2021.11.24

Points of this article

・It is deemed difficult to study thermal design in terms of θ_{JA} starting from the conditions of packaging used in recent devices.

・It is difficult to provide a definition of T_{A} that can be used uniformly for all recent hardware. Instead, separate definitions are sought.

・Actual measurements of T_{A} in equipment with high measurement densities are extremely difficult.

・In recent years, estimation of T_{J} using T_{T} and Ψ_{JT}, which are relatively easy to measure, has come into widespread use.

In the previous article, the definitions of the θ_{JA} and Ψ_{JT} parameters of thermal resistance data were explained. In this article and the next, we would like to consider how θ_{JA} and Ψ_{JT} are used, or can be used, in calculations to estimate T_{J}. We will later provide a separate explanation of calculations to estimate T_{J} using thermal resistance data.

This table is an excerpt relating to θ_{JA} and Ψ_{JT}, explained in the previous article. 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. The T_{T} included in the formula for Ψ_{JT} is the temperature at the center of the top surface of the package.

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

θ_{JA} |
Thermal resistance between the junction and the ambient environment | 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 actual equipment (actual heat dissipation environment) | Ψ_{JT}＝(T_{J}ーT_{T}) / P |

As applications, it is suggested that θ_{JA} may be used in “comparisons of dissipation performance of packages with different shapes”, and that Ψ_{JT} may be used to “estimate junction temperature for actual equipment”. The reasons for this are here considered.

There have been studies to determine whether θ_{JA} can be applied to thermal design. The upshot is that it is thought to be difficult to use θ_{JA} for thermal design. The main reason is as follows.

●T_{A} is the temperature at which site?

Ultimately, it is necessary to be able to estimate the value of T_{J}. In order to use θ_{JA} to determine T_{J}, the temperature T_{A} of the surrounding environment is needed.

The JEDEC Standards define the temperature T_{A}. The JEDEC standard used as reference is as follows.

▶JESD51-2A Integrated Circuits Thermal Test Method Environmental Conditions ? Natural Convection (Still Air)

In essence, T_{A} is measured at places conforming to the JEDEC standard, but there are also cases in which some manufacturers stipulate separate conditions for T_{A} measurement.

Moreover, the JEDEC standards define T_{A} in a space that is not affected by heat generation; but in recent years, there is highly dense component mounting in some equipment, and it is unclear whether there is a space at all that is not affected by heat generation.

●Trend towards high-density mounting

As indicated previously, due to high-density mounting, ICs and other components that emit heat are packed closely together on a PCB. Rises in temperature occur due to thermal interference from ICs and the like that are adjacent to the component in question, and so it is easy to understand the difficulty in deciding whether the temperature at a place one might consider appropriate for actual T_{A} measurement is in fact the correct T_{A}.

●When the effective heat dissipation range changes, θ_{JA} changes

The θ_{JA} indicated on the data sheet for an IC with a surface-mount package has as conditions a copper foil area for heat dissipation and the material and thickness of the PCB. Put otherwise, this can be taken to mean that θ_{JA} changes depending on the mounting conditions. The graph is an example of data showing the relationship between θ_{JA} and the surface copper foil area of the IC mounting site. It goes without saying that as the copper foil area increases, θ_{JA} diminishes; but the change in θ_{JA} is not linear, and if such a graph is not provided, it is considerably difficult to estimate θ_{JA} from the relevant area of the actual board. Unfortunately, a graph of this kind is not always provided.

Due to these circumstances, it is deemed difficult, particularly given current trends, to use θ_{JA} in thermal design. In recent years, what is becoming the most widely used method for estimating T_{J} involves measuring the temperature T_{T} at the center of the top surface of the package in question, and then calculating T_{J} from Ψ_{JT}.

Ψ_{JT} is a thermal characterization parameter representing the temperature difference between the junction and the center of the top surface of the package for a power consumption P of the entire device. The diagram below is a conceptual representation of T_{J} and T_{T}. Because T_{T} is the temperature at the center of the top surface of the package, it can be measured using a thermocouple or other means with actual equipment in the operating state.

If T_{T} can be acquired, the equation initially shown for Ψ_{JT} can be modified to determine T_{J}.

Ψ_{JT}＝(T_{J}ーT_{T}) / P ⇒ T_{J}＝T_{T}＋Ψ_{JT}×P

Here Ψ_{JT}×P is the temperature difference between T_{J} and T_{T}, and so by adding this to T_{T}, T_{J} is obtained.

In the next article, we will explain the relations between conditions in actual equipment and the parameters θ_{JA} and Ψ_{JT}, and will discuss the usefulness of Ψ_{JT} in estimating T_{J}.

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: Definitions of Thermal Resistance, Thermal Characterization Parameters
- 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
- Surface Temperature Measurements: Thermocouple Types