand similar across the assembly.

Verification on the Assembly

The verification combines the joint inspection with the thermal profile measurement, and it is worth planning both before the first production run. A pad with a cold joint is evidence that the relief is inadequate or that the profile needs to be extended, and the two possibilities should be distinguished with a thermocouple rather than guessed.

The measurement should include the plane connected pads specifically, since they are the slowest to heat. Recording the result makes the design rule falsifiable, which is the same approach that turns a quality control measurement into a design improvement.

Relief on Inner Layers and Heavy Copper

Where the plane is an inner layer and the copper is heavy, the thermal demand is greater and the relief geometry has to be adjusted. Spokes that are adequate on a one ounce layer can be marginal on a three ounce one.

The design should also consider the connection from the inner layer to the surface, because the heat path runs through whatever copper exists between them. That path belongs with the wider discussion of thermal management on the assembly, and the geometry should be agreed with the fabricator rather than assumed.

Relief and Rework

A pad with a relief is easier to rework than one connected solidly to a plane, because less heat is drawn away when a joint is re-flowed. That is a practical benefit that is rarely considered when the geometry is chosen.

The relief geometry should therefore be reviewed with the repair strategy in mind. Where a component is expected to need replacement, the pad design that allows it to be heated effectively is worth more than a marginal improvement in electrical performance.

FAQ

Do all ground pads need a relief? Every pad that connects to a large plane benefits from one. The requirement is strongest where the board is wave soldered.

Can the relief be omitted on a via? A via in a pad is different from a relief, and the two are often confused. The via is a connection through the board, while the relief controls conduction into a plane.

Does the relief affect signal integrity? It adds inductance and resistance to the connection. On a ground return for a fast signal the effect should be evaluated rather than assumed to be negligible.