I. Summary of Core Research

• The focus ratio (FR), defined as the sheath gas flow rate divided by the carrier gas flow rate, is established as the core control parameter. As FR increases, the beam becomes narrower, leading to a corresponding reduction in linewidth and an increase in film thickness; when FR exceeds 8, the feature size stabilizes.
• Quantization rules for all parameters: the smaller the nozzle, the finer the line; as platform speed increases, both line width and film thickness decrease simultaneously; increasing carrier gas flow rate only increases film thickness without widening the line.
• Ultimate single-pass printing performance: With a 200-μm nozzle and optimal FR = 6 conditions, a single deposition yields 20-μm‑wide conductive silver lines, an average film thickness of 2 μm, an aspect ratio exceeding 0.1, and a resistivity only twice that of bulk silver.
• A physical model of aerosol deposition mass conservation is established to elucidate the hyperbolic trade-off between line width and line height, thereby providing theoretical support for parameter prediction.
• Define a complete and stable process window, distinguish defect‑prone regimes such as low‑speed material overflow, high‑speed wire breakage, and severe FR airflow turbulence, and establish defect‑free mass‑production standard parameters. High‑precision grids and helical electrodes can be directly printed on silicon wafers, PI flexible films, and substrates with uneven topographies.

II. Standard Preparation Procedure for Aerosol Silver Conductive Tracks

(1) Complete Preparation Procedure

• Commercial 8–15 nm silver nanoparticle ink formulated with terpineol as a co‑solvent, maintaining a stable atomization state in a constant‑temperature 14°C water bath.
• Silicon/polyimide substrate cleaning, without the need for plasma pretreatment, exhibits excellent wettability and compatibility.
• Optomec M3D aerosol device, with ultrasonic atomization at a constant current of 0.6 mA and a fixed nozzle-to-substrate distance of 2 mm;
• Adjust the sheath gas, carrier gas, and stage speed, and perform single‑channel direct‑write of conductive patterns according to the focus‑ratio‑matched parameters.
• Nanoparticle sintering is completed by holding at 200°C in an oven for 1 hour.
• Profile meter, optical microscope, and four-point probe are used to characterize line width, film thickness, and resistivity per unit length.

(II) Five Core Advantages of Aerosol Technology (Comparison with Inkjet and Screen Printing)

• Single-pass fabrication of high-aspect-ratio fine lines, eliminating the need for multiple overprinting.

Traditional multi-layer wiring techniques inevitably result in wider traces; this solution employs a single‑step, narrow yet thick silver‑plated conductive trace, ensuring high integration and a large current‑carrying cross‑section.

• Focus on precision over precise shaping; dimensions are predictable and adjustable.

By precisely controlling the airflow ratio, micron-scale patterns can be accurately tailored without the need to change inks or masks, resulting in exceptionally high parameter‑iteration efficiency.

• Compatible with rigid silicon, flexible PI, and textured substrates.

Non-contact aerosol deposition enables continuous wiring on patterned substrates with trenches, making it well-suited for MEMS three-dimensional microdevices.

• Wide speed processing range, with a maximum printing speed of 100 mm/s.

It accommodates both small-batch lab prototyping and high-speed roll-to-roll mass-production lines, with linear and controllable resistance.

• Low material waste, localized additive deposition

Only the circuit areas are coated with silver ink, significantly reducing precious-metal consumption in large‑area coating processes and keeping R&D costs under control.

III. Five Key Technological Innovations

Innovation 1: Propose quantifiable regulatory indicators that focus on the FR core.

Innovation 2: A Physical Model of Mass Conservation in Forming Processes

Innovation 3: A complete, reusable, and stable process operating window

Innovation: 4‑channel printing achieves silver wires with a high aspect ratio exceeding 20 μm.

Innovation: A Universal Parameter System for Multiple Substrates

IV. Current Industry Challenges and Corresponding Solutions for Overcoming Them

Challenge 1: Conventional printing struggles to simultaneously achieve narrow line widths and high film thicknesses, leading to degraded overlay accuracy in multilayer stacking.

Challenge 2: Aerosol process parameters are numerous, and tuning relies entirely on experience, with no unified quantitative standards.

Challenge 3: With the increased printing speed, the conductive layer becomes thinner, leading to a significant rise in resistance.

