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Heat must be removed from electronic devices quickly and efficiently to ensure optimal performance, and to prevent premature component failure. CPUs and GPUs, LEDs, and photovoltaic inverters are areas that require excellent thermal management. Thermal interface materials (TIM) are available as pastes, silicone RTVs, epoxy adhesives, and epoxy/polyurethane-based encapsulants.
By Keith Carruthers, electrolube
Most components are low-power devices that produce negligible amounts of thermal energy. Many devices however – CPUs, power diodes, power transistors – produce significant amounts of heat. When heat is removed from the device quickly and efficiently, it helps prevent premature component failure. In addition, the ongoing product miniaturization trend has ensured that efficient thermal management is an essential part of modern and future electronics design.
CPUs and GPUs are perhaps the most obvious area where strong thermal management is essential. However, thermal management is becoming increasingly important in other markets.
The rapidly expanding industry of light emitting diodes (LEDs) is a good example. LEDs are replacing more traditional lighting methods (fluorescent and incandescent) in applications such as LCD TV backlights, electronic signs and displays, as well as new areas such as daylight automobile headlights. The heat generated by LEDs must be removed, just like that from a CPU, to ensure optimal performance.
Another example is photovoltaic inverters, which are known to be particularly sensitive to temperature. Continued thermal damage inflicted over a long time takes a toll, and solar applications are expected to withstand decades in the elements without failure.1 Dr. Sarah Kurtz of NREL states that "general reliability issues across all PV technologies are corrosion leading to a loss of grounding, quick connector reliability, improper insulation leading to loss of grounding, delamination, glass fracture, bypass diode failure, inverter reliability, and moisture ingress.2 The inverter is a key element for solar installations, and better functionality and lower costs are desirable. At present, one important factor is to increase the lifetime for inverters to match that of the panels, which is typically 20 years.3 Other applications include connections between the heat-pipe and water storage tank for solar-heating applications; hydrogen fuel cells; and wind power generators.
Newton's law of cooling states that the rate of heat loss is proportional to the temperature difference between the body and its surroundings. Therefore, as a component's temperature increases, the rate of heat loss per second equates to the heat produced per second within the component. For some components, internally produced heat may be high enough to significantly shorten the component's working life, and eventually cause it to fail.
Figure 1. Heatsinks are designed for maximum surface area in the allotted board real estate.
One heat-removal technique is to attach a heat-sink, artificially increasing surface area. The heat-sink is usually composed of a highly thermally conductive material (usually a metal) so the heat can be transferred away from the component. Heat is mainly lost from the surface of a body into its surroundings; therefore, heat-sinks generally are constructed with fins to maximize the surface area per unit volume (Figure 1).
The device and heat-sink are usually solid substrates that are mechanically bolted together. Ideally, the surfaces of these substrates should be perfectly smooth. As this is an engineering challenge, air gaps are generally present at the interface of the device and the heat-sink. Air is known to be an extremely poor thermal conductor (0.024 W/mK). This poor conduction makes the interface between the heat-sink and device a major bottleneck, resulting in inefficient heat transfer.
Assemblers fill these air gaps with a material that will significantly improve the thermal interface between the two substrates. This material can be a thermal paste, adhesive, RTV, thermal pad, or another medium. It is more effective to apply the minimum amount of product possible to fill the air gaps, thus keeping the thermal resistance as low as possible. Applying too much material will increase the thermal resistance of the interface.
In most cases, due to the increased filler content, high thermal conductivity means high viscosity. As a result, the possibility of air entrapment needs to be considered during the product application; a highly viscous material may trap more air in the interface, reducing the thermal conductivity performance. In such cases, it may be prudent to use a lower-viscosity material that will lower the risk of air entrapment.
Measuring Thermal Conductivity
Thermal conductivity measurement methods must be considered when qualifying heat-transfer products. Some techniques only measure the sum of the material's thermal resistance and the material/instrument contact resistances. When using a version of the heat-flow method that measures both of these values separately, values quoted are closer to the material's true thermal conductivity. Alternative methods that do not separate the material's thermal resistance and the material/instrument contact resistances may look impressive, but these higher readings are less accurate.
