Thermal Relief Design for Soldering
Why a Plane Makes Soldering Harder
A copper plane conducts heat better than the laminate, so a pad that is connected directly to a plane draws heat away from the joint during soldering. The result is a joint that reaches a lower peak temperature, that stays above the liquidus for a shorter time and that may not wet fully, which appears as a cold or an incomplete joint. A thermal relief is a connection that deliberately restricts the heat path, using a small number of narrow spokes rather than a solid connection, so that the pad can reach temperature while the plane remains electrically connected.
How the Relief Works
The relief is formed by removing the copper around the pad except for a few spokes, usually four, leaving the pad connected to the plane through a restricted path. The spokes’ width and number determine the balance: a wider or more numerous spoke conducts more heat and current, and makes the soldering harder; a narrower or fewer spoke restricts the heat but also restricts the current and adds resistance and inductance. The relief’s effectiveness depends on the layer and the process: a reflowed joint on a thin plane may not need a relief, while a hand soldered joint on a heavy plane usually does.
When Not to Use a Relief
A relief is not always desirable. On a power plane or a ground connection that carries a high current, the relief’s resistance and inductance are a real penalty, and a solid connection is the better choice provided the soldering can reach temperature. On a high frequency path, the relief’s added inductance changes the impedance, so the connection should be solid and the soldering managed by preheating rather than by restricting the path. Where a via is used for thermal conduction, a relief defeats its purpose, since the relief reduces exactly the conduction that the via was placed to provide.
Reflow and Wave Considerations
In a reflow process the whole board is heated, so the thermal mass of the plane is less of a problem than in a hand or a wave process, and the relief may be unnecessary. In a wave process the solder is in contact with the joint for a short time, so the relief matters more, and the design rules usually require it on a plane connected pad. Where the board has both processes, the relief should be applied to the side that is waved. The rules should state the requirement for each process rather than applying one rule to every board.
The Relief and the Joint’s Quality
The relief changes the joint’s shape as well as its formation. A pad with a relief reaches temperature faster, so the solder wets and forms a proper fillet, while a pad without one may leave the solder sitting on the surface. The relief also affects the solder’s drainage in a wave process, and an inadequate relief is a common cause of a joint that is filled but not wetted. Where a product has a joint that fails repeatedly at a plane connected pad, the relief is one of the first things to check, along with the preheat and the profile.
Design Rules and Consistency
The rules should be written and applied consistently, because an inconsistent relief makes the process unpredictable. A typical rule states the spoke width, the number of spokes and the annular gap for a given plane and process, and distinguishes between a signal pad, a power pad and a thermal via. The rules should follow the fabricator’s capability, since a very narrow spoke may not be manufactured reliably, and they should be checked against the assembly process, since a relief designed for hand soldering is unnecessary in a reflow oven. The two should be reviewed together rather than in isolation.
Verification
The verification is by examining the joints on the production board and by measuring the temperature at the pad during the process. A thermocouple on a plane connected pad and on an isolated pad shows the difference that the relief makes, which is the evidence that the design is correct. Where a joint is cold, the profile and the relief should be examined together, because a profile that is correct for an isolated pad may be inadequate for a plane connected one. The measurement is quick and it settles an argument that appearance alone cannot.
The Relief in a Mixed Assembly
A board that carries both a reflowed side and a waved side has two different soldering environments, and the relief should be applied where it is needed rather than uniformly. A pad on the reflowed side that is also waved, or a through hole pad that is waved after a reflow, sees the process that needs the relief. Applying a relief everywhere costs current capacity and inductance on the nets that do not need it, and omitting it where it is needed produces the cold joint. The design rules should therefore be per process and per side, and the assembly documentation should state which side is waved so that the two agree.
FAQ
What is a thermal relief for? To restrict the heat path from a pad to a plane so that the pad can reach soldering temperature.
When should a pad be connected solidly? On a high current path, a high frequency path or a thermal via, where the resistance and inductance matter more than the soldering.
Why is a relief less important in reflow? The whole board is heated, so the plane’s thermal mass is less of a barrier than in a hand or a wave process.
What does an inadequate relief cause? A joint that is filled but not wetted, appearing as a cold or an incomplete joint.
How is the relief verified? By measuring the pad’s temperature during the process and by examining the joints in production.
Conclusion
A thermal relief balances the soldering against the connection’s resistance, so set the spokes for the process and connect solidly where the current or the frequency demands it. Measure the pad’s temperature. Plane and pad design belongs to PCB design and layout, the process it serves is described in PCB assembly, and the acceptance is part of quality management. Soldering rules for a new product are set during prototype PCB assembly in 2026.
