Pattern Design
PCB pattern used to connect a component pad to a large copper plane using thin bridges rather than a solid connection. The thermal relief facilitates reliable soldering by preventing the large copper area from acting as a massive heat sink during the assembly process. It ensures that the heat from a soldering iron or reflow oven stays concentrated on the pad.
Soldering Quality
Cold joints occur when the pad cannot reach the liquidus temperature.
Current Constraint
The width and number of the bridges in a thermal relief must be sized to carry the expected electrical current without excessive heating. If the bridges are too narrow, they can act like a fuse and fail under high load conditions. Engineers must calculate the cross-sectional area of the relief to balance the thermal isolation with the required ampacity.
The resulting temperature rise is verified during full load testing to ensure it stays within the glass transition temperature of the board.
Design Rule
Automated checks in layout software verify that every connection to a plane includes the appropriate thermal relief structure. Exceptions are made for high current power paths where a solid connection is necessary for electrical performance. In these cases, specialized heating profiles are required during manufacturing to overcome the heat dissipation.
