Thermal Interface Material Choices

Phase Change Materials (Phase Change Materials (PCMs)
PCMs undergo a transition from a solid to a semi-solid phase with the application of heat from the operating processor and a light clamping pressure. The semi-solid PCM readily conforms to both surfaces. This ability to completely fill the interfacial air gaps and surface voids, usually under light clamping pressure, allows performance comparable to thermal grease. While less ‘runny’ than grease, PCMs contain wax and once the melt-on temperature is reached, they may flow out of tight areas. Recently introduced phase change type materials are not wax based and will not drip.
At room temperature these materials are firm and easy to handle. This allows more control when applying the solid pads to a heat sink surface. After installation, some phase-change pads create a strong adhesive bond between the processor and the heat sink. Exercise care when removing the heat sink from the processor. A slight twisting or rotating movement should help to remove the heat sink. Using strong force to remove the heat sink can damage the processor.

Thermal Greases
Thermal greases typically are silicones loaded with thermally conductive fillers. They don’t need curing and they can flow and conform to interfaces. They also offer re-workable thermal interface layers. It is important to ensure that the proper amount of paste or grease is dispensed prior to installing the heat sink. Too little grease may leave gaps between the heat sink and processor; too much might also cause air gaps and leak material outside the interface. On extended operation and over time, some greases can degrade, pump-out, or dry out, which affects thermal transfer performance. Despite these drawbacks, greases are the interface materials of choice in high performance processor applications. Thermal conductivity of high performance thermal greases is in the order of 10 W/mK, which is superior to other TIMs.

Gap Fillers
One of the largest segments of the thermal interface market, gap fillers are supplied in different thicknesses and can cover large segments of a board. Effective materials can fill gaps up to one-quarter inch with a soft, highly thermally conductive interface. Gap fillers can blanket over multiple components of varying height to conduct heat into a common heat spreader. These pads are often used when low compression forces are required, so high compressibility is an important feature. Gap fillers can be custom molded, and new form-in-place gap filler compounds are an option for high volume automation.

Thermal Films
Thermal films provide electrical isolation along with thermal transfer. Their film carriers give superior resistance to tear and cut-through from burrs on heat sinks. This category includes silicone, silicone free (e.g. ceramic-filled polyurethane) and graphite materials with a wide range of thermal performance and price points.

Thermal Pads
Thermal pads usually are fabricated by molding non-reinforced silicone with conductive fillers. Reinforcements for thermal pads can include woven glass, metal foils, and polymer films. Thermal pads are typically pre-cut in sizes to accommodate different size components. From a performance standpoint, they are inferior to phase change materials and thermal grease, but offer a practical, low cost TIM solution in many applications with less cooling requirements.

Graphite Films
These are electrically conductive, low cost and have been used for a long time. Graphite films are effective in very high temperatures (up to 500 ºC). Some manufacturers orient the fibers in a horizontal plane resulting in very different thermal conductivity measurements. For example, MH&W’s Keratherm 90/25 is rated at 7.0 W/mK on the x axis and 150.0 W/mK on the y-z axis.

Double Sided Adhesive Tapes
Most small heat sinks are attached to components with a double sided PSA thermally conductive tape. Factors for tapes include peel strength, lap- and die-shear strength, holding power, and thermal resistance. Thermally conductive adhesive tapes are considered to be convenient for heat­ sink attachment with mid-range thermal performance. While they replace mounting hardware, thermal tapes often have problems with the lack of flatness on component surfaces. Plastic IC’s are usually concave in the center and heat sink surfaces vary as well. This can result in air gaps in the interface. One thermal adhesive tape consists of a finely woven nickel coated copper mesh that conforms closely to irregular mounting surfaces varying up to 50% of its thickness.

Thermal Adhesives
Thermal adhesives are one- or two-component systems containing conductive fillers. They are typically applied via dispensing or stencil printing. These adhesives are cured to allow for cross-linking of the polymer, which provides the adhesive property. The major advantage of thermal adhesives is that they provide structural support, therefore eliminating the need for mechanical clamping.

Thermal Gels
Gels are low modulus, paste-like materials that are lightly cross-linked. They perform like grease with respect to their ability to conform to surfaces, while displaying reduced material pump-out.

Metal TIMs
Metal interfaces can be made in many different forms and are no longer limited to solder applications. In some applications metal TIMs are totally re-workable and recyclable. In recent years the need for better performing TIMs in such devices as power amplifiers and IGBT modules have prompted suppliers to explore other types of metal TIMs such as liquid metals, phase change metals, and SMA -TIMs (soft metal alloys). The soft or compressible metal thermal interface material (SMA -TIM) is the most easily adopted metal TIM because it does not need to be reflowed or contained in a gasket like a solder or liquid metal. Metal TIMs are very thermally conductive, reliable, and in the case of compressible metals, easily adopted.
A new hybrid material consists of a thermally conductive silicone film on one side bonded to a copper film. The advantage of this material is that it can be used to manufacture flex circuits as well as provide EMI and RFI noise protection.
Conclusion

Thermal interfaces are often considered late in the design stages of cooling systems. This isn’t the best practice as TIMs can be the limiting factors in the expense of thermal management designs. With more and more excess heat to be dealt with, there is a steady demand for higher performing TIMs. Used effectively, thermal interface materials can help reduce the size of heat sinks and the need for larger fans. The extended benefit is that an effective TIM is a faster, easier applied and less costly solution than changing heat sinks or redesigning a chassis.