2021.01.20 Thermal Design
In the previous article, titled "About Thermal Design," a summary explanation of the importance of thermal design was given. This article is devoted to a somewhat more detailed explanation.
Changes in Engineering Trends and Thermal Design
Engineering trends in recent years have placed emphasis on miniaturization, performance enhancement, and design properties. Below we consider how these trends affect heat and thermal design.
Due to demands for product miniaturization, efforts are underway to similarly miniaturize PCBs as well as capacitors and other components. With respect to miniaturization of semiconductor components, it is no longer rare for IC chips that previously had been provided in relatively large packages using through-holes, such as the TO-220 package, to be embedded in far smaller surface-mount packages.
Approaches involving higher integration levels are also taken. For example, two IC chips are provided in the same package in a dual configuration, or else a chip equivalent to two components is incorporated in a package, to raise the integration level, thereby raising the ratio of functionality to footprint.
Such component miniaturization and elevated integration levels act to increase heat generation. An actual example is presented below. The thermal images on the left are for an example of package miniaturization, comparing packages measuring 20×20×20 mm and 10×10×10 mm that consume the same power. It is clear that the red color, indicating higher temperatures, is concentrated on the smaller package; that is, heat generation is greater. The right side is an example of higher integration, comparing the same size package with one chip and with two chips; the difference in temperatures is obvious.
Further, high-density mounting methods are also used in which components that have been miniaturized or have higher integration levels are mounted at high density and on both sides of compact PCBs, and PCBs are packed tightly into housings.
In high-density mounting, the effective heat dissipation range of surface-mounted components that dissipate heat to the PCB is decreased, and heat generation increases. When the ambient temperature within a housing is high also, less heat can be dissipated. Consequently, whereas in the past it was hot only in the vicinity of heat-generating components, at higher densities the entire board reaches higher temperatures. For this reason, the temperature rises even for components that generate less heat.
In order to enhance equipment functionality, the number of devices may be increased, ICs with higher integration levels and greater functionality may be used, and it may be necessary to perform high-speed data processing or use higher-frequency signals, for example. These measures tend to require ever-greater power consumption, with the result that heat generation increases. Further, in many cases shields are necessary in order to suppress noise radiation when handling high frequencies. Heat builds up inside of shields, worsening the thermal conditions for devices within the shields. Moreover, it is difficult to increase the size of equipment in order to expand functionality, and the resulting high-density state described above means that temperatures are higher within housings and cases.
In order to differentiate products and appeal to aesthetic sensitivities, many products today emphasize design and even prioritize design. Unfortunate consequences of this include extremely high mounting densities and the inability to discharge heat appropriately, and there are cases in which housings become so hot as to cause problems. Put simply, some mobile devices have even felt hot when they are held. As explained above, components have been made smaller and shorter in the interest of design, that is, to achieve greater freedom in designing external shapes, but there are not a few cases in which design is given still greater priority.
Increased Heat Generation and Poor Heat Dissipation are Not the Only Problems
As explained up till this point, due to the changes in engineering trends with respect to miniaturization, performance enhancement, and design properties, although heat generation is increasing, it is becoming more difficult to ensure heat dissipation. Hence severe demands are being made on thermal design. And while this is certainly a major problem, there is another issue that should be examined.
In many cases, companies have probably established criteria for evaluation of thermal design as part of equipment design. If these evaluation criteria have been in place for some time, when they are not being reviewed in the light of recent engineering trends, the evaluation criteria themselves become an issue. If evaluation criteria that do not take into account the current state of affairs are followed without such examination, the possible occurrence of major problems must be considered.
In order to accommodate changes in engineering trends, it will also be necessary to review evaluation criteria for thermal design.
・As recent engineering trends, much emphasis has been placed on miniaturization, performance enhancement, and design properties.
・Because of these trends, heat generation has increased and heat dissipation has worsened, so that thermal design has become more difficult.
・It is important to examine whether existing criteria for evaluation of thermal designs can accommodate these current engineering trends.