Given the power dissipation issues associated with compact ICs and the increas-ing criticality of temperature control for minimizing failure rate, capable and innovative heat sink products and design software are forging ahead to meet the cooling challenge.
New heat sink architectures are helping to cool things down, as are new metal forming techniques for the extrusion-type sinks commonly seen atop ICs. Also in evidence are an increased dependence on liquid cooling and the use of copper and other materials for conducting heat away from such devices as IGBT modules. And laser applications are getting more efficient coolers and chillers.
Further, fans and blowers are often becoming integral parts of heat sinks. Also available for thorough analysis of thermodynamic problems, where the devil is in the details, is more complete design software-some of it Internet based-for the systems engineer.
The designs are often classified in two applications categories: IC specific, most often using extruded sinks-an aluminum die process that is frequently combined with separately bonded heat fins in place of one-piece extrusions-and general-purpose heat sinks. Custom design for both categories has become the rule. Choosing one architectural solution for dispersing heat in a given task is not a usual luxury. (Aside from the universal use of heat fins, the needs of Ics vs. discrete modules and lasers are often very different.) But designers have a broad selection of relatively low-cost and efficient extrusion sinks to choose from for chips and discretes.
These include moderately expensive, very efficient liquid-cooled modules for power-discrete modules, as well as expensive, highly efficient thermoelectric coolers for saving the most critical devices, such as lasers.
Among the latest heat sink solutions for ICs are cold-forged (KF-650) and extruded (KE-650) sink-plus-fan solutions of the "active cooler" type for the AMD K7 Athlon processors from Cofan (Fremont, Calif.). Cold forging, an extremely high-pressure metal-compression process in use for over a decade but relatively new to electronics heat sinks, improves molecular bonding. It also saves energy in the manufacturing process because the metal is formed at lower (or room) temperatures, and minimizes other machining and finishing costs.
The Cofan products dissipate the 50 to 54 watts generated by the current crop of 500-MHz-and-above processors.
Also available is the KC-500 solution for the Intel P2/P3 series of processors that uses the Secc2 cartridge format. The unit features a die-cast heat sink. While initially requiring much more expensive tooling equipment than extrusion types, the solution can cost less in high production runs.
The KC-500, measuring 136 x 48 x 24 mm, packs an integrated 12.7-cubic-feet/minute fan (50 x 50 x 12 mm) that uses coreless motor technology. A slide attachment clip allows fast and reliable installation as well as easy removal, according to Cofan.
Earlier, Wakefield Engineering (Beverly, Mass.) introduced three heat sinks for the Celeron CPU, one active and two passive, in its 866 series, along with the three-member 862 family for the Xeon processor. The 862 series has a thermal performance of 0.52 degrees C/W at an air velocity of 200 LFM. The 862X, with a cross-cut design to allow omnidirectional airflow, provides 0.44 degrees C/W, and the 862F active sink delivers 0.87 degrees C/W. In addition, Wakefield's series 869, 709, 710, 711 and 712 support all types of chassis configurations for AMD's K6-2 processor.
Wakefield Engineering's Hexfin product line for Xeon applications incorporates a convoluted fin pack bonded to an extruded base. The company says this scheme promotes an increased level of automation and serviceability and a 12 percent performance enhancement.
Aavid Thermal Products (Concord, N.H.) also has expanded its thermal-management solutions for sub-$1,000 PCs using AMD, Cyrix, IDT and Rise Technology CPUs, along with Intel's Celeron and Xeon. The heat sinks for those applications also feature cross-cut cooling fins and fans that exceed industry requirements for performance, cost, reliability and noise, the company said. Indeed, fans more and more are becoming an integral part of the heat sink product per se.
For example, Advanced Thermal Solutions' newly announced BalticCool line of "fansinks" are designed for general-purpose cooling and can be customized to power, microprocessor and telecom applications. First out of the chute is the ATS 97-09-039-wf, which provides a thermal resistance of 0.78 degrees C/W at a 12-V operating voltage, in a package measuring 40 x 40 x 20 mm and weighing 28 grams. The basic model is priced in the $6 to $7 range in OEM quantities.
