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2021.11.24 Thermal Design

Thermal Resistance Data: θJA and ΨJT in Estimation of TJ: Part 1

Thermal Design of Semiconductor Components in Electronic Equipment

In the previous article, the definitions of the θJA and ΨJT parameters of thermal resistance data were explained. In this article and the next, we would like to consider how θJA and ΨJT are used, or can be used, in calculations to estimate TJ. We will later provide a separate explanation of calculations to estimate TJ using thermal resistance data.

θJA and ΨJT

This table is an excerpt relating to θJA and ΨJT, explained in the previous article. 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. The TT included in the formula for ΨJT is the temperature at the center of the top surface of the package.

Symbol Definition Applications Formula
θJA Thermal resistance between the junction and the ambient environment 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 actual equipment (actual heat dissipation environment) ΨJT=(TJーTT) / P

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

Regarding θJA

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

●TA is the temperature at which site?
Ultimately, it is necessary to be able to estimate the value of TJ. In order to use θJA to determine TJ, the temperature TA of the surrounding environment is needed.

The JEDEC Standards define the temperature TA. The JEDEC standard used as reference is as follows.

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

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

Moreover, the JEDEC standards define TA 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 TA measurement is in fact the correct TA.

●When the effective heat dissipation range changes, θJA changes
The θJA 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 θJA changes depending on the mounting conditions. The graph is an example of data showing the relationship between θJA and the surface copper foil area of the IC mounting site. It goes without saying that as the copper foil area increases, θJA diminishes; but the change in θJA is not linear, and if such a graph is not provided, it is considerably difficult to estimate θJA 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 θJA in thermal design. In recent years, what is becoming the most widely used method for estimating TJ involves measuring the temperature TT at the center of the top surface of the package in question, and then calculating TJ from ΨJT.

Regarding ΨJT

ΨJT 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 TJ and TT. Because TT 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 TT can be acquired, the equation initially shown for ΨJT can be modified to determine TJ.

 ΨJT=(TJーTT) / P  ⇒  TJ=TT+ΨJT×P

Here ΨJT×P is the temperature difference between TJ and TT, and so by adding this to TT, TJ is obtained.

In the next article, we will explain the relations between conditions in actual equipment and the parameters θJA and ΨJT, and will discuss the usefulness of ΨJT in estimating TJ.

Key Points:

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

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

・Actual measurements of TA in equipment with high measurement densities are extremely difficult.

・In recent years, estimation of TJ using TT and ΨJT, which are relatively easy to measure, has come into widespread use.

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