» PCB & ASSEMBLY
» PCB & ASSEMBLY Alternative thermal management solutions Enhancing thermal management with IMS A little understanding can make a big difference when designing thermal management with insulated metal substrate (IMS). The first of a two-part series, this article examines how IMS can help control and dissipate heat in assemblies containing power semiconductors and LEDs. Chris Hanson, Global Head IMS Technology, Ventec International Group Proper thermal management is needed to ensure the reliability of components that exhibit significant self-heating. Typical examples include LEDs in lighting applications and IGBTs and FETs in power supplies, converters, and motor drives. Various techniques are available to extract and dissipate the heat from inside the component. The designer’s decisions can influence other important aspects of the end product, such as cost, size, and appearance. Attaching a heatsink, usually directly to the component itself, is a typical solution. There can be drawbacks, however. Depending on the power to be dissipated, the heatsink needed may be large and bulky. Active cooling, using a fan, may be required. In addition, attaching the heatsink during product manufacture may require special, or even manual, assembly processes that increase the cost of the end product. Cooling through the substrate Fortunately, there are alternatives. One is to dissipate heat into the substrate. This is a popular approach, particularly where surface-mount packages are used. Surface-mounted LEDs, for example, dissipate a high proportion of heat through the underside, to maximize utilization of the topside as the lightemitting surface. As far as power transistors are concerned, typical through-hole packages have design features that ease attachment and optimize dissipation from the die into a heatsink. Surface-mount packages, on the other hand, may be optimized for cooling through the topside, underside, or both. IMS structure and thermal dissipation Source: Ventec International Group 28 EPP Europe » 04 | 2023
Various grades of IMS are available, depending on the dielectric composition, layer thickness, and baseplate material and thickness Source: Ventec International Group Where heat is to be dissipated through the underside, the substrate must be capable of dealing with this. If the thermal properties of standard FR4 material are not sufficient, adding thermal vias filled with high-conductivity material can increase the ability to remove heat from the component package. However, this can increase the board cost and complicate circuit layout and routing. A thermally enhanced substrate can help overcome these challenges and, in some cases, can allow a heatsink-free design. Insulated Metal Substrate (IMS) circuit boards provide high thermal conductivity and are available in various grades. Some manufacturers also offer economical non-IMS products that deliver thermal performance several times better than FR4. IMS properties and parameters An IMS laminate consists of a metal baseplate separated from the circuit foil by a thin layer of dielectric, arranged to promote heat flow from the mounted component into the baseplate, from where it can be dissipated into the ambient environment. The dielectric is resin-based and serves to bond to the metal layers as well as providing electrical insulation between them. The baseplate is often aluminum, typically chosen for its light weight and DON’T MISS... The second article in this series will examine practical IMS applications and guidelines for material selection. relatively low cost. Various alloys are available that offer designers a choice of properties. Pure aluminum has superior thermal conductivity and is relatively low cost. On the other hand, aluminum/ magnesium/chromium mixes such as Aluminum 5052 – a medium-cost alloy – are better suited to processes such as bending, forming, and punching. The opportunity to dissipate large quantities of heat energy through the substrate helps ensure reliability while at the same time permitting a space-saving solution. Alternatively, the baseplate may be copper and, sometimes, steel. Despite increased cost and weight, copper may be chosen where superior thermal performance is needed. It can also be chosen if there is a need for the baseplate coefficient of thermal expansion (CTE) to be closely matched with that of the circuit foil, to minimize mechanical stresses at elevated temperature. The circuit layer is typically a standard HTE (High Temperature Elongation) ED (Electro-Deposited) foil, as used in regular copper-clad PCB laminates. Superelongation foil can be specified where extreme CTE mismatch is an issue. The dielectric layer may be a composite of epoxy resin with woven glass reinforcement, like a conventional laminate construction. Compared to conventional resins as used in ordinary FR4 laminates, its thermal conductivity is enhanced by loading with a thermally conductive ceramic filler. The conductivity can be controlled by adjusting the proportion of filler, up to a maximum of about 70%. EPP Europe » 04 | 2023 29
