All About Flex: Considerations for Impedance Control in Flexible Circuits

Estimated reading time: 4 minutes

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.

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