Design News, March 2013

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Mechatronics the sky's the limit NASA Low Outgassing Approved Epoxy Thermally Conductive EP37-3FLFAO • Electrically insulative • High flexibility • Excellent flowability • Cryogenically serviceable Hackensack, NJ 07601 USA +1.201.343.8983 • heat and failures may result. However, heavy copper circuits can help by reducing the I2R losses and by conducting heat away from valuable components, reducing failure rates dramatically. In order to achieve proper heat dissipation from heat sources in and on the surface of a circuit board, heatsinks are employed. The purpose of any heatsink is to dissipate heat away from the source of generation by conduction and emit this heat by convection to the environment. The heat source on one side of the board (or internal heat sources) is connected by copper vias (sometimes called "heat vias") to a large bare copper area on the other side of the board. Generally, classical heatsinks are bonded to this bare copper surface by means of a thermally conductive adhesive or in some cases, are riveted or bolted. Most heatsinks are made of either copper or aluminum. The assembly process required for classical heatsinks consists of three labor-intensive and costly steps. To begin, the metal serving as the heatsink must be punched or cut to the required shape. The adhesive layer must also be cut or stamped for a precision fit between the circuit board and the heatsink. Last but not least, the heatsink must be properly positioned on the PCB and the entire package has to be coated for electrical and/or corrosion resistance with a suitable lacquer or cover coat. Normally, the above process can't be automated and must be done by hand. The time and work required to complete this process is significant, and the results are inferior to a mechanically automated process. In contrast, built-in heatsinks are created during the printed circuit board manufacturing process and require no additional assembly. Heavy copper circuit technology makes this possible. This technology allows the addition of thick copper heatsinks virtually anywhere on the outer surfaces of a board. The heatsinks are electroplated on the surface and thus connected to the heat conducting vias without any inter- faces that impede thermal conductivity. Another benefit is the added copper plating in the heat vias, which reduces the thermal resistance of the board design, realizing that they can expect the same degree of accuracy and repeatability inherent in PCB manufacturing. Because planar windings are actually flat conductive traces formed on copper clad laminate, they improve the overall current density compared to cylindrical wire conductors. This benefit is due to minimization of skin effect and higher current-carrying efficiency. Onboard planars achieve excellent primary-to-secondary and secondaryto-secondary dielectric isolation because the same dielectric material is used between all layers, ensuring complete encapsulation of all windings. In addition, primary windings can be split so that the secondary windings are sandwiched between the primaries, achieving low leakage inductance. Standard PCB lamination techniques, using a choice of a variety of epoxy resins, can safely sandwich up to 50 layers of copper windings as thick as 10 oz/ft2. During the manufacture of heavy copper circuits, we are usually dealing with significant plating thicknesses; therefore, allowances must be made in defining trace separations and pad sizes. For this reason, designers are advised to have the board fabricator onboard early in the design process. Epec Engineered Technologies has developed a set of design guidelines for heavy copper circuits that give designers a basic overview of what is required. Power electronics products using heavy copper circuitry have been in use for many years in the military and aerospace industry and are gaining momentum as a technology of choice in industrial applications. It's believed that market requirements will extend the application of this type of product in the near future. Dave Basista is product manager - Heavy Copper/EXTREME Copper/PowerLink, Epec Engineered Technologies. For more information, go to Design News | march 2013 | www.d esign n –M8–

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