ELECTRONIC PACKAGING MATERIALS

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ADVANCED ELECTRONIC PACKAGING MATERIALS Heat Revolutionary advances have recently been made in advanced monolithic and composite packaging materials for microelectronics and optoelectronics.

1 ADVANCED ELECTRONIC PACKAGING MATERIALS Heat Revolutionary advances have recently been made in advanced monolithic and composite packaging materials for microelectronics and optoelectronics. Carl Zweben* Composite Materials Consultant Devon, Pennsylvania Integrated heat spreader (lid) TIM1 sink TIM2 High-performance packaging materials may be divided into two categories: those having thermal conductivities between 300 and 400 W/m-K, designated high-thermal-conductivity materials (Table 1); and those having thermal conductivities of at least 400 W/m-K, designated ultrahigh-thermalconductivity materials (Table 2). All of these materials have low coefficients of thermal expansion (CTEs), which are needed to minimize thermal stresses that can cause failure during manufacture and in service. Traditional packaging materials such as copper, aluminum, and copper/tungsten are shown in Table 1 for comparison. Note that some advanced materials are anisotropic, including copper-clad Invar (C-I-C) laminates and copper-clad molybdenum (C-Mo- C) laminates, and E-glass/epoxy printed circuit boards (PCBs). For these materials, Tables 1 and 2 present inplane isotropic properties, except for the Cytec ThermalGraph material, as discussed later. Particle-reinforced composites generally are isotropic in three dimensions. As the tables show, over twenty low-cte materials have thermal conductivities that are at least 50% higher than that of aluminum, and fifteen have thermal conductivities that are equal to or higher than that of copper. One material has a thermal conductivity that is in the range of four times that of copper. The tables clearly show that advanced materials *Fellow of ASM International Die Printed circuit board Solder ball Electronic packages require advanced thermally conductive materials in heat sinks and in heat spreaders. TIM1 and TIM2 are thermal interface materials. offer dramatic improvements in properties compared to aluminum, copper, and copper/tungsten. The advantages are particularly striking when density is considered. For many ultrahigh-thermalconductivity materials, specific thermal conductivity (defined as the ratio of thermal conductivity to specific gravity) is more than an order of magnitude higher than those of traditional materials, especially those with low CTEs, such as copper/tungsten. This is an important figure of merit for weight-sensitive applications. Arule of thumb in the history of technology says that when a critical parameter is increased by an order of magnitude, it has a revolutionary effect. This is true today in thermal management. However, we are in the early stages of a very dynamic technology. New materials may well emerge that will eclipse those considered here. This article discusses the five main categories of high-thermal-conductivity materials: monolithic carbonaceous materials, metal matrix composites (MMCs), carbon/carbon composites (CCCs), ceramic matrix composites (CMCs), and polymer matrix composites (PMCs). Composite packaging materials Many of the advanced materials of greatest in- ADVANCED MATERIALS & PROCESSES/OCTOBER

