Rth

Rth, short for thermal resistance, is a parameter used in electronics to describe how effectively a device or package conducts heat away from a hot region to a cooler region. It is usually expressed in kelvin per watt (K/W) or degrees Celsius per watt (°C/W). In datasheets, several forms are commonly specified: Rth(j-a) or RthJA, the thermal resistance from the junction (the semiconductor die) to ambient; Rth(j-c) or RthJC, from the junction to the case; and sometimes Rth(c-a) or RthCA, from the case to ambient, which depends on mounting and cooling.

The temperature rise of the junction due to power dissipation P is approximately ΔTj ≈ P × Rth. For example, with P = 2 W and RthJA = 25 K/W, the junction temperature would rise about 50 °C above ambient under the specified conditions. These values are conditional on the test setup and mounting, and they may change with airflow, board copper area, heatsink attachment, and other cooling methods.

Rth values are highly dependent on packaging, mounting, board layout, and cooling solutions. They are specification-sensitive and typically provided under defined test conditions; designers use them to determine whether a device will operate within its maximum junction temperature given expected ambient temperatures and cooling. In practice, achieving lower thermal resistance involves selecting appropriate heat sinks, reducing RthJC, and increasing heat spreading with copper area, vias, or other thermal management techniques on printed circuit boards.

Rth is a fundamental concept in thermal management, used in design calculations, simulations, and reliability assessments to ensure safe and reliable operation of electronic devices.