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JULY 2023 I DESIGN007 MAGAZINE 39 Roughness Not Just a Number RMS Rq Rz Rz (ISO), Rz(DIN), Rz( JIS), and a myriad other roughness metrics reveal a minefield of complexity to the SI engineer who may just ask, "How many microns of Cu is the roughness on this foil?" What is impor- tant is that the methods mentioned are mod- elled using specific roughness metrol- ogy, and there is n o t r u s t w o r t h y conver sion from one to the other. You may find some articles that say something like, "In a limited number of circum- stances a conversion is pos- sible," but it is prone to error and pitfalls. So, the bottom line is to use the gradient method; you need to use RMS (sometimes called Rq) roughness. For the Cannonball Huray, use Rz (DIN or JIS). ere is good reason for the variety, as in mechanical engineering you may be looking at a certain roughness to hold lubricant on a bearing or cylinder surface, or the measurement may simply be one of pro- cess control. The OEM Rules Regardless of the method you would like to use, or you feel is best for your application, if an OEM is specifying a particular model then you had best stick to that. It's fine to compare with other methods, but for product qualifica- tion, if the OEM is using roughness model "A," then that's the choice made for you. Future Directions You may have noticed earlier in the article that with improvements in resin systems and copper surface pretreatments, that profiles are getting smoother. Over time (and we are not there yet), the need for roughness modelling will ease. A Little Knowledge is… …a dangerous thing, as the saying goes. Whilst roughness data may be readily available from your foil or laminate supplier, remember that depending on where the material is in the stack, it is likely to undergo further treatment or plating on drill end layers. Not only do you need to have material data, but also stackup information on how the fabricator further treats it before bonding is valuable to form a com- plete picture. Conclusion Whilst at first sight roughness m o d e l l i n g m a y seem fearsomely complex, roughness is only part of the loss budget. Getting a reasonable figure in for roughness and choosing the correct model will enhance your predic- tion of insertion loss compared with solvers that simply model smooth surfaces. DESIGN007 Resources 1. "A Physical Surface Roughness Model and Its Applications," by G. Gold and K. Helmreich, IEEE Transactions on Microwave Theory and Techniques, vol. 65, no. 10, pp. 3720-3732, October. 2017. 2. "Fundamentals of a 3-D 'snowball' model for surface roughness power losses," P. G. Huray et al., in IEEE Electromagnetic Compatibility Magazine, vol. 9, no. 2, pp. 62-65, 2nd Quarter 2020. 3. Bert Simonovich's Design Notes: "Cannonball Huray model demystified," Lamsim Enterprises. 4. Surface roughness comparison of gradient and other methods: Polar Instruments Ltd. Click here. Martyn Gaudion is managing director of Polar Instruments Ltd. To read past columns, click here. Martyn is the author of The Printed Circuit Designer's Guide to… Secrets of High- Speed PCBs, Part 1 and Part 2. When it comes to generics, there is even more potential for variance.