Intelligent electronics is increasingly employed especially in large white goods appliances. This tendency is dictated by the markets as new devices are rated according to their energy efficiency, and as this rating is translated, for the consumer, into higher efficiency classifications. An additional driver for the increased use of electronics is operating convenience which improves user friendliness. With the increased use of semiconductors - and especially power semiconductors -, white goods developers must increasingly consider heat dissipation problems while simultaneously keeping production costs as low as possible, of course. One possible way to keep production costs reasonable is the use of pre-fabricated standard materials, such as transistor clips, cooling plates and other heat sinks, thermo-silicone films and phase-change materials (PCM).
Transistor clips
Power transistor clips may seem trivial at first sight, but they are based on long-standing expertise. Customized clip design may result in optimum thermal linkage between semiconductor clips, thermal interface materials (TIMs) and heat sinks. In white goods applications, transistor clips are usually screw mounted as they are often exposed to significant vibration, e.g. during the spin cycle of a washing machine. Force-displacement charts facilitate the ideal configuration within power drive or heating control applications. Developers should further take into consideration the fact that transistor clips are 100 per cent non-corrosive in electronic assemblies. This is achieved by the use of materials such as 1.4310 stainless steel. Semiconductor clips are usually available as standard clips for the most common transistor types such as TO-220, TO-126, but also for TO-3P, TO-247/248, multi-watt, 2 x TO-220 or TO-220/TO-3P, and optionally for low-profile bridge rectifiers. In addition, there are multiple transistor clips as well as electrically insulating mounting elements.
Thermo-silicone films
The power semiconductors and power modules in the electric drives of washing machines and dryers may serve as examples for the application of thermally conductive silicone films with the aim to achieve optimum thermal linkage as well as electric insulation of heat source and heat sink. In these cases, the thermal conductivity of the silicone is increased by adding thermally conductive ceramics (e.g. boron nitride, Al2O3, AlN) to the polymer structure of the elastomer. Under mechanical strain, the superior plasticity of silicone is an advantage: The material adapts very well to the contact surfaces, thereby minimizing total thermal contact resistance. These interface materials are available in a wide range of gauges and degrees of hardness as well as with different filler levels, which result in varied degrees of thermal conductivity that may be adjusted according to specific application requirements. When compared to phase-change materials (PCMs), silicone films are able to compensate larger distances between heat source and heat sink, ranging from, e.g., 0.1 to 0.45 or even 0.8 mm. As white goods applications are frequently subjected to shock or vibration, fibreglass filling or polyimide substrate carriers may insure additional mechanical stability. The ceramic-filled silicone materials also boast advantageous qualities such as high temperature resistance, chemical stability, durability and dielectric strength.
Phase-change material
Phase-change materials (PCMs) - thermally conductive materials with phase-change properties - are the ideal substitute for thermally conductive grease and are perfectly suited for compensation of distances in the µm range (up to around 70 µm). As opposed to solutions involving mica and thermal grease, the use of PCM affords guaranteed reproducibility. As the name suggests the material is characterized by performing a phase change at a certain temperature, normally between 51 and 60°C. Below that temperature, the material returns to its solid state. When turning liquid, the material expands actively by about 15 to 20 per cent, wetting out contact surfaces, eliminating air pockets from micropores and compensating surface irregularities. This significantly reduces total thermal contact resistance and allows for optimum heat transfer. According to application specifics regarding mechanical stability, PCMs are available on electrically insulating carriers (such as polyimides or other plastics) or on electrically conducting metal (e.g. aluminium) foils.
Heat sinks and cooling plates
The waste heat produced in power modules invariably needs to be dissipated toward a heat sink. This may consist of a profile heat sink or a cooling plate, the casing of a control unit, or the surface of the application casing, usually quite ample in the case of white goods. Extruded profile coolers are employed for the single transistors of the power amplifiers e.g. in the drives of washing machines and dryers. A low-cost solution is provided by die-cut heat sinks and cooling plates which are linked to several power semiconductors at once through mounting clips or screw fasteners. These cooling plates or angle plates, most commonly made of ALMG3 (or, alternatively, of copper, brass or the like) for superior heat dissipation, are also available with pre-cut holes for mounting with transistor clips.
Conclusion
There are specific heat management solutions for each requirement of the different power electronics applications, such as power semiconductors or power modules, in electric drives, in engine control and heating system control etc. in white goods. There is a wide range of options which take into consideration the special cost sensitivity in this area, from the „simple“ clip to heat sinks and cooling plates to the thermally conductive silicone films and PCM, the latter two being more or less equal in price. So far, electrically insulating, thermally conductive silicone caps and tubes which can be applied onto power semiconductors have not been mentioned. Tubes are cheap, possess great dielectric strength and, when combined with mounting clips, conduct heat to a heat sink. They can be cut to customer-specific length by the supplier or delivered as yards goods. Caps, in comparison to tubes, have the advantage of all-round electric insulation, may also be cut according to customer requirements, but are slightly more costly due to their superior insulating properties. In this industry, it is particularly difficult to provide best-quality products at reasonable prices - Kunze Folien faces up to the challenge.
