Thermal Management for Surface-Mount Devices

The origins of the ambient derating curve go back decades to when the U.S. military specified the performance requirements of components used in the design of their equipment. The purpose was to provide a high level of confidence that the components' performance would ensure system and equipment reliability. Some of these standards, such as MIL-PRF-39009 or MIL-PRF-39017, specified the performance requirements in various environmental conditions, such as power derating for increased ambient temperatures. The ambient derating curve was used because the resistor products available during that time were through-hole axial leaded devices, which dissipated approximately 80 % to 90 % of the power / heat energy to the air through convection. The lead provided a very small dissipation pathway to the PCB for power wirewounds, while the body provided a much larger pathway through air with the larger surface area and / or increased air velocity.
The higher power resistors have larger bodies that improve heat transfer to air, whereas smaller body through-hole parts operating at a lower power have a greater proportion of the power conducted to the PCB. This is indicated by the different points of derating for the different resistor technologies, as illustrated by the graphs below.
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