The junction-to-ambient thermal resistance, R_θJA, is the most commonly reported thermal metric and is the most often misused. R_θJA is a measure of the thermal performance of an IC package mounted on a specific test coupon. The intent of R_θJA is to give a metric by which the relative thermal performance of a package can be compared. Thus, the thermal performance of a TI device can be compared to a device from another company. This is true when both companies use a standardized test to measure R_θJA, such as that specified by JEDEC in the EIA/JESD51-x series of documents. Sometimes, however, JEDEC conditions are not followed and the excursions from the standards are not documented. These test variations can have a dramatic effect on the measured values of R_θJA. Therefore, unless test conditions are reported with the R_θJA value, they should be considered suspect.

The measurement of R_θJA is performed using the following steps (summarized from EIA/JESD51-1, -2, -5, -6, -7, and -9):

1. A device, usually an integrated circuit (IC) package containing a thermal test chip that can both dissipate power and measure the maximum chip temperature, is mounted on a test board.
2. The temperature sensing component of the test chip is calibrated.
3. The package- and test-board system is placed in either a still air (R_θJA) or moving air (R_θJMA) environment.
4. A known power is dissipated in the test chip.
5. After steady state is reached, the junction temperature is measured.
6. The difference in measured ambient temperature compared to the measured junction temperature is calculated and is divided by the dissipated power, giving a value for R_θJA in °C/W.