packaged for the
|By Robert R. Irving||The packaging of electronics
is being driven by requirements for increasing density and decreasing cost,
as well as for light weight in laptop computers, cellular telephones, electric
vehicles, and avionics. Thermal conductivity and expansion become key concerns
where space is at a premium and electric currents generate destructive heat.
Alloys containing copper and nickel have served well under these conditions, and now a number of companies are offering metal matrix composite materials as alternatives.
The composites consist of metals, such as aluminum, with varying content of particles of other matter, sometimes silicon carbide, for instance. The mixture weighs less than the pure metal and has different properties.
Carl Zweben, a consultant based in Devon, Pa., who specializes in composite materials, recommends them for use in electronics packaging. He said that composites can have higher thermal conductivity than traditional materials (more than twice that of copper), low thermal stress, and tailorable coefficients of thermal expansion. They can reduce weight by as much as 80 percent, and size by as much as 65 percent, he said. They also can be used in low-cost, net-shape fabrication processes.
When aluminum is reinforced with particles of silicon carbide, the result is called "alsick," from the abbreviation of the composite's elemental components, AlSiC. It is the volume material among the metal matrix composites for microelectronics applications, Zweben said.
Power module base plates and heat sinks of AlSiC cast by DMC2 Electronic Components. By controlling the silicon carbide volume, the company can adjust thermal expansion.
Zweben, who is a Distinguished Lecturer and Fellow of ASME, earlier in his career was engaged in materials research for General Electric Co. at Valley Forge, Pa. During the 1980s, he and his team focused on composites, particularly AlSiC, for parts in electronic packaging. The first part they made was a carrier component made of AlSiC, a metal matrix composite that was found to be 65 percent lighter than the material it replaced.
According to Zweben, the silicon carbide content can be adjusted to differing percentages to vary the composite's coefficient of thermal expansion. It can match the CTE of ceramics used in packaging, like aluminum oxide and aluminum nitride, with particle volume fractions of 0.7.
Composites with particle loadings of 0.2 have a CTE similar to those of the glass-reinforced epoxy composites of printed circuit boards, leading to their use in laptop computers.
AlSiC is used in such components as carriers, complex hermetic microwave packages, power semiconductor packages, heat sinks for integrated circuit packages, and support structures.
According to Zweben, the aluminum powder and the silicon carbide particles that constitute the composite can be inexpensive. "The price of some AlSiC composites, which make use of particle, not fiber, reinforcement, is now in the $2- to $4-per-pound range, in large quantities," he said.
Zweben said the difficulty in machining AlSiC leaves it most useful in near net shape production processes. Also, it is far less dense than some of the workhorse metals like tungsten and molybdenum.
Traditional microelectronic packaging materials used to achieve low coefficients of thermal expansion include blends of copper and tungsten (CuW) or of copper and molybdenum (CuMo), and a nickel-cobalt-iron alloy called Kovar, a trademark owned by CRS Holdings Inc., a subsidiary of Carpenter Technology Corp. of Wyomissing, Pa.
"Kovar has a low thermal conductivity and a density much higher than that of aluminum," Zweben said. "To obtain low-CTE materials with higher thermal conductivities than Kovar, copper is blended with tungsten or molybdenum. As the two constituents are not alloyed, these materials can be considered composites as well, although the term used here is metal-metal composites, not metal matrix composites."
The density of AlSiC is about 80 percent less than CuW and 70 percent less than CuMo. As an alternative to Kovar, AlSiC is less dense and more efficient in conducting heat.
Although AlSiC is generally considered less costly than the other materials, it could actually run higher, depending on considerations such as part complexity, particle volume fraction, and the size of the production run.
Ceramics Process Systems Inc. of Chartley, Mass., supplies the materials for making AlSiC parts and also manufactures finished products. According to Mark A. Occhionero, senior research scientist at CPS, one volume application for AlSiC materials is microprocessor lids, or heat spreaders. Not all assemblies and designs are the same. As a result, the thermal expansion requirement for each lid must be tailored to the manufacturer's design.
CPS has three different materials for this type of work. The materials are designated CPS AlSiC-9, AlSiC-10, and AlSiC-12. They have thermal expansion values of 9, 10, and 12 ppm/°C, respectively, from 20 to 150°C.
The manufacturer, DMC2 Electronic Components, says that its printed wiring board cores of AlSiC reduce thermal cycling and vibration fatigue, and weigh far less than conventional metal cores.
