Cooling Trends Direct Efficient Thermal Design

Jun 1, 2002 12:00 PM
By Sam Davis, Editor, Power Electronics Technology

Cooling solutions such as heatsinks, fans, heat spreaders, and thermal gap fillers help keep temperatures within the specified limits, ensuring efficient operation of power electronics systems.

With more electronic functions embedded into systems, power dissipation increases and makes thermal design more critical. To ensure system reliability, it's imperative to keep all components operating within safe temperature limits. Among the available thermal management products for cooling electronic systems are coolers, heatsinks, heat spreaders, thermal gap fillers, fans, temperature measurement devices, and CAD/CAE software that verifies the system's thermal design.

Heatsinks are among the most widely used thermal management products. A new series of heatsinks, Therma-Vent^™ by Thermacore, uses enhanced fin technology to increase the heatsink surface area and create turbulent airflow to disperse heat more efficiently. Using a new rolled and slit fin technology to maximize surface area, the flow of heat from the chip into the local ambient air increases (Photo 1). They reduce mass, have less system volume, and cost less, compared with traditional extrusions and folded fin products. Brazed attachment of the fins to base ensures mechanical and thermal joint integrity and minimizes the chance of cracks forming, as can happen with an epoxy bond.

The company is initially launching a standard active Therma-Vent^™ heatsink solution for the Pentium^® 4 processor (Socket 478). Derivative devices for desktop and notebook computers, embedded applications, communications equipment, and other electronics cooling applications can be rapidly customized for quick time to market.

Cool Innovations employs different technology with its UltraCool IV pin fin heatsinks that employ forged, highly conductive, oxygen-free copper (Photo 2, on page 48) intended to cool devices dissipating high heat loads. They are omni-directional, allowing efficient cooling because of their pin fin structure and use of highly conductive copper. The pin fin structure provides a large surface area that's effective with slow and moderate airflows (100 LFM to 400 LFM). The use of oxygen-free copper provides a thermal conductivity premium of 20% over pure aluminum and a 40% premium over other aluminum extrusion alloys.

These heatsinks' footprint range is 0.25 in. × 0.25 in. to 1.5 in. × 1.5 in. and from an overall height of 0.15 in. to 0.8 in. Pin diameter ranges from 0.06 in. to 0.125 in. The heatsinks are offered with a mechanical spring-clip suitable for various package types.

R-Theta's Fabfin series of aluminum heatsinks now has embedded heat pipes. This allows an engineered solution to common heat spreading issues created by higher power electronic devices. This process embeds heat pipes in the heatsink's baseplate, resulting in a cost-effective solution that doesn't significantly increase weight. With the added advantage of a patented swaging process for high-performance, high-ratio solutions, this represents an efficient heatsink when used with forced air for many thermal cooling applications. The fabrication process is identical for any aluminum FabFin heatsink and is offered on MF and AF fin spacing (3.43 mm and 5.49 mm), respectively.

Another variation for R-Theta's Fabfin heatsinks is an added copper inlay. This new technique improves heat spreading in high power electronic devices. The technique allows standard aluminum baseplates with embedded copper material in selected areas to accommodate specific heat spreading needs. The design allows selection of copper thickness according to dissipation requirements. Previously, engineers had to use expensive and heavy full-copper-based solutions to solve heat-spreading problems. These inlay solutions have the added advantage of a patented swaging process, for high-performance, high-ratio capability. This is an efficient FabFin heatsink when used with forced air cooling.

Coolers

Enertron offers a Compact Cooler for power electronics applications with limited space. It dissipates as much as 150W in a 155 mm × 45 mm × 25 mm space (Photo 3). Due to the space constraint, the heat is carried from the heat source to the heatsink using heat pipes. The heatsink consists of highly efficient, thin, copper fins through which the heat pipes pass. Two 40 mm × 40 mm axial fans provide the required airflow. The total thermal resistance of this cooler is 0.60°C/W.

