Estimated reading time: 6 minutes

Institute of Circuit Technology's 2014 Darlington Seminar

Martin Cotton, Ventec’s director of OEM Technology, can always be relied upon to deliver a dynamic and inspirational presentation. His down-to-earth discussion of practical design considerations for high speed PCBs was dedicated to the memory of the late Fred George Pochodnia, who had been a great motivator in the early days of his design career. With the emphasis on “practical," Cotton set out to build the basis of a design for a 36-layer backplane example, working at 10 Gigabits per Second data rate and 100 ohm impedance.

Initial questions to be answered were whether the data rate was for a single-channel differential pair or for a buss, buss speed being the total of all the channels in that buss. 100 ohm differential impedance used two tracks in harmony to keep losses to a minimum, and the geometry of those tracks and their positioning within the PCB build structure were critical to the realisation of a successful design, and the ability to manufacture it at a good yield. Setting-up the differential pairs in X, Y, and Z axes began with material selection and consideration of dielectric constant and loss factor, then the creation of “cells” to define trace geometry, trace separation, pair separation, layer to layer separation, taking material properties into account--particularly dielectric constant--before carrying out impedance modelling. The completed “cell” was the essential foundation of the design. Once it was established, the designer could proceed to initial stack-up and trace mapping, and sample simulations on maximum trace lengths. Knowing the material and the build enabled cost estimates to be made. Only then would the actual layout proceed, with periodic checking simulations to ensure acceptable signal integrity.

In Cotton’s example, 36 layers were required to realise the design, but post-layout simulation revealed signal integrity issues with 90 differential pairs of the 2,000 contained in the layout. The designer was then faced with a dilemma: To fix these by reviewing them individually, which would take time, or to move them from within the 36 layers and add four additional layers, plus two power planes for reference, making a total of 42, which would increase unit cost and could complicate the mechanical design. Another option might be upgrade to a more expensive material with better Dk and Df values, although Cotton stressed that in principle the material supplier should always be consulted in the first instance, rather than too late in the design process.

“Design once, make many times” was his message, “take the time to create what you set out to do,” adding that although CAD and simulation systems were very powerful tools the actual designing was done “up here”--pointing at his head!

A long drive for some, but well worth the journey, the Darlington Seminar was another very successful event in the ICT calendar, combining technical enlightenment with a great networking opportunity for the many members who attended.

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