2023.01.25

Points of this article

・When heat-generating ICs are mounted close together, thermal interference occurs, inducing rises in temperature.

・The required θ_{JA} is found from the allowed maximum T_{J}, and the resulting necessary heat dissipation area is estimated.

Up to this point, methods for estimating T_{J} using thermal resistance and thermal characterization parameters have been explained. In this article, a method is described for estimating the heat dissipation area in surface mounting in order to ensure that T_{Jmax} is obeyed, and important points relating to the thermal consequences of component layout are explained.

Demands for miniaturization in recent years have led to the need for circuit boards that are as small as possible, and component mounting densities are trending upwards. However, components that emit heat, which in this case are ICs, require use of the mounting board for heat dissipation, and to this end a certain area is needed. If this board area is not secured, thermal resistance is higher and heat generation becomes excessive. If heat-generating ICs are mounted in close proximity, interference by their generated heat may cause increases in temperature.

The figures below illustrate heat dissipation paths of a surface-mounted IC, and heat dissipation when heat-generating ICs are in close proximity.

Heat generated by an IC is conducted laterally (over a board area) and vertically (through the board thickness) and is dissipated. However, when ICs are densely mounted, there is mutual interference by heat dissipation in lateral directions in particular–the heat has nowhere to go. Hence temperature increases result.

**Estimating the Board Area Needed for Heat Dissipation**

In order to ensure that a surface-mounted IC can have a thermal resistance that remains within the T_{Jmax} limit, a corresponding heat dissipation area is necessary. In addition, it is also important to ensure that thermal interference does not occur. The diagram below illustrates the interval needed so that there is no thermal interference. After ensuring that this minimum condition is satisfied, the required heat dissipation area is estimated using a graph of the relationship between θ_{JA} and the copper foil area.

As the diagram indicates, the minimum interval is set such that there is no interference between straight lines inclined 45° from the edges of ICs to the board surface. Next, the required θ_{JA} for the conditions of use is determined.

Conditions example: IC power consumption=1 W, maximum ambient temperature T_{A(HT)}=85°C, maximum allowed T_{J}=140°C

From the graph on the right, we see that in order to set θ_{JA} to 55°C/W, a copper foil area of at least 500 mm^{2} is necessary.

In this way, IC intervals that avoid heat interference and a sufficient heat dissipation area are secured, and then the final placement of ICs is studied.

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 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
- Surface Temperature Measurements: Thermocouple Types