The heat collector and heatsink are mechanically joined with the aluminum housing, preventing stress on the heat pipes. The two fans are fastened to the housing to create a single thermal solution ready to be installed into the system. Use of heat pipes provides design flexibility. You can adjust heat pipe length and orientation to suit other applications, such as all-in-one desktop computers, enclosure cooling, telecom laser cooling, etc.

JMC's new SkyJet 60 CPU cooler features a patented Dual-Pass Airflow architecture from Agilent (Photo 4, on page 50). Designed specifically for future Intel^® Pentium^® 4 desktop and server processors, it can handle up to 75W. It's a radial fin heatsink that utilizes dual-pass airflow by taking air in from the top and all sides of the heatsink, creating a vacuum and expelling added airflow through the bottom of the heatsink. The cooler exhibits 0.32°C/W of thermal resistance with thermal interface material. It contains a top-mounted, high-speed 60 mm × 25 mm, 5000 rpm dc fan that supplies 25 cfm. The heatsink mounts to standard microprocessor sockets, including Socket 478 and Socket 603. The assembly is 57 mm (H) × 69 mm (D) and weighs 290 g, providing a compact design with efficient airflow and minimum noise levels.

Thermal Interface Material

The interface between the heated device and the heatsink are critical to thermal management solutions. This interface must provide optimized thermal conductivity while providing electrical isolation between the two mating surfaces. In addition, the material used in the interface must be able to accommodate surfaces that are not perfectly flat. Silicone grease has been used for this purpose, but new substances are easier and cleaner to handle while providing competitive thermal resistance.

Thermoset, Lord Chemical Products has developed a new thermal interface material: MT-315. This low stress adhesive has a bulk thermal conductivity of 7.3W/mK and can achieve bondline thicknesses of 1.5 mils to 2 mils. MT-315 exhibits very low thermal resistance, low contact resistance and highly effective conductivity when tested in package. The result is a lower contribution of the adhesive to the overall thermal budget.

MT-315 was specifically developed for high-power devices to bridge the gap between commercially available thermal interface materials and solder. It exhibits excellent adhesion to a variety of substrates including gold, silicon, ceramic, and nickel. Its minimal shrinkage and superior adhesion result in excellent resistance to delamination and degradation in the thermal pathway during reliability testing. This adhesive is engineered for lid attach on Flip Chip assemblies, to be used as the die to heat spreader thermal interface, as well as perimeter adhesive. Its excellent electrical conductivity makes MT-315 well suited for grounding and RF shielding on a high-speed, high-power semiconductor. MT-315 is engineered to provide a high-performance interface solution where a mechanical attach isn't possible or desired. At 25°C, it has a viscosity of 85,000 cps at 2 rpm and is suited for syringe dispense or printing operations.

Fujipoly of America Corp. supplies the SARCON^® XR-e and XR-j thermal gap filler pads that feature excellent thermal conductivity, thermal resistance, consistency, and elasticity. These silicone gel sheets are well suited for filling air gaps and uneven surfaces in a variety of electronic designs, including the computer, consumer, communications, automotive and industrial applications.

These thermal gap filler pads have thermal conductivity of 11W/m-K and 14W/m-K, respectively. The thermal conductivity produces thermal resistance of 0.11 in.²/W to 0.14 in.²/W for the SARCON XR-e and 0.09 in.²/W to 0.12 in.²/W for the SARCON XR-j. Both Thermal Gap Filler Pads have flame retardancy that satisfies UL94 V-0 class.

Made from silicone compound, the pads are flexible enough to conform to a variety of design considerations, such as tolerance backups and multiple component heights. They can easily adhere to components in a wide variety of shapes and sizes, including protrusions and recessed areas and are also available in custom die-cut pieces. When mounted on substrates or at heatsink junctions, they can virtually eliminate thermal impedance, yielding higher temperature transfer gradients.

