In PCB design, the choice between a thermal relief pad, also called a thermal pad or cross-shaped connection, and a solid copper connection is not arbitrary. It represents a trade-off among manufacturability, electrical performance, heat dissipation, and long-term reliability.
Why Use a Thermal Relief Pad?
Thermal relief is primarily used to prevent soldering difficulties. It is especially common for through-hole component pins connected to large copper areas, such as ground and power pins.
Reducing Heat Loss During Soldering
Large copper planes have a high thermal capacity and conduct heat away rapidly. If a through-hole pad is connected directly to a large copper plane, the heat supplied by a soldering iron or wave-soldering system can quickly spread through the copper. The hole and pad may then fail to reach the temperature required for proper solder wetting.
This condition can cause several manufacturing problems:
- The temperature inside the plated through-hole may remain below the solder melting or wetting temperature, resulting in cold solder joints, insufficient solder rise, and poor hole fill.
- The assembly process may require a higher soldering temperature or a longer heating time to compensate for the heat loss. This increases the risk of damaging components, the PCB substrate, or the copper structure.
- During reflow soldering, an SMD pad connected directly to a large copper area can heat more slowly than the opposing pad. The resulting thermal imbalance may contribute to tombstoning, component displacement, or an open solder joint.
Thermal relief pads connect the solderable pad to the surrounding copper plane through several narrow copper spokes. These spokes act as thermal bridges with limited heat-conduction capacity. The pad can therefore heat more quickly and form a more reliable solder joint, while still maintaining an electrical connection to the plane.
The thermal relief pattern does not eliminate heat transfer completely. Instead, it limits the heat flow sufficiently to balance solderability and electrical connection. The number, width, and geometry of the spokes affect both the thermal behavior and the current-carrying capability of the connection.
Reducing Rework and Repair Difficulty
Thermal relief also makes component removal easier. When a solderable pad is connected directly to a large copper plane, a standard soldering iron may not be able to supply enough heat to melt the joint efficiently. Operators may repeatedly heat the connection, increasing the risk of lifting the pad, damaging the plated hole, or overheating the component and laminate.
With a thermal relief connection, less heat is drawn into the surrounding copper. Through-hole components can generally be desoldered more easily, which is beneficial during repair, replacement, and production rework.
Improving Process Compatibility
Wave soldering is particularly sensitive to large copper areas connected to through-hole pins. If a power or ground pin is connected directly to a plane, the plane can remove heat from the solder joint faster than the process can replenish it. After the board passes through the wave, the hole may contain insufficient solder, or the solder may fail to rise adequately through the barrel.
For this reason, the ground and power pins of many conventional through-hole components, including connectors, pin headers, and switches, are commonly designed with thermal relief. This approach helps the soldering process achieve more consistent hole fill without requiring excessive heating.
Why Are Some Holes Connected Directly to the Copper Plane?
A solid connection is used when electrical or thermal performance is more important than soldering convenience. It is typically applied to vias that do not receive solder, or to nodes with demanding impedance, current-carrying, grounding, or heat-dissipation requirements.
Non-Soldered Vias
Many vias are used only for interlayer electrical connection. Examples include signal vias, ground-shielding vias, and thermal via arrays. Because these vias do not directly receive solder during component attachment, they do not have the same soldering heat-loss problem as component pads.
A solid connection provides the largest possible contact area between the via barrel and the surrounding plane. This reduces the parasitic resistance and inductance of the connection and can improve high-speed signal return paths and power integrity. A low-inductance ground connection is particularly valuable when the return current must transition between reference planes.
Solid connections can also provide higher current-carrying capability. A thermal relief pattern narrows the copper path through its spokes. For a high-current power connection, those narrow sections may produce additional heating or become a structural weak point. Connecting the via directly to the plane avoids this localized constriction.
Solid connections may also be more suitable for certain manufacturing structures, such as vias that require resin plugging or solder-mask plugging. A more complete copper structure can support hole filling and surface-planarity requirements, depending on the board construction and fabrication process.
