All About Flex: Considerations for Impedance Control in Flexible Circuits

Estimated reading time: 4 minutes

Impedance can be thought of as a system’s opposition to alternating or pulsing electronic current. The unit of measurement is ohms, the same unit of measurement in a direct current system. However, the components for calculating impedance are much more complex than DC resistance. For a direct current system, the resistance is related to the relative ease with which electrons can flow through the material.  Ohm’s law describes a fairly straightforward relationship between current and voltage (V=IR or R=V/I) where R is a constant number for any given material. Impedance is characterized by the equation including the DC resistance but also includes another component called reactance. Reactance is the ability of the system to store and release energy as current or voltage alternates. The equation for impedance is Z=R +iX, where iX is the reactance component. The reactance is a function of the capacitance of the system and the frequency of the alternating or pulsing current.

Why is impedance important?

Impedance is important for high-speed electronics. When frequencies become 200 MHz or higher, the impedance and impedance consistency becomes a significant factor in the system performance. During the last 20 years, electronic packages have become smaller, denser and faster. It is estimated that in 2000, only a small percentage of PCB and flexible printed circuit (FPCB) designs had an impedance requirement. As higher and higher frequencies continue their relentless march, impedance requirements today have become much more prevalent and important.

In a direct current system, when two components of different resistance are connected in series, the system resistance is simply equal to the two components added together (R1 +R2). The flow of the electrons is homogenous. The analogy is a garden hose where the flow of the water is the same throughout the hose.

In high-speed electronics, impedance does NOT behave the same way. High speed signals are like separate pulses propagating through the system. The current and magnetic pulses are affected by the impedance. When the pulses encounter a node of mismatched impedance, a flux of energy is induced which creates competing signals that can interfere with the main signal. The result is power loss and distortion of the signal. 

Many nodes of mismatched impedance can occur within a PCB system as attached components, conductor width, conductor spacing and dielectric thicknesses change. One way to deal with this issue is to isolate the signal traces so that the dielectric and geometries are identical throughout the signal path. This is called controlled impedance. In flexible circuits, there are two categories of designs that are typically used for controlled impedance: microstrip and stripline (Figure 1). Within the categories one can have single-ended transmission lines and differential pair transmission lines.

Figure 1: Designs for controlled impedance.

In both designs, the impedance is affected by the following:

• Dielectric constant (Dk) of the materials
• The DC resistance of the signal line
• Distance between the signal lines and ground planes or signal line pairs

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