The latest thermal management product from the Bergquist Co. is Hi-Flow^™ 225UT phase-change material. HF225UT is a non-reinforced phase-change composite offered in square or rectangular die-cut parts on a continuous roll of polyester carrier liner. A pressure-sensitive adhesive strip facilitates the room temperature applications to the heatsink and a clear covering protects the material during handling and shipping. A colored tab makes it easy to peel-place-press the heatsink to the power semiconductor.

A key advantage of Hi-Flow^™ 225UT is that it requires no heatsink preheating for application. This is an important benefit, as heatsink manufacturers don't need to add a preheat oven to their assembly line when using this material. As a phase-change material, HF225UT flows at 55°C and requires a moderate 10 psi to 20 psi to fully wet-out the interface.

AOS' new line of dry-to-the-touch thermal interface materials include Micro-faze^® thermal interface film and Sure-form^® gap filler materials. These materials offer thermal grease performance with low thermal resistance, but with the handling convenience of thermal pads and gap fillers. Micro-faze^® isn't a phase change material. It's a non-wax-based material that begins working at room temperature, and it's naturally tacky so material sticks to components without an adhesive — but it's easy to remove for reworking. Plus, it requires minimum force to achieve total surface wetting. It's available in two variations: Type A (0.018°C-in.²/W/mil) is thermally conductive and Type K (0.03°C-in.²/W/mil) is thermally conductive and electrically insulating. Both versions are offered in sheets and rolls and can be die-cut to any size and shape. Sure-form^® gap filler material is a non-silicone, dry thermal grease material developed for use on uneven or irregular surfaces. It conforms to any size or shape with minimum pressure and offers the performance of thermal grease.

Fan Cooling

New from Kooltronic is the Kooltray II tray of fans that fit in a 19-in. rack and operate from 115Vac or 230Vac, 50 Hz/60 Hz, or 12Vac, 24Vac or 48Vac. At 1¾-in. high, they fit under card racks or heat-producing sources (Photo 5). Standard units are available in 1, 2, or 3 row(s) of fans. Optional features include thermal fan speed control, fan failure indication and alarm output, dc power supply, and filtered fan tray. Another option monitors individual fan operation with an integral solid state alarm system that emits an alarm for power loss or low fan speed, as indicated by the associated LED for that fan. Also an option is a temperature-sensing control that modulates the speed of the fans as temperature varies; it includes an alarm that signals power loss or high temperature. Fans have ball bearing motors and are UL/CSA/VDE/TUV rated for -20°C to 70°C.

Parvus uses fans in a different way with its PC/104 Environmental Fan Card that's PC/104-system compatible. With an intelligent temperature sensor, the card closely controls the environmental characteristics of an enclosure by providing internally circulated air and monitoring internal chassis conditions, based on temperature and external analog inputs. It contains two 5V fans that operate in a push-pull configuration to move air in a circular cooling pattern around itself and adjacent PC/104 boards. If desired, each of the fans can be reversed to create a push-push or pull-pull movement of air. Together with the analog inputs, the card includes multiple digital inputs, outputs, and connections to external temperature sensors, allowing it to control complex environmental management schemes. In an enclosed chassis system, this thermal management card can cool up to four adjoining boards; in open air, it can cool two PC/104 boards.

If you want to measure the airflow in an air-cooled system, you might want to use the Cambridge Accusense CAFS XS series of air velocity and air temperature sensors that are temperature compensated from 0°C to 70°C. Their small size minimizes obstructions between components on a crowded circuit board. When positioning sensors in the flow, it's important that they don't obstruct or redirect the airflow. A small sensor, such as the CAFS series, minimizes obstructions and provides more precise readings. Shaped like a blade, the CAFS XS sensors may be positioned parallel to the flow of air. Their small size allows them to be inserted in the gaps between fins of wider heatsinks.