Thermal Management Solutions
Thermal management interface solutions are available as thermal pastes, silicone RTVs, epoxy adhesives, and epoxy/polyurethane-based potting compounds.
Thermal pastes. Thermally conductive pastes, sometimes referred to as greases, consist of thermally conductive fillers in a carrier fluid (Figure 2). The fillers can be a blend of one or more mineral fillers depending on the exact properties required; the carrier fluid can be a silicone or non-silicone based medium. Thermal pastes do not cure; therefore, they offer the best solution when rework is important. A thin layer of paste should be applied between the device and the heatsink to fill the air-gaps and improve the contact. The paste can be applied using a variety of methods: stencil printing, screen printing, automatic dispensing equipment, or even using an aerosol spray product such as a heat-transfer compound aerosol (HTCA).
Silicone and non-silicone thermal paste solutions are available.* Silicone products tend to exhibit lower oil bleed and evaporation weight loss, as well as offering a higher upper temperature limit in excess of 200°C. However, there are applications where silicones may be unsuitable; when devices are sensitive to silicone contamination, for example. Some new non-silicone thermal pastes possess oil-bleed and evaporation weight loss performance as good as, and in some cases better than, silicone-based pastes.
RTV and adhesives. For certain applications it may be desirable to use a product that cures into a solid, which may or may not require bonding two surfaces together. Three types of product that fall into this category: thermal bonding systems, and oxime- and ethanol-cure silicone RTV products.**
Figure 2. Silicone thermal pastes do not cure on the PCB assembly.
Thermal bonding systems can be single- or two-component systems, such as RTV's and high-strength epoxy adhesives, which are used to bond a heatsink to a component. In addition to the mineral fillers, the adhesives may contain small glass beads of controlled diameter; these allow for a set thickness (equal to bead diameter).
Epoxy/polyurethane-based potting compounds. For certain types of heat-generating circuitry, e.g power supplies, it may be beneficial to encapsulate the device in a heat-sink enclosure using a thermally conductive potting compound. Some two-part encapsulation solutions use epoxy and polyurethane technologies.***
Encapsulation resins possessing high thermal conductivity are usually resultantly high in viscosity. Lower viscosity versions tend to have reduced filler content, but do not necessarily suffer significantly detrimental effects on thermal conductivity (one example is 70% lower in viscosity, but only exhibits a 13% decrease in thermal conductivity). Polyurethane potting compounds can offer similar low viscosities with good thermal conductivity performance. SMT
* The silicone thermal pastes offered by Electrolube are called HTS and HTSP: HTS is the standard silicone thermal paste; HTSP is a higher thermally conductive version, and is useful where a high-degree thermal transfer is critical. Electrolube has also introduced two new non-silicone thermal pastes to the range called HTCX and HTCPX.
**Electrolube offers TBS, TCOR and TCER adhesives.
*** ER2074 is Electrolube's flagship thermally conductive encapsulation product. ER2183 is a lower viscosity version of ER2074 (5000 cPs). UR5097 is a polyurethane potting compound that possesses a similar viscosity to ER2183 (6000 cPs) with the added benefit of UL94V0 certification.
- Zielnik, Allen, "PV Durability and Reliability Issues," Photovoltaics World, September/October, 2009, pvworld.com.
- S.Kurtz, "Reliability Concerns Associated with PV Technologies," http://www.nrel.gov/pv/performance_reliability/pdfs/failure_references.pdf.
- Skumanich, Malik, et al, "The Other Half of PV: Balance of System," Photovoltaics World, January 2010.
Keith Carruthers, development chemist, Electrolube, a division of H K Wentworth Limited, www.electrolube.com.
Further Solar Reading
Read Is Solar Cell Production A Viable Business Venture for PCB Fabs? on SMT's Blog at http://tiny.cc/SMT495. Don Cullen, MacDermid, made the switch from circuit board to PV materials and processes provider.
Investigate more ways to save money during PV inverter assembly in Balance of Systems: The Next Step to Grid Parity on ElectroIQ.com at http://tiny.cc/PVW. Ted Sullivan from Lux Research explains how balance of system (BOS) components affect PV costs.