I. Core Principle of the Process

II. Schematic Diagram of the Core Process

Substrate Pretreatment

1. Substrate Selection

   Select suitable substrates: common flexible substrates (PET, PI films) and rigid substrates (glass, ceramics, silicon wafers). Substrates must meet requirements of high temperature resistance (for the sintering process) and surface flatness.

2. Key Surface Treatment Steps
   • Cleaning: Plasma cleaning, alcohol wiping, or ultrasonic cleaning is adopted to remove impurities such as oil stains and dust (impurities can cause reduced film adhesion and pinhole defects).
   • Activation: Flexible substrates require corona treatment to increase surface tension (≥38 mN/m). Rigid substrates can undergo sandblasting or coating pretreatment to enhance paste adhesion.

Screen Preparation

1. Screen Material Selection
   • Polyester mesh: Solvent-resistant and low-cost, suitable for patterns with a fineness ≤50 μm.
   • Nylon mesh: Good elasticity, suitable for curved surface printing.
   • Stainless steel mesh: Preferred for high-precision patterns, scratch-resistant, with a fineness of up to 20 μm.
2. Mesh Count Matching

   Film line width is inversely proportional to mesh count. A conventional range of 300–600 mesh is selected (300–400 mesh for line widths of 50–100 μm, 500–600 mesh for line widths of 20–50 μm).

3. Photosensitive Emulsion Coating and Exposure

   Coating thickness is 0.5–2 μm (affecting film thickness). Exposure energy must be precisely controlled (100–300 mJ/cm²) to ensure clear, non-serrated pattern edges. Residual photosensitive emulsion must be thoroughly rinsed off after development.

Metal Paste Preparation

1. Core Components
   • Metal powder: Particle size of 1–5 μm (e.g., silver powder with purity ≥99.9%).
   • Binder: Resins such as epoxy resin and polyimide.
   • Solvent: Turpentine, ethylene glycol ethyl ether (for viscosity adjustment).
2. Key Parameter Control
   • Viscosity: 2000–10000 mPa·s (adjusted according to mesh count; higher mesh count requires lower viscosity).
   • Solid Content: 60%–80% (higher solid content results in thicker films and better conductivity).
   • Stirring Process: Vacuum stirring for 30–60 minutes to avoid air bubbles (bubbles can cause film pinholes).

Screen Printing Operation

1. Equipment Selection
   • Manual screen printing table: For small-batch trial production.
   • Semi-automatic/full-automatic screen printing machine: For mass production, with a positioning accuracy of ±0.01 mm.
2. Core Parameters
   • Squeegee Angle: 45°–60° (smaller angles increase paste transfer volume and film thickness).
   • Squeegee Speed: 5–20 mm/s (excessively high speed may cause pattern defects; excessively low speed may lead to paste bleeding).
   • Screen-Substrate Distance (Off-contact Distance): 1–3 mm (too small a distance causes screen sticking; too large a distance leads to pattern distortion).
   • Printing Pressure: 0.1–0.3 MPa (pressure must be uniform to avoid local paste accumulation or deficiency).

Drying and Sintering

1. Drying Process

   Hot air drying (60–100°C for 10–30 minutes) to remove solvents and prevent film bubbling during sintering.

2. Sintering Process (Key Step)
   • Temperature Rise Rate: 5–10°C/min (to avoid substrate deformation or metal particle agglomeration caused by rapid heating).
   • Sintering Temperature: Adjusted according to paste-substrate compatibility (150–300°C commonly used for silver paste, 300–450°C for copper paste, 200–350°C for aluminum paste).
   • Holding Time: 30–60 minutes to promote melting and densification of metal particles, improving film conductivity and adhesion.
   • Atmosphere Control: Copper and aluminum pastes must be sintered in an inert atmosphere (nitrogen, argon) or reducing atmosphere (hydrogen) to prevent oxidation.

Post-Treatment and Inspection

1. Post-Treatment

   Remove edge ink bleeding (alcohol wiping or laser trimming) and apply film coating for protection if necessary.

2. Core Inspection Indicators
   • Film Thickness: Measured with a profilometer (error ±0.1 μm).
   • Conductivity: Sheet resistance measured by the four-probe method (sheet resistance of silver films is typically ≤0.1 Ω/□).
   • Adhesion: 3M tape peel test (no obvious peeling) and cross-cut test (≥4B grade).
   • Pattern Precision: Line width deviation (within ±5%) and edge roughness (≤2 μm) observed under an optical microscope.

III. Common Problems and Solutions

IV. Process Optimization Directions

1. High Precision: Combine electron beam lithography with screen fabrication to improve pattern precision to below 10 μm.
2. Functional Composite: Add nanoparticles (e.g., carbon nanotubes, graphene) to metal pastes to enhance the thermal and electrical conductivity of films.
3. Green Environmental Protection: Develop water-based metal pastes (to replace solvent-based ones) and reduce VOC emissions.
4. Flexibility Compatibility: Optimize binder formulations (e.g., using polyimide resin) to improve film bending resistance (no cracking when the bending radius ≤5 mm).