Category: Applied Thermal Engineering

Micro-drops of water channeled through the chip silicon looks like a promising way to keep chips cool and increase their performance.

Existing technology’s inability to keep microchips cool is fast becoming the number one reason why Moore’s Law may soon meet its demise.

In the ongoing need for digital speed, scientists and engineers are working hard to squeeze more transistors and support circuitry onto an already-crowded piece of silicon. However, as complex as that seems, it pales in comparison to the problem of heat buildup.

“Right now, we’re limited in the power we can put into microchips,” says John Ditri, principal investigator at Lockheed Martin in this press release. “One of the biggest challenges is managing the heat. If you can manage the heat, you can use fewer chips, and that means using less material, which results in cost savings as well as reduced system size and weight. If you manage the heat and use the same number of chips, you’ll get even greater performance in your system.”
Resistance to the flow of electrons through silicon causes the heat, and packing so many transistors in such a small space creates enough heat to destroy components. One way to eliminate heat buildup is to reduce the flow of electrons by using photonics at the chip level. However, photonic technology is not without its set of problems.

Microfluid cooling might be the answer

To seek out other solutions, the Defense Advanced Research Projects Agency (DARPA) has initiated a program called ICECool Applications (Intra/Interchip Enhanced Cooling). “ICECool is exploring disruptive thermal technologies that will mitigate thermal limitations on the operation of military electronic systems while significantly reducing the size, weight, and power consumption,” explains the GSA website FedBizOpps.gov.

What is unique about this method of cooling is the push to use a combination of intra- and/or inter-chip microfluidic cooling and on-chip thermal interconnects.
The DARPA ICECool Application announcement notes, “Such miniature intra- and/or inter-chip passages (see right) may take the form of axial micro-channels, radial passages, and/or cross-flow passages, and may involve micro-pores and manifolded structures to distribute and re-direct liquid flow, including in the form of localized liquid jets, in the most favorable manner to meet the specified heat flux and heat density metrics.”

Using the above technology, engineers at Lockheed Martin have experimentally demonstrated how on-chip cooling is a significant improvement. “Phase I of the ICECool program verified the effectiveness of Lockheed’s embedded microfluidic cooling approach by showing a four-times reduction in thermal resistance while cooling a thermal demonstration die dissipating 1 kW/cm2 die-level heat flux with multiple local 30 kW/cm2 hot spots,” mentions the Lockheed Martin press release.

In phase II of the Lockheed Martin project, the engineers focused on RF amplifiers. The press release continues, “Utilizing its ICECool technology, the team has been able to demonstrate greater than six times increase in RF output power from a given amplifier while still running cooler than its conventionally cooled counterpart.”

 

Moving to production

Confident of the technology, Lockheed Martin is already designing and building a functional microfluidic cooled transmit antenna. Lockheed Martin is also collaborating with Qorvo to integrate its thermal solution with Qorvo’s high-performance GaN process.

The authors of the research paper DARPA’s Intra/Interchip Enhanced Cooling (ICECool) Program suggest ICECool Applications will produce a paradigm shift in the thermal management of electronic systems. “ICECool Apps performers will define and demonstrate intra-chip and inter-chip thermal management approaches that are tailored to specific applications and this approach will be consistent with the materials sets, fabrication processes, and operating environment of the intended application.”

If this microfluidic technology is as successful as scientists and engineers suggest, it seems Moore’s Law does have a fighting chance.

 

Thermal imaging devices like night-vision goggles can help police, search-and-rescue teams and soldiers to pick out bad guys or victims through walls or in complete darkness. However, the best devices require cryogenic cooling, making them heavy, expensive and slow. Enter graphene, the semi-conducting material that’s 100 times stronger than steel — researchers from MIT have built a chip out of the material that may solve the problem. The resulting infrared sensors were small enough that they could be “integrated in every cellphone and every laptop,” according to the study’s co-author, Tomas Palacios.

Graphene is already one of the best infrared sensing materials, so the team first built a microscopic sensor chip out of the material. Further graphene was then used to carry the signals and suspend the chip over an air pocket, as shown below. That eliminated the need for external cooling, normally required by such devices to prevent internal heat from polluting the target’s infrared signature.

The compact sensor was able to detect a human hand and heated-up MIT logo, a promising first result. The goal is to further improve the resolution, so the tech can be used in everyday devices. For example, Palacios told LiveScience that the sensors could one day be integrated into car windshields, giving you “night-vision systems in real time without blocking a driver’s regular view of the road.” That said, we’re still waiting for a host of “promising” graphene-based technology to actually become usable products.

 

In the microelectronics world, the military and private sectors alike need solutions to technological challenges. Dr. Mustafa Akbulut, assistant professor of chemical engineering, and two students lead a project funded by DARPA to create thermal interface materials (TIMs) that have a superior ability to transfer heat and a strong capacity to keep cool.

In evaluating a central processing unit, as an example, there are many pieces that individually need temperature management. “As you get smaller and smaller, there is higher heat dissipation per unit area. Locally, you have higher temperatures…you have a harder time operationally—you need better thermal interface materials. This is especially important for radars, laser systems and also for military electronics,” said Akbulut.
Essentially and most critically, the device needs the ability to avoid overheating. As Akbulut asserts, “unless you cool it, it fails.”
In evaluating an electronic device and a cooling system that need to be placed together as they function, if there is an absence of thermal material in between, the heat created by the electronic device can potentially erode the device. According to Akbulut, non-soft materials are considered less effective as a TIM because they do not adequately cover all interior openings or gaps, even though the naked eye may not detect this space.
Akbulut explains why optimal contact is not achieved through current technology. “If you look at the very fine scale, [these two pieces] are not smooth. If you look at these with an electron microscope, you see they are like mountains. If you bring these surfaces together, they do not have perfect contact.” Thus, the objective of a traditional TIM is not fully met.
Soft materials, including paste, often minimize the gap, said Akbulut. The invention of his new metal-based, soft material leads to high thermal conductive activity and because of its malleable nature, consistent contact is achieved. His research group has recently developed TIMs with thermal conductivities greater than 100 W/m-k and elastic modulus values in the order of 20 GPa, signficantly advancing the current state of art for TIMs. As a comparison, this material is ten times softer than steel and three times more thermally conductive.
Using copper and nanomaterials together, Akbulut believes his new TIM can lead to greater optimization and large-scale implementation in the future.

A team led by Prof. Hanchen Huang of Northeastern University has discovered a new way to merge metals at room temperature, without heat by developing a new material called ‘MesoGlue’. Soldering techniques have improved a lot and in large scale circuit board production, most parts of it have been automated. But one thing has remained unchanged throughout the evolution of circuit boards; the hot solder. There has never been another way to attach electronic components to a PCB without melting iron. Not until now.

MesoGlue is a combination of metal and glue. The creators have used metallic nanorods with cores coated with elements Indium and Galium on either side of the two surfaces that need to be joined. When the two surfaces come in contact, the nanorods are interlocked, and form a liquid which is solidified by the core of the rods. The resulting bond acts like both, a conductor and a glue. The technology has been patented through Northeastern University.

The bond formed by the MesoGlue is matchless as it provides thermal and electrical conductivity like metal bonds, resistant to high temperature and air leaks. The applications are as wide as attaching miniature components to circuit boards, and attaching metal pipes without welding.

Small circuit components like processors, capacitors and resistors tend to lose their intended potential when heated to attach them to the circuit board. That is why soldering should be done very quickly, and with the smallest drop of iron as possible. This MesoGlue allows fusion of metals without having to heat them.

Though in its early stages, a company founded by Prof. Hanchen, Paul Elliot and Prof. Stephen Stagon has begun mass-producing this glue for commercial applications. By eliminating the need for heat, this metallic glue could improve the performance of every device that has circuit boards in them.

In the microelectronics world, the military and private sectors alike need solutions to technological challenges. Dr. Mustafa Akbulut, assistant professor of chemical engineering, and two students lead a project funded by DARPA to create thermal interface materials (TIMs) that have a superior ability to transfer heat and a strong capacity to keep cool.

In evaluating a central processing unit, as an example, there are many pieces that individually need temperature management. “As you get smaller and smaller, there is higher heat dissipation per unit area. Locally, you have higher temperatures…you have a harder time operationally—you need better thermal interface materials. This is especially important for radars, laser systems and also for military electronics,” said Akbulut.
Essentially and most critically, the device needs the ability to avoid overheating. As Akbulut asserts, “unless you cool it, it fails.”
In evaluating an electronic device and a cooling system that need to be placed together as they function, if there is an absence of thermal material in between, the heat created by the electronic device can potentially erode the device. According to Akbulut, non-soft materials are considered less effective as a TIM because they do not adequately cover all interior openings or gaps, even though the naked eye may not detect this space.
Akbulut explains why optimal contact is not achieved through current technology. “If you look at the very fine scale, [these two pieces] are not smooth. If you look at these with an electron microscope, you see they are like mountains. If you bring these surfaces together, they do not have perfect contact.” Thus, the objective of a traditional TIM is not fully met.
Soft materials, including paste, often minimize the gap, said Akbulut. The invention of his new metal-based, soft material leads to high thermal conductive activity and because of its malleable nature, consistent contact is achieved. His research group has recently developed TIMs with thermal conductivity greater than 100 W/m-k and elastic modulus values in the order of 20 GPa, significantly advancing the current state of art for TIMs. As a comparison, this material is ten times softer than steel and three times more thermally conductive.
Using copper and nano-materials together, Akbulut believes his new TIM can lead to greater optimization and large-scale implementation in the future.

The surprising discovery of a new way to tune and enhance thermal conductivity gives engineers a new tool for managing thermal effects in smart phones and computers, lasers and a number of other powered devices.

The finding was made by a group of engineers headed by Deyu Li, associate professor of mechanical engineering at Vanderbilt University.

Li and his collaborators discovered that the thermal conductivity of a pair of thin strips of material called boron nanoribbons can be enhanced by up to 45 percent depending on the process that they used to stick the two ribbons together. Although the research was conducted with boron nanoribbons, the results are generally applicable to other thin film materials.

