I. Core Research Summary

• Low-temperature densification of reactive silver ink yields electrical conductivity approaching that of bulk silver.

At 80°C, the low‑temperature substrate undergoes in‑situ decomposition to form a film with no significant particle gaps or porosity defects. The film’s effective electrical conductivity can exceed 70% of that of bulk silver, substantially reducing scattering losses associated with high‑frequency skin effects. The printing thickness of a single layer is 3 μm, and stacking two layers yields a total thickness of 6 μm.

• 10 GHz with ultra-low insertion loss, performance comparable to etched copper traces.

On the FR substrate, the per‑unit insertion loss at 10 GHz is only 0.038 dB/mm, comparable to the 0.033 dB/mm of conventional etched copper; across the entire 2–18 GHz frequency range, the loss variation remains stable and well‑controlled, effectively addressing the longstanding issue of high‑frequency performance degradation in additive‑manufactured RF circuits.

• The surface morphology of the substrate is a key factor influencing high-frequency loss.

On smooth FR4 substrates, the printed traces exhibit uniform impedance, with slight losses that increase linearly along the trace length. In contrast, on the woven‑texture TLX9 substrate, the single‑layer silver film shows thickness variations and edge notches, leading to abnormal loss characteristics. By contrast, a double‑layer 6-μm‑thick silver coating effectively evens out the substrate’s surface texture, eliminating distortion in the loss profile.

• Dual‑line THRU for de-embedding, enabling precise separation of substrate, fixture, and conductor losses.

A standardized RF test de‑embedding model has been established to eliminate the influence of connectors, transition sections, and dielectric losses, thereby isolating the intrinsic transmission characteristics of aerosol‑silver interconnects. The resulting data are of reference value for industry benchmarking.

• Digital Maskless 3D RF Freeform Fabrication

Eliminating the need for photolithographic masks, this technology enables direct‑write fabrication of curved, stepped, and multi‑layer heterogeneous RF structures, overcoming the conventional limitation of etching to planar processing and supporting the rapid development of lightweight aerospace RF components.

II. Complete Preparation Process and Process Advantages of Aerosol Sprays

(1) Complete RF Microstrip Printing Process

• Plasma cleaning of FR4/TLX9 RF substrates to remove surface oils and contaminants.
• The Optomec AJ300 is equipped with a 100 μm nozzle and uses ultrasonic atomization to process reactive silver ink.
• The substrate is maintained at a constant temperature of 80°C, and 3-μm microstrip lines are printed in ten stacked layers, with additional layers added as needed up to 20 layers (resulting in a 6-μm thickened film).
• Low-temperature in-situ reactive curing, eliminating the need for high-temperature sintering and causing no damage to the low-loss PTFE substrate.
• With standard copper pads and a ground plane, the vector network analyzer performs sweep‑frequency measurements from 2 to 18 GHz.

(II) Five Core Process Advantages (Comparison of Photolithography Copper vs. Inkjet Silver)

• The reactive silver ink is dense and free of pores, with extremely low high-frequency loss.

Unlike conventional nano-silver inks, which exhibit particle agglomeration and numerous pores, this formulation enables the in-situ formation of a continuous, dense metallic layer, significantly reducing millimeter-wave scattering loss at 18 GHz and meeting the stringent requirements of advanced radar systems.

• Low-temperature molding is compatible with various RF dielectric substrates.

Cured gently at 80°C, PTFE low‑loss laminates such as TLX9 do not deform under high temperatures, whereas photolithographic etching and high‑temperature sintering processes can damage flexible microwave substrates.

• Unique advantages of conformal printing for 3D curved surfaces and elevation differences

Photolithography is limited to planar surfaces, whereas aerosol jet printing—with a safe nozzle‑to‑substrate distance of 1–5 mm—enables direct writing of RF circuits on curved and stepped chip surfaces, enabling the integration of RF modules into a single monolithic structure.

• Digital rapid iteration, no need for pattern-making or mold creation.

By adjusting the width and length of CAD microstrip lines, we can produce a prototype on the same day, reducing the R&D cycle for radar antennas and filters from several weeks to just one day.

