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MAY 2022 I PCB007 MAGAZINE 59 and Polar Instruments supply- ing simulation tools for calculat- ing impedance. So how to make sure that the transmission path had the correct impedance? He took a familiar example of a coaxial cable, whose impedance was determined by its geometry and the permittivity of the insulat- ing material and demonstrated how the principles could be adapted to the planar structure of a PCB in the form of an off- set stripline. e geometry and the permittiv- ity of the material determined the impedance, and he showed a range of examples of "single- ended" structures. In contrast were "differen- tial" structures, widely used in digital transmis- sion systems, usually consisting of two traces running in parallel and coupled with a capac- itor. e advantages of differential signalling included improved noise immunity, lower voltage requirements, and higher data rates. Reischer listed the factors influencing imped- ance. Trace width was the most significant and was simple to modify. Substrate height, relative permittivity, and copper thickness were deter- mined by material selection, and etch-back was process-dependent. Sol- der mask thickness could have a significant effect on the proper- ties of edge-coupled microstrip designs, as could the influence of local resin-rich areas if similar features were embedded within the structure of the PCB. He discussed the practical aspects of resin distribution and thick- ness control during multilayer pressing. Vias gave some interesting effects; they were generally quite small compared with the wavelength of the signal, so they could normally be ignored except at extremely high frequencies. But on thick boards, as via stub-length approached a quarter of the wavelength of the signal, they could cause undesirable resonance. Back-drill- ing was a technique used to mitigate these effects. Future challenges included tighter imped- ance tolerance specifications, lower dielectric thicknesses and narrower trace widths. Fre- quencies beyond 5 GHz resulted in "lossy" transmission lines; dielectric losses could be reduced by using new base materials and smoother copper could reduce skin effects, both at increased material cost. From a design point of view, Reischer recommended keeping traces as short as possible and ensuring unin- terrupted signal return paths. For various rea- sons it was preferred to place critical lines on inner layers. And he emphasised the impor- tance of consulting with the PCB vendor on stack-up design and material selection before commencing the layout. Reischer's presentation delivered a compre- hensive theoretical background to the con- cepts of impedance control and signal integrity. His closing comments about consulting with the PCB vendor before commencing the layout provided a perfect opening for Michiel Op den Camp, engineering manager for NCAB Group Benelux, to give an insight into the technical Hermann Reischer