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BuyeRs’ guide To TheRMal ManageMenT


When a bill-of-materials lists a thermal interface material, buyers can be faced with a world of choice. Electrolube offer guidance and advice


The device/heatsink interface must be air free


Most electrical components produce negligible amounts of thermal energy. However, some devices, such as CPUs and power transistors, produce significant heatwhichmust be removed for optimal performance and to prevent premature failure.


Computer CPUs and GPUs are obvious applicationswhere thermal


management is essential. However, thermalmanagement is becoming increasingly important in other industries. Light emitting diodes and photovoltaic inverters are good examples.


Removing heat


Newton’s lawof cooling states the rate of loss of heat is proportional to the temperature difference between the body and its surroundings. Thus, as the component’s temperature increases, the rate of heat loss per second equates to the heat produced per secondwithin the component.


Oneway to remove heat is to attach a heat-


sink to artificially increase the surface area. Heat- sinks usually comprise a thermally conductive material (usuallymetal) to transfer heat away fromthe component. Heat ismainly lost from the surface into its surroundings so the heat-sink will usually be finned tomaximise surface area per unit volume.


The device and heat-sink are usually solid


substratesmechanically bolted together. Ideally, their surfaces would be perfectly smooth but this is not usually possible, leading to air gaps at the device/heat-sink interface. Air is an extremely poor thermal conductor (0.024W/mK) so the interface becomes a barrier to heat-transfer.


Consequently, the air-gaps need to


be filled with a material that improves the thermal interface. This material can be a thermal paste, adhesive, RTV, pad or other thermally conductive medium. To keep thermal resistance as low as possible the minimum amount of material should be used.


30 | December 2010


HTSP is Electrolube’s high conductivity silicone thermal paste


Too much material will increase thermal resistance, resulting in reduced performance.


Usually, due to increased filler content, high thermal conductivity


means thematerialwill be high viscosity. Thus, the possibility of air entrapment needs to be considered during the application. Sometimes it is prudent to use a lower viscositymaterial tominimise this risk of air entrapment.


Measuring thermal conductivity


Themethod used tomeasure thermal conductivity needs to be consideredwhen qualifying heat-transfer products. Some techniques onlymeasure the sumof the material’s thermal resistance and material/instrument contact resistance. Electrolube uses a version of the heat-flowmethod that measures both of these values separately. Values quoted using this heat-flowmethod are closer to the materials true thermal conductivity. Alternativemethods that do not separate thematerial’s thermal resistance and the material/instrument contact resistance may lookmore impressive, but these higher readings are less accurate.


Thermal pastes can be syringe applied


Thermalmanagement solutions Electrolube’s thermalmanagement


interface solutions include pastes, silicone RTVs, epoxy adhesives and epoxy/polyurethane- based potting compounds.


Pastes


Thermally conductive pastes (sometimes referred to as greases) comprise thermally conductive fillers in a carrier fluid. The fillers can be a blend of one ormoremineral fillers, while the carrier fluid can be a silicone or non-silicone basedmedium.


Thermal pastes do not cure, thus offering www.electronics-sourcing.co.uk


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