Unlike conduction and convection heat transfers, there is no medium involved between hot and cold bodies in thermal radiation heat transfers.
The emissive power of electronic devices is greater for thermal radiation heat transfers than for convection heat transfers.
In semiconductor electronic devices, heat transfers through thermal radiation occur mostly above the junction temperature.
Thermal radiation heat transfers
Thermal radiation heat transfer is a heat transfer method that plays an important role in the thermal management of certain electronic devices. The efficiency with which electronics cool can be improved by utilizing radiation heat transfers. Thermal radiation heat transfers contribute to low-air velocity and natural convection applications. Similar to other heat transfer methods such as conduction and convection, the temperature difference between an object and its surroundings matters in the radiation cooling of electronic circuits. Apart from the temperature difference, the surface texture of the electronic circuit or chassis and the view of the surroundings also influence thermal radiation heat transfers.
Let’s take a closer look at what thermal radiation heat transfers are and the factors that influence them.
Thermal Radiation Heat Transfers
In electronics, conduction and convection are the most commonly used heat transfer methods for cooling. In conduction and convection heat transfers, a medium is required to transfer heat from a hot body to a cold body. However, in thermal radiation heat transfers, there is no medium involved between the hot and cold bodies. In fact, the heat transfer caused by thermal radiation actually becomes more effective when there is no interfering medium in between the hot and cold regions. In electronics, thermal radiation can be utilized to transfer the heat from circuits to the ambient.
What Happens During Thermal Radiation Heat Transfers?
When an electronic component operates, it produces vibrational and rotational motions of the electrons, atoms, and molecules in the component materials. These internal motions result in thermal radiation emissions. Depending on the intensity of the vibrational and rotational motion, the temperature is experienced on the component’s surface. The temperature experienced depends on the intensity of the motional activities and holds a direct relationship with the thermal radiation emissions.
The thermal radiation emissions in radiation heat transfer correspond to the energy transfer between the hot body and cold body. Thermal energy is transported via electromagnetic waves that travel at the speed of light. The thermal radiation suffers no attenuation when traveling through vacuum. The electromagnetic waves that transport thermal energy in the vacuum or any other medium can be characterized by wavelength (λ) and frequency (v).
The frequency of thermal radiation electromagnetic waves that transport thermal energy is independent of the medium. It depends only on the source of thermal energy. The wavelength of thermal radiation electromagnetic waves is in the range of 10-1 to 102 µm. The thermal radiation electromagnetic wavelength and frequency share an inverse relationship (given in the equation below):
Note that c is the speed of light.
The speed of thermal radiation electromagnetic waves will be different in different mediums depending on the refractive index (n). If c0 is the speed of light in vacuum, then the speed of light in a medium (c) can be expressed as:
c=c0 / n
Thermal radiation heat transfer is a volumetric phenomenon. Let’s learn more about the factors that influence thermal radiation heat transfers.
Factors Influencing Thermal Radiation Heat Transfers
There are three main factors influencing thermal radiation heat transfers:
The temperature difference between hot electronic circuit components and their surroundings.
When there is a large temperature difference between an electronic component and its surroundings, the electronic component emits electromagnetic radiation to transfer heat energy. In semiconductor electronic devices, heat transfers through thermal radiation occur mostly above the junction temperature. The emissive power of electronic devices is more for thermal radiation heat transfers than convection heat transfers.
The texture of the object’s surface and its surroundings.
Device surface characteristics and their baseplate or chassis surface are important factors for effective thermal radiation heat transfers. The emissivity of electronic devices can be increased by focusing on surface texture. Materials used in electronics are usually opaque in nature. Therefore, the surface texture of the metals and non-metals used in electronic circuits needs to be given great attention up to a depth of 0.0001 and 0.02 inches, respectively. Normally, surface color does not influence the emissivity for the range of radiations coming from electronics heating.
The view that the object has of its surroundings.
The view that the object has of its surroundings is usually expressed using the view factor. It is the fraction of the thermal radiation that leaves one surface and intercepts by another surface. The view factor increases when two surfaces are completely overlapped. The maximum value of the view factor is unity. The view factor decreases to a low value when the surfaces are in different planes, making the angle between them almost equal to 180°.
Using Thermal Radiation Heat Transfers for Electronics Cooling
Utilizing thermal radiation heat transfer methods for cooling is an effective approach to thermal management in electronic circuits. To achieve thermal radiation heat transfers from electronic components, the temperature difference between the circuit and ambient needs to be high. For circuits operating in high-temperature conditions, thermal radiation heat transfer cooling will be the most effective. Cadence’s software can help you implement a suitable cooling system for your electronic circuits.
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