Rthja

RthJA, or thermal resistance junction-to-ambient, is a parameter used in electronics to describe how much the junction temperature of a semiconductor device rises for each watt of power dissipated when the device is in ambient conditions. It represents the overall heat path from the semiconductor die through the package and into the surrounding air, including any mounting surfaces, printed circuit board copper, heatsinks, and air movement.

RthJA is typically specified in device datasheets for a given package and mounting condition, usually under still air or natural convection with no heatsink. The value depends on several factors, including package type, die-to-package bond quality, the amount of copper area and surface contact with ambient, and the presence of any heatsinking or forced cooling.

RthJA is related to other thermal resistances. It can be viewed as the sum of junction-to-case resistance (RthJC) and case-to-ambient resistance (RthCA): RthJA = RthJC + RthCA. Because of this, reducing RthJA can be achieved by using better mounting, increasing copper area on the PCB, adding a heatsink, or improving airflow.

Design use: to estimate the junction temperature Tj from ambient temperature Ta and device power P, Tj ≈ Ta + P × RthJA. This helps determine whether operating conditions are safe and what cooling measures may be required. Values vary widely by package and board layout, typically ranging from tens to hundreds of degrees Celsius per watt.