In addition to microprocessor lids, Ceramics Process Systems also makes power substrates, insulated gate bi-polar transistor bases, microwave housings, and carrier plates. The power substrates make up about one-half of CPS's business. These units are used largely in cellular telephone base stations.
AlSiC substrates replace those made from machined copper plates. It is a classic case of finding a material having the same coefficient of thermal expansion as that of the product. Carrier plates from CPS are plasma sprayed with a copper-based coating, which enables users to solder the plates to other electronic components. AlSiC is a replacement for copper.
Microwave housings are plated with a nickel-gold surface, enabling the product to be brazed to a nickel-iron seal ring and a 30-pin header. CPS says that all features of this housing, including the 0.032-inch-thick inner septum walls, are held to a dimensional tolerance of 0.003 inch.
Ceramics Process Systems uses a net-shape fabrication pressure casting process for AlSiC components. This process allows complex geometrical features to be incorporated without machining.
Another producer of AlSiC composites, Polese Co. in San Diego, describes itself as a supplier of thermal management materials. It also markets CuW, CuMo, and a proprietary product, Silvar.
The recent purchase of technology from the Aluminum Co. of America has provided Polese with a near net-shape capability for producing AlSiC having 50 to 70 percent reinforcement. The process that enables Polese to do this is vacuum casting infiltration. Its cycle time is two minutes, Polese said.
Polese is also producing stamped AlSiC parts from continuous strip. Here, the material is 20 to 30 percent particulate volume fraction.
AlSiC, although it can be cheaper to use than copper-based materials, is not always so.
Triton Networks Systems, an Orlando, Fla., provider of high-speed broadband fixed wireless network products, worked with Polese to commercialize a technology developed originally for the military by Lockheed Martin.
Stu Weinshanker, Polese's product marketing manager, described the technology as "the guts of a radio" using gallium arsenide integrated circuits, which generate considerable heat. The challenge was to develop efficient packaging for a product that could be sold at a commercially competitive price. The final design, he said, uses a mix of copper and tungsten.
According to Weinshanker, the argument against using AlSiC in this case is that it requires dedicated tooling, which entails a large up-front investment. He added that, because of the tooling investment, AlSiC is best applied to designs that are fairly mature. Experimental ideas that can produce late design changes can be more costly with AlSiC than with many other materials, he said.
Metal Matrix Cast Composites Inc. of Waltham, Mass., is producing a discontinuous graphite fiber-reinforced aluminum product called GA 7-230. According to the company, this product is a fast-machining, lightweight material with low thermal expansion and high thermal conductivity.
The company uses a process it calls Advanced Pressure Infiltration Casting, a three-step procedure consisting of preform manufacturing, preform loading and preheating, and liquid metal infiltration. Different matrix metals, including aluminum, copper, and magnesium, can be used to infiltrate the wide range of preform materials and architectures.
As the company describes it, pressure in the final step forces the molten metal into the pore structure of the evacuated preform and the casting is cooled to promote directional solidification. The result is a porosity-free metal matrix composite, either a net-shape component or a block ready for machining.
Materials and Electrochemical Research Corp. of Tucson, Ariz., has developed two families of very high thermal conductivity composites with low and adjustable expansion for power electronics applications and other high heat flux environments. Both composites, noted J.C. Withers, the chief executive officer, use a form of carbon as the high thermal conductivity phase.
A cooler base of AlSiC made by Ceramics Process Systems Corp., as it appears disassembled. The assembly halves are net-shape fabricated, including the nearly 1,300 pin fin features on the top section. Hydraulic fittings for the cooler base are integrated in bottom section during material fabrication.
According to the company, one system, at 45 to 50 percent volume particle loading in an aluminum matrix, achieves a thermal conductivity about 650 W/mK, which is substantially isotropic with a thermal expansion of approximately 7.5 ppm/K. Expansion can be reduced to about 5 ppm without any appreciable change in the thermal conductivity.
The other aluminum matrix system is anisotropic, with the thermal conductivity of approximately 650 W/mK in the xy plane and 80-110 W/mK in the z direction. The coefficient of thermal expansion in the xy plane is in the 4.5 to 5 ppm/K range.
Either system is produced in near net shapes or plates in thicknesses that range from about 0.06 inch up to one or more inches. Metal skins such as aluminum, copper, nickel, or gold can be applied.
