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Advanced polyimides for more flexible high-speed 5G networks

February 11, 2019

Advanced polyimides for more flexible high-speed 5G networks

High-frequency, 5G radio-wave communication will augment today’s 4G networks and enable faster data downloads for end users along with quick response times for autonomous systems. As the communications infrastructure transitions from 4G to 5G networks, the materials in the associated hardware need to adapt to enable efficient, high-frequency, high-speed data transmission. Materials make a difference in the performance of various electronic components, and the best choices are not always the most intuitive ones.

The 5G network poses constraints on existing hardware that reach beyond the need to transmit large packets of data at high speeds. The multi-GHz transmission frequency of 5G limits the effective wireless transmission distance to a few thousand feet. This limitation, coupled with increased attenuation from walls, trees, and other obstacles, requires many individual antennas and small cells to provide effective, reliable coverage. Some of these new devices may be embedded in homes, streetlights, or cars, where space is at a premium and form factors and reliability impose stringent design restrictions.

Flexible substrate materials have brought advanced electronic circuits into products ranging from smartphones to medical devices. The ability of flex circuits to accommodate tight bending radii and eliminate the need for cables and connectors gives designers greater flexibility and enables products that would not otherwise be possible.

Flex circuits do not, however, immediately come to mind for high-frequency applications, where a low loss tangent is one of the most critical performance metrics. When the application requires a flexible circuit design, designers often select liquid crystal polymers (LCPs) because the dielectric properties of the films outperform those of many polyimides, leading designers to assume that LCPs are an obvious choice for 5G. This is not necessarily true. Not all polyimides used for flex circuits are alike, and the specific choice of material is critical. Moreover, the electrical performance of the complete flexible copper clad laminate (FCCL) should be considered when designing high-frequency flexible circuits.

DuPont’s Pyralux® AP, for example, exhibits electrical properties comparable to LCPs. The loss tangent of Pyralux® AP falls consistently between 0.002 to 0.003 when tested across a wide range of frequencies up to 60 GHz, making it a good choice for 5G mm-wave applications. Near the critical frequencies of 28 and 39 GHz that are common for small cells and beam formers, Pyralux® AP slightly outperforms LCPs (see Figure 1).

The total loss in the system is the sum effect from both the dielectric and the conductor (usually copper). For flexible circuits designed to operate at high frequency, the conductor losses contribute significantly to the overall dielectric performance of the chosen FCCL system.

The roughness of the copper in FCCLs plays a critical role. The smoother the copper surface, the lower the insertion loss. Switching from relatively rough electrodeposited (ED) copper to smoother rolled annealed (RA) copper improves performance up to 50%, and the effect is more pronounced the higher the operating frequency (see Figure 2). For copper lines with the same roughness, geometry makes a difference as well. Thicker, wider traces exhibit less loss because of reduced surface resistivity.

Figure 2. Comparison of insertion loss for microstrip test vehicles with electrodeposited and rolled annealed copper metallization of various thickness 


For FCCL applications that also require high reliability, proper adhesion between the copper and the laminate is critical. Fortunately, Pyralux® AP adheres very well to copper, so that even flex circuits made with RA copper exhibit excellent adhesion.

Pyralux® AP, when combined with high-performance RA copper, creates an FCCL that meets all the electrical requirements of high-frequency applications, making it an ideal flex circuit option in the race to 5G.


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