Thermal Interface Materials used in Electronics Cooling
Following the series of blogs that looked into details of thermal management devices, now we will look into materials that work in conjunction with these devices to help increase efficiency and productivity.These materials are known as thermal interface materials and one of the most common of these materials is known as thermal grease.
Thermal grease, which has a variety of aliases that include thermal gel, thermal paste and heat paste, is a material that has a high viscosity, similar to normal grease, and a high thermal conductivity that comes from its ability to bridge the gaps between microscopic air pockets because of its somewhat flat surface. Most thermal greases have a thermal conductivity of anywhere between 5 and 10 W/mK, while air has a thermal conductivity of about .034W/mK and metals such as aluminum have a conductivity of about 220W/mK. To relate back to our the thermal management series, thermal greases are commonly found in heat sinks to help increase the thermal intake of the device.
There are many basic types of thermal greases. Ceramic based thermal grease has a fairly good thermal conductivity, and usually consists of a ceramic within a gel like silicone compound. The most common of these ceramics range from beryllium oxide which has a thermal conductivity of 218 W/mK to silicon dioxide with a thermal conductivity of 1 W/mK. Metal based thermal grease contain pieces of silver or aluminum inside the gel to increase thermal conductivity, and while it conducts heat better than ceramic based grease it is also much more expensive. Metal-free thermal greases are commonly used in computer systems because they do not pass electrical current and thus do not short circuit. Carbon based thermal greases that include diamond powder or short carbon fibers can be used, and these types of greases are generally the most conductive, but are also the most expensive. Lastly are liquid metal base thermal greases which are made of liquid metal alloys of gallium and are extremely rare and expensive. Gallium is an element that is nonexistent in free form in nature and is a trace element in bauxite. With a solid state that is brittle at low temperatures, it has a low melting point and a high density liquid state like silicon and water. The primary uses for gallium are almost exclusively for microelectronics and the semiconductor industry.
Thermal greases mainly assist a heat sink inside of electronics and computer devices to draw heat away from a semiconductor component such as integrated circuits or transistors. To reiterate what I said earlier, the thermal grease helps fill in the gaps that occur from the imperfect heat sink surface, thus vastly improving the thermal conductivity and allowing for greater heat transfer capabilities. Due to manufacturing restraints and engineering limits on heat sinks, the surfaces of the parts cannot be made perfectly smooth and thus must be helped with thermal grease. Thermal greases all have a qualitative property of “conformability” that describes how well the grease conforms to an irregular surface. A higher level of conformability will lead to a higher thermal conductivity for the heat sink. Engineers can then use Computational Fluid Dynamics (CFD) software to determine the effectiveness of the cooling system being used.
Advances in Electronics Cooling Materials
As the technology advances and the need for greater thermal management is increased, research into improved thermal greases has advanced greatly. Arctic Silver TM, a company in Visalia, California, currently has 3 silver and ceramic thermal compound greases. One interesting product they manufacture, Arctic Silver® 5, is made with 99.9% silver and utilizes three unique shapes and sizes of pure silver particles to maximize the contact area. It contains over 88% thermally conductive filler by weight as well as thermally-enhanced ceramic particles that improve the compound’s performance and long-term stability. While the use of silver and copper greases have the capability to cause failure and disruption if allowed to come in close proximity of electrical paths, this formula was designed to not conduct electricity, only heat, allowing for less chance of this occurrence. Their other greases include the Arctic Alumina™ and the Céramique™ 2, both of which use ceramic fillers to provide a thermal conductivity that exceeds most metal based compounds. These greases are known for their stability and insulation, both in terms of heat and electricity.
As the demand for more efficient forms of thermal management and electronics cooling increases, advances in the technology and materials used in developing new types of thermal interface materials are showing their impact.