Knowing Your Application Needs is Key to Picking the Best TIM
As component powers continue to grow, so do their cooling requirements. One rule of thumb says that for every 10°C rise of the junction temperature the failure rate doubles. Thus, there is an urgent need to remove heat from hot chips to the surrounding air stream. Demand has led to a variety of new thermal management systems. But nearly all of these continue to use thermal interface materials, or TIMS, to effectively provide heat flow across the mating interfaces of cooling systems.
The essential purpose of TIMs is to maintain effective transfer of heat from hot chips to dissipating devices such as heat sinks or spreaders. As heat flows, it encounters thermal resistances that impede overall heat transfer. TIMs reduce the most problematic of these, the contact resistance between the mating parts (heat source – heat sink). Air gaps significantly limit heat flow from the hot component into the sink or spreader. An effective TIM replaces the gaps created by the non-smooth mating surfaces with a material whose thermal conductivity is much greater than that of air. Basically, it replaces poor conduction from point contacts and air to enhanced conduction through solids.
Most TIMs are polymer-based composites loaded with heat-conducting filler particles. Common fillers include aluminum oxide (alumina), boron nitride, aluminum nitride and magnesium oxide. Metal fillers, such as silver, can be used where electrical isolation is not needed. Some level of pressure is usually needed between the mating surfaces to compress filler particles and make the material flow into the surface irregularities to reduce contact resistance. Once in place, a TIM’s effective thermal resistance comprises the bulk resistance of the material and the contact resistance between the TIM and its mating surfaces.
Application Issues for Thermal Interface Materials
While thermal interfaces and TIMs are often considered well into the design process, several factors should be considered when it’s time to choose a thermal interface material:
- Thermal impedance is the single most important specification measured in degrees Kin2/W. Thermal impedance is an application-specific measure of the ratio of the temperature difference between two mating surfaces to the steady state heat flow through them. Thermal impedance usually decreases with added mounting pressure and contact area, but increases with the thickness of the TIM.
- Thermal conductivity, in W/mK, measures a material’s ability to conduct heat regardless of its thickness. A bulk measurement, thermal conductivity values can be used for comparing TIMs, but it does not describe a TIM’s ability to minimize contact resistance in an application.
- The gap space between the heat source and the heat spreader. As a rule, the thinner the TIM the better, but because mating surfaces are never perfectly flat, a minimum material thickness may be needed to accommodate non-flatness issues.
- Surface flatness of mating surfaces is important for determining the type of material. If both surfaces are flat, grease or thin films would be ideal choices, but that is seldom the case. Plastic IC’s are typically concave in the center and if the heat sink is extremely flat, the contact area would be limited to the periphery leaving an air pocket in the center.
- Electrical isolation, measured in kV, is sometimes needed. Silicone-based TIMs provide this property, along with thicker materials such as gap fillers. Thinner phase change materials and , greases may not be reliable electrical insulators. Graphite is electrically conductive.
- Compressibility is important when working with irregular surface as when covering a number of components. If heat and excess pressure are applied to a silicone-based TIM, silicone can escape and migrate along the PCB. Without sufficient pressure there may be excess thermal resistance across the interface.
- Temperature range in the interface determines which materials can be used. Silicone TIMS, e.g. gap fillers are rated to higher temperatures than silicone free interface materials.
- UL flame class rating. A UL flammability rating requirement is needed for many TIM applications. Most of these materials are available with V-0 ratings, which will meet most needs.
- Silicone or silicone free. Silicone is an excellent thermal material with a high temperature range but some applications, e.g. in space, can’t use it due to outgassing.
- Ease of application. The method of attachment is a cost and performance decision. Most small heat sinks are attached with a double sided thermal adhesive tape. Larger heat sinks require mounting hardware. Adhesives can be added to both or one side of the thermal material. However, with a layer of adhesive, thermal impedance will be increased.
- Utility. How easy are the materials to work with in a manufacturing environment? How easy are they to re-work when heat sinks must be removed? Some gap fillers can be re-used, but phase change materials and grease must be replaced.
- The long-term stability of the material depends on such factors as the usage temperature, time, application and material properties.
Thermal Interface Material Choices
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 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.
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 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 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.
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 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.
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 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.
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.