Power Electronics

Diverse Cooling Devices Pace Race To Get The Heat Out Of ICs

The need for heat sinks, heat pipes, temperature sensors, switches, and improved materials continues unabated as ICs shrink in size while providing higher performance, putting even more strain on cooling requirements.

There’s a constant demand for heat sinks, heat pipes, temperature sensors, switches and monitors, as well as thermal transfer materials. For suppliers of such components, the challenge is to push the performance levels of their products to meet new IC design packaging, layout and mounting requirements to “get the heat out.”

The heat management and removal task can be daunting when you consider the latest consumer electronic products like mobile phones, tablets, laptop and notebook PCs, and a host of other items that usually operate from battery power. That’s what Seiko instruments had in mind when it developed the S-5844 series of low-power dissipation temperature sensors/switches which it claims are the world’s lowest power dissipation devices at just 180 nA at 1.65 V. They fit into an ultra-small HSNT-4 package of just 1.0 by 1.0 by 0.4 mm. Moreover, they can withstand operating temperatures of -40 to 125°C.

The switches detect the specified temperature in a circuit, then invert the circuit’s output when that specified temperature is reached. The output voltage is restored once the temperature drops below the maximum specified level.

Texas Instruments also had mobile consumer electronics products in mind when it recently introduced a single-chip infra-red (IR) MEMS passive thermometer that provides contactless temperature measurement. The TMP006 integrates an on-chip MEMS thermopile sensor, signal-conditioning circuitry, a 16-bit analog-to-digital converter (ADC), and local temperature sensor and reference voltages on a single chip thatís just 1.6 mm2 (Fig. 1).

TI says this product is 95% smaller than other thermopile sensors. “It provides advanced thermal management of processors and allows application developers to creatively tap into the temperature measurement functionality,” says Steve Anderson, senior vice president of TI’s High Performance Analog Business Unit.

The thermometer features 240 µA of quiescent current and 1 µA of current in the shut-down mode. It operates over the -40 to 125°C temperature range with typical local sensor accuracy within ± 0.5°C and ±1°C typical accuracy. It includes an I2C/SMBus digital interface.

Some sensors measure more than just temperature. For example, Advanced Thermal Solutions’ MS 1000-CS-WC candlestick sensor measures air velocity as well. The use of a single sensor to measure both temperature and air velocity eliminates errors that can occur when air flow is non-isothermal.

This sensor is narrow and has a low profile to minimize disturbance of heat flow in the test domain. Its flexible plastic-sleeved 0.5-mm-diameter stem eases installation and re-positioning during the testing process. Its 9.5-mm plastic base eliminates any potential shorting issues. It can measure temperatures in the range of -30 to 150°C within ±1°C. Velocity measurements range from 0 to 50 m/s ±2% (10,000 ft./min.).

Advanced Thermal Solutions offers the iQ-200 thermal analysis system that tracks temperate, air velocity and pressure from multiple points to comprehensively profile heat sinks, components and pc boards. The iQ-200 simultaneously captures data from up to 12 J-type thermocouples, 16 air temperature/velocity sensors, and 4 differential pressure sensors to analyze electronic packages.

Others, like Sensitech, offer products like the TempTale4 (TT4) multi-alarm temperature monitors with an advanced and flexible range of configurable alarm options. It gives users the ability to program up to 6 independent time and temperature alarm thresholds for increased flexibility. This product is said to help customers mitigate risks in the global supply chain by allowing more precise and targeted monitoring of temperature-sensitive products.

Heat Sinks Come in All Shapes

You can get a heat sink in just about any shape to suit nearly every application. One of the more interesting shapes is the honeycomb structure from Jaro Thermal based on the science of natural honey bees (Fig. 2). This heat sink cools ball-grid array (BGA) packages in a thin-profile. It directs heat towards the outside of the device while producing a steady flow of cool air inside the heat sink. It is designed to be used with either plastic or metal/ceramic BGA packages, depending on the thermal interface material being used.

The heat sink is studded with holes, helping increase the surface area and cross ventilation. Ambient air is cooled even as heat is released. There’s also an adhesive-mounted version that eliminates the need for drilling mounting holes on the pc board.

Cool Innovations Inc. also makes some interesting heat sinks with a pin fin design some of which include a fan within and on top of the heat sink that is embedded in the heat sink’s pin array. They’re available in copper or aluminum and can generate 6.1 cubic feet/per minute (CFM) of air flow.

The heat sinks are designed for applications with restricted heights. They can accommodate a minimum overall height of 0.42 in. for PCI Express designs, and up to 0.63 in. for ATCA designs, as well as any other designs within this range. They range in footprints from 1.81 by 1.81 in. to 4.10 by 2.05 in., within the aforementioned height range. The high-reliability bracket fan that’s supplied with a heat sink operates from 12 V and consumes 200 mA.

Another interesting heat sink shape is the maxFlow heat sink from Advanced Thermal Solutions (Fig. 3). They include an integral push pin mounting system for fast and safe attachment of the heat sinks onto BGA packages and other hot components. Each extruded aluminum heat sink comes with a pair of either plastic or brass push pins that can be mounted in 3.00-mm holes on the pc board.

