Ask Electronics

Does anyone have any advice on routing some high speed signals on flex PCBs? I'm looking at jlcpcb because of the low cost but I'm having difficulty getting impedances to be decent.

My requirements are:

• 45 ohm single ended impedance (maybe? Def doable)
• 90 ohm differential impedance (usb spec)
• 5A current on power pins

But if we look at the capabilities: https://jlcpcb.com/capabilities/flex-pcb-capabilities

• 2 layer
• substrate thickness (PI) = 25 um
• 1 oz pour thickness = 35 um
• min trace width/spacing = 4/4 mil = 0.101/0.101 mm
• ε = 3.3

Is this feasible with this stackup? I'd like to do a 1 oz pour because of power traces, but there's also 0.5 oz (18um) and 0.33 oz (12um).

For the differential signals, when I'm doing impedance calculations, I can get to roughly 70 ohms using W=100um, S=200um. I don't think this is good enough. I think I can get away using a 0.33 oz pour but then I'm worried about the power pins.

And for the power traces, I'm needing 2.2mm, which is reasonable for the pins on a USB-C connector. But if I try using the 0.5 oz or 0.33 oz pour, it gets to be 4.2 mm and 6.3 mm, which seems impossible given the pins are tiny and very closely spaced together. Even with vias to the bottom layer, this seems problematic.

Anyone have any advice here? This is just for a hobby project, so I'm really not looking to change fabs because of costs.