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Beyond Design: Signal Integrity, Part 3

In last month’s column, I looked at the effects of crosstalk, timing, and skew on signal quality. This month, I will continue to discuss signal integrity, in particular where most designers go wrong and how to avoid the common pitfalls.

Digital designs become less forgiving as edge rates and frequencies increase. What used to work in the past may not work now, and a different approach to layout may be necessary. Also, there may be many issues that aren’t at first apparent, but affect the reliable performance of the product. Signal and power integrity issues, for instance, often manifest themselves as intermittent operation, which can be very difficult to nail. So it is best to find these issues during the design process and eliminate them at the source, rather than apply a Band-Aid solution after production.

I have analyzed many high-speed boards over the past 15 years and have established a process that I follow in order to achieve effective, consistent results. Not all assessments require expensive analysis tools, but rather common sense. I find that a large percentage of issues can be detected just by eye-balling the design--simulators don’t pick up everything.

The first thing to look at, of course, is the board stackup. The substrate is the most important component of the assembly and needs to be planned correctly to maintain consistent impedance across layers, avoid unintentional signal coupling and reduce electromagnetic emissions. In Part 1 of this series, I set out the basic rules for stackup planning that should be adhered to. The most important being: All signal layers should be adjacent to, and closely coupled to, an uninterrupted reference plane, creating a clear return path and eliminating broadside crosstalk.

This brings us to the next issue: Split planes and current return paths. With all signal layers turned on in your layout tool, it is difficult to see the wood for the trees.Read the full column here.Editor's Note: This column originally appeared in the December 2014 issue of The PCB Design Magazine.