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62 DESIGN007 MAGAZINE I DECEMBER 2020 Having a good understanding of the circuit material that a designer is working with, along with the potential PCB fabrication influences, is essential for having a successful first-time evaluation of a new circuit design. If a designer only considers the basic material properties of Dk and Df, that may not be enough informa- tion to have a successful first round of evalu- ations. The Dk and Df values that are found on a material datasheet are typically generated with a non-circuit test method, which is usually a raw material test that is using some type of fixturing. If that same material is evaluated in circuit form, it is very common that the Dk and Df values will be different because the propa- gating wave on the circuit may have different influences than in a test fixture. Understanding how a circuit material behaves in circuit form can be very valuable to the circuit design pro- cess for achieving better circuit performance for the evaluations of a new design. A topic that has been getting more attention over the past several years is copper surface roughness. The copper surface of concern is at the substrate-copper interface as a laminate is made and not the air side of the laminate. However, depending on how the circuit is made, the copper surface on the air side of the laminate can play a role in the impact that the surface roughness has on circuit performance. The copper surface roughness effect has been found to influence insertion loss, phase response, and propagation speed. In a very general sense, when the speed of an electromagnetic wave is analyzed, a slower velocity will indicate a higher Dk value of the wave propagation medium. If two similar cir- cuits are evaluated for the extraction of the material's Dk value, the circuit with the slower Understanding Material Interactions With PCB Fab Processes Lightning Speed Laminates by John Coonrod, ROGERS CORPORATION

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