2 Table 1 Properties of advanced materials with high thermal conductivities and low coefficients of thermal expansion (300 < k < 400) Thermal Specific thermal conductivity, Specific conductivity, Reinforcement Matrix W/m-K CTE, ppm/k gravity W/m-K None Aluminum None Copper Copper Tungsten Natural graphite Epoxy Continuous carbon fibers Polymer Discontinuous carbon fibers Copper SiC particles Copper Continuous carbon fibers SiC Carbon foam Copper Table 2 Properties of advanced materials with ultrahigh thermal conductivities and low coefficients of thermal expansion (400 < k) Thermal Specific thermal conductivity, Specific conductivity, Reinforcement Matrix W/m-K CTE, ppm/k gravity W/m-K None CVD Diamond None HOPG None Natural graphite Cytec ThermalGraph Continuous carbon fibers Copper Continuous carbon fibers Carbon Graphite flake Aluminum Diamond particles Aluminum Diamond & SiC particles Aluminum Diamond particles Copper Diamond particles Cobalt > >145 Diamond particles Magnesium Diamond particles Silver 400-> >103 Diamond particles Silicon Diamond particles SiC terest are composites, which Dr. Anthony Kelly has defined elegantly as two or more materials bonded together. The most important packaging composites consist of polymer, metal, ceramic, and carbon matrices reinforced with fibers, particles, or a combination of the two. The key reinforcements are continuous and discontinuous thermally conductive carbon fibers, and thermally conductive ceramic particles, such as diamond, silicon carbide, beryllium oxide (beryllia), and graphite flakes. Fiber-reinforced composites are usually strongly anisotropic. Particle-reinforced composites tend to be isotropic. The properties of the fiberreinforced composites shown in Tables 1 and 2 are inplane isotropic values. Most of the dozens of different types of carbon fibers have relatively low thermal conductivities. However, some commercial carbon fibers have nominal thermal conductivities as high as 1100 W/m-K. Experimental discontinuous fibers reportedly have thermal conductivities of 2000 W/m-K. Some sources put the thermal conductivity of carbon nanotubes (also called Buckytubes) as high as 6600 W/m-K. Carbon fibers serve as reinforcements in polymer, metal, carbon, and ceramic matrices. Tables 1 and 2 include inplane isotropic properties of carbon-fiber-reinforced composites. By orienting fibers, it is possible to develop much higher thermal conductivities in one direction. For example, the thermal conductivity of carbon-fiberreinforced copper with oriented fibers can be as high as 800 to 900 W/m-K in the fiber direction, making them competitive with heat pipes in some cases. 34 ADVANCED MATERIALS & PROCESSES/OCTOBER 2005

Tailoring thermal conductivity An important characteristic of fiber-reinforced composites is that their properties often can be tailored to a large extent.

3 Tailoring thermal conductivity An important characteristic of fiber-reinforced composites is that their properties often can be tailored to a large extent. For example, we can design materials with extremely high thermal conductivities in one direction that, as discussed, can compete with heat pipes over short distances. Alternatively, we can achieve high isotropic inplane conductivities to spread heat effectively. At the same time, we can tailor other properties, such as CTE. Because of the relatively high electrical resistivity of PMCs reinforced with discontinuous carbon fibers compared to metals, they are placed on video-chip heat sinks to reduce electromagnetic radiation. Advanced materials with ultrahigh thermal conductivity are applied in heat spreaders that are required in computers such as this Dell Inspiron laptop. Image courtesy Dell Inc. Carbonaceous materials Carbon, which comes in a wide variety of forms, is a remarkable material. Carbonaceous materials range from graphite lubricants to diamonds to high-performance structural carbon fibers in commercial and military aircraft and sporting goods. Among monolithic carbonaceous packaging materials, diamond films made by chemical vapor deposition (CVD), have been produced for some years. Materials that are new or newly applied to packaging include naturally occurring graphite, highly-oriented pyrolytic graphite (HOPG), Cytec s ThermalGraph (a fibrous panel), and carbon foam. Naturally-occurring graphite, also called natural graphite, has served as a thermal interface material for many years. Recently, it has been converted into a form useful for making heat sinks by bonding thin sheets together with an epoxy adhesive. Table 1 presents properties of these laminates, which are highly anisotropic. Through-thickness thermal conductivity is only about 6.5 W/m-K. Table 2 presents properties of natural graphite sheets, which are in notebook computers and plasma displays. These materials are also highly anisotropic. It is significant that one of the notebook computers in which natural graphite carries away heat is reportedly the lightest on the market, and has no heat pipes or fans. This is an obvious advantage in extending battery life, as well as reducing weight. Highly oriented pyrolytic graphite has been around for decades. However, only relatively recently has it been applied in electronic packaging. HOPG is a highly anisotropic, rather brittle and weak material, with an inplane thermal conductivity as high as 1700 W/m-K, and through-thickness conductivity of 10 to 25 W/m-K. The inplane CTE is 1.0 ppm/k, which is lower than needed for most packaging applications (many carbonaceous materials have negative axial or planar CTEs). However, HOPG can be encapsulated with a variety of materials that have different CTEs and also provide strength and stiffness. Encapsulated HOPG is in ground-based radars and aerospace printed circuit board cold plates (also called thermal cores, heat sinks, and thermal planes). It is under consideration for many other applications. Un-encapsulated HOPG also has been successful in particle physics research instruments. ThermalGraph panels, made by Cytec Engineered Materials, Tempe, Ariz., are made of oriented, carbon-bonded, highly conductive carbon fibers. The panels have axial thermal conductivities of 700 to 800 W/m-K. ThermalGraph can be infiltrated with polymers, aluminum, or copper to increase strength and through-thickness thermal conductivity. This material is under development for aerospace PCB cold plates, and other thermal management applications. Carbon foams have been developed for thermal conductivity in the last few years. Although their thermal conductivities are lower than that of aluminum, they can be infiltrated with thermally conductive materials. Table 1 shows properties of copper-infiltrated carbon foam. Metal matrix composites Tables 1 and 2 present properties of key highthermal-conductivity MMCs of interest. Matrices are aluminum, magnesium, copper, cobalt, and silver. Reinforcements include continuous and discontinuous carbon fibers; graphite flakes; and particles of silicon carbide, diamond, and beryllia. Thermal conductivities as high as 1200 W/m-K have been reported. Al/SiC is a family of materials having widely varying properties. Made by a variety of processes, the major limitation of Al/SiC materials is that their thermal conductivities are no better than those of aluminum alloys. However, incorporating CVD diamond or HOPG plates can raise thermal conductivity. Silicon-carbide-particle-reinforced aluminum ADVANCED MATERIALS & PROCESSES/OCTOBER

