The three critical factors for impedance and signal performance.

As demand for higher-resolution video, cloud computing, and other features drive data use higher, designers are pressed to find interconnect solutions for higher operating frequencies and reduced power consumption for increasingly thinner devices. I’m often asked, “Why are we having trouble designing a flex circuit to meet our impedance and signal performance targets?”

My answer always comes back to a combination of three critical factors: dielectric properties, copper roughness and circuit design. Here are a few key questions and considerations when designing a mass-producible, high-frequency flexible circuit:

What characteristics of the dielectrics determine high-frequency performance? What choices of materials do I have? The two main characteristics of dielectrics that impact signal performance are dielectric constant (Dk) and dissipation factor (DF). These factors allow you to achieve your desired dielectric thickness and meet a target impedance value. They help you choose materials that provide design options in copper trace width for higher-yield manufacturing. Finally, these factors predict whether the energy loss of the signal over the trace length given the operating frequency will be acceptable for the application.

Typical properties of a polyimide of Dk 3.2–3.4 and Df 0.001–0.009 often do not meet the performance requirements for insertion loss at higher frequencies. Designers and flex manufacturers are therefore evaluating several new materials options designed specifically for high-speed applications. Dielectric materials such as liquid crystal polymers (LCP) and fluoropolymers (PTFE) with lower Dk and Df values provide superior signal integrity performance, while maintaining more manufacturable line widths.

Matching your material choice and design to your specific application is a key factor in maximizing benefit to cost, as material prices and different processing requirements, such as higher temperature lamination, should be considered in advance. By modeling the Dk and Df characteristics by application, we make effective trade-offs between the different material sets on the market, and make cost-effective choices that achieve the desired performance objectives.

What role does copper roughness play in high-frequency performance? At high frequencies, electrons travel along the outer surface, or “skin depth” of the conductor, rather than through the entire volume of copper trace. To complicate matters, skin depth varies by the frequency of the signal. The calculated skin depth is 2.09µm at 1GHz compared to 0.93µm at 5GHz and 0.54 µm at 15GHz, which means that surface variations in the copper traces can have serious consequences on the performance of the circuit. This presents a problem because copper roughness is a desired surface-variation feature that acts as a grip for adhesives to bond the copper foil to the dielectric. Smooth copper with lower roughness is desired for higher frequency applications, which adversely impact the copper foil adhesion to the dielectric. The flex manufacturer should therefore optimize the manufacturing process for the appropriate copper foil material and roughness to meet the high-frequency requirements.

When designing high-frequency applications, what should I pay special attention to? Signal performance can be affected by a few key features and specifications. In particular, consider routing impedance-controlled circuits on a single layer, and design impedance reference planes with consistent dielectric thicknesses and uniform coverage over the traces. Other design specifications should include copper trace width and thickness control; however, the trace quality ultimately comes down to process control.

How do I know this will work? Figure 1 shows insertion loss data between an optimized circuit and a non-optimized circuit with identical circuit routing. Usable bandwidth increased from 5GHz to 10GHz simply by using a lower Dk and Df dielectric material and appropriate copper foil with reduced roughness.

Collaborative effort between the designer and flex manufacturer is critical for designs requiring high speed. Electrical simulation tools should be effectively utilized to model circuit performance to identify dielectric materials and thicknesses, circuit geometries and copper roughness. Manufacturing should commence after designer and manufacturer have confidence in the electrical simulation results. The flex manufacturer should optimize material selection and process parameters to obtain the desired results and correlate actual measurements to the electrical simulation.

Jay Desai is director, applications technology, at MFLX (mflex.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .