Crucial for future packaging
BY CARL ZWEBEN
Heat dissipation, thermal stress, and warping are critical issues in packaging of microprocessors, power semiconductors, high-power RF devices, laser diodes, LEDs, and MEMS. The severity of the problem is highlighted by Intel’s acknowledgement that it has hit a “thermal wall”, and evidenced by an automotive-like pumped liquid cooling system in the new Apple Computer Power Mac G5. Introduction of liquid cooling requires addition of new manufacturing and servicing infrastructures, and raises significant reliability and cost issues. New, high-performance materials developed in the last few years have ultra-high thermal conductivities, low coefficients of thermal expansion (CTEs), and low densities that can solve key packaging problems.
Thermal stress and warpage arise from CTE differences. Semiconductors and ceramics have CTEs in the range of 2 to 7 ppm/K. The CTEs of copper, aluminum, and glass fiber-reinforced polymer printed circuit boards (PCBs) are much higher. Traditional, first-generation, low-CTE materials, like copper/tungsten and copper/molybdenum, have high densities and thermal conductivities that are no better than aluminum. Even when liquid cooling is used, thermal stresses from CTE mismatch are still important.
Weight is a key consideration in most portable systems, including notebook computers, cell phones, hybrid automobile electronics, and avionics. Even if system weight is not an issue, low-density materials are needed for components like heatsinks to minimize shock load stresses during shipping.
Use of thermal management materials with high CTEs requires significant design compromises that reduce cooling efficiency and can increase cost. For example, it is common to use compliant polymeric thermal interface materials (TIMs) to attach aluminum and copper integrated heat spreaders (IHSs) to microprocessor chips. TIMs increasingly account for most of the total system thermal resistance . This problem can be overcome by direct solder attach, but this raises important thermal stress issues. A similar situation occurs in attachment of high-power laser diode heatsinks. At present, the solution is to employ “soft” solders, typically indium-based, which have low-yield stresses. However, these solders also have poor thermal fatigue and metallurgical characteristics. Use of materials with matching CTEs allows the packaging design engineer to select from a wider range of solders. In addition, low-CTE solders will further alleviate the thermal stress problem.
High-Performance Thermal Materials
An increasing number of high-performance advanced materials offer significant improvements, such as thermal conductivities up to more than four times that of copper; CTEs that are tailorable from -2 to +60 ppm/K; electrical resistivities from very low to very high, extremely high strengths and stiffnesses; low densities; and low-cost, net-shape, fabrication processes. Demonstrated payoffs include: improved and simplified thermal design; elimination of heat pipes, fans, and pumped fluid loops; heat dissipation through PCBs; weight savings up to 90%; size reductions up to 65%; reduced power consumption, and reduced thermal stress and warpage. CTE match allows direct attach with hard solders; increased reliability; improved performance; increased PCB natural frequency; increased manufacturing yield; and part and system cost reductions.
High-performance thermal materials fall into six main categories: monolithic carbonaceous materials, metal matrix composites (MMCs), carbon/carbon composites (CCCs), ceramic matrix composites (CMCs), and polymer matrix composites (PMCs).
Composites are nothing new in electronic packaging. For example, E-glass-fiber-reinforced polymer (E-glass/polymer) PCBs are PMCs; and copper/tungsten and copper/molybdenum are MMCs, rather than alloys. In addition, there are numerous ceramic-particle- and metal-particle-reinforced polymers used for thermal interface materials (TIMs), underfills, encapsulants, and electrically conductive adhesives. All are PMCs.
The first second-generation thermal management material, silicon carbide particle-reinforced aluminum (Al/SiC), is an MMC first used in microelectronic and optoelectronic packaging by industry experts at GE the early 1980s. As the technology matured and use increased, component cost dropped by several orders of magnitude. Microprocessor lids now sell for 2 - $5 in large volumes. Al/SiC has been used for some time in high-volume, commercial and aerospace microelectronics and optoelectronic packaging applications, demonstrating the potential for advanced materials.
Several of the newest , third-generation, high-performance materials discussed in this article are being used in production applications, including servers, notebook computers, plasma displays, PCB cold plates, and optoelectronic packages.
Figure 1. Thermal conductivities and coefficients of thermal expansion of semiconductors, ceramics and thermal materials used in microelectronic, optoelectronic, and MEMS packaging.
Figure 1, which plots thermal conductivity as a function of CTE, compares traditional and advanced thermal materials. Ideal materials have high thermal conductivities and CTEs that match those of semiconductors and ceramics such as Si, GaAs, Alumina, and Aluminum Nitride. By combining matrices of metals, ceramics, and carbon with thermally conductive reinforcements like carbon (C) fibers, SiC particles, and diamond particles, it is possible to create new materials with high thermal conductivities and a wide range of CTEs. Materials represented include a monolithic material - highly-oriented, pyrolytic graphite (HOPG) - and a number of composites: carbon fiber-reinforced carbon (C/C), carbon fiber-reinforced epoxy (C/Ep), carbon fiber-reinforced copper (C/Cu), silicon carbide particle-reinforced copper (SiC/Cu), and traditional copper/tungsten (Cu/W). HOPG and diamond-particle-reinforced metals, and ceramics have the highest thermal conductivities. One CMC - diamond-particle-reinforced SiC - is now in production IBM-server heat spreaders.
