All About Flex: Considerations for Impedance Control in Flexible Circuits
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
When designing a flexible circuit, the designer typically has the following parameters to work with in meeting impedance targets:
- Type of dielectric materials
- Thickness of dielectric materials
- Copper thickness
- Copper width and spacing
With these multiple factors affecting performance, the designer will have tradeoffs and limitations to consider. If cost is critical, it may be necessary to avoid specialty materials that might otherwise be attractive because of their low dielectric constant (Dk). Dielectric thicknesses can be adjusted upward and are often a desirable way to allow more manageable trace widths during fabrication, however thicker dielectrics can compromise flexibility as well as impact the overall space available. Reducing the copper profile (thickness and width) is an option, but the manufacturer’s fabrication capability is compromised as lines and spaces are reduced to the point of significantly affecting yields.
The more uniform and smooth the copper surface, the more ideal for impedance control, as the high speed signals travel on the surface of the conductors. Therefore, copper type and fabrication methods can be factors in meeting impedance targets. There are two basic copper types used for flexible circuits, electro-deposited (ED) and rolled annealed (RA). The topography of ED copper is granular and relatively rough. RA copper is often specified as it has a smoother surface, which becomes more important as signal frequencies increase.
The dielectric constant performs best if it is homogenous, but that does not prevent the use more than one material. Different layers of dielectric can be bonded together to create a dielectric constant. As long as this layering is consistent through the signal plane, the overall Dk remains consistent. Polyimide is the most common material for the core dielectric, but this can be wrapped with a bonding film and be comprised of adhesives or extruded materials like Teflon®. Adhesiveless materials have become popular in high-speed applications with their homogenous dielectric constant.
Does temperature affect impedance?
The biggest concern with impedance and operating temperature is the copper. Copper’s DC resistance changes as the temperature increases. The value is about 0.4% per degree C, so if the temperature of the copper changes 10°, the R changes 4%. If the temperature of the copper changes 100°, then the resistance changes by 40%! With narrow conductors in a flex circuit (this is generally the case in high speed circuitry) the DC resistance can be quite significant. A change to the DC resistance will travel along with any RF signal. This is something that should always be considered when dealing with higher temperatures. The dielectric can also contribute, but the copper effect is generally a much bigger change than any dielectric temperature effect.
Ultimately the impedance tolerance will depend on the design, materials used and process capability of the fabricator. The best solution to deal with impedance control is to take the time to review all the performance requirements with the flexible circuit fabricator using a robust design for manufacturing process.
Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.