Thermal Interface Materials - A Brief Overview
A variety of thermal
To be effective, all cold plates require intimate surface-to-surface contact with the component or board-to-be-cooled. Unfortunately, irregular surface areas, both on the electronic components and on the cold plate prevent good contact.
One industry expert from a major manufacturer of thermal interface materials notes, "Up to 99% of the surfaces are separated by a layer of interstitial air...a poor conductor of heat".  Another major manufacturer cites that two "mating surfaces may touch over only 1/10,000 of their surfaces. They may touch at only three points. Everywhere else is a air between them, forming a thermal barrier." 
Hence, some type of thermally conductive interface material is necessary to fill the interstices between the mating surfaces. Moreover, to ensure that electrical problems are not inadvertently introduced while solving the thermal problems, it is often essential that these same thermal interface materials simultaneously perform an electrical isolation function. 
Manufacturers of thermal interface materials offer a wide variety of products to suit the requirements of many different thermal management applications. These materials vary widely in terms of their performance (i.e., thermal, electrical, and physical properties), their general appearance, and their mode of application. Among the most commonly used classes of thermal interface materials are: thermal greases, cure-in-place thermally conductive compounds, gap filling thermally conductive elastomeric pads, thermally conductive adhesive tapes, and phase change materials. These are briefly described below.
Comprised of thermally conductive ceramic fillers in silicone or hydrocarbon oils, a thermal grease is a paste that is applied to one of the two mating surfaces. When the surfaces are pressed together, the grease spreads to fill the void. During compression, excess grease squeezes out from between the mated surfaces. Some form of clip or other mounting hardware is needed to secure the joint. Although it is comparatively inexpensive and thermally effective, thermal grease is not an electrical insulator. Another disadvantage is that it can be inconvenient to dispense and apply and it requires cleanup to prevent contamination problems. 
Cure-in-Place Thermally Conductive Compounds
A thermally conductive compound again incorporates thermally conductive ceramic fillers. However, unlike thermal greases, the binder is a rubber material. When first applied, the paste-like compound flows into the interstices between the mating surfaces. Then, when subjected to heat, it cures into a dry rubber film. Besides its thermal properties, this film also serves as an adhesive, allowing a tight, void-free joint without the need for additional fasteners. Thermally conductive compounds can successfully fill larger gaps in situations where thermal greases might ooze from the joint. Although application and performance is similar to that of thermal grease, cleanup is easier, simply involving removal of the excess cured rubber film. 
A sample gap pad -
Picture courtesy of
The Bergquist Company
Thermally Conductive Elastomeric Pads
A thermally conductive elastomeric pad consists of a silicone elastomer filled with thermally conductive ceramic particles and may incorporate woven glass fiber or dielectric film reinforcement. Typically ranging in thickness from 0.1 - 5 mm and in hardness from 5 to 85 Shore A, these pads provide both electrical insulation and thermal conductivity, making them useful in applications requiring electrical isolation. Thicker pads prove useful when large gaps must be filled. During application, the pads are compressed between the mating surfaces to make them conform to surface irregularities. Mounting pressure must be adjusted according to the hardness of the elastomer to ensure that voids are filled. A mechanical fastener is essential to maintain the joint once it is assembled.  
Thermally Conductive Adhesive Tapes
A thermally conductive adhesive tape is a double-sided pressure sensitive adhesive film filled with thermally conductive ceramic powder. To facilitate handling, aluminum foil or a polyamide film may support the tape; the latter material also provides electrical insulation. When applied between mating surfaces, the tape must be subjected to pressure to conform to the surfaces. Once the joint is made, the adhesive holds it together permanently, eliminating the need for supplemental fasteners. No bond curing is needed. One limitation of thermally conductive tapes is that they cannot fill large gaps between mating surfaces as well as liquids; hence, the user must trade off the convenience of tape mounting against a nominal sacrifice in thermal performance. 
Phase Change Materials
Solid at room temperature, phase change materials are melt (i.e., undergo a phase change) as the temperature rises to the 104°F to 158°F (40°C to 70°C) range. This makes the material (0.13 mm thick in its dry film form) as easy to handle as a pad, while assuring that it will, when subjected to heat during the assembly process, flow into voids between mating surfaces as effectively as a thermal grease. Ordinarily, applying power to the electronic component introduces the needed heat for the phase change to occur, establishing a stable thermal joint. These materials consist of organic binders (i.e., a polymer and a low-melt-point crystalline component, such as a wax), thermally conductive ceramic fillers, and, if necessary, a supporting substrate, such as aluminum foil or woven glass mesh. 
Where to Obtain or Learn More about Thermal Interface Materials
For a comprehensive listing of companies that provide thermal interface materials please visit Electronics Cooling, go to the buyers guide, and select interface materials.
de Sorgo, Miksa, "Thermal Interface Materials", ElectronicsCooling Magazine, Sept. 1996.
Orcus Technical Information
Hanson. Kevin, "Thermal Isolators, Material Properties that Determine Electrical, Mechanical, and Thermal Performance", PCIM Magazine, April 1999.
de Sorgo, Miksa, ibid.
de Sorgo, Miksa, ibid.
de Sorgo, Miksa, ibid.
de Sorgo, Miksa, "Understanding Phase Change Materials", ElectronicsCooling Magazine, May. 2002
Chomerics Technical Information, www.chomerics.com