In demanding industrial control and outdoor display applications, TES maintains specific requirements for PCAP (Projected Capacitive) sensor design. To achieve a longer service life and higher reliability—in addition to optimizing touch control circuits—a careful evaluation of base materials is essential.
PCAP Sensor Types (Out-cell)
There are various PCAP sensor designs available. The primary base materials used for touch sensor circuits are Inorganic Glass and Organic Film.
Fig.1 PCAP Sensor Types
What is the difference between Glass and Film bases ?
1. Thermal Expansion Coefficient (CTE) and ITO (Indium Tin Oxide) Micro-cracks
Glass: Glass has an extremely low Coefficient of Thermal Expansion (CTE). During extreme high and low temperature cycling (Thermal Shock), its dimensions remain stable. This ensures that the ITO (Indium Tin Oxide) conductive layer deposited on the glass remains intact.
PET Film: PET is prone to thermal expansion, contraction, and deformation under high temperatures or humidity. Since ITO (Indium Tin Oxide) is a brittle inorganic ceramic, the movement of the underlying PET often causes micro-cracking in the ITO (Indium Tin Oxide)layer (Fig 2). Over time, these cracks increase impedance, leading to touch disconnection or dead zones.
Fig.2 SEM of ITO Micro-Cracking
2. Anti-Ultraviolet (UV) Capability and Optical Attenuation
Glass: As an inorganic material, glass is inherently immune to UV rays. Even with prolonged exposure to intense sunlight, its light transmittance and clarity remain unchanged—a crucial factor for outdoor applications paired with high-brightness LCDs.
PET Film: As a high-molecular organic material, PET degrades under UV radiation. The most common symptoms are yellowing and decreased light transmittance, which hampers the visual experience and reduces the effective brightness of the display.
Fig. 3 Display differences after long-term outdoor use
3. Water Vapor Permeability and Outgassing
Glass: Glass is highly dense with a Water Vapor Transmission Rate (WVTR) of nearly zero. This provides a perfect barrier against external moisture, ensuring the integrity of the internal OCA (Optically Clear Adhesive) and preventing it from reacting with environmental humidity.
PET Film: PET absorbs moisture. In long-term high-temperature/high-humidity environments, an outgassing reaction can occur within the PET. These released gases can cause bubbles in the OCA (Optically Clear Adhesive) bonding layer, change the dielectric constant, or cause local delamination.
※. Degradation
Under prolonged outdoor conditions, PET undergoes photochemical and thermochemical degradation. This process breaks molecular chains, releasing VOCs (Volatile Organic Compounds). This outgassing is a primary cause of bubbles in the OCA (Optically Clear Adhesive) or micro-cracks in the XY circuit (Fig 4), resulting in unstable impedance.
Fig.4 Chemical constituents of the PET polymer
4. The "PET Crack" Side-Effect of Hard Coating
In many PET surface treatments, a hard-coating layer is added to improve yield. However, if the coating material is not properly defined, long-term sunlight exposure can cause the layer to develop cracks (Fig 5). This failure is often linked to hydrophobic materials using APTMS (Fig 6).
Fig. 5 PET Hard-Coating Cracks
Fig. 6 FTIR Analysis Results
Conclusion
Under harsh environmental conditions—such as high temperatures, high humidity, and long-term UV exposure—it is strongly recommended that users select PCAP sensors with a Glass Base to ensure long-term reliability and performance.
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