Pads Requiring Enhanced Thermal or Electrical Performance
Some solderable pads are intentionally connected to the plane with a solid connection or with wider spokes, even though this makes soldering more difficult. The exposed thermal pad of a QFN package is a typical example. Its purpose is to transfer heat from the package into the internal copper structure and, in some designs, into a larger thermal spreading area.
For such pads, the soldering challenge must be addressed through the overall assembly process. Preheating, the reflow temperature profile, and the stencil-aperture pattern all affect how effectively the pad can be soldered. These features remain important layout and process-design challenges because the same copper area that improves heat transfer also removes heat during soldering.
In some high-speed digital or RF circuits, a more direct ground connection may also be preferred for selected nodes. For example, a decoupling capacitor ground connection or an RF return path may benefit from reduced connection inductance. In these cases, some soldering convenience may be sacrificed to achieve a more direct electrical path.
Mechanical Strength and Internal-Layer Connections
Solid connections can also be selected where mechanical robustness or long-term current endurance is important. In certain high-voltage or electrochemical-stress conditions, narrow thermal spokes may be more vulnerable to long-term degradation than a continuous copper connection.
Internal layers generally do not require soldering. A via landing on an internal power or ground plane can therefore be connected directly to that plane without creating an external solderability problem. Thermal relief is mainly needed on external layers and on other layers that influence the heating of a solderable pad.
What Happens If Every Hole Uses a Solid Connection?
Widespread Soldering Defects
If all solderable pads and holes are connected directly to large copper areas, ground pins, power pins, and other plane-connected pins may suffer from insufficient solder rise, cold solder joints, voiding, and incomplete hole fill. After wave soldering, many through-holes may not contain enough solder, while the outer pad may have only a thin surface layer of solder.
The same problem can occur with SMD pads connected to large planes. If one side of a small resistor or capacitor dissipates heat much faster than the other side, the component may shift during reflow or lift at one end. This tombstoning or open-joint risk is especially significant for small passive components.
Higher Thermal Stress on Components and the PCB
When solid copper connections remove heat too aggressively, the production line may compensate by increasing the oven temperature or extending the soldering time. This approach can create additional risks:
- Temperature-sensitive components, including plastic connectors, LEDs, and electrolytic capacitors, may deform, age prematurely, or fail.
- The PCB may develop whitening, delamination, or warpage. In multilayer boards, repeated thermal stress can also affect the bond between inner-layer copper and the prepreg.
- Repeated thermal shock may increase the risk of cracking or separation in pad copper and plated-hole walls.
More Difficult Rework and Testing
Directly connected ground pins are difficult to desolder because the surrounding plane continuously absorbs heat. A conventional soldering iron may be unable to melt the joint efficiently, and repeated heating can damage the pad or the board.
Large plane-connected pads can also complicate in-circuit testing. Oxidation, uneven solder coverage, and the thermal behavior of the copper area may reduce the reliability of probe contact. Test-point design should therefore consider not only electrical accessibility but also the physical condition of the pad surface.
Design Guidance: Choose the Connection Based on Function
If a board uses only press-fit or compression connections, and all holes are non-soldered vias that are fully plugged, the soldering problems described above may not occur. However, on a board that includes conventional through-hole or surface-mount soldering, applying solid connections to every pad conflicts with basic design-for-manufacturability principles.
A practical rule is to use thermal relief for solderable pads connected to large copper planes, especially for through-hole power and ground pins. Use solid connections for non-soldered vias and for nodes where current capacity, return-path inductance, impedance behavior, or heat spreading has priority. For power-device thermal pads and other special structures, the copper connection must be evaluated together with the stencil design and the soldering profile.
In short, thermal relief prioritizes solderability, manufacturability, and repairability. A solid connection prioritizes electrical conduction and thermal performance. Neither pattern is universally superior. The correct choice depends on whether the feature must be soldered and whether its electrical or thermal requirements justify the additional manufacturing difficulty.
