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24 The PCB Magazine • December 2017 er to manage the rinses on the ENIG bath very carefully (Figure 4). Machining/Fabrication Scoring is the most common process used for square or rectangular shapes. The advan- tage of scoring is that it assists in maximizing material utilization since zero spacing is need- ed between parts to score them. In contrast, routing is the most expensive process since it is slower and requires spacing between parts and will likely reduce the material utilization. Make sure that your PCB fabricator has a scor - ing system that is specifically designed for scor- ing aluminum. The scoring machine should be equipped with a lubrication system. It is rec- ommended to use diamond-coated scoring blades and router bits when dealing with alu- minum base metal. Double-Sided/Multilayer IMPCB The PCB supplier manufactures a double- sided or multilayer IMPCB and then bonds it utilizing a thermally conductive prepreg to met- al (Figure 5). The bonding process is done in the same multi-layer press that is used to manufac- ture a multi-layer PCB. Many design factors and considerations that were discussed in the single-sided IMPCB section apply here, plus there are some addi- tional considerations to think about as listed here: Copper weights on all the layers The thicker the copper, the more expen- sive it is. Also remember that the outer two layers will receive additional copper since the vias will need to be plated. Lines and spaces should follow the design guidelines of the PCB shop based on the copper weights of each of the layers. Double-sided/multilayer construction It is important to decide whether you can use FR-4 for your multilayer construction or if you require thermally conductive prepregs and cores. If you need thermally conductive cores and prepregs, there are a number of options available—but core thicknesses are limited so it is best to work with a PCB supplier or a lami- nate supplier on constructions that make sense. The prepregs tend to be low flow; it is impor- THERMAL MANAGEMENT: A PCB MANUFACTURER'S PERSPECTIVE ON INSULATED METAL PCBS Figure 4: "Pinking" solder mask on ENIG board with multiple reflow cycles. Figure 5: Schematic of a double-sided IMPCB.