Application of Spray Coating Machines in the Encapsulation of Flexible Electronic Devices

Process principle

Based on jet deposition technology, the spray coating machine uses pneumatic or ultrasonic excitation to form a micron-sized droplet solution of the packaging material, and then sprays it evenly on the surface of the flexible substrate at a set angle and speed. The droplets quickly volatilize part of the solvent during flight and form a continuous film layer when they reach the substrate. During the packaging process, the spraying flow, spray pressure, scanning trajectory and atomization parameters can be adjusted in real time to adapt to the changes in surface tension and curvature of different flexible substrates (such as polymer films, ultra-thin glass, non-woven fabrics, etc.). This method avoids the mechanical damage that may be caused by contact coating, and is especially suitable for non-contact packaging of brittle and thin-layer electronic devices.

Material Adaptability

Common packaging materials include paraxylene derivatives, polyurethane resins, organopolysiloxanes, polyimide precursors, and acrylic copolymers. During spraying, the carrier gas flow rate and nozzle pore size should be matched according to the viscosity of each material (usually range 0.5–300 mPa·s) and evaporation rate. For materials with high moisture sensitivity, dry nitrogen can be introduced as a buffer gas in the spraying chamber to control the relative humidity below 10%. The following table lists the main parameters and applicable scenarios of typical packaging materials.

Optimization of process parameters

The main factors affecting coating uniformity and density include spray pressure (typically 0.1–0.5 MPa), nozzle-to-substrate spacing (50–200 mm), scanning rate (200–800 mm/s), single-layer wet film thickness (5–50 μm), and interlayer curing conditions. For multi-layer coating, each layer is subjected to preset infrared baking (temperature 50–90 °C, time 3–10 min) to eliminate sagging and promote interlayer fusion. According to the polymer diffusion kinetics model, the cross-linking density at the coating interface can be adjusted by adjusting the baking temperature T and timet Describes the relationship between

D_eff = D₀ · exp[−E_a/(R·T)]

In the formula D_eff for the effective diffusion coefficient,D₀ is the pre-index factor,E_a For diffusion activation energy,R is the gas constant. Through the fitting of experimental data, the optimal baking window can be obtained for the specific material system.

Process control essentials

In order to ensure the stable electrical performance of the packaging layer, the porosity rate in the coating should be controlled to be less than 0.5%. The real-time coating thickness deviation can be monitored by online laser triangulation and the spray flow rate can be adjusted with feedback. The substrate temperature should be kept about 15 °C below the glass transition temperature of the material. In addition, gradient spraying parameters (flow attenuation to 80% of the center area) are designed along the edge area to compensate for the thickness accumulation caused by edge effects. For the shaft area of flexible devices, it is recommended to use multiple slow scans (200 mm/s) to form a gradient thickness distribution at the stress concentration of the coating layer to improve the bending fatigue life.

Quality verification methodology

The packaging quality can be evaluated by the following items: 1. Film layer integrity inspection - using fluorescent dye penetration method to observe the location of coating defects under ultraviolet excitation; 2. Barrier performance measurement - measured by water vapor transmittance (WVTR) tester under 40 °C/90% relative humidity conditions, requiring WVTR ≤ 10 for flexible devices⁻³ g/(m²·day)； 3. Determination of bonding force - using 90° peel test, the peel strength needs to be ≥ 0.5 N/cm; 4. Insulation resistance - apply 100 V DC voltage between the two electrodes, and the insulation resistance should be ≥ 10 after 1 minute¹² Ω。 In addition, it is recommended to perform dynamic bending tests (bending radius of 5 mm, ≥ 1000 times) per batch, followed by WVTR and insulation performance change rate.

Common defect countermeasures

Defects such as orange peel marks, pinholes, and tailing occasionally appear during the spraying process. Orangery peel can be eliminated by increasing the atomization air pressure or reducing the viscosity of the paint to less than 50 mPa·s. Pinholes are mostly caused by bubble entranging, which can be prevented by adding a vacuum degassing module (vacuum ≤0.05 MPa) in the feeding pipeline. The tailing phenomenon is usually related to the fast scanning speed or slow solvent volatilization, and it is recommended to reduce the scanning rate to 300 mm/s or choose a fast-drying solvent (boiling point 70–85 °C) to improve it. If there is interlayer peeling in a multi-layer structure, the bottom layer can be treated with air plasma (power 100 W, treatment time 30 s) to reduce the surface contact angle to less than 15° to enhance adhesion.

Extend application scenarios

This technology has been extended to non-medical fields such as transparent conductive film protective layer, micro sensor array passivation layer, and photovoltaic module backplate moisture-proof coating. In the field of photoelectric display, spray coating machines can achieve ultra-thin protective layers below 15 μm, so that flexible screens can maintain complete water and oxygen barrier under the condition of curvature radius of 3 mm. In the field of functional textiles, conductive circuit encapsulation and fabric surface finishing are combined into one, and a single equipment can continuously spray coils for more than 30 m, increasing production efficiency by 2 times compared with traditional lamination methods. For high-temperature application environments (such as engine peripheral sensors), polyimide precursors are sprayed and cured at 350 °C, and the coating temperature resistance level can reach more than 260 °C. It is worth noting that the spraying process is more adaptable to large-area special-shaped parts (such as curved light panels in automotive interiors) - through the linkage of the six-axis robotic arm, the three-dimensional curved conformal coating can be achieved with a positioning accuracy of ±0.1 mm.

Citation Notes:
1. The rheological control principle of the coating process is derived from the relevant chapters of "Polymer Material Forming and Processing Technology".
2. The test method of water vapor transmittance refers to GB/T 30426-2013 "Determination of water vapor transmittance of flexible materials".
3. Optimization of plasma surface treatment parameters Based on the revision of the contact angle model in Surface and Interface Engineering.