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26 The PCB Design Magazine • March 2015 are minimized. The geometry of those tracks and their positioning with the PCB structure are critical to realizing a good design that can be manufactured with a good yield. The design is only good when the specifica- tions are met and the PCB can be manufactured with a good yield. Now is the time to consider materials, be- cause setting up differential pairs in the X, Y & Z axis requires we know and understand the Dk (dielectric constant) and Df (dissipation fac- tor) of the material. Materials specifications are a good starting point. After all the suppliers will have designed the material with a purpose, so they will have a particular data rate in mind. But not all material suppliers are the same. Some will have design experience and some will not. Some are distributors, whilst others will have direct access to the staff at the laminator who designed the materials to meet a particular need in the market. Choose the vendor carefully and work closely with them to get the right material designed in, and agree design rules appropriate to that material and to your needs. They'll need to have experience of the fabrication process as well as design and be able to support you in working with the PCB manufacturer through- out the prototype and volume manufacturing process. You're now ready to create the specification of the 'cells'—the traces, trace separation, pair separation, and the layer-to-layer separation. Again, the materials team should be able to sup- port this and you should certainly be sharing it with them before starting the design. Imped- ance modeling can be performed at this stage using modeling software, such as that offered by Polar Instruments and other companies. The material Dk will have a substantial effect at this point impacting upon layer separation MAkE ThE RIGhT DECISIOnS AT ThE RIGhT TIME In ThE PCB DESIGn PROCESS continues feature Figure 1: Typical supplier specification.