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PCBD-Oct2014

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46 The PCB Design Magazine • October 2014 Conductive adhesives have been around for many years. Some are electrically conductive, others are thermally conductive, and some have both properties. Additionally, the conductive adhesives can be a pressure-sensitive adhesive (PSA) or a thermoset adhesive. To narrow the scope of this month's column, only thermoset thermally and electrically conductive adhesive (TECA) will be discussed. TECA is often used for heat sink attachment of PCB assemblies when thermal management is a concern. In the RF industry, power ampli- fiers often generate a lot of heat, and the PCB supporting the power amp is generally attached to a large metal heat sink. The attachment be- tween the PCB and the heat sink is usually done by mechanical attachment with screws, sweat soldering or TECA. Each attachment method has its own set of capabilities and limits. The mechanical attachment may have air gaps between the circuit and the heat sink, and if these are in critical areas they can make a less efficient heat flow path from the PCB to the heat sink. A less efficient heat flow path can cause the PCB assembly to have a higher tem- perature than desired and sweat soldering can have similar issues due to voiding. Air gaps are not an issue with TECA when parts are properly bonded using vendor supplied parameters. The drawback to TECA, compared to the other two technologies, is that TECA is usually not as ther- mally conductive as a metal-to-metal contact. And for RF applications, the heat sink is often used as the system ground and the electrical LIGhTNING SPEED LAMINATES column by John Coonrod rOGErS COrPOraTIOn Chilling Out with Conductive Adhesives

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