Effective thermal management must be employed at the component level and the system level (see "Shrewd Thermal Management Helps Defeat The Heat," ED Online 16767). The system may employ good circuit design practices and reliable components, but system reliability will suffer if appropriate temperature controls aren't properly implemented. Available devices, techniques, and software that provide thermal management at the system level include fans, liquid cooling, thermally enhanced circuit boards, and thermal analysis software.
Fans for electronic systems are usually attached directly to a motor's output, without gears or belts. Motors for ac-powered fans usually use mains voltage, while motors for dc-powered fans use low voltage, typically 24, 12, or 5 V. Fans for electronic systems usually use brushless dc motors to minimize electromagnetic interference (EMI). Table 1 lists the characteristics of fans employed in electronic systems.
A fan inside an electronic system draws cooler air into the case from the outside, expels warm air from inside, or moves air across a heatsink to cool a particular component. The use of fans and/or other hardware to cool a system is called active cooling. The width and height of these usually square fans are measured in millimeters, with common sizes including 60, 80, 92, and 120 mm. Round fans are also available.
Airflow ratings are in cubic-feet per minute (CFM) for a rotation speed in revolutions per minute (RPM). Fans with a higher CFM rating may produce less noise (in decibels, or dB). Some fans come with an adjustable RPM so they can produce less noise if the system does not need as much airflow. The type of bearing used in a fan affects performance and noise output (Table 2).
System packaging is important when employing fans for cooling. That is, components mounted on the pc board should not restrict air flow, particularly higher-power devices. Heatsink fins should be in line with the flow of air. In addition, if the cooling system uses air filters, they should be accessible for cleaning.
Liquid Cooling A liquid cooling system usually consists of a cold plate, pump, heat exchanger, and pipes or hoses (Fig. 1). This thermal management system employs a pump to move a continuously flowing liquid in a loop that cools a pc board. In operation, the system's pc board generates heat that transfers to a thermally conductive cold plate, heating the liquid coolant flowing through it.
This heated liquid coolant is then pumped through the heat exchanger, which moves the heat from the liquid coolant to either the ambient air or, in the case of a liquid-to-liquid heat exchanger, to another liquid coolant. The cooled liquid coolant then flows through the pipes or hoses back to the cold plate. Most liquid coolants also use a small amount of additives to inhibit corrosion and lubricate the pump.
Cold plates come in different forms. One approach consists of copper or stainless steel tubes pressed into a channeled aluminum extrusion. A second technique uses flat aluminum or copper tubes with an internal fin that increases their performance. A third type consists of two aluminum or copper plates metallurgically bonded together with an internal fin.
Coolant compatibility with a wetted surface is a factor in selecting a specific cold-plate technology. A copper cold plate is compatible with water and most common coolants. Aluminum performs well with an ethylene glycol/water mixture, oils, and other fluids, but is not compatible with untreated water. Stainless steel is necessary when using de-ionized water or other corrosive fluids. Table 3 lists the possible problems that can occur with liquid cooling systems and the appropriate corrective measures.
Another liquid-based approach to combat heat is spray cooling with pinhole-sized openings that shower a semiconductor with droplets of a special liquid that won't damage electronic circuits. Cooling occurs in two ways. Heat is thought to disperse most efficiently when bubbles of vapor form as the coolant evaporates from the surface in a process known as boiling. The coolant also removes heat by simply warming as it flows over the semiconductor toward a drain. The liquid and vapor are then collected and recycled, and a heat exchanger with a slow-moving fan dissipates the heat they carry.
Thermally Enhanced Circuit Boards Some electronic systems, such as power modules, exhibit high power and small size where it's not convenient to use a heatsink. These modules usually carry high current and need a high voltage-isolation rating. In some applications, they may have to operate over a temperature range that reaches up to 200°C.
One type of thermally enhanced circuit board is the direct-bond copper (DBC) substrate commonly used in power modules. It consists of a ceramic base with a sheet of copper bonded to one or both sides of the ceramic material. In the manufacturing process, a copper-oxygen eutectic bonds to the copper and ceramic base. The top copper layer can be chemically etched in a manner similar to conventional printed-circuit technology. The bottom layer is usually soldered to a heat spreader.
One type of DBC uses alumina (Al2O3) as the ceramic base because of its relatively low cost, though it isn't a good thermal conductor and is relatively brittle. The aluminum-nitride (AlN) DBC is more expensive than alumina, but has better thermal performance. Beryllium oxide (BeO) can also be used for the ceramic base, but is usually avoided because of its toxicity.
A major advantage of DBC substrates is their low coefficient of thermal expansion (CTE), which is close to that of silicon. This ensures relatively good thermal cycling performance for semiconductors mounted on the substrate.
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