Watch Lee Ritchey of Speeding Edge as he discusses using low-Dk materials in high-speed digital designs.
The merits of using low DK (dielectric constant) laminates in PCBs that have very high data rate paths:
- For a given trace width selected to produce a given impedance, the laminate used between the trace and its adjacent plane can be thinner. This results in an overall thinner PCB.
- For a given thickness of laminate used to produce a given impedance, with a lower Dk laminate the trace width can be wider resulting in a lower signal loss due to skin effect loss in the trace.
- For larger, high layer count backplanes, using a lower Dk laminate can reduce overall board thickness.
The drawbacks of using low Dk laminates:
- Lower Dk laminates cost more and provide less flexibility in terms of material choices. In some instances, using a lower Dk laminate will create a situation wherein the material used is only available as a single-source.
- In terms of loss vs trace width, a much greater improvement in loss is achieved by using a lower loss laminate than by increasing trace width. Also, increasing trace width increases over board thickness. Example: a 22-layer board with 4 mil wide stripline traces produces a board that is 106 mils thick. Increasing the stripline traces to 8 mil wide produces a board that is 160 mils thick—a 60% increase in overall board thickness.
Conclusions: increasing trace widths does not provide enough of a benefit in loss to warrant the use of a low Dk laminate. Use of a lower loss laminate, such as Isola’s Tachyon, yields a much better improvement in terms of loss. In addition, there is 60% less material used which can pay for the added cost of a lower loss laminate.