Article updated on 18 February 2021
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Printed circuit boards (PCBs) are mechanically supported electrical circuits, where a series of components and electrically conductive tracks are fabricated onto the PCB to enable electricity to flow. PCBs are used in all electronic devices. However, whilst their use is now ubiquitous, a lot of designing goes into the PCB before processing and large-scale manufacturing can occur. One of the key aspects of any PCB design is the thermal design, i.e. the heat transfer processes, due to the amount of heat given off by multiple components in a localized area. In this article, we look at how a thermal analysis approach helps to thermally design PCBs.
Thermal Analysis of PCBs
Thermal analysis is the process of measuring how a material/device behaves at different temperatures. For many applications, especially single materials, a thermal analysis is a physical analytical method. However, in the design of PCBs, thermal analysis methods are usually computational methods that help to define the thermal module of various components and set simulation control parameters to measure the effects of temperature (on both components and the whole device) in various operating conditions. Because many PCBs are used in high-tech environments, which often have a varying temperature range (including high temperatures), the PCB needs to undergo a computational thermal analysis at the concept stage. This not only helps to optimize the efficiency of the PCBs and improve its safety, a thermal analysis protocol can deduce any thermal problems with the PCB, temperature dense areas can be identified and removed, and the heat conduction path, position of key components, the radiator shape, heat dissipation rate, and the heat transmission efficiency can all be optimized before processing begins.
Thermally Designing PCBs
There are many areas that need to be considered during the thermal analysis to realize an efficient and optimized PCB. The whole of the thermal design process is based around the theory of heat transfer and fluid mechanics, and thermal analysis helps to break down to the fundamental property levels and analyzes the efficiency of the PCB. Because thermal analysis is concerned with different temperature environments, one of the key considerations in the thermal design of PCBs is the transfer of heat from a high-temperature zone to a low-temperature zone, so analyzing this aspect is crucial, as is the mechanism of heat transfer (heat conduction, heat convection and heat radiation)—which can also be modelled.
Overall, many parts of the actual design process rely on the results of the thermal analysis to better understand how the heat flow and heat transfer processes work in the PCB; and these results and simulations can then be used in the design process to build more efficient circuits whilst determining if other components, such as a heat spreading pad or thermal vias, are required to transfer the heat more efficiently/across a wider area.
The thermal design of a PCB is technically defined as the “the process in which the thermal resistance from heat source to heat consumption space is decreased to minimum through cooling measures by thermal transmission attributes or density of heat fluid is controlled to be within an acceptable range”1. Therefore, to ensure that any design process is reliable, thermal analysis methods on the rates of natural cooling, forced air cooling, fluid cooling, evaporation cooling, alongside lesser important cooling approaches—such as thermotube, cold plate, thermoelectric refrigeration methods—are performed at the concept stage.
PCBs will use one, if not more, of the above cooling mechanisms, hence the need to model and analyze the potential of each one. Natural cooling is the PCBs ability to conduct heat without external strength and often involves the heat transfer mechanisms of heat conduction, radiation heat transfer, and natural convection transfer. On the other hand, forced air cooling is the transfer of heat from the source to the sink, often via a cooling airflow provided by the components on the PCB. There are two different approaches for fluid cooling—direct and indirect—and both involve a coolant, where direct and indirect cooling mechanisms are determined by whether the components directly touch the coolant or not (if not, it is often through a heat exchanger or cool plate). The final cooling mechanism is evaporation cooling, which is the most common method and is facilitated by the ebullition of a cooling medium.
Through modeling all the different cooling mechanisms, the most efficient one for that specific PCB design can be chosen at the manufacturing stage. Additionally, there are also some rules which are followed in any PCB design and these can be used in conjunction with the thermal analysis to better define the parameters of the PCB and help with the final optimal design of the PCB.
Overall, thermal analysis methods are critical to the optimal design of PCBs, as they not only improve the safety due to minimizing localized heat regions that could become problematic, they also help to design the most optimal system. Whilst thermal regulation optimization is only part of a PCB design, it can result in a lower energy device, due to a lower amount of heat and power loss, and this can result in a PCB with a greater longevity and lower energy consumption (i.e. cheaper to run).
References
- PCB Cart: https://www.pcbcart.com/article/content/principles-of-thermal-design-for-PCB.html
Sources and Further Reading
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