In the previous blog on PCB Insertion Loss and High-Speed Buses, we described how this loss increases with greater link speed. Designing for higher link speeds may, as a consequence, drive up PCB cost. This blog explains some techniques to mitigate this cost increase.
In PCB Insertion Loss and High-Speed Buses, four main PCB approaches to addressing higher insertion loss were described:
- Using wider traces
- Providing for looser coupling (that is, greater spacing between traces)
- Delivering thicker dielectrics (more laminate/prepreg layers and thicker layers)
- Lowering the PCB material dielectric constant
The first two items tend to decrease routing density, which in turn usually requires more board layers, which can drive board cost up. The use of thicker dielectrics also increases cost. And lowering the board dielectric constant implies moving to more exotic materials, which is also more expensive.
Moving from single stripline to dual-stripline routing can help reduce PCB cost and allow for highly-dense board form factors. But, this comes at the expense of broadside coupling and additional crosstalk between signals, which has an impact on signal integrity. A graphical representation of single stripline versus dual stripline looks like this:
Field solvers will determine the stackup needed to achieve a given insertion loss. Violating these stackup guidelines is risky because of the chance of reduced system stability due to enhanced crosstalk. There are design practices, however, which can mitigate this risk, among them angled routing and zigzag routing.
Minimizing crosstalk between two differential pairs, a differential pair and a single-ended trace, or two single-ended traces that cross over each other on adjacent layers requires that the angle should be no less than 30 degrees and no more than 150 degrees:
Inter-layer crosstalk is exacerbated by dual striplines running parallel to each other, especially near pin fields, where it is not easily avoided. Follow the Angled Routing rules above for each cross-over: