Thermal Management Effective cooling using heatsinks
In the innovation area to improve the level of effectiveness of electronic components, the different approaches for cooling these components have not provided a solution. The only things that have changed are how, where and the way various cooling concepts are used. The safe function of the components with a long service life thereby entails great effort for cooling and comprehensive knowledge about the inner- relationships. Jürgen Harpain, development manager at Fischer Elektronik tells us more
T Heatsinks and fan heatsinks
he continuously growing integration and performance densities of electronic components is an area which has made progress in the semiconductor industry. The resulting higher losses consequently cause an increasing component temperature. This is why thermal management is becoming more and more important but is also a frequently underestimated design characteristic. The following rule of thumb can be noted as a statement on the service life of electronic components in conjunction with their temperature: the assumed service life increases by approximately 50 per cent for each 10°C temperature increase. The flawless and, most importantly, long-life function of electronic components requires effective thermal management.
The two generic concepts of "soft and loud" heat conduction should be mentioned for the different cooling possibilities for electronic components. The "quiet" version, heat conduction through free convection and radiation, e.g. through passive heatsinks, is a proven means for cooling small as well as large dissipation. The heatsink is the best known and most used element in electronic cooling and entails a mechanical part which is connected to the cooling electronic components with a heat conducting effect. According to the second main principle of thermodynamics, the heat flow is always in the direction of the lower temperature, thus from the warmer to the cooler body. The heatsink therefore absorbs
the thermal energy of the component to be cooled and conducts it to the ambient air using the principle of surface enlargement. This system, which works according to the principle of exclusive surface enlargement, in addition to limitations in terms of size, volume, weight also have physical limitations. They are selected based on the special heat conductivity ( ) of the materials used for the heatsink, usually aluminium but less frequently due to the costs. To cool only small-scale, high-performance electronic components, an analysis of the surface size of the heatsink alone is not enough to definitively say whether the parts to be cooled can be brought to the desired temperature. The heat flow within the heatsink, in its floor plate and in the ribs, is, as previously mentioned, limited by the special heat conductivity, i.e. increasing the surface would not result in an effective improvement. If, however, it is necessary to improve heating properties, then the "loud" version for cooling, the use of fan motors (fan coolers) or various types are used to support the cooling effect of the heatsink, offers some good approaches. From free convection, through the activated air, also what is known as forced cooling occurs which can naturally conduct a greater amount of heat due to the increased air throughput. Using activated air makes it possible to further increase the effectiveness of cooling. A heatsink with a fan, depending on the application, also achieves an improvement in heat conductivity by approximately 40 per cent. The heatsinks
Figure 3: Affordable production of aluminium heatsinks in the extrusion pressing process
used are especially designed for good heat conduction using activated air (Pin Fin). For active cooling, it should be noted that it is not odour-free. Fan motors and also air motion create sound waves which have an impeding effect for all applications. However, there are also positive aspects of air cooling in addition to the low overall temperature. Other benefits include decreased soiling (dust deposits) and also a more consistent heat distribution with more frequent activation and deactivation. Like selecting a heatsink for free convection, the correct heatsink is to be selected for activated air, based on the corresponding heat resistance diagrams which indicate a dependence on the air speed.
Heatsink production The materials used for heatsinks are mainly aluminium materials because they offer the best price/performance/weight/volume ratio and can also be mechanically processed. The material size of the specific heat conductivity of a material ( ) is a strong determining factor for a good heat conduction process, as previously described. The aluminium alloys used have values of >200W/m∗K and can be used in various
Figure 2: Determination of the heat resistance for free and forced convection
sub-classifications. Created in the extrusion press process, heatsinks are pressed out of what is known as modelling alloys, e.g. when shaping the warmed aluminium material through a matrix, with the integrated heatsink geometry in negative. A wide range of profile types can be implemented with this process but extensive geometries can be created affordably. In addition, there is good availability due to the innovative warehousing of standard types which are cut, processed and surface- treated in terms of length.
The thermal resistance In order to find the right heat extraction method, I first recommend determining what is known as the thermal resistance (heat resistance). This, conversely, is proportional to the heat conductivity, i.e. the better a component abducts the heat, the smaller its thermal resistance is. It also makes it possible to stake which type of cooling must be considered: positive heat extraction using a heatsink, active heat extraction using fan motors (fan units) or heat extraction using other media, such as liquids.
Conclusion The challenge to reduce the heat in electronic components remains despite many new and good approaches to solving the problem. Through the miniaturisation and the greater complexity of the electronic components, the summary of individual function components into one component, the effort for the "appropriate" heat reduction will continuously grow in the future. The possibilities to develop the influencing parameters to improve heat conduction have been virtually achieved and will only bring limited success with disproportionately high effort if continued. Physical improvements to semiconductors to reduce heat loss, e.g. increasing the level of efficiency of the electronic structural elements and moderate miniaturisation are not essential from the heat reduction perspective.
Figure 1: Passive heat reduction of LEDs using special extrusion heatsinks
42 September 2018 Components in Electronics
www.fischerelektronik.de/en www.cieonline.co.uk
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