Issue link: https://iconnect007.uberflip.com/i/525318
26 The PCB Design Magazine • June 2015 b) The impedance of the signal trace reduc- es as the resin flows out of the prepreg around the traces and makes the prepreg thinner. This results in the trace becoming closer to the plane. c) The edge of the PCB can have less resin than the centre (and therefore slightly differ - ent impedance) due to resin flowing out of the edge of the lamination book. The resin/glass percentage, across the entire panel, determines the impedance—the more resin the higher the impedance. d) Buildup layers—the outer most prepreg and copper—are etched last in the fabrication process so the resin does not flow around the outer layer copper traces. In this case, the trace thickness is added to the total board thickness and the impedance does not change. The stackups of Figure 4 are typical stackups for 62 mil substrates, although they may vary between PCB fabricators as they may stock dif - ferent materials. Please note the following: • The outermost dielectric is prepreg. Figure 3: Total board thickness. Figure 4: Typical stackups. (source: Advanced Circuits) STACkuP PLANNING, PART 1 continues beyond design