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DECEMBER 2024 I DESIGN007 MAGAZINE 43 nect density. Electrical interconnect on inter- nal layers for the board enables significantly greater circuit routing density. e circuit path between key components can be more direct as well, providing greater circuit speed and lower resistance. To ensure the greatest intercon- nect efficiency, the designer should alternate the overall direction of the circuit path from one layer to another, using plated via-holes to accommodate direction change as needed. With the circuit width and space requirement already established in the CAD system, the auto-router function can quickly complete the initial interconnect process. ree technical levels of capabilities for cir- cuit conductor routing on the inner layers of the circuit board are presented in Table 1: standard, advanced, and capability classified as emerging technology. e capability shown in the emerging tech- nology column may be a standard capability for some fabricators but, for others, the pro- cess will require improvement in process con- trol and chemistry refinement. e individual circuit board supplier's standard level of com- plexity will generally represent what is rec- ommended to achieve their highest yield and most favorable unit quality. e circuit rout- Table 2: External layer circuit routing from an independent industry capability survey ing complexity and copper foil thicknesses on the external surfaces of the multilayer printed circuit board fabrication may differ from those routed on the inner layers of the stackup. e final process stage for the multiple layer circuit board is lamination of all the circuit layers, including the two external layers. Fol- lowing lamination, via holes will be drilled or formed, followed by a number of plating steps to fill holes, and in some cases, build up the conductor pattern thickness. A series of imaging, plating, and chemical etching pro- cesses takes place for the two external layers. Depending on the board fabricator's capabil- ity, conductor routing on the external surfaces of the circuit board may require a slightly dif- ferent requirement for conductor routing. e data furnished in Table 2 may be required by the supplier, and although it will have an impact on circuit routing density, fabrication yield may be a factor. Stackup Planning for High Density Multilayer Circuit Boards A PCB stackup plan must be developed to define the order, thickness, and materials used for each layer of the circuit board. e circuit layers are either dedicated to routing signal