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42 DESIGN007 MAGAZINE I NOVEMBER 2022 element. However, in high frequency designs the signal largely moves to the surface of the copper, and here it engages with the surface fin- ish. For boards to be useful they must bond and survive assembly, possibly some rework, and then the environment that they will be used in. is means that the copper must bond suf- ficiently to the epoxy in the prepreg to ensure that the board does not delaminate during either assembly or its working life. If you are using Gigabit speeds, then you will know that the roughness impacts insertion loss. e com- plex nature of electroplated surfaces means that it is far from practical to field solve the EM fields and associated losses at the surface, so empirical models are used to make predictions of loss owing to skin effect which are "good enough" for the SI engineer to work with when considering the loss budget. Chemistry suppliers are gradually improv- ing the pre-treatments in use to permit the use of ever smoother copper, which is easing this modelling situation, but most designs need to take care of roughness modelling. Design- ers new to the subject may think that they can obtain all the roughness data they need from a datasheet. However, more seasoned pro- fessionals will know that, depending on the stackup and the drill arrangements, only some of the surfaces are as in the datasheet; some will be pre-treated for bonding, and others will be plated as part of the plated through-hole pro- cess. is adds a layer of electrodeposited cop- per on the foil or core material and awareness of the stackup and the PCB fabrication process. It also adds the knowledge that maybe one fab- ricator prefers one approach over another and that for good modelling the SI engineer needs Figure 4: Surface roughness is difficult both to measure and model.

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