High TG Flex PCB for Motor Control

Flex PCB Controlled Impedance

I am absolutely new to flex PCBs. I saw some from time to time but never designed one myself.

Now I'd like to get into this topic and the first task would be to create a ~ 10cm board to board interconnect flex PCB with 90 ohm (USB) and 100 ohm (LVDS) controlled impedance traces on it.

I used the KiCAD PCB calculator, asked the PCB manufacturer about the tangent loss (0.01) and dielectric constant (3.3) to calculate trace width and spacing, as I am used to from designing standard PCBs with FR4 material.

I realized pretty quickly that the trace widths and spacing I'd need to achieve e.g. 45 ohm differential would be way out of spec.

I assume that my assumptions are completely wrong. How do you guys do that? Just ignore the impedance of the flex PCB?

Answers

Answer 1

If you need impedance control, don't use such thin core. You can request them to make what you need.

From the PCB Way page you have linked, at the top: "The following is the usual FPC stackup information. If you need custom FPC stackup, please make note and we will manufacture according to your requirement. Polyimide (PI) is the most commonly used thermal curing insulating material in flexible circuit processing. The thickness range of the material is generally 12.5μm (0.5mil) and 125μm (5mil). Divided into with glue and without glue, the DK with glue is 3.5, and the DK without glue is 3.3."

In the past, I have used 0.075mm core for USB flex.

Comments

• thanks for your answer. I just calculated your example with the KiCAD PCB Calculator, using 75 um core, 0.25mm track width and 0.25 mm spacing and the result is Zodd=~19 Ohm (=> Zdiff=~38 Ohm). Does this mean you simply ignored the impedance requirement for USB and it worked anyway, because the frequency is relatively low (12 Mhz)?
• No, do not ignore the impedance requirement. I use Allium for impedance calculations. 0.25/0.25mm seems wrong with this stack-up. 75um core, differential 90R (USB). I get 0.125mm track 0.2mm gap. If your supplier can't make this work, have go with something even thicker then. This was just an example from a real-world design that works at USB 3.0 speeds.
• ok, that helped! looks like I can create 90 Ohm and 100 Ohm (both differential) with 100 um core, 0.25mm width and 0.3mm spacing. I guess I start understanding that topic ...

Answer 2

I'm also learning about flex and impedance control. I've asked jlcpcb about their capabilities.