PCB007 Magazine

PCB-Mar2015

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46 The PCB Magazine • March 2015 Feature this heat can then be dissipated into a second- ary heat sink. The holes around the LED pads do limit the potential LED density, and from our experience we find that holes placed further than 5 mm from the LED have a much reduced effect on the junction temperature. Obviously, the use of via-in-pad technology will allow for higher LED packing densities but this does cre- ate other assembly issues (and if this means us- ing hole-filling, then any cost-savings for using FR-4 will be eroded); however, via-in-pad will improve the thermal performance when com- pared to having vias around the LED (Figure 1). To obtain the maximum thermal performance from this PTH approach will require the use of an isolating thermal interface material (TIM), which will eliminate the risk of electrical leak- age and help considerably with heat dissipation (into a secondary heat-sink). Ideally, the non- LED side should have no solder resist coating as this provides the best transfer of heat (i.e., us- ing the TIM to provide the electrical isolation); however, many applications use a solder resist in order to ensure the PCB is electrically isolated from the heat sink. When it comes to mid- to high-power or high-density LED applications, many companies turn to insulated metal substrates (IMS) because it provides a convenient and reliable thermal solution as it comes with an in-built heat-sink (Figure 2). The IMS is a relatively simple materi- al which comprises of a copper foil bonded to a metal base with a thin dielectric. The copper foil provides the circuit image, and because the heat dissipation is primarily routed directly through the dielectric, then the copper weight is less of an issue (as with FR-4 products) and this helps when tracking high-density designs. The metal base is usually aluminium because of its light weight and relatively low cost, and because it is a well-established heat-sink material (thermal conductivity 140–200 W/mK, depending on the grade). For more demanding applications, cop- per is used (thermal conductivity ~400 W/mK) even though it is heavier and more expensive. It is in the dielectric layer where we see the main difference between suppliers (and their product range), although they all tend to be thin layers (sub 0.20 mm) with a varying level of thermal properties. Typically, the thermal performance of these dielectrics is enhanced by the addition of ceramic materials (such as aluminium oxide, aluminium nitride and boron nitride), increas- ing the thermal conductivity of the base resin from around 0.25W/mK to upwards of 5W/mK. Most LED applications are low voltage systems, so electrical breakdown performance is not nor- mally a major concern, although most of the figure 1: fr-4/CEM pTh using thermal vias. THERMAL MANAGEMENT FOR LED LIGHTING APPLICATIONS continues

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