One of the remarkable aspects of the effect Li discovered is that it is reversible. For example, when the researchers wetted the interface of a pair of nanoribbons with isopropyl alcohol, pressed them together and let them dry, the thermal conductivity was the same as that of a single nanoribbon. However, when they wetted them with pure alcohol and let them dry, the thermal conductivity was enhanced. Then, when they wetted them with isopropyl alcohol again, the thermal conductivity dropped back to the original low value.

One of the first areas where this new knowledge is likely to be applied is in thermal management of microelectronic devices like computer chips. Today, billions to trillions of transistors are jammed into chips the size of a fingernail. These chips generate so much heat that one of the major factors in their design is to prevent overheating. In fact, heat management is one of the major reasons behind today’s multi-core processor designs.

A thermal invisibility cloak that can make objects thermally invisible by redirecting incoming heat has been developed by Singaporean researchers.
Based on carefully engineered metamaterials – materials with properties that can’t be found in nature – the technology could potentially help improve the thermal performance of various electronic systems by fine-tuning thermal dissipation.
The team from the Nanyang Technological University (NTU) that developed the system has previously experimented with the so-called passive thermal cloaks capable of guiding conductive heat around a hidden object.
The team’s latest invention is the first active thermal invisibility cloak with an on/off switch and the ability to be adapted to objects of varying geometries.
“We considered the question of whether we can control thermal cloaking electrically, not by guiding heat around the hidden object passively with traditional metamaterials, but by ‘pumping’ heat from one side of the hidden object to the other side actively, with thermoelectric modules,” said Professor Baile Zhang, the lead researcher behind the project. The work is described in an article featured on the cover of the latest issue of the journal Applied Physics Letters.
Zhang said the device could help optimise the thermal performance of a large variety of electronic devices including high-power engines, magnetic resonance imaging instruments and thermal sensors.
The active thermal cloak consists of 24 small thermoelectric semiconductor heat pumps controlled by an external input voltage. These heat-pumps are distributed around a 62-millimeter diameter air hole in a carbon steel plate just 5mm thick.
When electrical current runs through the junction between two modules, the so-called Peltier effect kicks in and removes or generates heat.

Researchers have determined that gallium nitride (GaN) could become the next best semiconductor for electronics because it would immensely cut energy usage.

Cambridge Electronics Inc. (CEI) has announced a new line of GaN transistors and power electronic circuits. This line promises to reduce energy usage by 10 to 20 percent in consumer electronics, data centers and electric cars by 2025. CEI plans to use these transistors to make data centers use less energy, electric cars more powerful and cheaper to build and power adapters one-third of the size, according to Phys.org.

“CEI’s GaN transistors have at least one-tenth the resistance of such silicon-based transistors, according to the company. This allows for much higher energy-efficiency, and orders-of-magnitude faster switching frequency—meaning power-electronics systems with these components can be made much smaller,” according to Phys.org.

Typically GaN transistors have not been in the market because of safety issues and high manufacturing costs. “Power transistors are designed to flow high currents when on, and to block high voltages when off. Should the circuit break or fail, the transistors must default to the ‘off’ position to cut the current to avoid short circuits and other issues—an important feature of silicon power transistors. But GaN transistors are typically ‘normally on’—meaning, by default, they’ll always allow a flow of current, which has historically been difficult to correct,” according to Phys.org.

Researchers have addressed these issues by modifying the GaN transistor structure and developing transistors that are ‘normally off.’

“To make traditional GaN transistors, scientists grow a thin layer of GaN on top of a substrate. The MIT researchers layered different materials with disparate compositions in their GaN transistors. Finding the precise mix allowed a new kind of GaN transistors that go to the off position by default,” according to researchers.

OKI Circuit Technology in Japan has developed a mass-production technology for multi-layer copper-coin printed circuit boards that supports high speeds and high frequencies.

The technology is a T-Coin (Technology of copper coin insert) structure that has the shape and thickness of copper coins; it improves heat dissipation, prevents damage, and increases reliability and thermal conductivity. The technology is available in a wide range of copper coin diameters from 3.0 mm to 6.0 mm and printed circuit board thicknesses from 1.0 mm to 2.0 mm.

“Materials designed for use at high speeds and high frequencies tend to have lower structural strength than conventional printed circuit boards. Inserting copper coins, which subjects through-holes to loads, was previously considered difficult for multi-layer boards. The conventional mainstream method for ensuring heat dissipation has been “through-hole construction,” whereby many through-holes are drilled in the printed circuit board and their surfaces are copper-plated to dissipate heat. However, with electronic components packed at ever-greater densities to handle ever-higher data volumes, the amount of heat to be dissipated has increased, while the space available for providing through-holes to dissipate heat has shrunk. This trend has made measures for achieving adequate heat dissipation an increasingly urgent issue,” according to Business Wire.

“The T-Coin increases the area available for heat conduction for a limited number of through-holes by inserting cylindrical copper (copper coins) into through-holes without leaving gaps using a specially-developed method that minimizes pressure loading. This technology improves heat dissipation performance 20-fold while ensuring high reliability and long service life. Heat-generating components are in direct contact with large areas of copper with high thermal conductivity, ensuring high heat dissipation performance.