In recent electronic equipment design, in addition to miniaturization and efficiency enhancements, noise-related EMC (electromagnetic compatibility) measures and the like have also been addressed; apart from these, there has been a focus on heat-related problems, and thermal design has become an important issue. Heat is related to the performance and reliability of components and equipment, and is also connected to safety, and as such as been an important area of study for some time. We asked Mr. Ippei Yasutake of the System Solutions Engineering Headquarters at ROHM about the reasons for the current emphasis on heat issues.
―Today I'd like to ask about problems in thermal design. To begin with, could you please summarize thermal design, and explain why it is necessary. That will serve as a foundation for other things I'd like to ask you.
Very well then. In essence, I will be talking about semiconductor components, such as ICs and transistors. Absolute maximum ratings are established for all semiconductor components; among those that are related to temperature, the junction temperature, that, is, the temperature Tjmax of the chip within the package, is specified. Of course it is necessary to use a design such that Tjmax is not exceeded. When considering the equipment as a whole, there are absolute maximum ratings not only for semiconductor components, but also for capacitors, resistors and the like, relating to temperatures and to power losses, and care must be taken to ensure that these ratings likewise are not exceeded.
For this reason, thermal calculation is performed, to reduce thermal resistance or boost heat dissipation performance so that Tjmax is not exceeded, and values are kept within the maximum ratings. This is the essence of thermal design.
If heat issues are not dealt with properly in the design stage, problems originating in heat issues may be discovered in the product prototyping stage or, in some cases, immediately before mass production. While this is not an observation limited to thermal issues, countermeasures incur greater time and cost penalties the closer they are to mass production, and if product shipment is delayed, there may be the enormous problem of lost business opportunities. In a worst-case scenario, problems in the marketplace may arise, leading inevitably to recalls and to loss of trust. Thus thermal design is extremely important. And precise thermal design starting from the initial stage is essential.
―I understand what you are saying. But, aren't these things basic and universal things that have been known since long ago? Why have they emerged as important issues in recent years?
Put simply, this is because the density of heat generation has been rising in recent years, due to the miniaturization and more sophisticated functionality of equipment and components. In an increasing number of cases, such problems cannot be addressed adequately using conventional thermal countermeasures or thermal calculation methods.
―I understand the gist of what you are saying, but could you please be a little more specific.
What I would like for you to understand first of all is that changes in engineering trends are creating headwinds for thermal design environments. The previously mentioned miniaturization and sophisticated functionality of equipment and components, as well as design properties, are also related.
―Well then, please start with miniaturization.
Due to requests for smaller product sizes, miniaturization not only of PCBs and ICs but of capacitors and other components have likewise proceeded. In component miniaturization, it is no longer rare for an IC chip that previously had been provided in a comparatively large through-hole-mount package, such as theTO-220 package, to be embedded in a far smaller surface-mount package.
There are also methods that use higher integration levels. Including two IC chips in a package intended for a single chip, or enclosing a chip equivalent to two chips, are also possible. Such component miniaturization and higher integration levels increase heat generation. These are images of temperature distribution, comparing the cases of a 20×20×20 mm package and a 10×10×10 mm package of components that consume the same amount of power. The smaller package clearly results in greater heat generation. Moreover, there is also a clear difference in the cases of one chip and two chips housed in a package of the same size.
Moreover, when such components are mounted at high density on a PCB, the heat generation density rises further, and because the footprint is decreased, the effective heat dissipation area is also diminished, and the PCB as a whole reaches higher temperatures, causing the temperatures of other components with smaller heat generation to rise as well. The graphics below represent this visually, showing how, when heat-producing components are in close proximity, the temperature of the PCB overall rises.
―What kind of effect does "sophisticated functionality" have?
In order to enhance equipment functionality, devices are added, or more capable ICs with a higher degree of integration are used, and faster processing or higher frequency operations is employed, so that power consumption increases, and consequently there is a tendency for more heat to be generated. At higher frequencies, shields are needed to suppress noise, but heat resides within the shields. Moreover, it is difficult to increase the sizes of housings in order to expand functionality, so that the density within a housing increases, the temperature within the housing rises, and heat dissipation is further impeded.
―And what are "design properties"?
An increasing number of products are placing emphasis on design in the interest of differentiating the products and achieving higher ratings. In some cases, design takes precedence, so that holes for air intake and discharge cannot be provided at appropriate positions, in some cases resulting in problems due to elevated temperatures within housings. In the interest of design properties, that is, for a higher degree of freedom in determining the outer shape of a product, components are being made smaller and shorter, but even so, there are cases in which design is given priority.
Thus heat generation is increasing due to these three trends--miniaturization, sophisticated functionality, and design properties--and so thermal design is tasked with daunting problems, such as the greater difficulty of dissipating heat.