Thermal Management
necessarily result in the most efficient heat transfer, however. Thermal resistance, measured in K
m2/W, is the reciprocal of thermal conductivity. It takes into account the interfacial thickness and although it is dependent on the contact surfaces and pressures applied, some general rules can be followed to ensure thermal resistance values are kept to a minimum and maximise the efficiency of heat transfer. For example, a metal heat sink will have a significantly higher thermal conductivity than a heat transfer compound used at the interface and therefore it is important that only a thin layer of this compound is used; increasing thickness will only increase the thermal resistance in this case. Therefore, lower interfacial thicknesses and higher thermal conductivities give the greatest improvement in heat transfer. In some cases, however, utilising a material with a higher bulk thermal conductivity could be to the detriment of contact resistance and thus, no improvement will be accomplished.
An example of this difference can be
drawn from the comparison of thermal compounds or pastes and thermal pads, as shown in the Table on page 18. Thermal pads are solid, polymerised materials of a fixed thickness which are available in a variety of thermal conductivities. Thermal compounds or pastes are non-curing compounds and as a result, their viscosity can alter slightly as the temperature increases. This allows for a further reduction in interfacial resistance. In the case of thermal pads, high pressures are needed to achieve an adequate
Significant variations in thermal conductivity values for the same product can be achieved by utilising different test methods or parameters. This can result in bulk thermal conductivity values that appear very high when quoted but in use have a dramatically reduced efficiency of heat dissipation. Some techniques only measure the sum of the materials’ thermal resistance and the material/instrument contact resistance. At Electrolube we use
thermally conductive medium will result in a reduction in the rate of heat dissipation. For interface materials, the viscosity of a product or the minimum thickness possible for application will have a great effect on the thermal resistance and thus, a highly thermally conductive, high viscosity compound that cannot be evenly spread onto the surface, may have a higher thermal resistance and lower efficiency of heat dissipation when
interface, thus, a paste and pad of similar bulk thermal conductivity may have very different thermal resistance measurements in use, and as such a difference in the efficiency of heat transfer will be observed. Another concern with using bulk thermal conductivity values alone for product selection is that there are a number of different techniques available.
a version of the heat-flow method that measures both of these values separately, giving a much more accurate bulk thermal conductivity measurement. This leads us to another important factor in product selection, the application of thermal management materials. Whether it is an encapsulation compound or an interface material, any gaps in the
compared to a lower viscosity product with a lower bulk thermal conductivity value. For encapsulation resins, this could be expressed in a similar way; the higher the viscosity, the more difficult it is for the resin to flow evenly around the unit and therefore, air gaps are formed in the potting compound reducing the rate of heat dissipation. The figure on this page shows the potential differences in heat dissipation by measuring the temperature of a heat generating device in use. These results have been based on work completed by an end user, where all products were thermal interface materials, applied using the same method, at the same thickness. It is clearly evident that a higher bulk thermal conductivity value, in this case 12.5 W/m K, does not necessarily result in more effective heat dissipation when compared to products with lower values, such as the above at 1.4 W/m K. The reason for this could be due to the processing method not being suitable for the product, for the product not being easy to apply or possibly the product was not designed for this particular application. With such rapid advances in the
electronics industry and more specifically, LED applications, it is imperative that materials technology is also addressed to meet the ever demanding requirements for heat dissipation.
Electrolube |
www.electrolube.com
Jade Bridges is European Technical Support Specialist, Electrolube
www.cieonline.co.uk
Components in Electronics
February 2014 19
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