In some special devices & equipment, esp. in communication, signal processing & transferring, transmission lines and cable inductance, RF products, and so on, a fixed impedance value will be need. Under this situation, designer will need to control the impedance in Flexible PCB (flex printed circuit board) or Rigid PCB, such as 50 ohm or 100 ohms are commonly used.

Here we will discuss focused on Flex Circuit. Normally the impedance is decided by following elements:

- Dielectric Constant of Material. (er)

Material is referred to core base such as Polyimide (PI) and PET (seldom used in FPC industry) and PP in PCB. The common value for Polyimide is from 3.5~4.0, and special one can be 5.0 and so on. - Dielectric Thickness (mm) (H)
- Trace Width (mm) (W, or W1 and W2 if need more precision result)
- Finished Copper Weight (mm) (T) (1oz. = 1.4 mils = 0.035 mm)
- Trace Spacing (mm) (S)

For examples, for single layer flex circuits, if the material dielectric constant is 3.7, thickness is 0.1mm, use 1/2 OZ (0.018mm) copper, both trace width and space are 0.15mm, and then you get can 100 ohms impedance value for that circuit.

You can find more information across the internet and follows are useful information links:

*Cable
impedance* (richful information about flex cable impedance)

*Eddy
Current Inspection Formula - Impedance* which has visual figure so that you can
understand it more clearly.

*Impedance*
(A PDF file includes near all related information for impedance)

*PCB
Impedance and Capacitance Calculator* (You can calculte impedance for seven
different structures)

*PCB
Impedance Calculator* (Another tool, can be used for almost all types of structures)

Currently we use the most famous impedance tool: Polar Si8000, to calculate the impedance value, using our different materail to get a suitable value for customer.

Also we have a simple tooling (in excel format) to get the similar purpose. You only need to input value of each element and then you can get the result. We always try to be your best partner in Flex PCP/FFC/PCB, so please contact us today and we can send this useful tool to you, without any fee, only an email! Total freely!