First, you've got ICs and power semiconductors operating at higher-than-ever power rates. Now add in the rising power densities of their associated systems and power-management devices generating more heat to support higher power loads. What you wind up with is heat dissipation reaching stratospheric levels.
To keep pace, thermal management must work harder to cool these systems and maintain an operating temperature that optimizes reliability—in other words, the lower the operating temperature, the higher the reliability.
Implementing thermal management in an electronic system involves a combination of electronic engineering and mechanical, chemical, and metallurgical disciplines. Most designers understand that thermal management requires fast-switching, low on-resistance MOSFETs to minimize heat dissipation. Yet many designers aren't as aware of the other disciplines required to optimize thermal management. Several non-electronic thermal-management solutions exist.
Many systems use convection cooling with either a fan or centrifugal blower. For instance, there's Comair Rotron's 7.9-in. Mixed Flow DC fan. Targeted at telecom applications, it also cools computer cabinets and racks. Cabinets and racks have more equipment packed in to deliver increased capabilities, raising the resistance to airflow within the limited space that surrounds the equipment. This airflow resistance makes it essential to cool the electrical equipment efficiently, to eliminate overheating and equipment failure.
The compact Mixed Flow fan combines characteristics of fans and blowers to deliver increased pressure and airflow. It measures 7.90 in.2 by 2.75 in. deep and weighs 4.6 lb. A powerful and efficient cooling unit, the fan delivers 375 to 480 cubic feet per minute (CFM) and a static pressure of 1.4 to 1.9 in. of water while generating 59- to 72-dBA noise as a function of speed.
This fan meets UL approval ratings and is polarity protected. Its die-cast housing and the polypropylene impeller both meet the UL 94V-0 flammability rating. Operating temperature ranges from -10°C to 75°C. Also, the fan runs at either the standard 24 V dc (nominal) or 48 V dc (nominal). It's specified for a continuous duty life of 50,000 hours at 25°C.
When coated with Comair Rotron's EnviroShield, the Mixed Flow also can withstand degradation caused by harsh environmental conditions, moisture, and salt. Thus, it's well suited for telecom applications.
Other product offerings from Comair Rotron use the ThermaPro-V technology, which automatically adjusts airflow based on current temperature needs. This leads to minimized power consumption and noise, and will eliminate expensive circuit redesigns. Also available is an automatic restart feature that ensures resumption of operation after power failure or overheating. In addition, a tachometer output can be incorporated to monitor performance and protect essential components.
THERMAL INTERFACE MATERIALS
Thermal interface materials improve thermal conduction and electrical isolation between a heat-producing semiconductor and its associated heatsink. When heated or compressed between two surfaces, the interface material flows and displaces air gaps, providing better heat conduction between the two joined surfaces.
One form of interface material, phase change material (PCM), is usually found in sheet form with melting points above room temperature, typically between 40°C and 65°C. At that temperature, they change from a solid to the semi-liquid state to fill in any air gaps between the heatsink's surface and semiconductor.
PCMs came into prominence when neither the dry joint nor the thermal attachment tapes provided sufficient thermal performance to cool semiconductor-heatsink combinations. Manufacturers wanted an interface material applied by the heatsink manufacturer to avoid handling messy silicone grease.
Chomerics' Multiphase T557 and T558 are both PCMs (Fig. 1). T557 is an inherently adhesive film of 0.005 in. thickness. T558 is the same PCM as T557, except for a conformal metal-foil carrier coating on one side. Thermflow T558 is recommended for applications requiring rework and ease of disassembly. This new material offers the high-performance properties that are typical of free-film phase change materials, with the added benefit of easy removal.
Another type of thermal interface material is the gap filler, such as Chomerics' Therm-A-Gap 570 and 580 highly conformal, thermally conductive gap-filler pads. The materials consist of an ultra-soft silicone elastomer filled with ceramic particles. The conformability of these materials enables them to blanket highly uneven surfaces, wetting out mating surfaces to efficiently transfer heat away from components.
