2021.08.18

Points of this article

・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.

In this article, we define θ_{JA} and Ψ_{JT}, parameters of actual thermal resistance data presented in the previous article.

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.

Symbol | Definition | Applications | Formula |
---|---|---|---|

θ_{JA} |
Junction-to-ambient thermal resistance※1 | Comparison of heat dissipation performance of packages with different shapes | θ_{JA} = (T_{J} ? T_{A}) / P |

Ψ_{JT} |
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) | Ψ_{JT} = (T_{J} ? T_{T}) / P |

θ_{JC-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 |
θ_{JC-TOP} = (T_{J} ? T_{C-TOP}) / P |

θ_{JC-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 |
θ_{JC-BOT} = (T_{J} ? T_{C-BOT}) / P |

- ※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).

Downloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.

- About Thermal Design
- Changes in Engineering Trends and Thermal Design
- A Mutual Understanding of Thermal Design
- Fundamentals of Thermal Resistance and Heat Dissipation: Heat Transmission and Heat Dissipation Paths
- Fundamentals of Thermal Resistance and Heat Dissipation: About Thermal Resistance
- Fundamentals of Thermal Resistance and Heat Dissipation : Thermal Resistance in Conduction
- Fundamentals of Thermal Resistance and Heat Dissipation : Thermal Resistance in Convection
- Fundamentals of Thermal Resistance and Heat Dissipation : Thermal Resistance in Emission
- Thermal Resistance Data: JEDEC Standards, Thermal Resistance Measurement Environments, and Circuit Boards
- Thermal Resistance Data: Actual Data Example
- Thermal Resistance Data: θJA and ΨJT in Estimation of TJ: Part 1
- Thermal Resistance Data: θJA and ΨJT in Estimation of TJ: Part 2
- Estimating TJ: Basic Calculation Equations
- Estimating TJ: Calculation Example Using θJA
- Estimating TJ: Calculation Example Using ΨJT
- Estimating TJ: Calculation Example Using Transient Thermal Resistance
- Estimation of Heat Dissipation Area in Surface Mounting and Points to be Noted