Editor’s note: These questions were asked in response to Electronics Cooling’s recent webinar by Carl Zwebin. To view the webinar, click here.
Question: Al/SiC is an attractive material. Is there a need for other new composites?
Answer: There is no one universal material. There is a need for low-CTE materials with thermal conductivities higher than those of Al/SiC.
Question: For the data from Eupec / Infineon, were the Si chips soldered directly to the AlSiC, or was the AlSiC coated or plated with another material to enable soldering?
Answer: The chips were soldered to Aluminum Nitride substrates. The substrates were soldered to Al/SiC base plates, which matched the CTE of AlN. This greatly reduced thermal stresses that were a problem with traditional copper base plates.
Question: What is the size of graphene sheets seen in the market?
Answer: 210 by 297 millimetres are standard at present. Larger specimens can be obtained by special order.
Question: Are there developments in low/tailorable CTE + high thermal conductivity but machinable composites?
Answer: Machinability depends on several factors, including type of particle, particle volume fraction, type of matrix, and machining method. For example, we used EDM with AlSiC, which is electrically conductive. Aluminum and copper reinforced with various types of carbon are non-abrasive, and relatively easy to machine.
Question: What is the usual particle size in the state of the art thermal interface materials?
Answer: My specialty is materials used as heat spreaders. I don’t know the answer to your question. However, the best thermal conductivity is obtained with thermally conductive reinforcements that go all the way through the TIM, like carbon fibers, etc.
Question: I’m very interested in anisoptropic materials. Is there some place I can get comprehenisive information on anisotropic thermally conductive composite materials?
Answer: There are many good publications on composite materials that cover anisotropy. For example, Comprehensive Composite Materials from Elsevier (in the interest of full disclosure, I am co-editor-in-chief). I can send papers on anisotropic thermal materials (c.h.zweben@usa.net).
Question: I missed the material that had 1000 W/m-K isotropic thermal conductivity. What is its composition and who manufactures it?
Answer: Diamond particle-reinforced silver. This particular composition is in the laboratory stage. The work was done by Professors L. Weber and R. Tavangar at the Laboratory for Mechanical Metallurgy, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Lausanne, Switzerland. As I mentioned, materials range from R&D to high-volume production. We are still in the early days of thermal materials.
Question: Which type AlSiC cheaper than Cu?
Answer: The material is made by MC-21. Contact Dr David Schuster: (schuster@mc21inc.com). You may use my name, if you like.
Question: What are your thoughts on new flip chip technologies and elimination of the IGBT base plate (i.e. soldering the flip chip directly to the PCB).
Answer: The proof of the pudding is in the testing. If it works, fine.
Question: How do the fatigue life properties of these materials compare to traditional materials like aluminum alloys?
Answer: There are many different materials. The mechanical fatigue properties have not been evaluated for some. The fatigue life of some forms of Al/SiC is better than that of monolithic aluminum. Carbon fiber-reinforced polymers also have excellent fatigue properties. Most packaging applications have been evaluated by thermal cycle testing of actual assemblies.
Question: How do you use the specific heat?
Answer: I am not sure what you mean. Specific heat of composites is used the same way as for monolithic materials.
Question: I noticed that the thermophysical properties of thermal conductivity and density were addressed during the discussion. What about specific heat. Also, how does temperature dependency affect the thermal performance of these materials?
Answer: It is not possible to present all properties in a one-hour webinar. Further, for many materials, thermal conductivity is more readily available than specific heat. Thermal properties of all materials are temperature dependent.
Question: Would you have a supplier name and material name for the carbon fiber injection molded material used in a laptop that replaced copper for its EM properties?
Answer: The company is Cool Polymers. I believe the material was E2. For more information, contact Dr. James Miller (jim@coolpolymers.com). You may use my name if you wish.
Question: What specific material suggestions for high thermal conductivity and high electrical resistivity?
Answer: Unfortunately, current advanced thermal materials are all electrically conductive to some extent. As I mentioned in the “FUTURE DIRECTIONS” section. I am hopeful that thermally conductive, electrically insulating materials will eventually be developed. At present, we have to make do with things like Diamond films, BeO, AlN and other ceramics.
Question: I had a question on the thermal conductivity of diamond-Ag composites, which was quoted at 983W/mK. I was under the impression that an electrical insulator(diamond) was not compatible with an electrical conductor (Ag) in the conduction of heat because of the phonon scattering phenomena. What am I missing?
Answer: Test data for diamond/silver, diamond/copper and diamond aluminum all show thermal conductivitives above that of copper for many formulations. There is some phonon scattering at interfaces, but this can be managed. Note that many thermal interface materials consist of metal particles in a polymeric matrix.
-Carl Zwebin
