EMC & Thermal Management Produce heat sinks individually
Heat sink solutions, which are individually adaptable to customer-specific applications in terms of geometry and design, are more and more in demand and sought-after. When the packing densities of the electronic components become smaller, with simultaneously increasing power densities, this often complicates the implementation of efficient thermal management
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or every user of electronic components or even systems, it is clearly recognisable that a trend towards steadily decreasing packing densities has set in. In the consumer sector, the terminals are getting smaller, more compact and handier. However, the increasing packing density of the electronic components, with an often also simultaneous increase in power loss, inevitably lead to higher component temperatures. For temperature control and thus, also for extending the overall service life of such systems, efficient thermal management is indispensable. When selecting thermal management, a distinction is always made between passive or active cooling. For the pure, free convection, so-called extruded aluminium heat sinks (Figure 1) are often used, in numerous applications. As with all other types of cooling types, the heat sink is connected directly to the component to be cooled. The heat flow then takes place from the warm to the cold area, i.e. the heat sink absorbs the thermal energy of the component to be cooled and directs it via the principle of surface enlargement, the rib structure, into the ambient air. Extruded heat sinks are produced in the so-called extrusion process. In this case, a heated block of aluminium material is pressed, by means of an extruder, through a die with introduced heat sink geometry in the negative. According to the physical principles of action, the larger the heat-transferring surface, the better the heat transfer from solid bodies to a surrounding fluid (or heat sink to ambient air). Therefore, customers always try to achieve the largest possible heat exchange surface with the heat sink design. Often, however, the risks inherent in the physical and production processes will be ignored. With the heat sink design, the intended use must always be taken into account in the application, i.e. whether it is a free or forced convection. This pre-selection is extremely important because in free convection, too small a rib distance leads to an overlapping of boundary layers. This overlapping causes individual ribs and surfaces to influence one-other negatively. Between too closely adjacent cooling ribs, a resistance builds up against the natural buoyancy behaviour, the so-called chimney effect, whereby the efficiency of the heat sink in the heat dissipation will become significantly impaired. The extruding production process provides another limiting reason for too narrow rib spacing. A heat sink with very small rib-distance in order to achieve the highest possible surface area requires an extreme tongue ratio. In the world of extrusion, the tongue ratio describes the relationship between the rib height and -distance, in relation to the associated heat sink width. Depending on the extruder, profiles with a tongue ratio of 4: 1 to 16: 1 are given. Since the actual heat sink in the tool die is
always to be regarded as negative, too high tongue ratios must be viewed more closely from the standpoint of feasibility. If the rib distances are too high, they will now be displayed in the tool as a narrow bar. Depending on the press force and the size of the extruder, a few hundred Newton per mm² now acts on this bar. Incorrect dimensions of the heat sink profile or too high pressing force and flow speed of the aluminium material, which makes the bar vibrate, could cause a tool breakage, in the worst case. A good heat sink design must therefore always consider the economic aspects and longevity of the extrusion tools.
Large heat sink surface in forced convection The disadvantages of a narrow rib distance in the heat sink design, however, also have their right to exist from a technical point of view, when it comes to forced cooling. In this case, the air is thus conveyed in directional form through the rib geometry with the aid of an additional cooling fan motor. With forced convection, the so-called Bonded Fin heat sinks (Figure 2) provide excellent and very effective solutions. Complicated application specifications that do not require a standard heat sink often require custom heat sink solutions. The close-meshed lamellar heat sinks are frequently in demand and sought-after, in the case of difficult heat-related technological relevance or installation conditions.
Lamellar heat sinks of the KTE series are made of a highly heat-conductive aluminium material and can be realised for semiconductor mounting with one or two base plates. Standard extruded profiles are available for the base plates or floor profiles utilised. If the geometric dimensions of these profiles do not fit into the application, they can also be produced from a solid material. In all variant, the opposite side of the component mounting surface contains a special groove- geometry, into which aluminium or copper fins are pressed through an additional processing method. Additionally, in order to optimise the heat transfer resistance or the imprinting zone of the individual lamella in the bottom of the heat sink, a special thermal adhesive is used to compensate for any air pockets. Furthermore, the thermal adhesive, in addition to the thermal engineering connectivity, functions as mechanical stabilisation of the individual fins on the bottom of the heat sink.
Figure 1: Numerous different extruded heat sinks provide very good and efficient solutions for the cooling of electronic components
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Figure 2: KTE series heat sinks can be customised according to customer specifications and requirements
Components in Electronics February 2019 33
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