• Precise Localized Deposition of Precious Metal Consumables

Only the RF signal‑related areas are silver‑plated, eliminating the need for full‑area copper or silver plating, thereby achieving weight reduction while lowering the manufacturing cost of RF circuit boards.

III. Five Key Technological Innovations

Innovation 1: An aerosol–reactive silver ink combination enables low-loss millimeter-wave conductors.

Innovation 2: Substrate Texture—Mechanism for Controlling Coupling Losses as a Function of Film Thickness

Innovation: A Three-Channel Dual-Line De-embedding Standardized RF Characterization Framework

Innovative 4-temperature in-situ metallization forming process

Innovative 5‑D planar and 3‑D integrated RF manufacturing roadmap

IV. Current Industry Challenges and Corresponding Solutions for Overcoming Them

Challenge 1: Conventional nano-silver printed films have numerous pores, leading to a sharp increase in millimeter-wave loss above 10 GHz.

Challenge 2: The woven PTFE microwave substrate has an uneven surface, resulting in gaps in the single-layer silver traces and non-uniform thickness.

Challenge 3: Interference from RF test connectors and transition sections prevents the independent assessment of the conductor’s true loss.

Challenge 4: Conventional etching processes are limited to planar surfaces and cannot fabricate curved surfaces or stepped RF structures.

Challenge 5: Long photolithography mask‑making cycle times and high iteration costs for radar filters and antennas.

V. Future Diversified Industrial Application Scenarios

• Airborne/Satellite Lightweight RF Components

• Automotive 77 GHz millimeter-wave radar module

• 5G/6G RF Front-End and Small Cells

• High-frequency microwave devices for medical imaging

• Rapid Prototyping Platform for University RF Laboratories

Figure 1. Circuit board design, dimensions, and modeling approach employed in this study. (a) Schematic of microstrip elements on an FR4 substrate; (b) Microphotograph showing three sets of copper‑clad layers and printed silver microstrip structures; (c) Microscopic images of the copper‑clad layers and printed silver traces. The silver microstrips are formed by overlapping adjacent sub‑traces until the desired width is achieved; (d) Data‑processing workflow using a computational model to extract the circuit board’s unknown parameters, including insertion loss and the complex dielectric constant components.

Figure 2. RF S‑parameter measurement data for FR4 board test samples. S11 represents the reflection characteristics of the microstrip line, while S12 denotes its transmission characteristics. For each set of microstrip line length combinations, the printed silver layer exhibits a slope offset of 0.2 dB/GHz relative to the etched copper layer in the raw measurement data (this offset accounts for fixture effects).

Table 1. Plate Material Properties and Device Dimensions

Figure 3 shows the relationship between the CST Studio parameter‑scan results and the measured S12 data for a 6‑inch silver microstrip. To generate curves of different colors, the model was adjusted to account for surface roughness and electrical conductivity. Each color band corresponds to a specific surface‑roughness value and is defined by the associated percentage of relative conductivity: the lower limit of each band is 20% of the bulk conductivity of silver, while the upper limit is 100%.

Figure 4. Characteristic parameters extracted from CST Studio simulations. The dielectric constant and loss tangent of the FR4 substrate were simulated across the entire frequency range and fitted to the known values listed in the substrate’s datasheet; the resulting fit accounts for these properties, yielding the material’s insertion loss at various frequencies.

Table 2. Summary of Insertion and Deletion Values in the Main Report

Figure 5. Effect of metal thickness on RF signal attenuation in the TLX9 board. When only 10 layers are printed (3 µm), a loss‑inversion phenomenon occurs: as the microstrip line length decreases, the insertion loss initially increases. However, after increasing the number of layers to 10, surface effects that previously caused instability in the signal characteristics begin to be mitigated. At 20 layers (6 µm), the insertion losses of the three microstrip lines become essentially identical, and the overall loss is lower than that observed for the 10‑layer configuration.

Figure 6 shows microscopic images of the surface roughness of a TLX‑9 silver‑halide photo with an initial thickness of 3 µm. The “grooves” in the 6‑inch track (left) and the “ripples” in the 8‑inch track (center) both correspond to the weave pattern of the printing plate, indicating that the surface requires further treatment to achieve a smoother photographic finish. The image on the far right depicts a 10‑inch printing track with improved uniformity, featuring sharp, well‑defined edges and a smooth surface.