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26 The PCB Magazine • December 2017 tant to work with a PCB supplier that fully un- derstands the lamination press cycles to be used with these materials. Thermally conductive prepreg Choose the prepreg to bond the PCB (dou- ble-sided or multilayer) to the metal based on thermal conductivity required and thickness of the copper circuitry. From a PCB manufacturing perspective, a number of different factors need to be accounted for in the process of bonding the PCB to base metal: • Ensure there is no delamination between the PCB and the metal. There are design factors that can impact this and process conditions in the lamination process • Have a method to control the flow of pre- preg through the plated through-holes to the top side; have a method to remove any flow that ended up on the top surface of the PCB • There are number of mismatched CTEs in this package. It is important to try to bal- ance the copper in the construction as much as possible from a PCB perspective and have a press cycle that helps to mini- mize warpage Base metal Aluminum is the most common metal in use, however there are many applications that will also use copper as the base metal. In gener- al, when aluminum is the preferred metal, the 6061T6 alloy is the best choice for this type of construction. Metal Core Boards Conceptually, a metal core board is exact- ly what it sounds like: the metal is in the mid- dle of the PCB, sandwiched between layers on both sides. Metal core PCBs usually have blind via layers located on both sides of the metal core substrate. There are also plated through- holes (PTH) going through the entire package. From a PCB perspective, it is important to iso - late the metal from the through-hole, other- wise the board would short out completely. To accomplish this, one has to start by drilling the metal core approximately 40–50 mils larg- er than the plated through-holes, slots or cut- outs. These then need to be filled with a non- conductive epoxy filler and then pressed. After pressing the metal core will need to have the filler compound removed from the surface and then prepared for lamination with the inner - layer cores. After lamination, the PTH is drilled and processed in the normal manufacturing process. While thermal management is a factor in these PCBs, another reason that metal core boards are used is to help with vibration reduc- tion so that components don't fall off the PCBs in high-vibration applications. THERMAL MANAGEMENT: A PCB MANUFACTURER'S PERSPECTIVE ON INSULATED METAL PCBS Figure 6: Schematic of a multilayer metal core board.