Issue link: https://iconnect007.uberflip.com/i/1285883
SEPTEMBER 2020 I DESIGN007 MAGAZINE 29 mils in Figure 3) if necessary. Close coupling will provide the most routing space. 4. Trace Coupling Although trace coupling is not strictly speak- ing an impedance variable, it should be con- sidered during the planning stage. The combi- nation of trace clearance and dielectric thick- ness provides the point of coupling (12 mils, in the case of Figure 4). At this point, differential pairs are no longer coupled, and individual parallel traces become vulnerable to crosstalk. Use the point of coupling to define the cross- talk trace clearance constraints. 5. Dielectric Thickness The main considerations for dielectric thick- ness (Figure 5) include: a. All signal layers should be adjacent to and closely coupled to an uninterrupted reference plane. This provides the shortest loop area and the lowest inductance for the return current path. b. The minimum dielectric thickness is determined by the selected material glass style. c. A wider weave fiberglass may cause skew in differential pairs at high frequency (>10 GHz), converting differential mode current to common mode— hence radiation. Glass weave skew and pile unevenness can be prevented by using two plies of 1067 glass prepreg combined. Do not use 106 or other wide gap glass styles. 1067 glass is typically 2.5–3 mils. With regard to stripline (inner layers), the thinnest dielectric should be closest to the GND plane return path. In a dual stripline con- figuration, the dielectrics closest to the planes should be thin, and the dielectric between the signal traces should be thickest, which also reduces broadside crosstalk. Figure 3: Impedance vs. trace clearance. Figure 4: Impedance vs. coupling. Figure 5: Impedance vs. dielectric thickness.