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84 DESIGN007 MAGAZINE I APRIL 2021 statistician, George Edward Pelham, "All mod- els are wrong—but some are useful." is is a piece of advice that engineers and fabricators need to take to heart; while the mathemati- cal models we use for modelling save time, make better product, and increase yields, we only get the best from them by applying them with a good understanding of the input data and the limitations of the base materials used, and the manufacturing processes of pressing, plating, curing, and chemical adhe- sion promotion treatments applied to the finished article. DESIGN007 References 1. The Works of Isambard Kingdom Brunel, edited by Sir Alfred Pugsley, Institute of Civil Engi- neers, London/University of Bristol. Martyn Gaudion is managing director of Polar Instruments Ltd. To read past columns or contact Gaudion, click here. containing the drill ends to connect the vias in the PTH pro- cess. erefore, the roughness needed for simulation is that of the plated layer and not of the base mate- rial. Simulation can only be as good as the input data fed to the simulator. e data does exist, but it is the job of the designer to know where to look, and when to take that data from a material data sheet, and when it needs to come from the fabricator. Once the correct input data is available, then even the roughness may be fed into the simulator and the effects on signal integrity modelled with ease. Figure 7 presents the classic modelling and simulation dilemma. e Huray model is an excellent way of predicting the effect of copper roughness on signal losses along a transmis- sion line, something which is inherently chal- lenging. However, to "feed" the Huray model with good input data requires an SEM image of the finished plated or unplated copper sur- face. Not everyone has a scanning electron microscope on hand, and so a compromise in estimating the parameters is needed for day to day as opposed to laboratory use. Fortunately, Bert Simonovich of Lamsim Enterprises has provided a method which translates the matte and drum side roughness into Huray input parameters which are "good enough" to make a very usable model for predicting roughness impacted insertion loss. Simulation Models Whenever simulating, keep in mind the words of Polar's favourite eminent British Figure 7: Modeling roughness with SEM or Cannonball model.