High-Quality Microprintable and Stretchable Conductors for High-Performance 5G Wireless Communication

Abstract

With the advent of 5G wireless and Internet of Things technologies, flexible and stretchable printed circuit boards (PCBs) should be designed to address all the specifications necessary to receive signal transmissions, maintaining the signal integrity, and providing electrical connections. Here, we propose a silver nanoparticle (AgNP)/silver nanowire (AgNW) hybrid conductor and high-quality microprinting technology for fabricating flexible and stretchable PCBs in high-performance 5G wireless communication. A simple and low-cost reverse offset printing technique using a commercial adhesive hand-roller was adapted to ensure high-resolution and excellent pattern quality. The AgNP/ AgNW micropatterns were fabricated in various line widths, from 5 μm to 5 mm. They exhibited excellent pattern qualities, such as fine line spacing, clear edge definition and outstanding pattern uniformity. After annealing via intense pulsed light irradiation, they showed outstanding electrical resistivity (15.7 μΩ cm). Moreover, they could withstand stretching up to a strain of 90% with a small change in resistance. As a demonstration of their practical application, the AgNP/AgNW micropatterns were used to fabricate 5G communication antennas that exhibited excellent wireless signal processing at operating frequencies in the C-band (4−8 GHz). Finally, a wearable sensor fabricated with these AgNP/ AgNW micropatterns could successfully detected fine finger movements in real time with excellent sensitivity.

A Guide to Printed Stretchable Conductors

“…In a composite, vertically stacked flakes or 2-D structures help in maintaining electrical conductivity with minimal electrical fluctuations during straining by sliding action . High aspect ratio structures help improve the conductivity (for a given mass loading of Ag) by forming better percolated networks. , Nanoparticles have the ability to move (rotate, slide) to preserve the percolating pathways that get distorted under strain. , Combining two different Ag particle morphologies further helps in improving the electrical and mechanical compliance under strain. , Another strategy to tune the performance is varying the volume fraction of silver fillers and additives like phosphotungstic acid (PTA) that changes the viscoelastic behavior of the composite before and after printing. Additives like PTA can form dynamic hydrogen bonds with the polymer matrix to allow spatial movement or reorganization of Ag flakes in the matrix under strain.…”

“…For reverse offset printing, a semidry film with sufficient cohesive strength is needed for reliable transfer of pattern from an ink-coated roll to the substrate. A combination of a low and a high boiling point solvents maintains the semidry state, while binders like polyacrylate ensure sufficient cohesion . For direct ink printing, the viscosity of the composite ink can be tuned by adding a lubricant, which can improve the dispersibility of Ag filler, leading to improved conductivity under strain …”

Liquid Metal Patterning and Unique Properties for Next‐Generation Soft Electronics

“…RF electronics is a kind of electronics for realizing high frequency and speed communication technology, which can be applied to various fields requiring long-distance and precise realtime signal transmission. [54,124] Utilizing LM in RF electronics makes the device soft which is expected to dedicate 5G communications, internet of things, and healthcare monitoring technology. One of challenges when fabricating RF device stems from the adhesion between substrate and conductor.…”

“…RF laminates—a group of high dielectric performance polymers used as substrates for RF circuit—are mainly used for its benefits on realizing high signal quality. [ 124 ] Unfortunately, such materials are difficult to be integrated with LM circuit due to low surface energy and roughness, which lower patterning quality and eventually leads to unreliable electrical performance. [ 54 ] Another challenge is achieving high surface smoothness of conductive path.…”

“…In 5G communication, rough conductor surface can enlengthen the conductive path and reduce the skin depth, which results in low performance of the circuit. [ 124 ] LM can be an effective solution to these challenges due to its high surface tension induced smooth surface and liquid‐originated various patterning techniques mentioned in this review. For example, Watson et al presented electrowetting‐assisted LM patterning technique which successfully realized stable LM pattern onto wetting‐unfavored substrates and controlled pattern width by tuning applied voltage while maintaining constant thickness.…”