Energy Mismatch
Heat distribution refers to the difference in the amount of energy required to raise the temperature of various parts of a circuit assembly. Thermal capacitance differential occurs when massive components like transformers or heat sinks are placed near small chips with very low mass. This variation causes some parts of the board to heat up much faster than others during the reflow process.
Equilibrium Requirement
Solder joints on high-mass parts may remain cold while the surrounding areas reach peak temperature. To overcome a large thermal capacitance differential, the reflow profile must include a longer soak period to allow the temperature to equalize across the entire assembly. Without this equalization, the flux on the smaller components may burn off before the larger components are ready to solder.
This imbalance often leads to poor wetting on the heavy parts and charred residues on the light parts.
Layout Mitigation
Layout engineers can reduce this effect by using thermal reliefs on large copper planes and spacing heavy components away from sensitive ones. While oven tuning can compensate for some of the thermal capacitance differential, extreme cases require specialized carriers or localized heating. The physical limit is reached when the necessary soak time exceeds the life of the solder flux.
