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JUNE 2019 I DESIGN007 MAGAZINE 49 to promote chemical rather than mechanical adhesion are becoming available though at a premium (Figure 6). Whilst Huray for "purists" requires images processed from SEM images of the copper foil, there is a very useful empirical short cut to entering viable Huray parameters from simple Rz figures. This is the Cannonball Huray input developed by Bert Simonovich at Lamsim Enterprises (Figure 7). Once you have completed modelling, it is always good to close the loop by comparing measured with modelled. For clarity, Figure 8 shows one model versus the measured, but you can imagine overlaying a measurement result over a group of five roughness simulations to see which is the best fit as previously shown. Conclusion In summary, you may recall the question my colleague asked Dr. Bogatin, "Which is cor- rect?" and his answer, "Neither." You should look at both and at the correlation, and then use your mind to test the validity of either. Ask yourself, "Does the measurement result look credible at the frequency of interest or has it become unstable as the signal power drops into the noise floor? Does the phase align at the frequency of interest?" And regarding the modelling, ask yourself if you have accounted for the roughness in an appropriate way. Have you rounded off the loss tangent figure and lost precision? Have you studied a microsection of the trace under test? Being aware of these items and applying mindful common sense will give you confidence in aligning modelling and mea- surement in a mindful way. Glossary Dielectric constant: The ability of how well a substrate can store electrical charge, often referred to with a variety of symbols such as ε r, Er, Dk, or K. Loss tangent: The (undesirable) ability of a substrate to turn useful signal energy into heat (unless you are making a microwave oven.) Loss tangent is also referred to with a variety of terms, including tan δ, Tan D, or Df. From an SI perspective, the lower, the better, but you need to factor in the cost. DESIGN007 Martyn Gaudion is managing director of Polar Instruments Ltd. To read past columns or contact Gaudion, click here. Figure 8: Modelled and measured data overlaid.

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