Increasingly Dense Mobile Devices Present Cooling Challenges in Product Design

April 23, 2014

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As mobile devices get ever more sophisticated in their functionality, they are packing greater amounts of internal electronics -- heat-producing electronics. It's up to mobile device designers to find ways to dissipate heat efficiently. While there are several passive-cooling methods, active cooling might be essential in the future.

Your cell phone needs charging again, and it seems that you just did it. Frequent charging is a direct result of the power consumption of the phone, which is largely due to heat.

Of all the challenges the mobile device industry faces, keeping components cool is the most important, since overheating causes significant reductions in the operating life of a device and leads to device failure. Devices such as cell phones are becoming increasingly complex, with larger screens and more functional capabilities, which makes heat dissipation even harder.

Device designers have to manage several components that consume a lot of power, such as the display, DRAM (dynamic random-access memory) chip, and power amplifiers. Today’s device displays are often the most power-hungry components, so it is essential for device makers to reduce display subsystem power through innovative techniques. The power-dissipation levels found in mobile devices are heading to 5 W (watts) and higher.

Knowledge of consumers' desires, needs, and behaviors -- in the way they are using their mobile devices today as well as in the future -- is guiding system designers at device manufacturers to enhance the effectiveness of their designs based on performance, power, and thermal parameters. They are being challenged to make the right tradeoffs among these specifications, keeping in mind the overall effect on the system.

As these demands increase, the challenges of beating the heat produced by increasingly dense printed-circuit boards continue to emerge. Multiple microprocessors, along with logic elements, are reaching into the gigahertz range of operation, which makes cost-effective thermal management the highest priority among engineers in the design, packaging, and materials fields.

Adding to those problems is the current trend of manufacturing integrated circuits (ICs) for greater functional densities. Simulations have shown a 10°C rise in temperature to potentially double an IC chip’s heat density and thus reduce performance by more than one-third.

Cooling Methods

Cooling comes in two forms: passive and active. The advantages of passive cooling include longer product life and lack of noise. However, the biggest challenge still remains: to efficiently and rapidly dissipate heat. The heat transfer in passive cooling generally happens from conduction and convection.

Convection can be defined as the heat energy that is transferred between a moving fluid and a surface at different temperatures. When the density of a fluid changes due to factors such as rising hot air or sinking cooler air, natural cooling occurs. Convective heat transfer can take the form of forced convection in addition to natural convection.

A method that is often used with passive cooling is software in the operating system that reduces the operating parameters in one or more components in the thermal zone to reduce the heat being generated. For example, this might involve reducing the frequency of the clock that drives a device, lowering the voltage supplied to the device, or turning off a part of the device. As a rule, this limits device performance.

A common passive heat transfer method in electronic devices is to use heats sinks, or heat spreaders. A heat spreader transfers heat between a source of heat and a secondary heat exchanger whose surface area and geometry are larger than the source. This type of spreader is often just a plate made of copper that has a high thermal conductivity. In a passive system such as this, conduction or convection is relied upon to remove the heat.

Another effective method of passive heat transfer is the use of heat pipes. They offer reliable and simple operation with a host of advantages, including no moving parts, high effective thermal conductivity, vibration-free operation, and the ability to transport heat over sizable relative distances. Heat pipes transfer heat more proficiently than solid conductors such as aluminum or copper, since they have a lower total thermal resistance.

A heat pipe is filled with a small amount of working fluid, which could be water, nitrogen, acetone, sodium, or ammonia. When the working fluid is vaporized, it absorbs the heat. The vapor then carries the heat to the condenser region. Here, the condensed vapor releases heat to some form of cooling medium.

Either gravity or the heat pipe’s wick structure then returns the condensed working fluid to the evaporator, creating capillary action. Cylindrical and planar heat pipe variants both contain inner surfaces lined with a capillary wicking material.

Printed circuit board design is also used to reduce device temperatures. Use of additional layers of solid ground or power planes connected directly to heat sources with multiple vias, which are usually hollow and cylindrical copper metal transfers between layers, increase the effective surface area. The use of thermally conductive planes to spread the heat evenly will noticeably lower the temperature by increasing the area used for heat transfer to the atmosphere.

Thermal-interface materials are another part of the potential solutions for heat reduction. Thermal greases are extremely effective in transferring heat. They have properties that are similar to grease, which increases the thermal conductivity of a thermal interface by filling microscopic air gaps present due to the uneven surfaces of the components.

These compounds have much greater thermal conductivity than air but are much less efficient than metal. In electronics, they are often used to aid a component's thermal dissipation via a heat sink or heat spreader.

While passive cooling can be effective, active cooling is the way of the future due to its ability to transfer heat more quickly and efficiently. The design issues with active cooling, however, are tremendous, with size being the major one.

The most common form of active cooling is the use of fans. To perform active cooling, the device operating system turns on a cooling device, such as a fan, and this increases power consumption. Additionally, when fan cooling is employed, entrance and exit points in the device are needed in order to move the heated air from the device. Blockage of these vents will cause the temperature in the device to rise.

Mobile device designers are relying on various methods to reduce the heat in their products, but the future will be advancing the technologies associated with active cooling due to its superior performance, as devices get more complex and heat-reduction demands rise.

Sam Pelonis is president of Pelonis Technologies, a leading manufacturer of axial AC and brushless DC fans and motors specializing in high technology and original equipment manufacturing (OEM) solutions, based in Exton, Penn. With over 25 years of product development and manufacturing experience, Pelonis Technologies’ customers come from a variety of markets, including medical equipment, aerospace and defense, heating and air-conditioning, automotive, and appliances.
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