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40 DESIGN007 MAGAZINE I MAY 2023 materials as an example, this lumps 185HR, 370HR, IS550H, IS415, FR408HR, and I-Speed all together. So, if a designer needed high-frequency performance with something like I-Speed, but allowed a substitution with /126, their fabricator would likely swap the material to something else. at other material could likely be a 185HR or 370HR, which have significantly differing Dk/Df values as well as differing glass weave types. A designer who wants to control the materi- als in their stackup is oen pressed to commu- nicate specific acceptable alternative supplier laminates, such as "Panasonic Megtron 6 (or Isola I-Speed)," to their fabricator. To use shoes as an analogy: Shoes gener- ally all have a sole, laces, breathable top, etc. A slash sheet reference would help identify "shoes," but doesn't help identify "running shoes" vs. "hiking boots." Nike and Adidas both make running shoes, so a designer could use "/ shoes" (slash sheet reference), but they may be in trouble if they are running a marathon and want "Nike or Adidas running shoes." A limita- tion of the slash sheets is that there are signifi- cant differences between the brands and the types of shoes, and unfortunately they could all be lumped together in the same slash sheet for "shoes." A designer should clearly communicate with the fabricator regarding the desired glass weave, resin type, etc., going into their board if they wish to ensure their signal integrity char- acteristics will be met. Don't take unnecessary shortcuts when a few more words can save a world of headache. DESIGN007 Geoffrey Hazelett has a bachelor of science degree in electrical engineering, and was vice president of sales at Polar Instruments for several years. He is currently home with his new baby daughter, Sage. Chapter 5: Control of Electrical Performance With many things influencing electrical perfor- mance, like resin system, glass weave, copper treat- ment roughness and impact of alternative oxide, what options are available for the user to make sure that the PCB works as intended? Electromagnetic field simulations have improved significantly in recent years, but they all depend on defining the structures correctly. Geometries can be found quite easily by cross-sectioning. For Dk/ Df values, it's important to understand the methods that have been used at the CCL supplier to deter- mine them. There really is no magic Dk number to plug into a simulation tool to get everything right. Even so-called 'design-Dk' values are just measure- ment by a different test method that may or may not reflect field orientation in a given design. As covered above, we know that the issue of copper roughness is making things very com- plicated. The roughness created by the alterna- tive oxide will be differ- ent from PCB supplier to PCB supplier, so there are no good rules of thumb that one can apply in a model with 100% confidence. The only way to be sure is to measure characteristics on the final PCB. Download this book to continue reading. BOOK EXCERPT The Printed Circuit Designer's Guide to... High Performance Materials