Another entry applicable to microprocessors, RF transmitters and insulated-gate bipolar transistors is the Therma-Base from Thermacore (Lancaster, Pa.). This design is based on the company's "vapor chamber" technology to alleviate the so-called "thermal spreading resistance" of heat sinks that today are typically larger than the devices they cool.
Ideally, heat sinks are most efficient when the heat flux is uniform and heat travels directly through the baseplate to its fins. When heat distribution is not uniform, there is an increase in the spreading resistance (a function of the difference between the average source and average baseplate temperatures), which results in a lateral flow of heat along the baseplate and thence to the fins.
The usual brute-force solution is to increase airflow or the size of the heat sink. In contrast, the base of a Therma-Base sink is a vapor chamber or "vacuum vessel" whose inside walls are lined with a metal wick saturated with a working fluid (water). As heat is applied to the base, the fluid at that location vaporizes and the vapor rushes to fill the vacuum.
Vapor coming into contact with a cooler wall surface condenses, releasing its latent heat of vaporization. The condensed fluid circulates toward the heat source via capillary action in the wick structure, and thus the system is unaffected by orientation. According to Thermacore, this technique provides a proper means of distributing heat from a concentrated source to a large surface.
While some applications are designed for the relatively slow removal of heat using just the sink's baseplate, fins remain the major means by which heat is released to the environment in the quickest, most efficient manner. Thus, while heat fins are generally part and parcel of the sink, heat fin thermodynamics and manufacturability is a science unto itself.
Fin density, height and width, and baseplate thickness for a given airflow are the major design parameters. The constructional attachment of fin to heat sink proper also comes into play.
The latest advances in that regard include Aavid's AallMetal bonded-fin heat sinks. They do not require epoxy or other filler material for attaching to the baseplate and thus are said to improve overall cooling performance without a price premium. The heat sinks use what Aavid calls a proprietary "cold forming" technique that locks the fins in place and forms an intimate metal-to-metal contact. Therefore, they operate as a solid piece of aluminum, and Aavid said the conductivity throughout the part is equal to that of a conventional extrusion.
The resulting parts can be handled and finished as a single-piece aluminum extrusion.
The AallMetal construction "drives all of the air out of the fin-to-base joint, providing a pull-out force of over 650 pounds per linear inch for each fin," said Chris Soule, Aavid marketing manager. "This pull-out performance is equal to or better than that of our industry-leading epoxy-bonded parts."
AallMetal parts range slightly lower than Aavid's bonded-fin parts. For example, the smallest size, suitable to cool microprocessors, is $10 each per 500 pieces; the largest size, suitable for cooling motor drives of hundreds of horsepower, is $150 in the same quantities.
Along the same lines, R-Theta (Mississauga, Ont.) manufactures the FabFin and Aquasink line of heat sinks with a patented process that uses no glue and allows fin-to-baseplate ratios that exceed traditional extrusion-based designs. The company's existing Miccroseal process for protection against corrosion provides a threefold improvement over conventional chromate finishes, according to R-Theta. Another enhancement to FabFin is the Helicoil threaded inserts, which eliminate stripping threads in IGBT installation.
When it comes to the larger "power board" applications, such as cooling IGBT output stages, the focus shifts to so-called cold plates. As in the Thermacore device, a circulating fluid comes into play, with a power module placed on a flat plate with associated plumbing (external pump required) that circulates water to carry away heat from the semiconductors.
The plumbing in this automobile-radiator-like setup is usually copper. While a more expensive and weighty solution than aluminum, and thus not optimally suited to extruded baseplates, copper has about twice the conductivity and serves better as a heat pipe.
