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 GUID-XXXXXXXX-SF0T-XXXX-XXXX-000000318287.html#GUID-XXXXXXXX-SF0L-XXXX-XXXX-000000318287 table is determined by the JEDEC standard (#T5355625-110), 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 8-7 JEDEC Standard 2s2p PCB

#T5355625-111 and #T5355625-112 illustrate the functions of R_θJA and ψ_JB versus copper area and thickness. These plots are generated with a 101.6-mm × 101.6-mm × 1.6-mm PCB of two and four layers. For the 4-layer board, inner planes use 1-oz copper thickness. Outer layers are simulated with both 1-oz and 2-oz copper thickness. A 4 x 5 (DBV package) array of thermal vias with a 300-µm drill diameter and 25-µm copper plating is located as close as practical to the GND pin 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.

As illustrated in #T5355625-112, ψ_JB increases with increasing connecting copper area. The reason for this increase is that the board temperature is measured at the copper near the GND pin, and because the GND pin is fused to the die pad, more heat escapes through the GND pin when more copper is connected to the pad, and thus the temperature at this point is higher. Consequently the ψ_JB increases. This increase does not imply that heat sinking for the device is reduced when more connecting copper is added. Increasing connecting copper area always increases board-level heat sinking for the device. Furthermore, the boards used for #T5355625-112 have vias connecting to internal copper planes. Therefore, ψ_JB is much higher than what is specified in the GUID-XXXXXXXX-SF0T-XXXX-XXXX-000000318287.html#GUID-XXXXXXXX-SF0L-XXXX-XXXX-000000318287 table, which uses the high-K board layout specified in JESD51-7 that has no thermal vias.