Therm-A-Gap T630 is a thermally conductive gap filler that suits applications where typical gap-filling pads cause too much stress on component solder joints and leads, resulting in damage to the pc board. T630 is a highly conformable, one-component pre-cured silicone that can be dispensed to fill large and uneven gaps in electronics assemblies.
PCMs require heat to melt and flow. Different interface materials, gap fillers, insulating pads, and other factors only require pressure to displace the air. This is because they tend to be soft rubbers.
HOW HEATSINKS WORK
A heatsink is a metallic device with high thermal conductivity. It increases a heated component's cooling surface area, such as a semiconductor. The heatsink attaches to the component, usually with some form of thermal interface material. Using its large surface area, the heatsink lowers the component's temperature by radiating its heat into the surrounding air. Heatsinks are made from an aluminum or copper alloy that has fins either shaped as parallel plates or with round, square, or elliptical pins.
Round pin-fin heatsinks are an emerging technology finding significant applications in high-end electronic systems. Cool Innovations' pin-fin heatsinks consist of a base plate with an array of round pins (Fig. 2).
These pin-fins provide effective cooling due to their unique, thermally oriented structure with an omnidirectional configuration and large surface area. The round pins create turbulence inside the pin array, and the omnidirectional configuration maximizes air intake into the pin array. These two factors combine to expose the pin-fin's large surface areas to the highest possible volume of air. As a result, pin-fins produce substantial cooling power.
Copper pin-fin heatsinks offer two advantages over identically structured aluminum pin-fins. First, they offer a cooling premium of 10% to 15%. Second, they can spread the heat in a more rapid fashion over their base plate. The copper pin-fins act as a combined heatsink and heat spreader. These pin-fins suit devices that contain small and focused heat sources. They also fit well into IC packages that don't contain embedded heat spreaders, such as flip chips.
Proper airflow management is especially important for boards that contain a dense array of devices, where airflow turns into a scarce resource in board area located further from the air source. Pin-fin heatsinks offer low pressure drop per square inch of surface area. Pressure drop can be defined as the difference between the airspeed entering the pin array versus the airspeed exiting the array. As a result, pin-fins create less airflow disturbance and leave more of the available air active for cooling other devices.
Originally used for spacecraft and satellites, you can now find heat pipes in notebook and desktop computers and game consoles. A heat pipe exhibits a very high thermal conductivity for heat transfer, and it doesn't consume energy or produce heat itself. However, it can't cool a device below ambient temperature.
A heat pipe consists of a vacuum tight envelope, a wick structure, and a working fluid. The heat pipe is evacuated and then back-filled with a small quantity of working fluid—just enough to saturate the wick. The atmosphere inside the heat pipe is set by equilibrium of liquid and vapor. As heat enters at the evaporator, it upsets this equilibrium and generates vapor at a slightly higher pressure. This higher pressure vapor travels to the condenser end. Here, the slightly lower temperatures cause the vapor to condense, giving up its latent heat of vaporization. The condensed fluid is then pumped back to the evaporator by the capillary forces developed in the wick structure.
Heat-pipe cooling is gaining popularity due to its advantages over extruded heatsinks that rely on pure conduction to move heat to extended surfaces for dissipation. Also, heat pipes provide more packaging flexibility than extruded heatsinks. On top of that, heat-pipe operation is completely passive; there are no moving parts, pumps, or valves.
Thermacore's Therma-Sink uses heat-pipe technology to move the heat from an attachment plate mounted to the component to a location within the system where enough air volume exists for adequate heat removal. Fins are stacked on the heat pipes to provide adequate surface area for heat dissipation to the air stream. Such cooling lets designers place the components close to each other for lower signal loss and higher bus speeds. Multiple Therma-Sinks can be used to transfer the heat from an entire system into a common air stream.
In notebook computers, heat pipes direct the heat from the processor into an air stream to remove heat from the case. This doesn't affect other sensitive components, such as the hard drive. This style of heat-pipe assembly is referred to as a "Remote Heat Exchanger" (Fig. 3).