One of the newer releases for power boards is Aavid's 6-Pass Liquid Cold Plate for "six-pack" IGBT modules. The aluminum water-cooled cold plate uses 3/8-inch copper tubing formed to provide six cooling channels that are placed directly under the heat-generating area of an IGBT array. The arrangement ensures plus/minus 2 degrees C temperature uniformity over the IGBT substrate in base or mobile applications.
The copper, initially a round section that flattens as the tubing is pressed against the IGBT substrate, minimizes the thermal path between the IGBT base and the cooling liquid, Aavid said. About half the area of the plate is taken up by the cooling passages to provide thermal resistance as low as 0.003 degrees C/W at a water flow of 1.5 gallons/minute. That heat-dissipating capability, according to Aavid, represents a 50 percent improvement over that of competing heat sinks, including those cooled by water.
The cold plate is available in three lengths: 6, 12 and 24 inches. Pricing is $42.74 each in lots of 100 for the 6-inch model, $66.80 for the 12-incher and $104.38 for the 24-inch version.
Also in this vein, Aavid's Cool Covers family of cooling solutions for Rambus in-line memory modules is available for a basic price of $1.38 apiece in quantities of 25,000.
For highly efficient operation, such as protecting costly laser equipment, designers can turn to thermoelectric coolers. Some of the more recent devices, which use the Peltier p-n junction effect for cooling, include the ThermaTec from Melcor (Trenton, N.J.), a patented module for high-temperature (225 degrees C) applications. It offers heat pumping capacity to 51 W.
In addition, Melcor's new UltraTec family provides heat-pumping capacities up to 78 W and temperature differentials to 79 degrees C for both limited and larger heat loads. Melcor has also released a 270-W liquid chiller that includes a pump, reservoir and fluid detector for laser, medical and laboratory applications. Its built-in digital temperature controller is said to stabilize liquid temperature to within 2 degrees C.
The chiller is designed to use water, distilled water or coolant solutions and offers a variety of benefits over traditional compressor-based chillers, according to the company: no CFCs, compact design, low maintenance, accurate temperature control and relatively low cost. The fully packaged unit is $1,565.
Another liquid cooler for general applications, from Lytron (Woburn, Mass.), provides a 150-W/ degrees C cooling capacity for a 400-CFM air flow. It includes a water flow regulator, an adjustable thermostat and a level sensor. Pricing starts at about $900.
Whether for extrusion heat sinks or coolers, system designers are seeing a growing number of more capable tools-of which the Internet itself is one-for modeling thermodynamic performance more accurately and in less time. Among the new software is Melcor's Aztec (A to Z Thermoelectrics), for thermoelectric-cooling and heat sink design. Designed to run on a PC, the Windows 3.x/95 program includes thermal-load estimation for active and passive components and enclosures. The program selects suitable thermoelectric-cooling (TEC) products based on the design parameters.
They are then modeled for a given heat sink resistance and input power, and data sheets with dimensioned drawings, graphics and design specs are printed out.
Additionally, the program can specify appropriate high-density bonded fins, heat sinks or liquid heat exchangers matched to the selected TEC. The Aztec program
disk can be purchased from the company for $49.95 or downloaded free from www.melcor.com.
At the Unix and NT workstation level, Aavid/Fluent's Icepak, updated this year to version 3.0, is now a fully interactive computational fluid dynamics software tool with faster model building capabilities and more comprehensive physical models for accurate simulation. It accurately represents complex nonrectangular geometries, and it boasts parallel and postprocessing capabilities, Aavid said. Icepak offers enhanced CAD geometry import in IGES and DXF formats for easy integration with other CAD tools and EDA software.
Carrying design directly to the Internet, R-Theta last spring launched R-Tools, which it calls the industry's first online thermal-modeling tool. Users can quickly and accurately model the company's heat sinks free in their application and view an instant simulation of the results, including a full 3-D color thermal plot of multiple power sources and locations.
Aavid Thermal Products Inc.
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