Mobile devices are hotter than ever in both good and bad ways. While sales of smartphones, tablets, and other types of mobile devices continue to set records, a growing demand for faster processing speeds and smaller, thinner form factors makes reliable thermal management increasingly challenging. Fortunately, even as thermal management demands skyrocket, designers can take heart in the fact that they can now choose from a rapidly growing pool of technologies and design choices aimed at keeping mobile devices cool, safe, and functional.
In addition to the thermally-aware power management algorithms frequently used to scale back performance in the interest of lower temperatures, designers are relying on a variety of hardware-based approaches to ensure that mobile device temperatures don’t soar to levels that adversely affect performance or reliability.
Cooling Choices
Thermal management products are designed to minimize or dissipate the heat generated by the regular operation of electronics in order to improve their reliability and prevent premature failure. Mobile device cooling comes in two basic forms: passive and active. Passive cooling is great, due to the fact that it generates no noise, usually isn’t liable to sudden failure, and requires no power source. Active cooling technologies, while generally offering more rapid and efficient temperature reduction, typically present disadvantages in terms of size, power requirements, noise, and/or cost.
The oldest and most widely used passive heat transfer methods are heat sinks and heat spreaders. A basic heat sink is simply a heat exchanger that transfers heat from a device, such as a CPU, to another medium. Heat sinks come in many different sizes and forms. Typically, a heat sink uses a thermally conductive metal, such as aluminum or copper, which is physically mounted to a heat generating component. Heat is transferred from the component to the heat sink through conduction and then dissipated through the heat sink’s surface into the ambient air via natural convection.
A more sophisticated type of heat sink, a heat spreader, works by transferring heat from its source to a secondary heat exchanger featuring a larger surface area and geometry. Typically, a flat plate that’s formed out of a thermally conductive material, such as graphite, is used to draw heat and pass it to the mobile device’s external metal chassis and, ultimately, into the environment. Most current smartphones rely on some form of heat spreader to dissipate heat.
Heat spreaders are frequently applied as a backing plate to printed circuit boards (PCBs) containing heat generating components. Thermal vias embedded in the PCB function as conduits between component packages and the heat spreader to improve heat flow. Some PCB technologies eliminate the need for an external plate by integrating a metal core layer directly into the board, enabling the PCB itself to serve as a heat spreader.
All types of heat spreaders provide excellent temperature management. Yet for even more powerful and effective cooling it’s possible to send a current through a graphene heat spreader, which instantly transforms the device into an active cold plate via thermoelectric cooling. Thermoelectric coolers (TECs) are thin and compact devices typically inserted between a heat source and a heat sink to enable rapid heat dissipation. When voltage is applied to a TEC, a temperature differential is formed between each side of the device, allowing heat transfer to take place via conduction.
Heat pipes provide yet another way to reliably lower the internal temperature of various types of mobile devices. An attractive — although more expensive — alternative to solid conductors, such as copper or aluminum, heat pipes take advantage of their lower total thermal resistance to shift heat from internal hot spots to less critical areas, quickly and reliably. Heat generated by a component boils the liquid at one end of the pipe, which then travels down to the cool end. The vapor quickly condenses (emitting latent heat) and returns back to the heated end of the heat pipe via capillary action or gravity. Heat pipes are typically constructed out of a thermally conductive metal that can either lay flat or be bent into complex shapes, which makes the approach highly suitable for use in space-restricted PCBs.
Historically, liquid cooling was mostly used to manage temperatures inside large stationary systems, such as servers. But current leak-proof heat pipes make the approach practical in mobile devices as compact as smartphones. Samsung, for instance, has successfully used water-filled copper pipes inside its S7, S8, and S9 smartphone models to effectively transport internally-generated heat to the device’s case.
Design also plays an important role in effective heat mitigation. Carefully planned PCBs, configured to optimize thermal management, can lower internal temperatures with little or no cost or performance overhead. Adding extra layers of solid ground or power planes to a PCB, connected directly to heat sources with multiple vias (typically hollow and cylindrical copper metal transfers sandwiched between layers) is a technique that’s frequently used to increase the effective surface area for heat dissipation.
The most popular active cooling technologies, fans and blowers, are widely used inside larger mobile devices, such as laptop computers, to continuously manage internal temperature. Cooling is accomplished by using the fan or blower to increase the flow of air directly above a heat generating component or a passive cooling device, such as a heat sink.
The downside to both fans and blowers is the fact that the hot air transported by the moving blades must be directed toward one or more external vents. If these vents are accidentally blocked by the end user, such as by placing the device on top of a pillow, internal temperature can quickly rise to an unacceptable level, resulting in a spontaneous system shut down or, in the worst case, component damage or destruction.
Additionally, even the smallest fans and blowers are generally too large and power hungry to deploy inside smartphones and tablets. There is at least one exception, however. Earlier this year, Chinese phone manufacturer Nubia introduced the Red Magic 3. This smartphone, targeted at mobile gamers, incorporates both a liquid-cooling copper heat pipe and an internal cooling fan. This fan reportedly runs quietly at speeds up to 14,000 rpm inside its own isolated chamber, protecting it from dust and liquid contamination. The fan is reportedly rated for over 30,000 hours of continuous use.
Final Word
Arrow.com offers a wide variety of thermal management products and accessories from the industry’s top manufacturers, including Sanyo Denki, Orion Fans, ebm pabst, Delta, ATS, Aavid Thermalloy, and more, including fans and blowers, heat sinks, heat spreaders, TECs, heat pipes and other heat management accessories for all applications.
