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48 The PCB Magazine • March 2015 materials on the market will easily withstand 1000V DC (and most will withstand 3000V). When selecting an IMS for a particular ap- plication, most companies seem to select based on the claimed thermal conductivity; however, it is better to select based on the thermal im- pedance, which is basically the relationship be- tween the thermal conductivity and the thick- ness of the dielectric. This is, more or less, a lin- ear relationship, so if the thermal conductivity is doubled, or if the thickness is halved, then the thermal performance is improved by a factor of two. This fact is used by some suppliers offering dielectrics that have no ceramic fillers but are very thin (e.g., 25 microns) in order to maintain a good thermal performance; and there is also a trend for thin, ceramic-loaded materials that are providing superior thermal performance (i.e., very low thermal impedance). As demand increases, new materials are continually appearing, with one in particular that is aimed strictly at the high-performance sector. This material differs from the standard IMS because the ceramic is actually deposited directly onto the aluminium base, providing a thin dielectric layer coupled with a high ther- mal conductivity, resulting in a very low ther- mal impedance substrate. Formable versions of IMS are also available, and they are similar in appearance and construction. They use rolled and annealed copper foils, thin, non-reinforced dielectrics, and a malleable grade aluminium, all designed to bend without cracking. As mentioned previously, many customers select materials using the thermal conductiv- ity as stated on the technical data sheets (TDS); however, from evidence gained during simple product evaluations it would indicate that this information should only be used as a guideline, as materials often with similar stated thermal performance appeared to perform differently when tested under similar conditions. Original- ly, the in-house product approval test at Spirit Circuits used a PCB made from the material to be tested, with a simple star design. A standard LED was attached, with a thermocouple con- nected to its thermal pad, and its temperature was logged over a range of drive currents (100, Feature THERMAL MANAGEMENT FOR LED LIGHTING APPLICATIONS continues figure 2: Standard IMS MpCB.