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Hot stuff, cool solutions

Hot stuff, cool solutions

Heat sinks cool power packages without contact

For high-power surface-mount devices, Aavid Thermal Technologies has designed a compact surface-mountable heat sink. The new components, designed for D2Pak and D3Pak devices, consist of stamped copper plated with 60/40 tin/lead solder for easy surface mounting.

The heat sinks work by conducting heat from the power IC's pins instead of from the IC package. Heat from the pins transfers to pc-board thermal vias, which run vertical to the board, or to a buried copper heat spreader underneath the package and parallel to the pc board. One version of the heat sink solders on the opposite side of the pc board to conduct heat from the vias. Another solders right over the IC package to attach to the edges of the buried heat spreader.

Using the heat sinks results in a 30% improvement in thermal resistance from case to ambient air, say Aavid engineers. D2 Pak heat sinks reduce device case temperatures by 20C; D3 Pak versions by 12C. Designers would need to add at least 1.5 inches of extra copper around the device on the pc board to achieve similar results.

The heat sinks do require the addition of a 1/16-inch copper pad to both sides of the device's mounting pads. Maximum height is 0.4 inch. Price: 20 to 25 cents each in 10,000-unit quantities.

Aavid Thermal Technologies Inc., 1 Kool Path, P.O. Box 400, Laconia, NH 03247, (603) 528-3400, FAX (603) 528-1478, Internet http://www.aavid.com .

Stick-on bandage spreads chip heat

An easy way to cool ICs for less than a dollar is by sticking on one of Chomerics' T-WING(TM) heat spreaders. The result: reduced package temperatures by up to 25C.

T-WINGs look like small bandages, but have the sticky portion in the middle instead of on the sides. In addition to being inexpensive, they're much smaller than conventional heat sinks.

A pressure-sensitive, thermally conductive silicone adhesive provides a strong bond at temperatures as high as 125C. It takes less than 5 psi of force to apply them, minimizing the risk of damage to the component. The T-WING itself consists of 5-oz copper with 0.0015-inch-thick electrically insulating black film laminated on both sides. Total thickness measures about 0.015 inch.

Because they can bend, the "thermal-wing" heat spreaders achieve nearly 100% adhesive contact with concave or non-flat packages. You can bend the T-WING's sides to avoid contact with other components, and even cut away sections to make room for other pc-board components. Standard sizes range from 0.5x2 to 1.5x4 inches. Custom configurations are also available.

Chomerics Inc., 77 Dragon Court, Woburn, MA 01888, (617) 935-4850, FAX (617) 933-4318.

Thermoelectric modules offer precision cooling

Need to cool your electronics below ambient temperature or stabilize temperatures to within 0.1C with no moving parts, electrical noise, or environmental worry? Thermoelectric coolers (TECs) offer these capabilities and more. Melcor's TECs are modular and come in more than 150 size and power varieties.

As direct current flows through the TEC's p-n semiconductor junctions, heat is absorbed on the cold-side ceramic plate, then pumped to the hot-side plate. Reversing the direction of current reverses the direction of the heat being pumped. The rate of cooling is determined by the amount of current, so users can easily control temperature by controlling the current supply. The TEC transfers heat from the hot-side plate to a heat sink for dissipation to the environment.

You can use TECs side by side to increase the amount of heat pumped. If the temperature differential between the hot and cold faces needs to be more than 60C, consider stacking the devices on top of one another in a "cascade." Cascades, or multistage TECs, can achieve temperature differences up to 131C.

Small and lightweight, TECs are available in different shapes and sizes--some with center holes for feeding through wires or other hardware, for example. Since they have no moving parts, the devices are silent and reliable. They cool quickly, even in a vacuum or weightless environment, or in any physical orientation. Another benefit: TECs generate little electrical noise and can provide precision temperature control when used with an appropriate controller. However, TECs are more expensive than some other cooling methods and tend to lose their competitive advantages when cooling loads exceed 200W.

The solid-state design of the TECs results in MTBFs greater than 200,000 hours. Melcor's CP series is suited for applications requiring up to 125W of cooling; the FC series of subminiature, low-current TECs offers spot temperature control for heat loads under 10W. Melcor engineers also can configure custom assemblies to meet special size and performance requirements.

Melcor Corp., 1040 Spruce St., Trenton, NJ 08648, (609) 393-4178, FAX (609) 393-9461.

