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20 The PCB Magazine • March 2015 The right decisions are not always the easi- est decisions, but making them well and as ear- ly as possible often avoids errors and addition costs. This is certainly the case in PCB design and a key decision influencing the design pro- cess and the eventual outcome is the selection of material and of the materials vendor. This is even more important when the PCB requires significant performance parameters to be met, such as high speeds. What I am not trying to do is teach hard- ware or PCB design. What I am seeking to do is to consider the processes that lead to successful by Martin Cotton VEnTEC InTErnaTIOnaL GrOup Make the Right Decisions at the Right Time in the PCB Design Process design and the errors that lead to risk, potential problems and compromises. Let's take an example of a back plane design that has been specified at 10Gbs with 100ohm impedance with a design target of 36 layers. First, we need to ensure that we fully under- stand what the specification means. Are we sure we mean 10Gbs, which is a data rate in gigabits per second, and not 10Ghz, which is a frequen- cy? Are we seeking that data rate for a single channel, or do we need that rate for a buss? The buss speed is the total of all the channels in that buss. The next task would be to consider the de- sign rules we'd like to apply based on the need for 100 ohms of differential impedance using two tracks that are in harmony so that losses Feature

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