A long-time developer of composite materials, DMC2 Electronic Components Inc. of Newark, Del., manufactures AlSiC metal matrix components with a range of silicon carbide content. According to Kevin J. Dempsey, sales manager, DMC2 is embarking on a new program aimed at high-quality, cost-effective production of AlSiC microprocessor lids, having volume fraction reinforcements of 20 to 70 percent.
DMC2 Electronic Components uses two manufactur-ing methods, which the company has trademarked as Primex and Primex Cast, for making AlSiC parts.
Primex is a process in which silicon carbide is mixed in a slurry and cast in a mold. The casting, a near net shape part, is removed from the mold and fired to the green state. Eventually, it is placed on a sheet of aluminum alloy in a nitrogen atmosphere at 800°C.
The aluminum melts and infiltrates the porosity of the silicon carbide part by capillary action. This process results in particle fractions of 50 to 70 percent, Dempsey said.
Coating Saves a Step
A coating sprayed onto the part before the infiltration stage prevents the forming of a solid aluminum skin that would have to be removed in a separate process. Another coating makes it easier to separate the finished AlSiC part from the remaining aluminum base.
In the Primex Cast method, component fabrication is performed from an AlSiC slurry using conventional foundry processes. This method is used for particle volumes of about 20 to 40 percent, Dempsey said.
According to Dempsey, the high-volume composites, at 70 percent silicon carbide, have a coefficient of thermal expansion of about 7 ppm/°C. At the other extreme, a composite with 20 percent silicon carbide might have a CTE of 16 ppm/°C.
The company recommends its AlSiC components for a variety of thermal management and structural applications, such as pin fin heat sinks, racks and chassis, and microprocessor lids. It supplies AlSiC base plates for use with the heavy-duty ICs in the engine of Toyota's hybrid car, the Prius.
An early AlSiC electronic package developed by Carl Zweben's group at GE is lighter and more heat-conductive than Kovar. The composite, however, can pose manufacturing challenges.
A new company, ThermAluminum Materials LLC of Murrysville, Pa., is distributing two relatively new materials to the microelectronics industry. One is a controlled expansion alloy of aluminum and silicon from Osprey Metals Ltd. of Neath in the United Kingdom. The other material is an AlSiC composite sheet rolled by MC-21 Inc. of Carson City, Nev.
Warren H. Hunt Jr., president of ThermAluminum, said that Osprey Metals' controlled-expansion alloys contain varying aluminum content; 30, 40, 42, and 50 percent are available. Under development are materials with 12 and 25 percent silicon for lids. The materials are machinable and platable by conventional methods. Hunt said samples are being evaluated by more than a dozen companies in the United States and as many more in Europe.
Potential applications include heat sinks, as well as microwave, power IC, and RF packages and bases.
The composite sheet from MC-21 is aluminum with 20 percent silicon carbide material. The company reports its thermal expansion as 15 ppm/°C and ther-mal conductivity as 185 W/mK. Produced as rolled sheet, this material is suitable for stamped or electro-discharge-machined heat spreaders, thermal planes, and other 2-D parts requiring limited expansion and good thermal conductivity.
According to David Schuster, president of MC-21, a special roll-casting technology has been developed for the production of the AlSiC sheet. Potential uses are for integrated circuits and hard disk drives.
That the sheet is stampable, he said, makes it easier for fabricators to achieve parts having final dimension. "This product does not require a lot of downstream fabricating," he said.
MC-21 handles sales directly with customers as well as through ThermAluminum.
Another composite material of interest is beryllia, particle-reinforced beryllium. It has been used in production, but nowhere near as much as AlSiC, largely because of its high cost. Other metal matrix composites include carbon fiber-reinforced copper, diamond particle-reinforced copper, and diamond particle-reinforced aluminum.
Nonmetallic materials applicable to electronic packaging applications include carbon fiber-reinforced polymer matrix composites and carbon/carbon composites. Some of these materials are experimental in packaging; some are in production.
Another nonmetallic material, pyrolytic graphite, is a plate-like material that has a reported thermal conductivity of about 1,600 W/mK.
A key firm involved in promoting pyrolytic graphite in packaging is Advanced Ceramics Corp. of Cleveland.
Zweben predicts that, in the future, many of these materials will also be used in the optoelectronics industry. This will be a new market for the materials, but one with enormous potential.
© 2001 by The American Society of Mechanical Engineers