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38 The PCB Magazine • December 2017 sional, wrapping itself around the finished bare PCB. For some more complex designs we have seen the use of the external heatsink as a center core between two opposing PC boards bonded together like a sandwich. Internal Heatsinks Placing the heatsink within the PC board brings the mass of metal closer to the devices, as well as opens both sides for component mount- ing, but again has its limitations. Yes, it brings the heatsink closer to the device level, but it is still a slow and mass method of trying to dissi- pate the heat. Typically, this method has been done through the years by using CIC (copper/In- var/copper), solid copper or aluminum. The issue becomes that now the fabricator must etch and/or machine the inner heatsinks so as to not short out to the metal core. Once the removal of the metal is complete, the heatsink is typically backfilled with a non-conductive resin like that of the buildup material of the PCB. In recent years there have been many ad- vances in materials such as the use of carbon fi- ber within the PC board. This too can be used as a thermal highway to draw the heat into the board and out to a source of thermal connec- tion draw such as a chassis. Again, one must consider that having too much resin in a PTH could create its own set of problems. The fabri- cator must consider using a stable resin system so that Z-axis expansion does not create other problems. Filling and planarization need to be done prior to the lamination to ensure that no air entrapment is evident. This method as with external heatsinks does require some extra pro- cessing (Figure 5). Coin Technology Coin technology is fast becoming an alter- native to the internal heatsink. It is widely used for the component heat management and re- moval in RF/microwave designs on a regular basis. It is probably the costliest, but it is also the most effective and direct method of pull- ing heat away from a specified location. Using coin technology allows the designer to place the component directly on top of or within the solid "coin," which allows the thermal draw directly downward and away from the de- vice to the backside, where it can be dissipated (Figure 6). Not all suppliers are willing to offer this technology, but in the RF design world it is used often. THERMAL MANAGEMENT CONSIDERATIONS AT THE BARE BOARD LEVEL Figure 5: Visual of basic external and internal heatsink methods.