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Emerald Group Publishing Limited
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Keeping cool in hot applications
Keeping cool in hot applications
Keywords: Thermals, Materials, Aerospace technology
Cutting-edge composite technologies that combine the best properties of both metals and ceramics are making a breakthrough in thermal management and structural engineering applications. Enabling the finished product to be tailored to meet multiple performance requirements such as thermal expansion, weight, stiffness and thermal conductivity, this technique eliminates the need for the compromises often associated with traditional material solutions.
Manufactured and developed by Ixion Thermal Materials and represented in the UK by ATEK Technology Limited, these innovative products are now widely used in aerospace technology and aircraft engineering.
Military. The F-22 Raptor is the United States Air Force's next generation air- superiority fighter. Using a combination of stealth, speed and advanced weapons and electronic counter-measures, the F-22 provides first-look, first-shot, first-kill capability. The F-22 makes use of Ixion's AlSiC components in several key systems, ranging from the fly-by-wire controls to power generation unit, as well as the heads-up-display panel and electronic counter-measures array.
Many US and foreign military aircraft, such as the EA-6B Prowler, F-18 Hornet and Eurofighter Typhoon, also utilise Ixion materials. Chosen for their outstanding high quality and performance, they are ideal for use in incredibly severe environments. In fact, one of the little known heroes of the Kosovo conflict in Yugoslavia in 1999 was the ALE-50 towed decoy. The system, often called a lifesaver by pilots and flight crews, effectively saved numerous aircraft by causing the incoming ground to air missiles to hit the decoy and not the aircraft. In this instance, AlSiC materials form one of the key components of this system.
Aerospace.The Iridium system is the most ambitious communications system ever launched by a non-governmental agency. Designed to provide worldwide wireless telephone service regardless of location, the constellation consists of 66 low-earth-orbit (LEO) satellites. Due to the need to employ several different types of printed wiring board materials, AlSiC printed wiring board cores were selected, based on the ability to tailor the core properties to that of the board materials. The cores provided mechanical support that minimised vibration stresses during launch and they continue to ensure long-term system reliability through effective temperature control during rotation of the satellite in and out of direct solar exposure.
In addition to LEO type satellite applications, AlSiC is also in use in the Lockheed-Martin (now BAE Systems) RAD6000 Flight Computer assembly, for many deep space probes launched over the last several years, including the Mars Pathfinder and the Cassini spacecraft (mission to Saturn).
Known as PRIMECOOL and PRIMEFLO, Ixion AlSiC components are designed for systems requiring low to moderate thermal dissipation through direct-contact conduction cooling.
The PRIMECOOL range incorporates the following.
Power module base plates and sinks. The reliability of high performance IGBT and MOSFET power modules can be significantly improved by replacing standard copper base plates with AlSiC base plates that closely match the thermal expansion of the ceramic substrates carrying the power die.
Microprocessor lids and heat spreaders. Considerable cost savings and substantial package weight reduction can be achieved in microprocessor packaging applications through the replacement of copper/ tungsten and copper/molybdenum. In addition, AlSiC lids can be provided with coefficient thermal expansion (CTE) values that range from 6.9 to 16 ppm/C.
Printed wiring board cores. Able to improve surface mount assembly reliability by minimising thermal cycling and vibration fatigue, AlSiC PWB cores also provide excellent thermal performance while saving up to 70 per cent of the weight of conventional metal cores.
Electronic chassis and enclosures. Weight savings of more than 45 per cent and improved dimensional stability over those manufactured from machined aluminium have been achieved with AlSiC chassis and enclosures.
Carriers and hybrid package bases. AlSiC carriers and package bases have demonstrated weight savings from 33 to over 80 per cent, while matching or exceeding the thermal performance of Cu/W and Fe/Ni alloy-based components.
PRIMEFLO components are designed for systems requiring high-thermal dissipation through active air or liquid cooling.
Ideal for severe environment and mission critical applications, AlSiC components are designed as rugged one-piece AlSiC heat exchangers with internal pin or fin cooling features. Printed wiring board cores, power module heat-sinks/base plates and custom designed heat-sinks are found in numerous aerospace and defence applications.
Ixion AlSiC composites consists of fine silicon carbide particles dispersed in an aluminium alloy matrix. Composites combining these two materials are ideal for many electronic thermal management applications, since the resultant properties are isotropic and can be tailored for specific applications by adjusting the percentages of the SiC particles and the aluminium. For the base-plate application (and many other electronic thermal management applications), desirable properties are achieved at about 55-70 per cent silicon carbide by volume, the balance being aluminium. Representative properties for a 60 per cent SiC, 40 per cent aluminium alloy composite, in comparison to aluminium alloy 6061 and copper, are given in Table I.
