Estimated reading time: 4 minutes

Flexible Thinking: Power and Thermal Management—Dealing With the Heat

Without power, electronics are useless. With power, miracles happen. Managing that power is critical in both design and operation in terms of heat generation and energy conservation, especially for battery-powered devices. Moreover, often in electronic products, designers find themselves providing power to an electronic module or system at multiple different voltages and currents. This brings to fore one of the things that is often underappreciated until after the fact, and that is power’s omnipresent by-product—heat. 
 
Heat is a reality in the operation of every electronic device, no matter the size. The amount of heat produced by some devices, such as digital watches and small calculators, may be nearly immeasurable, but where there is resistance, there is also heat generated. However, for larger and more powerful electronic devices and systems, heat is much more apparent; anyone who has used a modern laptop on their lap for any length of time has likely felt the accompanying discomfort. 
 
With the industry’s unceasing effort to increase product performance by shrinking transistors and circuits, an unfortunate increase in thermal energy densities on ICs has resulted. Today, the matter of elevated temperatures on the chip is an increasingly important issue for chip, package, and system designers for a very important reason: there’s an inverse relationship between long-term reliability of electronics (specifically transistors) and higher temperatures. 
 
The simple fact is that as the chip gets hotter and/or spends more time at an elevated temperature, the reliability of the IC die, and the product in which it is used, tends to worsen. This is due in large part to the shrinking material gap for the diffusion of metals to an inevitable short and failure as transistors shrink with each new node, which is now on the cusp of 4–5 nanometers. Earlier transistor nodes could be expected to last for decades or even centuries; today, the expectations can be measured in a few years or even months. Armed with this knowledge, technologists have directed significantly more attention toward the thermal management of electronic systems. Once an afterthought, management of the thermal effects is increasingly moving up in the design process. 
 
When it comes to the task of cooling, two, staged modes of thermal transfer are used: primary thermal transfer modes and secondary thermal transfer modes. The primary modes are generally based on conduction, the first of which is direct thermal transfer normally through a solid conductive material as a metal since metals are generally good thermal conductors. However, they can vary widely in terms of their thermal conductivity. 
 
To interface with the device requiring heat removal and the feature that affects that removal, a thermal interface material (TIM) may be used. These specialty materials bridge the gap between the two surfaces completely to assure there are no “hot spots” or points where the heat generated by the device would otherwise be excessive. Moreover, they often serve to mitigate the difference in rates of thermal expansion between the heat source and the heat removal device. 
 
The method of heat energy transfer to the local environment for most electronics is by convection. (Thermal engineers frequently use the axiom, “At the end of the day, the heat all goes back to air.”) An example of a conventional  ?release structure would be a finned metal which acts as a radiator. This can be done in concert with a heat transfer accelerator, such as a heat pipe, a sealed system with micro-channels and a fluid. 
 
When one end of the heat pipe is placed on a hot surface (e.g., an IC chip), there is the evaporation of the internal liquid at the interface with the chip and cooling and condensation at the distal end. A heat pipe offers much better thermal performance than solid metal and can be very low-profile (~0.5 mm), self-contained hollow metal device filled with a liquid that cycles from liquid to gas and condenses back to liquid to remove more heat from the device. 
 
If you have ever looked at a recent computer motherboard, you will see a finned metal device that looks like it was designed more for a modern Formula 1 racing car than a computer motherboard (Figure 1). This high-performance CPU cooler has a horizontal vapor chamber, eight heat pipes, and can reportedly dissipate up to 250 watts from its microprocessor-sized footprint—think cooking stove heat densities. 
 
While voltages and operating currents have been dropping steadily, due to shrinking transistor sizes, watt densities have been increasing dramatically over the last decade because of the huge increase in transistor counts on some ICs that in some applications exceed one billion. One billion transistors times billions of on-off switches, and even a minuscule amount of energy per cycle, adds up. 
 
Summary
In summary, electronics continue to find applications in ever-increasing areas of daily life. Power management by design, both before and during use, is going to be critical. When making decisions, product designers need to consider many relevant and important issues. Power management will likely only increase in importance. In the future, it will be more necessary to think about managing the thermal aspects of power usage upfront rather than after the fact. Also, microprocessor cooling structures will resemble cooling systems used on high-performance racing cars.
 
This column originally appeared in the February issue of Design007 Magazine in the FLEX007 section.