Thermal Phenomenon
Printed circuit board regions containing large copper planes or thick component leads absorb thermal energy rapidly during the soldering process. This heat sink effect prevents these specific joints from reaching the required liquidus temperature at the same time as the surrounding low-mass areas. Consequently, soldering such areas requires careful profiling to avoid localized defects.
Defect Generation
Inadequate heat distribution during manual or automated soldering leads to incomplete solder melting. Due to the heat sink effect, the solder paste may fail to wet the pad properly, resulting in dry or cold joints that are susceptible to mechanical and electrical failure under stress. When assembly lines run too quickly, these high-mass areas fail to reflow completely, while the low-mass regions of the same board are overheated.
This mismatch in thermal demand across a single board can cause uneven solidification and tombstoning of adjacent small components.
Mitigation Method
Thermal relief pads are integrated into the circuit design to isolate heavy copper regions. Adding these thin copper spokes limits the heat sink effect by restricting the rate of thermal dissipation during the reflow or wave soldering cycle. Adjusting the thermal profile to include a longer soak time also allows the entire assembly to reach a uniform temperature before the final liquidus spike.
