The most used thermal resistance parameter R_θJA is highly dependent on the heat-spreading capability built into the particular PCB design, and therefore varies according to the total copper area, copper weight, and location of the planes. The R_θJA recorded in the Thermal Information table is determined by the JEDEC standard (Figure 7-8), PCB, and copper-spreading area, and is only used as a relative measure of package thermal performance. For a well-designed thermal layout, R_θJA is actually the sum of the package junction-to-case (bottom) thermal resistance (R_θJCbot) plus the thermal resistance contribution by the PCB copper.

Figure 7-8 JEDEC Standard 2s2p PCB