Duo delivers compact cooling

Thermal-management products often work better together. One especially popular combination involves a fan and a heat sink. Thermalloy's new Powergainseries of forced-air coolers integrates the two into one unit. They give designers a way to reduce the weight and volume needed to cool power modules, thermoelectric devices, and IGBTs (insulated-gate bipolar transistors). Six models dissipate 200 to 1,000W.

Thermalloy's patented pin fins in combination with impingement fan air results in volume and weight savings of as much as 60%, say company engineers. The savings in volume results from the increase in the convective heat-transfer coefficient due to the turbulence within the heat sink and shorter paths for air to travel. Reducing heat-sink volume allows for shorter buses, smaller circuit boards, reduced product size, and lower weight.

Conventional cooling methods use forced air blown through extruded shapes, or bonded fin assemblies, in the same direction as the fins. This strategy generally results in long airflow paths. The air temperature increases in direct relation to the length of time it contacts the hot metal surfaces.

Also, air, as it passes along conventional fins, loses cooling efficiency due to the buildup of a boundary air layer along the surface. Boundary layers are stagnant layers of air caused by skin friction between flowing air and the fin surface. This layer serves as a thermal insulator between the heat-sink fin and the cooling air stream.

In the Powergain system, boundary layers are broken up at each pin. In addition, a swirling action within the pin structure helps increase convective heat-transfer coefficients.

Because of the fan's placement relative to the heat-sink pins, the Powergain mounting surface is evenly cooled, ensuring consistent performance from hot devices. For instance, Thermacool Inc., Louisville, KY, used two Powergain heat sinks to cool a thermoelectric-based, 1,000 BTU/hr refrigeration system. "Using Powergain devices let us evenly cool the thermoelectric modules, while reducing product volume by a third," says Dennis Kinsfather, Thermacool's director of engineering.

Thermalloy Inc., 2021 W. Valley View Ln., P.O. Box 810839, Dallas, TX 75234, (214) 243-4321, FAX (214) 241-4656.

Pipes carry the heat out

Sometimes you don't have room for a heat sink and, even if you did, the device would weigh too much. In that case, consider a heat pipe from Noren Products.

A heat pipe consists of a sealed aluminum or copper container with inner surfaces that have a capillary wicking material. Inside the container, a liquid under its own pressure enters the pores of the capillary material, wetting all internal surfaces.

Applying heat at any point along the heat-pipe surface causes the liquid at that point to boil. When that happens, the liquid picks up the latent heat of the vaporization process. The gas, which has a higher pressure, moves inside the sealed container to a colder location, where it condenses, giving up the latent heat of vaporization. This action, in turn, moves that amount of heat from the input to the output end of the heat pipe.

Heat pipes have an effective thermal conductivity thousands of times that of copper. Designers can save weight and space by replacing heat sinks with these pipes in large applications, such as cellular-phone base stations, but will increase cost.

The pipes can be built in almost any size or shape, then bent or shaped for an application's geometry. Flat pipes are typically used for cooling pc boards or heat leveling to produce an isothermal plane. Another option: flexible heat pipes for an application that requires a range of motion.

Noren also offers special functional units, including heat pipes that maintain a constant temperature or temperature range, as well as pipes that allow heat to transfer in only one direction.

Noren Products Inc., 1010 O'Brien Dr., Menlo Park, CA 94025, (415) 322-9500, FAX (415) 324-1348.

Fans that spot-cool

For high-power surface-mount devices, Aavid Thermal Technologies has designed a compact surface-mountable heat sink. The new components, designed for D2 Pak and D3 Pak devices, consist of stamped copper plated with 60/40 tin/lead solder for easy surface mounting.

The heat sinks work by conducting heat from the power IC's pins instead of from the IC package. Heat from the pins transfers to pc-board thermal vias, which run vertical to the board, or to a buried copper heat spreader underneath the package and parallel to the pc board. One version of the heat sink solders on the opposite side of the pc board to conduct heat from the vias. Another solders right over the IC package to attach to the edges of the buried heat spreader.

Using the heat sinks results in a 30% improvement in thermal resistance from case to ambient air, say Aavid engineers. D2 Pak heat sinks reduce device case temperatures by 20C; D3 Pak versions by 12C. Designers would need to add at least 1.5 inches of extra copper around the device on the pc board to achieve similar results.

The heat sinks do require the addition of a 1/16-inch copper pad to both sides of the device's mounting pads. Maximum height is 0.4 inch. Price: 20 to 25 cents each in 10,000-unit quantities.

Aavid Thermal Technologies Inc., 1 Kool Path, P.O. Box 400, Laconia, NH 03247, (603) 528-3400, FAX (603) 528-1478, Internet http://www.aavid.com .

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