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The right decisions are not always the easiest decisions, but making them well and as early as possible often avoids errors and addition costs. This is certainly the case in PCB design and a key decision influencing the design process 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 hardware or PCB design. What I am seeking to do is to consider the processes that lead to successful design and the errors that lead to risk, potential problems and compromises.
Let’s take an example of a backplane 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 understand 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 frequency? 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 design 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 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 specifications are met and the PCB can be manufactured with a good yield.
This article originally appeared in the March 2015 issue of The PCB Design Magazine. To read the rest of this article, click here.