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Process Technology and Engineering – From Theory to Flawless Production
Even the best choice of materials is worthless if the manufacturing process lacks the necessary precision and cleanliness. Optical bonding is a high-precision process. This is particularly true for HMI systems, where optical quality, mechanical robustness, and reliable touch and display functionality must all be ensured simultaneously.
Cleanroom Environment: The Need for Cleanliness
A single dust particle measuring 50 µm that becomes trapped between layers is clearly visible to the naked eye, rendering the product unsuitable for sale.
Typical cleanroom requirements:
- ISO Class 7 for general process areas
- ISO Class 5 for critical lamination and inspection areas
The following are required:
- HEPA/ULPA filters
- Special cleanroom clothing
- Strict cleaning protocols
Optical bonding or lamination must therefore be performed under controlled conditions in a cleanroom to reliably prevent particles, air bubbles, and contaminants.
Surface Preparation: The Foundation for Perfect Adhesion
The surfaces of glass, touch sensors, and displays must be absolutely clean and chemically activated prior to bonding to ensure maximum adhesion. The process involves several steps: wet cleaning with deionized water and special cleaning agents to remove coarse contaminants, followed by drying in a particle-free oven. The most critical step is the plasma treatment. During this process, the surfaces are exposed to low-pressure plasma, which removes the last organic contaminants at the molecular level and increases the surface energy. This improves wetting by the adhesive and creates additional chemical bonding sites, resulting in significantly higher and more durable adhesion.
Lamination and Curing:
Precision Under Vacuum
OCA Lamination
The actual lamination process requires the highest precision. In OCA lamination, the pre-cured adhesive film is first applied to the cover glass, touch sensor, or display. The two components are then aligned in a high-precision tool with an accuracy of ±0.1 mm or better. A stamp or roller then presses the components together with a defined pressure and temperature. After lamination, the bonded part is exposed to high pressure (approx. 5 bar) and a moderate temperature (approx. 50°C) in an autoclave (vacuum chamber) to remove all air bubbles, including small micro-air bubbles, and maximize adhesion.
OCR - Optical Bonding
In OCR bonding, the liquid adhesive is first precisely dispensed onto the cover glass, touch sensor, or display. The second component to be bonded is then slowly lowered to evenly distribute the adhesive. Curing takes place, depending on the material type (acrylate or silicone), by irradiation with UV light of a defined wavelength and intensity, at room temperature or a moderate temperature. Depending on the process and material, vacuum chambers may also be used. The curing process must be precisely controlled to minimize shrinkage and stresses in the adhesive.
The goal of OCR bonding is to create an air-free, homogeneous, and permanently stable optical bond. Unlike OCA, OCR allows for complete adaptation to complex geometries, large surfaces, and curved substrates.
Quality Control: 100% Inspection
After the bonding process, each individual unit must undergo rigorous quality control. Automated Optical Inspection (AOI) uses high-resolution cameras and specialized lighting to detect defects such as particles, bubbles, scratches, or delamination that are invisible to the human eye. In addition, functional tests are performed to verify optical performance (transmission, haze, color accuracy) and, where applicable, the functionality of the touch sensor. Only products that pass this 100% inspection are shipped to the customer.
Note on the technical information in this chapter
The technical values, test profiles, and performance data stated in this chapter are provided for technical classification purposes. Actual results depend on the specific display configuration, material system, bonding process, application profile, and scope of qualification. Product-specific values are validated on a project basis using data sheets, measurements, test reports, and customer specifications.