Table 1. Properties of high-performance and traditional thermal materials.
Table 1 presents properties of selected high-performance materials. Copper, aluminum, copper/tungsten, E-glass/epoxy (PCB) and Al/SiC are shown for reference. For anisotropic materials, inplane and through-thickness values are presented.
A number of third-generation advanced thermal materials are now in commercial and aerospace production applications, including servers, plasma displays, notebook computers, printed circuit board cold plates, avionics, and optoelectronic packages. Low-CTE, high-stiffness, lightweight carbon fibers with thermal conductivities as high as 1100 W/m-K can be used to reduce the CTE and increase the thermal conductivity and stiffness of PCBs. These applications are historic packaging milestones.
Figure 2. Diamond particle-reinforced silicon carbide ceramic matrix composite heat spreader with a silicon coating that improves surface roughness. Courtesy of Skeleton Technologies
Figure 2 shows a diamond particle-reinforced silicon carbide CMC heat spreader with a silicon coating to improve surface roughness. This composite is now in IBM heat spreaders.
Figure 3. Ultra-lightweight notebook computer uses natural graphite heat spreaders, eliminating heat pipes and fans.Courtesy of Graftech
Figure 3 shows an ultra-lightweight notebook computer that uses natural graphite heat spreaders, eliminating the need for heat pipes and fans.
Figure 4. Carbon fiber-reinforced aluminum spacecraft phased array antenna microwave packages. Courtesy of Metal Matrix Cast Composites
Figure 4 shows carbon fiber-reinforced aluminum microwave packages that are used in a spacecraft phased array antenna. Carbon fiber-reinforced copper, which has a higher thermal conductivity, is now available.
The high CTE of PCBs is a key source of thermal stress and warping. One solution has been use of copper-Invar-copper constraining layers. A new, lightweight approach uses thermally conductive carbon fibers that, in addition to reducing CTE, allow heat removal from the bottom of chips, as well as the tops. The high stiffness of these fibers also reduces warping and increases PCB natural frequency. Thermally conductive carbon fibers are also being used in high-performance TIMs.
Solving Manufacturing Problems with Composites
CTE mismatch also causes thermal stress and warping that can result in failures during manufacturing, greatly reducing yield. In one case involving a complex, expensive ceramic package, the yield was less than 5%. Modeling the many process steps using finite element analysis enabled experts to define the required base-plate CTE that would produce an acceptable level of warping. This increased yield to over 99%, saving over $60 million.
Cost is a complex issue involving many factors. Component and system cost are both important. For example, an expensive material may well be cheaper than a pumped liquid cooling system when all costs - component, manufacturing, servicing and warranty - are included. Several high-performance materials, including diamond particle-reinforced SiC, HOPG, natural graphite, carbon fiber-reinforced aluminum, and diamond particle-reinforced copper, are being used in commercial and aerospace applications, demonstrating their cost effectiveness.
We are in the early stages of a thermal materials revolution. Al/SiC, the first second-generation thermal material, was developed only about two decades ago. Most of the new, third-generation high-performance materials were developed in the last few years. It seems reasonable to expect that in the future there will be significant developments in both materials and processes, leading to improved properties and reduced costs. Decreasing cost and increasing awareness will stimulate use in an increasing number of microelectronic, optoelectronic, and MEMS applications.
One intriguing area of interest is nanocomposites. Estimates of carbon nanotube thermal conductivity run as high as 6600 W/m-K. Values over 3000 have been measured. Graphite nanoplatelets, which are much cheaper than nanotubes, are another candidate nanoscale reinforcement, as are nanoparticles of diamond and other thermally conductive materials. While the small size of nanoscale reinforcements results in a large number of interfaces that reduce effective thermal conductivity, the materials are worth exploring. It may be these materials can be used with other reinforcements, such as thermally conductive carbon fibers, to produce hybrid composites with attractive properties. Another potential advantage of nanocomposites is reduced CTE.
Composite solders with low CTEs are under development. Combined with the materials discussed in this article, the packaging engineer will be able to match thermal expansions throughout the system, improving manufacturing yield, reliability, and thermal performance. 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.
Some of the data in this paper were taken from the following publications, and appear courtesy of the publishers:
A. Kelly and C. Zweben, Editors-in-Chief., Comprehensive Composite Materials, Pergamon Press, Oxford, 2000.
C. Zweben, “Composite Materials And Mechanical Design”, Mechanical Engineers’ Handbook, Book 1: Materials and Mechanical Design, Third Edition, Myer Kutz, Ed, John Wiley & Sons, Inc., New York, 2005.
C. Zweben, Chapter 5, “Metal Matrix Composites, Ceramic Matrix Composites, Carbon Matrix Composites and Thermally Conductive Polymer Matrix Composites”, Handbook of Plastics, Elastomers and Composites, Fourth Edition, A. Harper, Editor-in-Chief, McGraw-Hill, New York, 2002.
Contact the author for a complete list of references.
Carl Zweben, Ph. D., advanced thermal materials consultant, may be contacted at 62 Arlington Road Devon, PA 19333-1538; /610/ 688-1772; E-mail: [email protected]