The influence of SiC on the properties of the resultant AlSiC composite are demonstrated in the composite that was chosen for this study, i.e. MCX-693 silicon carbide reinforced aluminium. The addition of SiC, which is an excellent thermal conductor (270W/mK), to the aluminium alloy results in a composite having a thermal conductivity equivalent to 6061 aluminium. Moreover, SiC has a low CTE (3.7ppm/K) and its incorporation into the composite results in a dramatic reduction in the CTE of the unreinforced aluminium alloy, thereby bringing the CTE into a range (about 6.4ppm/K) that is well matched to typical ceramic substrate materials such as Al2O3, BeO and A1N.
The effect of the SiC is also observed in the elastic modulus of the composite, producing a material that is stiffer than steel. One benefit of the enhanced modulus is that the thickness of composite base plates can be less than those of copper, thereby partially compensating for the lower thermal conductivity of the composite versus copper. Further, the addition of SiC results in only a marginal increase in the density of the composite over the aluminium alloy, an important factor in weight-critical applications.
AlSiC can be machined by electrodischarge machining (EDM), diamond tooling, water jet and laser machining (on sections 1mm or less). The type of machining process used, however, will vary depending upon the SiC content. It can also be plated with nickel, gold and tin and, where appropriate, anodised and chemical conversion coated. Methods for bonding organic printed wiring board cores to AlSiCs include adhesives, epoxies, thermoplastics and thermoset plastics. Metallised ceramic substrates can be soldered to plated AlSiCs (Tables II and III).
|Thermal expansioncoefficient (ppm/K)||6.4||23||18|
|aThe alloy used in MCX-736 is an aluminium/silicon/ magnesium alloy|
This is one of the key properties to be considered in high performance design. By matching the CTE's of the materials in an electronic assembly, stresses developed between them during power cycling are minimised. Lower stress means less fatigue in the bond joint between the assembly layers, resulting in a significant improvement in the long term reliability of the system. With CTE values ranging from 6.4 to 9.9ppm/K, Ixion's PRIMEX-based AlSiC composites provide an excellent match for most ceramic and semiconductor materials.
For applications where heat dissipation is a critical requirement to maintain system performance and reliability, the thermal conductivity of the heat-sink material is one of the first design parameters to be considered. With thermal conductivity values ranging from 163 to 192W/m/K, Ixion's PRIMEX- based AlSiC composites are an ideal choice for most high performance applications where the combination of CTE and thermal conductivity are primary system design drivers.
The density of all of Ixion's PRIMEX-based AlSiC composites closely matches with that of aluminium alloys commonly used in electronic assemblies. When compared to many other thermal management materials, weight savings of as much as 80 per cent can be achieved. For applications where weight and thermal performance are important, Ixion PRIMEX materials are prime candidates.
For electronic systems operating in severe vibration environments, a high natural frequency is desired for long-term mechanical reliability. Since the natural frequency of a structure is proportional to the square root of its specific stiffness, this property becomes important in such applications. Each of Ixion's PRIMEX-based AlSiC composites combine high elastic modules with low density to provide some of the highest specific stiffness values currently available for electronic structures.
|Power module base plates||*||*||*|
|Printed wiring board cores||*||*||*||*|
|Carriers and package bases||*||*||*||*||*|
|Microprocessor lids/heat spreaders||*||*||*|
|Custom heat sinks||*||*||*|
|Printed wiring board cores (air cooled)||*||*|
|Printed wiring board cores (liquid cooled)||*|
|Power module base plates||*||*||*|
|Custom heat sinks||*||*||*||*|
Table II Typical properties
|Thermal conductivity (25°C) (W/m/K)||180||175||192||165||179|
|Thermal expansion coefficient( 55 to +125°C), (ppm/K)||6.4||6.8||6.9||7.2||9.4|
|Young's modulus (Gpa)||255||235||262||235||196|
|Flexural strength (4pt bend) (Mpa)||300||480||517||330||530|
|Specific heat (25°C) (J/gK)||0.72-0.76||0.72-0.76||0.72-0.76||0.72-0.76||0.72-0.76|
|Electrical resistivity, (mÙcm)||30-50||30-50||30-50||30-50||30-50|
Advanced manufacturing technologies
Through many years of experience, Ixion engineers have taken the AlSiC technologies from the laboratory and implemented them as key parts of high-volume processes. For example, these novel processes are capable of meeting the demands of the automotive and microprocessor packaging markets as well as those of the military and aerospace industries.
The pressureless metal infiltration process: This process infiltrates ceramic particle performs with molten aluminium to reduce components typically ranging from 50 to 70 vol. per cent silicon carbide.
Providing systems and product engineers the ability to solve thermal and structural problems associated with high performance material based solutions, this process also offers maximum design flexibility. In the past, such problems were typically solved with metal or ceramic solutions, often compromising product performance and/or design. However, Ixion approaches the problem differently by combining metals and ceramics to create materials having properties that can meet multiple performance and design specifications, while improving overall system reliability. The result is a product that has the unique combination of thermal conductivity, tailorable thermal expansion, low mass and high stiffness – in other words, a no compromise solution.
Dan WhiteIxion Thermal Materials