Diamond particlereinforced copper composites reportedly are being placed in production laser diode packages. (Al/SiC) is the most widely applied advanced MMC packaging material.

4 Diamond particlereinforced copper composites reportedly are being placed in production laser diode packages. (Al/SiC) is the most widely applied advanced MMC packaging material. The author was the first to use this material in microelectronic and photonic packaging, beginning in the early 1980s. Millions of piece parts are now produced annually. Al/SiC microwave packages and solid and flowthrough PCB heat sinks are in numerous avionic systems. Commercial applications include optoelectronic packages; light-emitting diode (LED) packages; servers; notebook computers; cellular telephone base stations and handsets; and power supplies for trains, wind turbine generators, and hybrid vehicles. The International Space Station includes Al/SiCencapsulated highly oriented pyrolytic graphite PCB heat sinks. An Al/SiC optoelectronic package for the telecommunications industry has an HOPG insert to improve heat spreading. Aluminum reinforced with discontinuous carbon fibers, (carbon/aluminum) is operating in spacecraft phased array antenna RF packages. Continuous-carbon-fiber carbon/aluminum cold plates were chosen for a spacecraft electronics system. Copper reinforced with discontinuous carbon fibers recently has been commercialized. Diamond particle-reinforced copper (diamond/ copper) composites reportedly are being placed in production laser diode packages. These and other diamond-reinforced MMCs are being evaluated in several development programs. Carbon/carbon composites Carbon/carbon composites consist of carbonaceous matrices reinforced with carbon fibers. They are stronger, stiffer, and less brittle than monolithic carbon, although they cannot be considered highperformance structural materials. Some CCCs have high thermal conductivities. Table 2 includes properties of one formulation. CCCs have served in a limited number of production thermal management applications, including expendable launch vehicle PCB cold plates, spacecraft radiator panels, and thermal doublers for PMC spacecraft radiator panels. Ceramic matrix composites It was recently announced that diamond-particle-reinforced silicon carbide (diamond/sic) heat spreaders are being placed in production for IBM computer servers. This is an historic milestone. As Table 2 shows, this material has a low CTE, low density, and a thermal conductivity that is 50% higher than that of copper. Diamond/SiC was originally developed for cutting tools and rock drills, so a production base exists. Polymer matrix composites Although the thermal conductivities possible with PMCs are not as high as those of other advanced materials, they have a number of advantages, including ease of fabrication, low density, and for some materials, relatively low cost. 36 For more information visit

