A thermal relief pad is a technique used in PCB design to minimize thermal stress issues. It consists of copper spokes that extend from the pad to the surrounding copper plane.
Figure 1: Thermal Relief Pads
Functions of Thermal Relief Pads
Thermal relief pads primarily address uneven heat distribution during the soldering process. Copper pours are large areas of copper on a printed circuit board that are not part of a trace or component pad. When large copper pours are present, they can act as heat sinks, absorbing heat from the soldering iron and making it more difficult for heat to reach the component pad, which can hinder the formation of a strong solder joint. This can result in soldering issues such as tombstoning, cold solder joints, and solder bridging. Thermal relief pads are specifically designed to mitigate these issues by creating a small gap between the copper trace and component pin, allowing heat to be effectively directed to the component pad for a reliable connection.
In addition to improving heat distribution during soldering, thermal relief pads also help prevent overheating and potential component damage. Directly connecting a component pad to a copper pour can restrict heat flow, causing the component to overheat and potentially fail. By using a thermal relief pad, heat can be more efficiently conducted away from the pad and into the surrounding copper areas, thereby improving heat dissipation and preventing overheating.
Whether or not to include thermal relief pads in a PCB design depends on the specific requirements and constraints of the project. For instance, if the design doesn’t involve high-power components, large copper pours, or wave soldering, thermal relief pads may not be necessary.
Considerations in Thermal Relief Pad Design
Below are key design considerations for thermal relief pads.
For through-hole components, which typically have larger pins requiring more heat during soldering, thermal relief pads are commonly used to ensure proper heat dissipation. When designing thermal relief pads for through-hole components, it is crucial to determine the appropriate pad size and shape based on the component size and lead diameter. A larger pad typically requires more spokes to ensure efficient heat dissipation. Other factors such as trace width, layer thickness, and copper weight also influence the size and shape of the thermal relief pad.
For surface-mount components, thermal relief pads are usually designed as in-via pads. The via serves as the thermal relief mechanism, allowing for proper heat dissipation while maintaining clearance around the edge of the pad. It is important to ensure that the via is properly sized and positioned to provide sufficient heat dissipation. The orientation of the pad is also a critical factor, as it can influence the amount of heat transferred to the component during soldering.
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