Unfortunately, R_θJA has often been used by system designers to estimate junction temperatures of their devices when used in their systems. The equation usually assumed to be valid for calculating junction temperature from R_θJA is:

This is a misapplication of the R_θJA thermal parameter because R_θJA is a variable function of not just the package, but of many other system level characteristics such as the design and layout of the printed circuit board (PCB) on which the part is mounted. In effect, the test board is a heat sink that is soldered to the leads of the device. Changing the design or configuration of the test board changes the efficiency of the heat sink and therefore the measured R_θJA. In fact, in still-air JEDEC-defined R_θJA measurements, almost 70%–95% of the power generated by the chip is dissipated from the test board, not from the surfaces of the package. Because a system board rarely approximates the test coupon used to determine R_θJA, application of R_θJA using Equation 1 results in extremely erroneous values.

Table 1-1 lists factors that can influence R_θJA for a given package outline when all materials are held constant. The first column lists the factor while the second column gives a rule of thumb estimate as to the impact of the factor.

In light of the fact that R_θJA is not a characteristic of the package by itself but of the package, PCB, and other environmental factors, it is best used as a comparison of package thermal performance between different companies. For example, if TI reports an R_θJA of 40°C/W for a package compared to a competitor's value of 45°C/W, the TI part will likely run 10% cooler in an application than the competitor's part.