Tests done on maxFlow heat sinks with an air flow rate of just 100 linear feet/minute (LFM) show that the junction temperature of the device being cooled (Tj) can be reduced by more than 40% below the temperatures achieved using conventional straight-fin heat sinks. Heat sinks are available in sizes ranging from 37.4 by 37.5 by 10.0 mm up to 41.4 by 45.74 by 24.5 mm. Each MaxFlow heat sink is pre-assembled with a Chomerics T766 phase-change thermal interface material for improved component-to-sink thermal transfer. Pads are centered on the heat sinkís base.

Coolchip Technologies, an MIT spinoff, has decided to devote its resources to improving CPU computing performance by focusing on the insulating layer of air that forms on critical heat-transfer surfaces on conventional heat sink coolers. According to Coolchip Technologies, their heat sink is a revolutionary air-based cooler that is capable of rejecting more heat and improving processor performance better than any air-based cooler (Fig. 4).

The heat sink slashes the insulating boundary layer and greatly decreases the largest thermal resistance found in conventional heat sinks, which is found on the static metal fins. In the case of fins, increased air flow rates by the use of a fan results are increased, diminishing improvements in cooling capability. Coolchip’s design won the MIT $200,000 grant Clean Energy Prize this year.

Some companies use nanotechnology to improve heat sink performance. The patented NanoSpreader technology from Celsia Technologies makes use of liquid and vapor layers that act as thermal interface materials between the heat sink and the fins (Fig. 5). The technology makes use of a copper-encased two-phase vapor chamber in which pure water is vacuum-sealed. The liquid is absorbed by a copper-mesh wick and is passed as vapor through a micro-perforated copper sheet where it condenses and returns to the wick as liquid.

According to Celsia Technologies, NanoSpreaders feature one-half the weight of solid copper yet can transfer heat roughly ten times their thermal conductivity rate. This technique is said to minimize the thermal resistance from the device to the fins, which can be as much as 30% for some designs.

Heat Pipes Promise Big Improvements

Heat pipe technology is increasingly being used in laptop computers, high-powered electronics and spacecraft thermal controls. Their use is expected to increase with rising energy savings requirements. Much of the work in heat pipe technology is under development and is expected to emerge in the commercial market soon.

Heat pipes are basically vacuum-sealed tubes that move or spread heat from a source to a region where it is more readily dissipated. It does this with a minimal drop in temperature.

A typical heat pipe is a sealed and evacuated tube that contains a porous wick and small amount of working fluid. This wick is usually a sintered-powder metal that lines the tube’s internal circumference. The tube’s central core is open to permit vapor flow (Fig. 6). One of the companies working on developing heat pipes is Thermacore Inc. It is the lead partner in an €8.3 million Euro NanoHex research project looking to produce next-generation liquid-coolant solutions that incorporate nano-particle technology.

Researchers at Purdue University have developed a heat pipe that uses tiny copper spheres and carbon nanotubes (CNTs) to passively wick a coolant toward hot electronics, and that can handle roughly 10 times the amount of heat generated by conventional ICs. The research team is hoping to develop heat pipes about one-fifth the thickness of conventional heat pipes and covering a larger area.

Better Thermal Materials

A key effort to getting the heat out of ICs is the development of better thermal materials. Earlier this year, Anomax Corp. Ltd. announced the invention of a breakthrough high-dissipation substrate they call Integrated Plated Circuit Heat Sink (IPCHS) that incorporates an aluminum base with a filled anodized dielectric layer and plated patterned circuits (Fig. 7). Anomax says that its invention provides extremely high thermal transfer levels which are many times higher than currently available methods.

The IPCHS approach overcomes a key difficulty in using alumina, the gold standard for thermally conductive materials. It is difficult to plate alumina for it to be useful as a dielectric with high thermal conductivity and its porosity traps moisture and leaks electricity. The IPCHS process fills the porosity of alumina and metalizes it, allowing a circuit to be plated on a true dielectric alumina surface. Substrates can be made in either 2-dimensional or complex 3-dimensional circuit configurations.

“IPCHS allows devices to be mounted directly on a circuit that’s plated on alumina, enabling thermal transfer between device to the heat sink to be direct and effective, at up to 30+ mW/K, which is more than 8 times today’s best capability,” says H. A. Chan, Anomax’s director. The thermal performance of IPCHS was benchmarked against a leading metal core substrate of equal size. The results show a far superior thermal performance for IPCHS for 65-µm alumina. Applying the same amount of heat, the temperature at the circuit was 81°C for the metal core and only 65°C for IPCHS which was 47% cooler.

Material improvements

Ultimately, new materials like silver-diamond composites may be required to help cool small high-performance ICs. That’s what researchers at the Georgia Institute of Technology (GaTech) have determined as they develop such a composite for military applications. Their research is focused on producing a silver-diamond thermal shim of unprecedented thinness —about 250 µm or less (Fig. 8). The ratio of silver to diamond in the material can be tailored to allow the shim to be bonded with low thermal-expansion stress to the high-power wide-band-gap semiconductor materials slated for next-generation phased-array radars.

Thermal shims are needed to pull heat from high-power semiconductor ICs and transfer it to heat-dissipating devices such as fins, fans or heat pipes. And the shims need to be very thin and must exhibit high thermal conductivities to fit into tiny structures.

“We have already observed clear performance benefits—an estimated temperature decrease from 285°C to 181°C using a material of 50% diamond in a 250-µm shim,” says Jason Nadler, a GaTech research engineer leading the project. The project’s aim is to approach diamond percentages as high as 85%.

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