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 in the Section 6 section is determined by the JEDEC standard (see Figure 8-1), 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 8-2 and Figure 8-3 depict the functions of R_θJA and ψ_JB versus copper area and thickness. These plots are generated with a 101.6-mm x 101.6-mm x 1.6-mm PCB of two and four layers. For the four-layer board, the inner planes use a 1-oz copper thickness. Outer layers are simulated with both a 1-oz and 2-oz copper thickness. A 4 x 4 array of thermal vias of 300-µm drill diameter and 25-µm Cu plating is located beneath the thermal pad of the device. The thermal vias connect the top layer, the bottom layer and, in the case of the 4-layer board, the first inner GND plane. Each of the layers has a copper plane of equal area.