Thermal Resistance Data: Definitions of Thermal Resistance, Thermal Characterization Parameters

In this article, we define θ_JA and Ψ_JT, parameters of actual thermal resistance data presented in the previous article.

Definitions of θ_JA and Ψ_JT

We begin by reviewing the previous article.

• ・θ_JA（℃/W）：Junction-to-ambient thermal resistance
• ・Ψ_JT（℃/W）：Junction-to-top of the package thermal characterization parameter

The following schematic diagram enables the reader to conceptualize θ_JA and Ψ_JT.

θ_JA is the thermal resistance from the junction to the ambient environment; heat dissipation occurs over multiple thermal paths. Ψ_JT is the thermal characterization parameter from the junction to the center of the top surface of the package.

Moreover, the thermal resistance from the junction to the package top surface (θ_JC-TOP) and the thermal resistance from the junction to the package bottom surface (θ_JC-BOT) are also defined, and are illustrated below. Where θ_JC-TOP and Ψ_JT are concerned, it should be noted that there is a slight difference between the “package top surface” and the “center of the package top surface”.

These are defined by the JEDEC standard JESD51. Below we have summarized the definitions, applications, and calculation formulae.

• ※1： The ambient temperature (T_A) is the temperature of the air in the vicinity of a position where there is no effect from the component being measured, that is, outside the boundary layer of the heat source.
• ※2：θ_JA and Ψ_JT　are data at the time of mounting on a JEDEC board.
• ※3：θ_JC-TOP and θ_JC-BOT are measured in conformance with JESD51-14 (TDI method).