Also intended for 19-in. rack-mounting are Lytron's systems that provide up to 2100W of cooling capacity (Photo 6). Rack mounting reduces the floor space requirement and improves the overall system compactness. They are available with copper, stainless steel, and aluminum heat exchangers and are compatible with a variety of cooling fluids, such as water, deionized water, EGW, and oil. You can insert the unit into a rack, make the electrical and fluid connections, and it's ready for operation. Three different cooling capacities, 500W, 1300W, 2100W, are available with electrical and pump options to customize the system. These systems are used in a variety of applications including cooling lasers, semiconductor capital equipment, analytical instruments, and medical equipment.

Cool Plastics

Effective thermal heat management in power electronics has become a key consideration. New thermally conductive plastics, introduced by Cool Polymers^® are growing in popularity because they are 40% lighter than aluminum, injection moldable into a wide variety of parts, and between 100 to 500 times more conductive than conventional plastic. Parts made with thermally conductive plastics manage heat and lower overall temperatures. In addition, these plastics provide dimensional stability, low thermal expansion, low thermal resistance, and intricate design features. The thermally conductive plastics can be developed with any base resin (commodity or engineered) and are electrically conductive and insulative. These plastics also provide excellent EMI/RF shielding.

Power resistors, made with thermally conductive plastic (Photo 7), provide low thermal resistance, manage and dissipate high temperatures, provide electrical insulation, and can be manufactured in reel-to-reel volume production.

Software Solutions

CAD/CAE software can simplify thermal management design. New Dynamic Soft Analysis is the BETAsoft Integrated Component/Board Thermal Analysis. It allows for simultaneous thermal design of the component and analysis of the board environment. The design of the component includes the variations of material, configuration, and the use of heatsinks. At the same time, the board thermal analysis considers the component placement, board structure, air speed, and cooling enhancements. Very accurate results of simulation provide advanced component/board design with assured performance.

Fluent Inc.'s IcePak 4.0 provides thermal management CAD/CAE software, which employs an assembly level meshing approach that groups objects into assemblies, meshing IC packages and heatsinks, and meshing the assembly and the rest of the solution domain separately. It introduces Model Manager that manages the creation, edits, replication, and other object functions; assemblies; libraries; and problem/project configurations and settings. The Advanced Object Wizards creates heatsink configurations, IC packages (PBGAs, TBGAs, QFPs, etc.), fans with 3-D housings and hubs, heatsinks, enclosures, and p. c. boards. Four-Window viewing, a third new feature, simultaneously displays complex 3-D models from four viewpoints, with on-screen view controls.

AOS, Eatontown, N.J.
CIRCLE 335 on Reader Service Card

Bergquist Co., Minneapolis
CIRCLE 336 on Reader Service Card

Cambridge Accusense, Shirley, Mass.
CIRCLE 337 on Reader Service Card

Cool Innovations, Willowdale, Ontario, Canada
CIRCLE 338 on Reader Service Card

Cool Polymers, Warwick, R.I.
CIRCLE 341 on Reader Service Card

Dynamic Soft Analysis, Pittsburgh
CIRCLE 350 on Reader Service Card

Enertron, Mesa, Ariz.
CIRCLE 351 on Reader Service Card

Fluent, Lebanon, N.H.
CIRCLE 352 on Reader Service Card

Fujipoly, Kenilworth, N.J.
CIRCLE 353 on Reader Service Card

JMC Products, Austin, Texas
CIRCLE 354 on Reader Service Card

Kooltronic, Pennington, N.J.
CIRCLE 355 on Reader Service Card

Lytron, Woburn, Mass.
CIRCLE 356 on Reader Service Card

Parvus Corp., Salt Lake City
CIRCLE 357 on Reader Service Card

R-Theta, Missisauga, Ontario, Canada
CIRCLE 358 on Reader Service Card

Thermacore, Lancaster, Pa.
CIRCLE 359 on Reader Service Card

Thermoset, Lord Chemical Products, Indianapolis
CIRCLE 360 on Reader Service Card