Thermal Relief Spoke Geometry for Through-Hole Wave Soldering

A thermal relief is a deliberate interruption of copper between a pad and the plane it belongs to, and every dimension of that interruption changes how the joint fills in the wave. This article sets out the spoke count, spoke width and air gap values that survive fabrication, and explains what happens to the joint when each of them is chosen badly.

Why a Thermal Relief Exists in a Plane

A pad connected to a copper pour on all sides conducts heat away from the joint during soldering. The plane behaves as a heat sink with very low thermal resistance, and the iron or the wave has to replace that heat faster than the plane removes it. Where the connection is solid, the pad may never reach the flow temperature of the alloy and the joint forms cold.

The relief also controls how much solder the joint can draw. Molten alloy follows the copper, so a pad with four wide spokes wicks paste and wave solder away from the barrel and into the plane. Narrow spokes limit that wicking path and keep the alloy where the fillet needs it, which is why the geometry is a soldering parameter rather than a mechanical one.

Spoke Count, Width and the 45 Degree Rule

Four spokes at 90 degrees is the usual default, but the count should follow the barrel size. For a 0.8 mm to 1.0 mm finished hole in wave soldering, four spokes of 0.25 mm to 0.30 mm are common; below 0.6 mm, two or three spokes of 0.20 mm are easier to etch and produce a more predictable joint. Spoke width below 0.15 mm is unreliable on 1 oz copper because the etcher cannot hold it, and the spoke then becomes the narrowest feature on the panel.

The spokes belong on the 45 degree diagonals rather than on the orthogonal axes. Diagonal spokes present the same copper area to the wave regardless of how the board sits on the conveyor, and they leave the quadrants between them open so that solder can reach the barrel. Orthogonal spokes create two wide shadow lanes that trap flux and delay fill.

Air Gap Dimensions and Fabrication Limits

The air gap is the annular clearance between the pad and the plane, and 0.20 mm to 0.25 mm per side is a practical range for 1 oz copper. The gap has to be wide enough for the etcher to clear the copper and narrow enough that the plane still heat sinks the joint through the spokes alone.

Below 0.15 mm the gap sits at the limit of standard etching, and the slivers of copper that break away become loose contamination on the panel. Above 0.35 mm the gap consumes routing area and weakens the plane around a via farm, so the value is stated on the drawing rather than left to the shop; our fabrication notes describe how those limits are written and checked.

How the Spokes Behave in the Wave

In the wave the spokes decide how quickly the pad reaches temperature and how long the alloy stays molten. A pad with four 0.25 mm spokes reaches wave temperature inside the contact time of a standard pallet, while a pad tied to the plane by a full annulus does not, and the result is a fillet that has formed on the component side but not in the barrel.

The contact time available is set by conveyor speed and wave width, so a change to either changes the fill. Where the panel runs at 1.2 m per minute against a 60 mm contact width, the dwell is roughly three seconds, and a heavy plane on a 2 mm board can consume most of it.

Shadowing From Adjacent Planes and Tall Components

A relief does not protect a joint from shadowing. A connector body or a large electrolytic next to the barrel blocks the wave, and the joint then fills only from the secondary wave, which may not reach it. The layout should keep at least 3 mm of clear travel between the leading edge of a through-hole pad and the nearest tall component.

Inner-layer planes make shadowing worse, because the heat that does arrive is drawn down the barrel and away from the fillet. Where an inner plane belongs to the same net, the connection should use the same spoke geometry on every layer so that the thermal path stays predictable.

Solder Volume, Fillet Formation and Barrel Fill

Barrel fill is the percentage of the hole height wetted by solder, and the Class 3 requirement in IPC-A-610 is 75 percent on a supported hole. The relief influences that figure because it decides how much paste and wave solder is available; a pad whose spokes are too wide pulls alloy into the plane and leaves the fill short.

Measuring it is destructive. A microsection is cut through the barrel axis, polished and examined at 50 to 200 times, and the fill percentage is read from the section. Our solder joint acceptance criteria set out how the same section is judged for voids, fillet shape and wetting angle.

Nets That Must Not Have a Relief

High-current nets, ground returns and any pad carrying more than about two amperes should not be relieved. The relief adds resistance and creates a local hot spot, and the same interruption that helps the iron hurts the current path. A relief placed on a power pad is a design error that only shows up as heat, which is why our thermal management notes treat the two requirements together.

Radio-frequency ground pads and shield cans behave in the same way, because the gap and the spokes become an inductor at the frequencies involved. Where a pad has to be both solderable and electrically solid, the usual answer is to keep the solid connection and to add preheat, a longer dwell or a second pass instead.

Verification by Section and Thermal Imaging

First-article verification uses a cross-section through one relieved pad and one solid reference pad so that the fill figures can be compared under the same process settings. An instrumented board with thermocouples on both pads, or an infrared image taken during the wave, shows whether the relief is meeting the wetting window.

The thermal data is often more useful than the section, because it needs no destruction and it shows the ramp rather than the endpoint. A pad that peaks 20 degrees below its neighbours is a candidate for a wider air gap or fewer spokes, and the change is made before the balance of the batch is released.

Documenting the Relief in the Fabrication Notes

The drawing should state the spoke count, the spoke width, the air gap and the net classes that are exempt. Stating only the air gap leaves the spoke count to the shop, and the shop will normally choose four spokes because that is the default in the CAD library.

The notes should also name the connections that are solid and say why, so that a later revision does not quietly relieve them. That documented exception list is what keeps the panel consistent from lot to lot, and it is also what an auditor looks for first.

FAQ

Does every through-hole pad on a plane need a thermal relief? No. Pads carrying significant current, radio-frequency grounds and pads that will not be hand soldered can be left solid. The relief exists to make soldering possible, so where the process already supplies enough heat the relief only adds resistance.

Can the air gap be reduced to gain routing area? Only to about 0.15 mm on 1 oz copper, and only if the etcher can hold it. Below that value the gap closes on some panels and the relief disappears, which is worse than no relief because the design assumes the heat path is restricted when it is not.

How is spoke width chosen for a small barrel? Scale it with the hole: a 0.6 mm barrel usually takes two or three spokes of 0.20 mm. The objective is the same as on a large hole, enough copper to manufacture reliably and little enough to let the joint reach temperature.