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March 2014 • The PCB Magazine 19 THERMALLY CONDUCTIVE SUBSTRATES & THERMAL MANAGEMENT continues mal impedance is proportional to the ratio of thickness to CTE, and is probably a more practi- cal parameter. So, rather than just demand a high W/mK value, consider whether a lower W/mK at a low- er dielectric thickness will give the same thermal impedance, at lower cost, provided that the di- electric strength is sufficient, and it's really only at high operating voltages that this becomes a serious consideration. So for most LED, and low voltage applications, the lower W/mK materials are perfectly adequate. Halve Dielectric Thickness, Halve Thermal Impedance As discussed above, thermal conductivity is only part of the equation … halving the di- electric thickness will in turn halve the thermal impedance and so in effect double the thermal capacity, but this also brings challenges: • Thickness control of the dielectric • Risk of dielectric breakdown and hi-pot failure • Stability We are now producing commercial quanti- ties of IMS materials with a dielectric thickness of 50 µm and below, but this has only been pos- sible following a lengthy investment program, including, but not limited to: • Proprietary treaters and coaters • The introduction of ultra purity fillers • Introduction of super fine filters • Proprietary modifications to the resin supply lines With the above modifications we are now able to produce, in commercial quantities thin cores down to 35 µm guaranteed against hi-pot failure. IMS materials are currently available with thermal conductivity values from approximate- ly 1.0 watt per metre Kelvin (W/mK) suitable for low-power LED applications, and up to about 8W/mK for power electronics. For reference, copper is 380W/mK, aluminium 200W/mK and FR-4 laminate around 0.4W/mK. It is important to systematically evaluate and characterise an IMS material for a particular application, rather than rely on data-sheet information, which may overstate certain critical parameters or, at the very least, be difficult to use comparatively due to a variety of different test methods being used to generate critical datasheet values, such as thermal conductivity and electrical breakdown strength. Insulated metal substrates are generally compatible with lead-free soldering processes, although construction has a considerable influ- ence on reliability, the resin chemistry deter- mines characteristics, such as T g (glass transition temperature) and T d (decomposition tempera- ture), which, likewise, affect reliability under thermal stress and thermal shock conditions. In typical LED street lighting applications, the main reliability issue for insulated metal substrates is not dielectric breakdown, since op- erating voltages are relatively low, but the effect of shear stress resulting from CTE differences between copper, dielectric and aluminium dur- ing the severe thermal cycling between power- on and power-off, together with day-night and seasonal ambient temperature and humidity variation. Some key points to consider in selecting di- electric type and thickness for IMS: • Understand the thermal, electrical and mechanical performance requirements of the design – performance/reliability/cost trade off • Thermal Conductivity of the Dielectric – Lower thermal conductivity—thinner dielectric required? – Higher thermal conductivity—thicker dielectric possible? • Copper Thickness – Heavy coppers have a larger treatment profile – Consider the peak-to-peak dielectric separation between copper treatment and the treated surface of the aluminium/copper/steel backing – this directly influences electrical breakdown performance & reliability: <=105 µm copper—75–100 µm dielectric >=140 µm copper—100–150 µm dielectric