Advanced Thermal Management Materials
By Carl Zweben, PhD, Advanced Thermal Materials Consultant
Heat dissipation, thermal stress, warping, weight, and cost are critical microelectronic and photonic packaging issues. Traditional thermal management materials, which date from the mid 20th century, all have major deficiencies. In response, material suppliers are continually introducing a new generation of materials, providing the packaging engineer with more options.
Deficiencies of Traditional Thermal Management Materials
Traditional thermal management materials include copper, aluminum and materials with low coefficients of thermal expansion (CTEs), such as "Kovar", Alloy 42, Titanium, tungsten/copper, molybdenum/copper, copper-Invar-copper (C-I-C) and copper-molybdenum-copper. All have significant deficiencies.
Thermal stress and warpage arise primarily from CTE differences. Semiconductors and ceramics have CTEs in the range of about 2 to 7 ppm/K. The CTEs of copper, aluminum and glass fiber-reinforced polymer printed circuit boards (PCBs) range from 16 to 23 ppm/K, requiring use of greases or compliant polymeric and solder thermal interface materials (TIMs). CTE mismatch also dictates the need for underfills in flip-chip attachment. Note that even when liquid cooling, thermoelectric cooling, etc. are used; thermal stress caused by CTE mismatch is an issue.
Polymeric TIMs increasingly account for most of the total system thermal resistance. An alternative, "soft solders" typically Indium based, have poor thermal fatigue and metallurgical characteristics. Use of materials with matching CTEs allows use of hard solders, which provide the lowest thermal resistances, and lack the problems of greases and compliant TIMs.
Traditional low-CTE materials, like tungsten/copper, molybdenum/copper and C-I-C, which are decades old, have high densities and thermal conductivities that are no better than that of aluminum. "Kovar", Alloy 42, and titanium have very low thermal conductivities. C-I-C constraining layers have been used for many years to reduce the CTE of PCBs.
Weight is a key consideration in most portable systems, including notebook computers, cell phones, hybrid automobile electronics, avionics, etc. In addition, low-density materials minimize shock loads during shipping and in service.
Advanced Thermal Management Materials
In response to traditional thermal materials deficiencies, more advanced materials have been developed that offer significant improvements (see Table). Advantages include: thermal conductivities up to more than four times that of copper; CTEs that can match those of semiconductors, ceramics, and PCBs; electrical resistivities ranging from very low to relatively high; low densities; high strengths and stiffnesses; and low-cost, net-shape fabrication processes.
Table: Properties of selected advanced thermal management materials. Asterisks indicate in-plane values for anisotropic materials.
Demonstrated payoffs include improved and simplified thermal designs; elimination of heat pipes, fans and pumped fluid loops; weight savings up to 90%; size reductions up to 65%; reduced cooling power consumption; reduced thermal stress and warpage; direct attach with hard solders; increased reliability; improved performance; increased heat dissipation through printed circuit boards (PCBs); tailored PCB CTEs that can potentially eliminate underfills; increased PCB natural frequencies; increased manufacturing yield; and part and system cost reductions.
Advanced thermal materials fall into six main categories: monolithic carbonaceous materials (i.e., various forms of carbon), metal matrix composites (MMCs), carbon/carbon composites (CCCs), ceramic matrix composites (CMCs), polymer matrix composites (PMCs), and metallic alloys, some of which can be considered MMCs (Table). Note that diamond particle-reinforced MMCs and CMCs are used in many industrial applications, such as rock drills and grinding wheels.
Composites are nothing new in electronic packaging. For example, E-glass fiber-reinforced polymer PCBs and many TIMs and underfills are PMCs. Copper/tungsten and copper/molybdenum are MMCs, rather than alloys.
The Table presents properties of selected advanced materials: highly-oriented pyrolytic graphite (HOPG); pyrolytic graphite sheet, natural graphite, silicon carbide (SiC) particle-reinforced aluminum (commonly called Al/SiC or AlSiC), beryllium oxide particle-reinforced beryllium, continuous carbon fiber-reinforced epoxy; natural graphite/epoxy; discontinuous carbon fiber-reinforced copper; diamond particle-reinforced aluminum, copper and SiC; and carbon fiber-reinforced carbon. For those materials that are not isotropic, inplane thermal conductivities are presented.
High-thermal-conductivity, low-CTE, high-stiffness, lightweight carbon fibers are being used to reduce the CTE and increase the thermal conductivity and stiffness of PCBs, serving the same function that C-I-C traditionally did.
To illustrate the advantages of advanced thermal materials, replacement of traction IGBT copper base plates with Al/SiC, which has a low CTE, eliminates solder joint failure and reduces weight, increasing lifetime from ten to thirty years. The same result could have been achieved with a traditional low-CTE material like tungsten/copper, but the penalties would have been greater cost and weight.
The number of commercial and aerospace convection-cooled and liquid-cooled advanced thermal management materials applications is increasing, including servers, notebook computers, PCB cold plates, aircraft, and spacecraft avionic systems, phased array antennas, power modules, sensors, displays, laser diodes and photovoltaics.
Figure 1 shows IGBT modules with Al/SiC base plates and Al/SiC base plates for IGBTs and other power devices.
Figure 1. IGBT modules with silicon carbide particle-reinforced aluminum (Al/SiC) base plates and Al/SiC base plates for IGBT and other power devices. (Courtesy Thermal Transfer Composites)
The ultra-lightweight Sony Vaio PCG-X505/S-P Extreme notebook computer, which weighs only 768g (1.7 pounds), uses SpreaderShield™ natural graphite heat spreaders (Figure 2). It has no heat pipes or fans. Natural graphite is lighter than aluminum and has thermal conductivities up to 1500 W/m-K.
Figure 2. Use of SpreaderShield™ natural graphite to dissipate heat eliminates the need for heat pipes and fans in the Sony Vaio PCG-X505/S-P Extreme notebook computer (Courtesy Electronic Thermal Management & Advanced Energy GrafTech International Holdings Inc.).
The thermal materials revolution is still in its early stages. Al/SiC, first used in packaging at GE, is only a few decades old. Historically speaking, this is barely the blink of an eye. It is reasonable to expect significant future developments in materials and processes, improving properties and reducing costs. Decreased cost and increased awareness will stimulate more microelectronic and photonic applications.
Nanocomposites are an intriguing area of development. The estimated carbon nanotube thermal conductivity is 6600 W/m-K. Values over 3000 have been measured. While the small size of nanoscale reinforcements results in a large number of interfaces that reduce effective thermal conductivity, the materials are certainly worth exploring.
Another important trend is combining materials to create hybrids; for example aluminum-encapsulated diamond/SiC and HOPG.
The number of advanced thermal management materials with thermal conductivities up to more than four times that of copper, low CTEs and low densities is increasing, providing the design engineer with a greater range of options. Some are cheaper than traditional materials.
For More Information
These materials are covered in the author's papers and in public short courses that he teaches for IMAPS, Semitracks and other organizations and in in-house short courses that have been presented at Fortune 500 companies and government organizations.
The author can be reached at: email@example.com