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36 The PCB Magazine • December 2017 THERMAL MANAGEMENT CONSIDERATIONS AT THE BARE BOARD LEVEL Thermal Vias Thermal vias can also be a great method to aid in the movement of heat from a specific location(s) or device into the circuit board. The use of thermal vias for heat transfer is a wide- ly used and proven method that does not im- pact the cost too dramatically. When using the thermal via in conjunction with a heavy copper plane layer or internal heatsink, one can create a thermal Z-Axis highway of sorts. One of the gains of this method is that it can be used in specific problem locations or components ver- sus trying to do it in a mass event. The design- er can create a via field under the specified loca- tion and simply plate heavier than normal cop- per, copper fill or plate shut stacked vias to help draw temperature. Figure 4 shows thermal vias with various plating thickness and/or internal fill, whether conductive or non-conductive. Today, we tend to see about 65% of all designs using stacked or blind via technology. This allows the design- er to utilize HDI plated closed stacked vias for density but can also be used to aid in creating a thermal highway directly to the internal heat- sink. External Heatsinks Probably the most common and widely used method of heat removal is the external heat- sink. Simply noted as a metal plate, it is typi- cally made of copper or aluminum placed on the outside of the PCB. These are usually bond- ed to one side of the PCB using either a b-stage material or some type of thermally conductive bond ply. The heatsink is then either connect- ed to the chassis, box or fanned from some air- flow source. This method is commonly used today as it is one of the simplest, most cost-effective meth- ods. However, you are now drawing the ther- mal, not from a specific location, but through the entire thickness of the PCB itself and then to the opposing side, dispersing the heat through a mass method. It is not always the most effi- cient, nor most effective way of accomplishing the thermal dissipation, though it is tried and true. We have seen this method being used typ- ically in a box level where weight and/or size is not an issue. As stated, the external heatsink can be bonded using a thermally conductive adhesive as well as being somewhat 3-dimen- Figure 4: Various via configurations.