SMD Resistor Thermal Analysis

The ability of a surface mount resistor to dissipate a given amount of power is ultimately limited by the temperature of the active resistor film. Assuming that the effects of convection and radiation are negligible, this film temperature is determined by the accumulation of increases in temperature due to conduction through each thermal impedance between the film and the ultimate "perfect heatsink." This relationship is shown in the block diagram titled "Figure 1." A typical example of this relationship applied to a surface mount resistor is shown in "Figure 2." A detailed analysis of this example and some comments on the extension of the model to the generic problem follow. Figure 2 depicts a typical mounting design for a surface mount resistor. The resistor is soldered to a PC board using Sn62 solder. The PC board has copper traces and a land area for the ground terminal of the resistor that is populated with an array of filled vias. The density of this array is designed at the maximum permitted by the design rules, for the associated PC board manufacturing process, to minimize the thermal impedance of the area under the resistor.