Wave Soldering Thermal Relief Design

A thermal relief is the pattern of copper spokes that connects a plated through hole to a plane without attaching it solidly. It exists to slow the rate at which heat is conducted away from the joint during soldering, so that the barrel and the pad reach a temperature high enough for solder to flow through the hole. Getting the relief right is a balance: too little copper and the joint does not fill, too much and the connection loses thermal and electrical performance.

What a Thermal Relief Does

Copper is an excellent conductor of heat, which is a virtue in service and a problem during soldering. A hole connected to a ground plane by solid copper is thermally tied to the whole plane, and a wave or a hand iron cannot raise the barrel temperature enough to melt solder through the hole. The relief reduces the cross-section of the connection, which raises its thermal resistance and allows the joint to heat locally.

The relief also affects the electrical and thermal behaviour of the finished board. The same reduced cross-section that helps soldering adds resistance and inductance to the connection and reduces the heat that can be conducted out of a component through the plane. On a power or thermal path, that penalty has to be accounted for. Thermal relief design practice covers the geometry options.

Connection Geometry

The relief is defined by the number and width of the spokes, the diameter of the clearance in the plane and the size of the pad. Four spokes at right angles are the conventional pattern and give a predictable thermal resistance with a balanced mechanical connection. Two spokes are easier to fill but weaker mechanically and less symmetrical, which can matter for a heavy component or a connector.

Spoke width is the main adjustment. Wider spokes conduct more heat away and make filling harder; narrower spokes fill more easily and increase the resistance. The correct width depends on the plane’s copper weight, the hole size and the soldering method, so a value copied from another product may be wrong. Layout for soldering yield describes how the surrounding copper changes the requirement.

Reliefs for Wave Versus Reflow

Wave soldering benefits from a relief on every plane-connected hole, because the wave contacts the joint for a short time and the plane would otherwise draw the heat away faster than it is supplied. Where a hole is on the thermal land pattern of a large connector, the relief is often essential to achieve a consistent fill across all the pins.

Reflow soldering of a through hole part, either by pin-in-paste or by a dedicated process, has different thermal dynamics because the whole assembly is heated. A relief is still useful, but the constraint is looser, and the design can afford a more generous connection. Where the same board is soldered by both methods, the relief should be sized for the more demanding one. Process comparison helps decide which requirement dominates.

Pad, Hole and Solder Drain

The pad size interacts with the relief. A pad that is large holds more solder and produces a stronger joint, but it also presents more area to the wave and can bridge to a neighbour. The hole diameter sets the volume that has to be filled, and a large hole with a narrow relief is the combination that produces incomplete fill most often.

Drain is the other half of the problem. While the relief slows heat loss, the solder must still be able to flow out of the hole and form a fillet on the top side, and a pad that is too small or a hole that is blocked by the component lead restricts that flow. On a multilayer board with thick copper, the relief and the hole size may both need to be increased to achieve a complete fill. Alloy condition also affects flow, since a pot that has drifted from specification wets differently.

Plane Connection in Multilayer Boards

On a multilayer board, a hole may connect to several planes, and each connection contributes its own heat loss. A joint with three plane connections needs a more restrictive relief than one with a single connection, and the innermost planes are the ones that draw heat away from the barrel most effectively. Where a design has been transferred from a simpler stackup, the reliefs often need re-sizing.

The thermal performance of the finished board should be checked after the reliefs are chosen. A plane connection intended to carry current or to conduct heat away from a component may lose more capability than expected when it is reduced to four narrow spokes, and the effect should be evaluated rather than assumed. Where the connection is critical, a solid connection with an additional via nearby is sometimes the better arrangement.

When to Omit the Relief

There are cases where a solid connection is correct. A mounting hole that carries no solder joint does not need one. A hole that is soldered by a method with ample heat, such as selective soldering with a long dwell, may fill adequately with a solid connection and benefit from the lower resistance. A hole carrying high current or high frequency may need the solid connection for its electrical properties, and the soldering process must then be adapted to suit.

The decision should be made explicitly rather than by default. Some layout tools apply a relief globally, and the result is a design that solders well and performs below its thermal or electrical intent. Reviewing the plane-connected holes as a group, and recording which ones have solid connections and why, catches that case before the board is fabricated.

Verifying the Fill

Filling is verified by inspection of the top-side fillet and, where the requirement is strict, by X-ray or cross section. A hole that fills from the bottom but leaves a concave or absent top-side fillet is a partial fill, and it usually means the relief is too restrictive or the preheat is too low. Because the defect is not always visible from the solder side, the inspection plan should specify where to look.

Where a fill problem appears on one hole of a connector rather than all of them, the cause is usually local: a shadow from a taller part, a ground plane on that net only or a component lead that blocks the flow. Comparing the good and the bad holes identifies the difference, and that comparison is usually faster than adjusting the machine. Plating thickness in the barrel should be confirmed as part of the same check, since a thin barrel changes the fill behaviour.

Documenting the Design Rules

The relief geometry should be captured in the layout rules rather than re-decided on each board. A rule that states the spoke width and the clearance diameter for the plane, the copper weight and the soldering method gives the designer a starting point that is known to work. Where a product needs a different value, the reason should be recorded, and the exception reviewed so that the standard is not eroded.

The rules should also connect to the process: if the wave solder machine or the preheat settings change, the relief requirement may change with them. Reviewing the connection geometry when the process changes is cheaper than discovering that a board which used to fill reliably no longer does, and it keeps the two sides of the design conversation in step.

FAQ

How many spokes should a thermal relief have? Four at right angles is the usual default. Two are easier to fill but weaker; six or more are rarely needed.

Can a relief be too restrictive? Yes. Too little copper means the joint fills but the connection carries less current and conducts less heat, so the electrical and thermal duty has to be checked.

Do reflowed through holes need reliefs? They benefit from them, but the constraint is looser than for wave soldering. Size the relief for the more demanding process.

How is an incomplete fill detected? By inspecting the top-side fillet, since a bottom-only fill may look acceptable from the solder side. X-ray or microsection confirms it where the requirement is strict.