Distributing heat evenly in two dimensions, heat spreaders eliminate "hot spots" while simultaneously reducing touch temperature in the third dimension. They come with custom-designed thermal properties and various form factors.
One of the newest types of heat-spreader materials is produced from graphite. At 40% the weight of aluminum and 18% the weight of copper, graphite offers excellent thermal conductivity. GrafTech International produces the material from expanded natural graphite flake, which is pressed and rolled out into long sheets and then cut to the required size for heat spreaders. Price-wise, it's competitive with other heat-spreader materials, so it has made its way into laptop computers and other consumer applications. A graphite-epoxy combination also is used as fins in heatsinks.
Sony's VAIO 505 series notebook, which is only 9.7 mm thick at the front and grows to 21 mm at the back with no fans or heat pipes, uses GrafTech International's eGraf SpreaderShield natural graphite heat spreaders. Two variants of the material were employed to spread heat from both the graphics and Pentium M processors into the notebook's case.
The Panasonic Toughbook Y2 semi-ruggedized notebook computer also uses a graphite heat-spreader application: GrafTech's eGraf Fredda natural graphite product (Fig. 4). Fredda material lets designers create complex 3D forms by distributing heat evenly in two dimensions. Custom-designed Fredda components come in a range of thicknesses, shapes, and sizes. Fredda technology combines with the SpreaderShield heat spreader as the only thermal-management solution used to dissipate heat from the 1.3-GHz Pentium M processor, as well as the graphics chip in the Toughbook.
Samsung SDI takes advantage of SpreaderShield graphite heat spreaders to eliminate hot spots and improve temperature uniformity in its large-screen plasma display panels. Hot spots can occur in a plasma display when an image remains stationary for a prolonged time. SpreaderShield natural graphite heat spreaders offer thermal conductivity of 240 to 500 W/mK, compared to about 1 W/mK for the silicone and acrylic materials previously used in plasma panels. SpreaderShield also weighs 45% less.
INSULATED METAL SUBSTRATES
Thermal-management systems may include packaging techniques for conduction, convection, and radiation cooling. Conduction can be the most efficient mode of heat transfer. About 80% of the heat generated in a chip can be dissipated through the pc board. However, conventional pc-board materials don't guarantee sufficient heat dissipation because the polymer materials typically used to manufacture circuit boards are very poor thermal conductors.
The insulated metal substrate (IMS) offers a more thermally conductive alternative to conventional FR-4 boards. This board consists of a circuit layer of copper, which is a dielectric layer that provides insulation with minimum thermal resistance. Underneath the dielectric is a base layer of aluminum or copper. Aluminum base metal offers excellent heat dissipation ability, mechanical integrity, low cost, and lightweight construction.
The Bergquist Company's Thermal Clad is an IMS. The board consists of 1 to 10 oz of copper circuit layer (Fig. 5). The dielectric layer, which has Underwriters Lab (UL) recognition, bonds the base metal and circuit metal together. Although the base layer is often aluminum, other metals such as copper also may be used. The most widely used base material thickness is 0.062 in. in aluminum, though many thicknesses are available. Some applications might not need the base layer of metal.
A number of thermal-management methods using both air and liquid have been developed by the Georgia Institute of Technology. Synthetic jet-augmented heatsinks and two-phase thin heat-spreader technologies were created for portable electronics applications. Also, liquid cooling schemes utilizing single-phase and two-phase transport were developed for heat fluxes above 50 W/cm2. These include stacked microchannels, two-phase thermosyphons, and vibration-induced droplet atomization methods.
Efforts have focused on both a fundamental understanding of these techniques, and prototype demonstrations on candidate electronic systems. A parallel effort has focused on crafting thermal characterization techniques through computational modeling. Of particular interest are methodologies for multiscale modeling, multimode transport predictions, and reduced models for thermal design and optimization.
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