Thermal Materials Solve Power Electronics Challenges

Feb 1, 2006 12:00 PM
By Carl Zweben, Ph.D., Advanced Thermal Materials Consultant, Devon, Pa.

High-Performance Thermal Materials

In response to the well-documented needs previously described, an increasing number of high-performance advanced materials that offer significant improvements have been and are continuing to be developed. Advantages include thermal conductivities up to more than four times that of copper, CTEs that are tailorable from -2 to +60 ppm/K and a wide range of electrical resistivities. They also have extremely high strengths and rigidity, low densities, and low-cost, net-shape fabrication processes. Demonstrated payoffs include the following: improved and simplified thermal design, elimination of heat pipes, fans and pumped fluid loops, heat dissipation through pc boards, weight savings up to 90%, size reductions up to 65%, reduced cooling power, reduced thermal stresses, direct attach with hard solders, increased reliability, improved performance, increased pc board natural frequency, increased manufacturing yield, and part and system cost reductions. These materials are being used in a rapidly increasing number of commercial, aerospace and defense applications.

High-performance thermal materials, which are at various stages of development, fall into five main categories: monolithic carbonaceous materials, metal matrix composites (MMCs), carbon/carbon composites (CCCs), ceramic matrix composites (CMCs) and polymer matrix composites (PMCs).^{[4, 6-9]} A composite material is two or more materials bonded together. They are nothing new in electronic packaging. For example, E-glass fiber-reinforced polymer (E-glass/polymer) pc boards are PMCs. Cu-W and Cu-Mo are MMCs, rather than alloys. The numerous ceramic particle-reinforced and metal particle-reinforced polymers used for TIMs, underfills, encapsulants and electrically conductive adhesives are all PMCs.

The first second-generation thermal management material, silicon carbide particle-reinforced aluminum, commonly called Al/SiC in the packaging industry, is an MMC that was first used in microelectronic and optoelectronic packaging by me and my colleagues at GE in the early 1980s.^[9] The first parts cost hundreds to thousands of dollars. As the processing technology matured and use increased, component cost dropped by several orders of magnitude. Microprocessor lids using this material now sell for $2 to $5 each 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 of advanced thermal management materials.

At present, we are in the early stage of the third generation of packaging materials. Several of the new high-performance materials discussed in this article are being used in production applications, including servers, notebook computers, plasma displays, aircraft and spacecraft electronics, and optoelectronic systems. Considering that these materials were only commercialized in the last few years, this is remarkable progress.

Fig. 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 like Si, GaAs, alumina, aluminum nitride and low-temperature cofired ceramics (LTCCs). As the figure shows, by combining matrices of metals, ceramics and carbon with thermally conductive reinforcements like special carbon fibers (abbreviated C), SiC particles and diamond particles, it is possible to create new materials with high thermal conductivities and a wide range of CTEs.

Materials presented include monolithic metals, highly oriented pyrolytic graphite (HOPG) and a number of composites. The composites include 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 Cu-W. HOPG, also called thermal pyrolytic graphite and annealed pyrolytic graphite by various manufacturers, and diamond particle-reinforced metals and ceramics have the highest thermal conductivities.

Tables 2 and 3 present properties of several dozen selected second-generation and third-generation high-performance materials, respectively. Table 3 includes diamond made by chemical vapor deposition (CVD) for reference. For anisotropic materials, inplane isotropic and through-thickness thermal conductivity (k) values are presented. The absolute and specific thermal conductivities of the advanced materials in Table 2 and especially in Table 3 are significantly higher than those of the traditional materials in Table 1.

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