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2021.08.18 Thermal Design

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

Thermal Design of Semiconductor Components in Electronic Equipment

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

Definitions of θJA and ΨJT

We begin by reviewing the previous article.

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

The following schematic diagram enables the reader to conceptualize θJA and ΨJT.

θJA is the thermal resistance from the junction to the ambient environment; heat dissipation occurs over multiple thermal paths. ΨJT 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 (θJC-TOP) and the thermal resistance from the junction to the package bottom surface (θJC-BOT) are also defined, and are illustrated below. Where θJC-TOP and ΨJT 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.

Symbol Definition Applications Formula
θJA Junction-to-ambient thermal resistance※1 Comparison of heat dissipation performance of packages with different shapes θJA = (TJ – TA) / P
ΨJT Thermal characterization parameter representing the temperature difference between junction and package top center for a power consumption P of the entire device Estimation of the junction temperature for an entire equipment (actual heat dissipation environment) ΨJT = (TJ – TT) / P
θJC-TOP Junction-to-package-top thermal resistance. Heat dissipation path is at the package top surface only, thermal insulation elsewhere. Used in simulations of heat conduction, thermal fluids, etc.
Can also be applied to thermal resistance network method
θJC-TOP = (TJ – TC-TOP) / P
θJC-BOT Junction-to-package-bottom thermal resistance. Heat dissipation path is at the package bottom surface only, thermal insulation elsewhere. Used in simulations of heat conduction, thermal fluids, etc.
Can also be applied to thermal resistance network method
θJC-BOT = (TJ – TC-BOT) / P
  • ※1: The ambient temperature (TA) 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:θJA and ΨJT are data at the time of mounting on a JEDEC board.
  • ※3:θJC-TOP and θJC-BOT are measured in conformance with JESD51-14 (TDI method).

Key Points:

・Thermal resistance and thermal characterization parameters are defined in JESD51, which is a JEDEC standard.

・There are basic applications for the different thermal resistances and thermal characterization parameters; the relevant thermal resistances and thermal characterization parameters are used in calculations.

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