Hatched ground plane
Problems with thin flexible PCB
- The base polyimide dielectric core has a thickness of 25um.
- This means that we need extremely thin traces with a solid ground plane if we want to reach our PCIe impedance target.
- Thicker dielectric cores are provided but they increase cost substantially and trace width barely increases.
- Even worse the calculated transmission line trace width is calculated as 0.05mm which is below JLCPCB’s manufacturing minimums.
Hatched ground plane
- Hatched ground plane can allow for wider traces while matching impedance.
- No easy equation to get impedance measurement (fill factor approximation does not model this adequately).
- Design of hatched ground plane must meet manufacturing capabilities of JLCPCB.
- Requires parametric search with simulation software.
- Refer to this section about simulating circuits with openEMS.
Determining parameters
There are three major parameters to consider:
- Trace width.
- Hatch width.
- Hatch gap.
Since we need to be above the minimum trace width for JLCPCB to manufacture it, we should select a fixed trace width of 0.1mm.
- This is similar the trace width of 0.13mm for our transmission line on the FR4 substrate for both the M.2 cad and Oculink port board.
- This means we will have an easier time designing a taper geometry when connecting the flex connector to our boards (discussed here).
- Means we only need to perform a parametric search with two variables (the hatch width and gap) which is less time consuming.
Additional design considerations
Refer to minimising intra-pair skew for hatched ground planes and differential pairs.