5 As discussed, glass-fiber-reinforced polymer PCBs, which are PMCs, have been applied in electronic packaging for decades. However, these materials have low thermal conductivities and relatively high CTEs. In many cases, low-cte materials such as C-I- C have served as constraining layers to reduce the effective CTE of the printed circuit board assembly. Now, thermally conductive carbon fibers are replacing the C-I-C. The advantages are higher thermal conductivity, higher stiffness, reduced weight, and ease of processing. As mentioned earlier, the development of thermally conductive carbon fibers was a major breakthrough. Table 1 presents properties of polymers reinforced with continuous fibers. Continuous thermally conductive carbon fibers can produce PMCs with inplane thermal conductivities over 50% greater than that of aluminum, combined with high modulus and low density. These materials have functioned in a number of production thermal management avionics applications, including aircraft PCB thermal planes and electronic enclosures. Other thermal management applications include spacecraft battery sleeves, radiator panels, and thermal doublers. A major advantage of PMCs reinforced with short discontinuous fibers is that they can be formed into complex shapes by injection molding. Although thermal conductivities are much lower than those of PMCs with continuous fibers, they are adequate for many applications. A significant and increasing number of production components are now made of these materials, including integrated circuit pin-fin heat sinks and heat spreaders. Annual production volume reportedly is in the range of millions of piece parts. An important consideration is that the injection molding process can orient fibers, resulting in anisotropic properties, which are undesirable in some applications. However, other processes for discontinuous fibers do not have this drawback. Properties of composites with long discontinuous fibers and high fiber volume fractions can approach those possible with continuous fibers. However, the associated processes tend to be more expensive than for injection molding, and shapes are more limited. As discussed earlier, PMCs reinforced with discontinuous carbon fibers have much higher electrical resistivities than metals, reducing electromagnetic radiation. This has led to their application in video chip heat sinks. Future materials We are in the infancy of the development of advanced composite and monolithic materials that are tailored to meet the specific needs of the electronic packaging industry. As the technology matures, materials with better properties and cheaper processes undoubtedly will continue to emerge. Nanocomposites are particularly intriguing. The history of advanced materials has shown that as production volume grows, costs typically drop, making them increasingly attractive. We have seen this with many advanced materials, including Al/SiC. Because of the unique ability of advanced materials, especially composites, to meet future packaging requirements, they can be expected to play an increasingly important role in the 21st century. For more information: Carl Zweben, Ph. D., Composites and Advanced Thermal Management Materials Consultant, 62 Arlington Road, Devon, PA ; tel: 610/ ; fax: 610/ ; c.h.zweben@ usa.net. Acknowledgements Some of the data in this paper were taken from the following publications, and appear courtesy of the publishers: Composite Materials And Mechanical Design, by Carl Zweben: Mechanical Engineers Handbook, Second Edition, Myer Kurtz, Ed., John Wiley & Sons Inc., New York, Metal Matrix Composites, Ceramic Matrix Composites, Carbon Matrix Composites and Thermally Conductive Polymer Matrix Composites, by C. Zweben: Handbook of Plastics, Elastomers and Composites, Fourth Edition, Charles A. Harper, Editor-in-Chief, McGraw- Hill, New York, Comprehensive Composite Materials, A. Kelly and C. Zweben, Eds., Pergamon Press, Oxford, For more information visit 37

High-Performance Carbon-Based Thermal Management Materials September 25, 2013

High-Performance Carbon-Based Thermal Management Materials September 25, 2013 ADVANCED CARBON-BASED THERMAL MANAGEMENT MATERIALS AND APPLICATIONS SPONSORED BY: Carl Zweben Ph. D. Life Fellow, ASME; Fellow ASM and SAMPE; Assoc. Fellow, AIAA Advanced Thermal Materials Consultant Presenter