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10 SMT Magazine • March 2015 cally an air-cooled heat sink, liquid cold plate, or the metal surface of some other component. application Interface conditions and Impact on Thermal Performance The ideal interface consists of metal-to-met- al contact across the contact area, which would require precision machining and polishing of the two surfaces to a degree and also add sig- nificant manufacturing costs to those compo- nents. In lieu of a set of ideal polished surfaces, the efficient TIM provides a very thin thermally- conducting material which, given variation in metal surfaces, may vary in thickness through the interface. The thickness of the metal TIM at various points across the interface would be de- termined by several factors: the type of mechan- ical fasteners used to attach the device to the heat sink or cold plate, the amount of clamp- ing force exerted, the location of the fasteners, and the degree of roughness and flatness of the two manufactured surfaces. The surface of a liq- uid cold plate, for example, may be a machined surface of a casting (which may expose internal voiding within the casting), the machined sur- face of an aluminum or copper cold plate, or the raw extruded surface of an aluminum cold plate, if no machining is specified. The mating surface of the heat sink or cold plate may also have variations due to warpage or bending (de- pending on the thickness), the care exercised when handling during manufacturing and as- sembly, and the relative clamping force applied versus the stiffness and strength of the heat sink or cold plate. If mechanical fasteners such as screws or bolts are located only at the periphery of a large module, the flatness of the module metal base- plate can be altered as fasteners are torqued into place. This can change the physical characteris- tics of the interface when measured at a greater distance from the locations of the fasteners. For instance, standard power semiconductor mod- ules, known as isolated gate bipolar transistors (IGBTs), are very common components used in electrical drives and machine tools, control- ling wing flaps and actuators for aircraft, and switching devices within electrical inverters for propulsion powertrains in vehicles. Standard IGBT module footprints have industry-standard dimensions, with specified locations for fasten- ers, which are typically at the periphery of the device. There are also some industry designs for small modules, which include one or more fas- teners in the center of the device. characteristics of Well-Performing TIms The highest-performing TIMs must be ca- pable of adapting to varying surface conditions and the specifications of a given application. These factors include: • Surface flatness of the mating surfaces • Surface roughness of the mating surfaces • Type, number, and placement of fasteners (screws, bolts, clips, use of ancillary components such as springs) • Clamping force applied by the fastener mechanism • Ambient temperature, humidity, and other anticipated environmental conditions • Temperature variation during the operation, cyclical and non-cyclical • Mounting attitude A common statement in the application of TIMs is that the best interface is achieved with no TIM at all, indicating the value of metal-to- metal contact. In reality, this statement must be modified such that the best-performing thermal interface materials are: • Applied in a relatively thin layer by design, sufficient to fill the specified gap • Exhibit very high ability to move and conform to minor surface variations and imperfections • Exhibit a high degree of thixotropicity of the paste or compound to avoid flowing out of the interface • Operate for the intended life of the assembly without drying, hardening, flaking, or otherwise deteriorating The above expectations of operational per- formance all involve the use of TIMs using mechanical fastening to join the two surfaces. Thermally conductive adhesives also must func- tion under similar conditions, but generally without the benefit of high clamping forces ap- DEvELOPmEnTS WITh mETaLLIc ThErmaL InTErFacE maTErIaLS continues Feature

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