The temperature range of individual electronic components impairs or prolongs the lifetime of an entire system setup enormously. With increasing density, more compact designs and ever-increasing computing power, there is a growing need for selective cooling that is carefully chosen in advance to counter the rising power losses in components. Martin Unsöld, senior marketing manager, Relays, Batteries, Fuses, Mechanical Switches, Thermal Management at RUTRONIK Elektronische Bauelemente GmbH and Andreas Plate, team leader, Product Management, ASSMANN WSW examines how to optimise the cooling of MOSFETs, IGBTs, transistors and other components
Active components only output a small proportion of the power they consume as a signal. Depending on the electronic component and application, high power losses occur despite continuous improvements to semiconductor processes. For ohmic loads, heat generation is virtually unavoidable. The term – power loss – describes the difference between power consumption and power output. This condition should and must be taken into account during the development phase, both with regard to individual components and to complex, electronic systems. This is because the reaction speed, stability, and lifetime of electronic components falls rapidly as temperatures start to rise. If a critical temperature is exceeded, component failure is inevitable. Conversely, it is possible to increase component lifetime significantly by consistently reducing sources of heat. Heatsinks enlarge the surface of the components to be cooled many times over and the resulting increase in convection prevents the assembly from overheating. Particularly when operating electronic assemblies at high ambient temperatures, it is essential that the specified permissible working temperatures of components of e.g. +85°C or as high as +125°C in the extended temperature range are strictly adhered to. Even short-term transgressions of the maximum junction temperature in semiconductors will inevitably result in damage or even the destruction of the component.
Among the smaller heatsinks with a heat resistance range of >16 K/W are the stamped, rod-based, plug-in, soldered, SMD and U-shaped heatsinks. They are available from ASSMANN WSW in a variety of materials, ranging from punched aluminum with a plate thickness of between 0.6 and 1.5 mm up to C1100 copper alloy, and can be anodised or partially or fully tin-plated if required. Copper, with approx. 380 W/(m*K), offers better heat conductivity than aluminum alloys, which offer approx. 220 W/(m*K). However, this does not mean that copper alloys are always the preferred option. That is because the specific requirements of the respective application must be taken into account.
Extruded heatsinks for the area of BGAs, PGAs and automotive applications
There is also the option of using extrusion technology to manufacture heatsinks. The cold aluminum is subjected to high pressure, which causes it to flow. In the final analysis, the advantages of this production process are extremely close adherence to surface tolerances, homogeneous material composition and a micro-structure in the direction of the heat flow, which ensures swift, uniform heat dissipation both at the heatsink base and at the pins or fins. The specially shaped pins or fins enable higher airflow speeds than extruded fins.
To achieve lower heat resistance of between 6 K/W and 16 K/W, punched lightweight heatsinks are not sufficient. For this purpose, heatsinks made of extruded profiles are available in standard lengths, with and without soldering pins. Only extruded profiles made of AL6060 and AL6063 alloys are processed for this. The alloy AL6063 mostly used in Asia is somewhat “softer” than the typically European version AL6060 and therefore offers an advantage in creating profiles and during subsequent processing, depending on the profile shape or fin geometry. A distinction is made between ridged, double-ridged, corrugated fin, star, threaded duct, angle section, and fin profiles with assembly surfaces and customer-specific aluminum profiles. The “extruded heatsink in standard lengths” from ASSMANN WSW includes a broad spectrum of profiles with standard hole patterns, pressed solder pins, threaded holes or previously extruded threaded ducts. In the case of heatsinks with soldering pins, there is no need for a thread, as they can be soldered to the component. Profiles with extruded, threaded ducts on the other hand offer the advantage that the electronic components can be mounted with the existing duct without the need for separate machining (boring and threading). The application ultimately determines which variant is preferable. With regard to the extruded profiles, various standard profiles are also available with profile widths of up to 600 mm, for example, for use with multiple transistors or MOSFETs that are connected in series. With their large-surface, grooved, fin geometry, they can achieve heat resistance values ranging from <6 to <1 K/W.
Special cases require special solutions
In some applications, however, such standardised products are unable to provide adequate cooling. Special solutions are available to choose from here, with alternative materials, specific profile lengths, modern CNC machining where milling work is necessary (punching, boring, threading), special profile cross-sections, plate fin profiles or welded heatsinks, anodised aesthetic surfaces or special packaging for manual, semi-automatic and fully automated fixtures.
In these cases, it is even more important than with standardised products to accommodate as much information as possible in the design of the solution. In this regard, Rutronik collaborates closely with suppliers such as ASSMANN WSW in order to achieve the best outcome for the customer both in technical and financial terms. Adequately dimensioned profile and machining drawings with tolerance specifications for this purpose are an absolute necessity. When handling complex, customer-specific profiles, 3D data is also very useful. If it is specified in the machining drawing which surface is required for the heatsink, for example whether it must be free of scratches or sanded, this can be accounted for in the selection.
The image “special profile” depicts a successful example of a heatsink. This profile combines a whole range of special features, from plate fin profiles to grooved fin geometry (for increasing the surface area), from extruded threaded ducts to aesthetic surfaces with natural aluminum color and an anodised layer of >15 µm.