Challenge 4: High mold cavity pressure can easily lead to overflow and edge‑blurring defects.

Challenge 5: Lack of a standardized mass-production process window, leading to significant variations in tuning outcomes across different equipment.

V. Future Diversified Industrial Application Scenarios

1 Flexible Foldable-Screen Micro-Drive Circuit

2 MEMS surface acoustic wave and pressure sensors

3 Flexible Circuits for Wearable Physiological Sensing

4 Micro‑photovoltaics, thin‑film transistor arrays

5 Laboratory Platform for Printed Electronics R&D

Figure 1. Schematic diagram of an aerosol jet printing head, illustrating the aerodynamic focusing effect achieved by the sheath gas inside the print head.

Figure 2. Cross-sectional profile of the printed silver line obtained by surface profilometry. The data indicate an average thickness of 2.41 μm; the width measured under an optical microscope is 45 μm.

Figure 3. Schematic illustrations of various printed silver patterns: (a) printed pads and traces on a flexible polyimide substrate for electrical characterization; (b) line‑shaped morphology images of lines printed on a silicon substrate with etched grooves 10 μm deep and slightly tapered walls; (c) printed spiral pattern on a polyimide substrate; (d) lines with a pitch of 30 μm and a width of 40 μm; and the University of Minnesota logo printed at focus ratios of (e) 3, (f) 5, and (g) 7.

Figure 4. (a) Effect of focus ratio on line width and thickness. (b) Two-dimensional profile measurement data of printed lines for focus ratios ranging from 1 to 7. The carrier gas flow rate and stage travel speed were fixed at 20 sccm and 2 mm/s, respectively.

Figure 5. Effect of nozzle size on (a) line width and (b) thickness. The carrier gas flow rate and the sample introduction speed were kept constant at 16 sccm and 2 mm/s, respectively.

Figure 6. Effect of carrier gas flow rate on (a) linewidth and (b) thickness at different focusing ratios. The stage velocity used was 2 mm/s.

Figure 7. Effect of stage velocity on (a) line width and (b) line thickness at different focus ratios. The carrier gas flow rate was held constant at 24 sccm.

Table 1. Independent Effects of Each Process Variable on the Geometry of Printed Circuit Patterns

Figure 8. Electrical properties of printed traces. (a) Variation of the resistance per unit length as a function of the focusing ratio and the carrier gas flow rate (stage speed = 2 mm/s). (b) Effect of the stage speed on the resistance per unit length (carrier gas flow rate = 40 sccm, FR = 3).

Figure 9. Magnified view of the nozzle, illustrating the physical characteristics of the aerosol jet as it exits the nozzle and contacts the moving substrate to form printed lines. Following impact, the aerosol stream coalesces, and the carrier gas is expelled.

Figure 10. Based on the mass conservation equation, this schematic illustrates how the line width and thickness vary with the focusing ratio under different carrier gas flow rates (at a fixed phase velocity) or different phase velocities (at a fixed carrier gas flow rate). The values on the axes represent estimated dimensions of the printed lines prior to drying and sintering.

Figure 11. Over-spraying phenomenon. (a) Optical microscopy image of the over-sprayed regions on either side of the printed and sintered silver lines on the silicon wafer; (b) Scanning electron microscopy (SEM) image of the same area, showing that the over-spraying manifests as clusters of fine sintered silver nanoparticles.

Figure 12. Operable window and ranges of process parameters. (a) Lower and upper limits corresponding to each carrier gas flow rate at a fixed focus ratio (FR = 3); (b) the operable window at a specified carrier gas flow rate (24 sccm), with the high‑and‑narrow line region highlighted. Optical microscopy images show: (c) blurred edge lines printed at an extremely low stage speed (1 mm/s); (d) discontinuous lines printed at an extremely high stage speed (40 mm/s); (e) incoherent lines produced when the focus ratio is less than 1; and (f) lines printed under optimized conditions. Each bar represents 50 μm.

Figure 13. Characterization of sintered silver lines printed under optimized conditions. SEM micrographs show: (a) a 20-μm-wide silver line grid fabricated at a carrier gas flow rate of 24 sccm, FR = 6, and a stage speed of 5 mm/s; (b) a surface morphology image revealing the sintered silver nanoparticle structure.