Electrical – Routing USB3 signals

• This is determined by copper thickness, gap, dielectric constant, d and length and thus there is incremental C and L with resulting impedance , Zo for a coplanar microstrip and thus C can be computed from this. For Zo=50R it is about 3.3pF /" or 1.3pF/cm

• So compute Zo from a coplanar stripline calculator (complex) then compute C from Zo as shown here. This assumes other conductors are >10x further away. If not then you have a mesh of calculations ;)

• If you can't find a coplanar stripline tool, try this formula.

\$ C [pf/cm] = 0.12 * \dfrac{t}{w} + 0.09 * (1+d)*log_{10}(1 + \dfrac{2w}{g} + \dfrac{w}{g}^2)\$

• w = width of track [mm]
• t = thickness of track [um]
• d = dielectric constant ~ 4.2 FR4
• g = gap [mm]

hopefully, I got the units correct...you do the math on a 50R pair.

The second term dominates. Using k=3.5 (kapton), w=10 mm, s = 0.5 mm, t = 2 microns, the capacitance is 1.1 pf/cm

Reference https://www.physicsforums.com/threads/capacitance-of-coplaner-adjacent-plates.130876/

I've done a 1 mm thick board with dimensions about 3 x 15 cm. This design was noticeably more flexible than a similar 1.6 mm board, and flexible enough that I worried about the possibility of board flex causing damage to mounted components or the solder joints holding them to the board.

Given that the flexibility of a board depends on the cube of the board thickness (because it depends on the area moment of inertia of the cross-section of the board), and your proposed stackup is less than half the thickness of mine, I expect you will need to take special care to mount this board at multiple points, or provide some other stiffening mechanism, to avoid damaging the board or the parts on it.