Junction-to-Air Thermal Resistance

The thermal resistance parameter (RJ-A) characterizes the heat transfer (or heat flow) between the silicon die, where the heat is generated, and the external ambient air surrounding the IC. As the heat flows from the IC in all possible directions, one part of it is conducted (by physical contact) to the PCB and then dissipated in ambient air, and the other part is conducted directly to the package edges where it is dissipated in ambient air as well.

For this reason, RJ-A strongly depends on the following parameters:

The lower the RJ-A, the more efficient the heat transfer between the silicon die and ambient air. For example, RJ-A = 30°C/W means that for 1W of dissipated power on the die, TJ settles 30°C above the ambient air temperature. With a 20°C/W value, TJ settles only 20°C above the ambient temperature for the same power dissipation.

In applications without heatsink attached to the top of the device case, and in still air conditions, it is commonly assumed that about 70% of the heat generated by the processor is conducted to the PCB. In these cases, the PCB design has a major impact on RJ-A.

The SAMA7G5 Series data sheet provides one RJ-A value for a very specific measurement condition specified by the JEDEC JESD51-2 standard (a 10x10 cm, four-layer PCB, with two signals and two power layers). This RJ-A value only aims at making device comparisons for one given PCB condition. It cannot be used for a rigorous thermal study on a specific PCB.

Any system design should include a thermal study to minimize RJ-A as much as possible. An efficient thermal design (with low RJ-A value) brings the following benefits:

On the contrary, a poor thermal design (high RJ-A value) may raise the following issues: