Thermal Relief and Copper Balance in PCB Design
Two decisions about copper are made on almost every board and are almost never written down. The first is how a pad is connected to the plane around it, which decides how easily the joint can be soldered. The second is how evenly copper is distributed across each layer, which decides how the board behaves during lamination and reflow. Both are usually left to a default setting in the layout tool, and both have a measurable effect on yield and flatness.
Why Pads Are Connected by Spokes
A pad connected to a large copper plane by solid copper conducts heat away from the joint almost as fast as a soldering iron can supply it. The result is a joint that never reaches temperature, a weak fillet or a cold solder joint, and it is the reason thermal relief exists. A thermal relief connects the pad with a small number of narrow spokes, which carry the electrical and thermal current while limiting the rate at which heat escapes during soldering.
The spoke geometry is a compromise. Narrow spokes restrict heat loss and make soldering easier, while wide spokes carry more current and spread heat better in service. Where a pad carries a signal only, the relief can be narrow; where it carries current or dissipates heat, the spokes have to be sized for that duty rather than left at the default width.
When a Solid Connection Is Better
There are cases where a solid connection is the right choice. A pad carrying substantial current should be connected with as much copper as possible, because the relief spokes become the bottleneck and can heat up under load. The same applies to a ground pad whose purpose is to conduct heat into the plane, where a thermal relief would defeat the object of the connection.
Where a solid connection is used on a pad that must also be soldered, the assembly process has to compensate. A larger iron, a longer dwell, preheating the board or using a reflow profile with more soak time all help, and the requirement should be noted on the assembly drawing rather than left for the operator to discover. Our notes on thermal management in PCB design cover the in service side of the same trade.
Copper Balance and Lamination
Copper balance describes how evenly the copper is distributed across a layer. During lamination the resin flows away from areas of heavy copper and accumulates in areas with little, so a layer with uneven distribution ends up with uneven dielectric thickness. The board can also bow as the two sides of the stackup expand differently when heated.
Balancing the copper improves both problems. Large empty areas are filled with a copper pour or a grid pattern connected to a suitable net, so that the resin has nowhere to accumulate and the two sides of the board behave similarly. The pattern matters: a solid pour that is not connected to anything can act as a resonator, so it should be stitched to the plane or removed.
How Balance Affects Warpage
Warpage is the visible result of an unbalanced build. If one side of the stackup has considerably more copper than the other, the two sides expand by different amounts when the board is heated during lamination or reflow, and the board bows toward the side with less copper. The effect is larger on thin boards and on large panels, and it appears as a flatness problem during stencil printing long before it is noticed anywhere else.
The remedies are structural as well as electrical. Balance the copper between layers, keep the stackup symmetric about the centre line, distribute the heavy copper evenly rather than concentrating it in one corner, and allow the fabricator to add a balanced copper thieving pattern on outer layers where the design is sparse. Our notes on warpage causes and fixes cover the process side.
Heat Spreading Versus Soldering
Heat spreading and solderability pull in opposite directions, and the design has to choose per pad rather than per board. A thermal pad under a power device wants maximum copper for spreading, while a small passive on a ground plane wants a relief so that it can be soldered reliably. The distinction should be made by the function of the pad, not by the default setting of the tool.
Where a pad must serve both purposes, a compromise is available: a moderately sized thermal relief with four spokes can still be soldered comfortably and still conducts adequately, and the plane behind it provides the spreading. Sizing the spokes deliberately, rather than accepting the default, is what turns a compromise into a decision. Our notes on PCB thermal design rules describe how the path behaves once the heat is in the plane.
Practical Rules for Copper Distribution
Four rules cover most cases. Keep the copper on opposite layers broadly similar in area. Keep the stackup symmetric, with the same dielectric thickness and foil weight on either side of the centre. Fill unused areas with a pattern that is connected rather than floating. And place the heavy copper features, such as large pads and bus bars, away from the board corners where their effect on flatness is greatest.
It also helps to review the copper from the fabricators point of view before release. A layer that is nearly empty in one half and dense in the other is a warning sign for both warpage and dielectric thickness variation, and the fix at design time costs nothing while the fix after lamination costs a batch of boards. Our notes on component placement show how placement contributes to the distribution.
Relief on Connector and Shield Pads
Connector pads and shield pads are a common source of soldering difficulty, because they are often large and connected to a plane on several layers. A connector shell pad tied solidly to ground on four layers will soak heat away faster than a hand iron can replace it, and the resulting joint is one of the hardest to inspect. A thermal relief applied to the inner layer connections while the outer layer stays solid gives a solderable surface and a good electrical path at the same time.
Shield cans and metal frames present the same problem on a larger scale. Where several pads must be soldered and the can itself is a heat sink, preheating the assembly and using a larger tip are practical measures, and the layout can help by reducing the number of layers that each pad is bonded to. Recording which pads are solid and which are relieved, on the assembly drawing, prevents the operator from discovering the difference at the bench.
FAQ
Should every pad have a thermal relief? No. Signal pads on a plane benefit from one, but pads carrying significant current or intended mainly to conduct heat are better connected solidly, with the assembly process adjusted to suit.
How is copper balance measured? By comparing the copper area on each layer and on each side of the stackup centre line. Suppliers check it as part of the design for manufacture review and can add a balancing pattern where it is needed.
Does copper balance matter on a thin board? It matters more. Thin boards and large panels warp more readily for the same imbalance, so the distribution of copper is a more significant design variable than on a thick build.
