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Transparent PET material flexible FPC (flexible printed circuit board) is a flexible electronic interconnect component based on transparent PET (polyethylene terephthalate), which combines transparency and flexibility. It has important applications in consumer electronics, wearable devices, intelligent displays, and other fields. The following provides a detailed introduction from the aspects of definition, core characteristics, structure and manufacturing, performance parameters, and application scenarios:
1、 Basic Definition
Transparent PET material flexible FPC refers to a flexible and foldable printed circuit board formed by using transparent PET film as the substrate, preparing conductive lines (usually copper layer) on its surface, and combining with transparent insulation layer (such as transparent cover film). Its core feature is that both the substrate and key structural layers have high transparency, while maintaining the wiring function and mechanical adaptability of flexible FPC.
2、 Core characteristics of transparent PET substrate
PET, as a substrate, is the foundation of its transparency and flexibility, with key characteristics including:
High transparency: The visible light transmittance usually reaches 85% -92% (wavelength 400-700nm), close to glass, and the haze is low (<2%), visually almost not blocking the background;
Flexibility and mechanical properties: The elongation at break can reach 150% -300%, and it can be repeatedly bent (with a minimum bending radius of 0.5mm). The tensile strength is about 50-70MPa, meeting daily bending needs;
Temperature resistance: Short term temperature resistance can reach 120 ℃, long-term use temperature range -40 ℃ to 80 ℃ (lower than PI substrate, limiting high-temperature applications);
Electrical insulation: dielectric constant of about 3.0-3.5 (1MHz), dielectric loss<0.02, insulation resistance>10 ¹⁴Ω· cm, suitable for high-frequency signal transmission;
Cost advantage: Compared to the polyimide (PI) substrate commonly used in traditional flexible FPCs, PET prices are 30% -50% lower, significantly reducing production costs.
3、 Structural composition of transparent PET flexible FPC
Its structure is similar to traditional FPC, but each layer needs to meet the "transparency" requirement. Typical structures include:
Substrate layer: Transparent PET film (usually 25-125 μ m thick), serving as the load-bearing foundation for the circuit;
Conductive layer: an extremely thin copper layer (thickness 5-35 μ m, mostly rolled copper or electrolytic copper), formed by etching to form circuit patterns (line width and spacing can be as small as 30 μ m/30 μ m);
Note: Copper itself is opaque, but due to its thin line width and low proportion (usually<10%), it still maintains high transparency overall;
Transparent covering film: an insulating layer (mostly made of transparent PET or transparent epoxy resin, with a thickness of 12-50 μ m) covering the surface of the conductive layer to protect the circuit and enhance insulation;
Surface treatment layer: Localized gold/tin plating (thickness 0.1-1 μ m) to prevent copper layer oxidation and ensure solder joint reliability (only in the pad area, not affecting overall transparency).
4、 Manufacturing process characteristics
The manufacturing of transparent PET flexible FPC is based on traditional FPC technology, but optimization is needed to ensure transparency and performance:
Substrate pretreatment: Clean the PET surface through plasma or chemical treatment to improve the adhesion with the copper layer (avoid delamination affecting transparency);
Preparation of conductive layer:
The mainstream method is "chemical copper plating+electrolytic copper plating": first deposit an ultra-thin copper seed layer (0.1-0.5 μ m) on the surface of PET, and then electrolytically thicken it to the target thickness;
In a few scenarios, "pressed copper foil" is used: ultra-thin copper foil (5-12 μ m) is hot pressed and bonded with PET (special adhesive is required to ensure transparency and no bubbles);
Graphic transfer: high-precision lithography (exposure accuracy ± 5 μ m)+etching (alkaline etching solution, avoiding PET corrosion), ensuring clear circuit patterns and no copper residue (residue will reduce transmittance);
Transparent cover film bonding: Press the transparent cover film onto the circuit layer at high temperature (120-150 ℃) (pressure 0.5-1MPa) to avoid bubbles (bubbles can cause uneven light transmission);
Post processing: laser cutting (accuracy ± 10 μ m), electrical performance testing (conductivity/insulation resistance), reliability testing (bending, temperature and humidity resistance).
5、 Key performance parameters
Transmittance: Visible light (400-700nm) transmittance ≥ 80% (when the proportion of copper lines is less than 10%);
Bending performance: Static bending radius ≥ 0.5mm, dynamic bending (180 ° reciprocating) ≥ 100000 times (resistance change<10%);
Electrical performance: minimum line width/spacing of 30 μ m/30 μ m, conduction resistance ≤ 50m Ω/cm (when the copper layer thickness is 18 μ m), insulation resistance ≥ 10 ¹¹ Ω (500V DC);
Environmental resistance: After being placed in an 85 ℃/85% RH environment for 1000 hours, the insulation resistance remains ≥ 10 ¹⁰ Ω without delamination;
Dimensional stability: When the temperature changes from -40 ℃ to 80 ℃, the thermal shrinkage rate is less than 0.1% (MD direction).
6、 Typical application scenarios
The core advantage of transparent PET flexible FPC is "transparency+flexibility", which is suitable for scenarios with high requirements for visual effects and spatial adaptability:
Consumer Electronics:
Folding screen phone: transparent ribbon cable connects the screen to the motherboard (does not obstruct the display in the folding area);
Transparent keyboard/touchpad: The circuit is hidden under the transparent panel to maintain a simple appearance;
Wearable devices:
Smart watch/wristband: a flexible and transparent circuit that fits the skin (without affecting the appearance of the device);
Smart glasses: transparent wires inside the frame (connecting sensors and processors);
In car electronics:
Vehicle Head Up Display (HUD): Transparent wiring connects the projection module to the control board (without obstructing the projection image);
Transparent window sensor: flexible circuit inside the window (detects light/temperature, does not affect line of sight);
Medical equipment:
Adhesive transparent physiological monitoring patch: The circuit is hidden in a transparent substrate (not protruding when attached to the skin);
Internal wiring of endoscope: flexible transparent wiring adapts to narrow and curved spaces without interfering with imaging;
Smart Home:
Transparent refrigerator panel: internal wiring connected to touch module (keeping the panel transparent as a whole);
Smart mirror: a flexible circuit behind the mirror surface (connecting the display and sensor without affecting reflection).
7、 Advantages and limitations
Advantages:
Visual friendliness: Transparent properties are suitable for scenes that require "invisible lines" to enhance product aesthetics;
Lower cost: PET substrate+simplified process, cost 40% -60% lower than transparent PI flexible FPC;
Convenient processing: PET hardness is lower than PI, making it easier to cut and bend, suitable for large-scale production;
Lightweight: Density of 1.3-1.4g/cm ³ (lower than PI's 1.4-1.5g/cm ³), reducing equipment weight.
limitations:
Insufficient temperature resistance: Long term use temperature ≤ 80 ℃ (PI can reach 120 ℃), not suitable for high temperature environments such as car engine compartments;
Limited mechanical strength: The tensile strength is lower than PI (about 100-150MPa), and it is prone to fatigue after long-term dynamic bending;
The bonding strength of the copper layer is weak: special treatment processes (such as plasma activation) are required, otherwise delamination may occur under high temperature and high humidity.
8、 Development Trends
High performance substrate development: By molecular modification (such as introducing aromatic monomers), the temperature resistance of PET is improved (with the goal of long-term temperature resistance of 100 ℃);
Ultra fine wire wide manufacturing: Developing laser direct etching technology to achieve a line width and spacing of 10 μ m/10 μ m, improving wiring density;
Multi functional integration: embedding transparent sensors (such as ITO conductive layers) in circuits to achieve the integration of "circuit+sensing";
Green technology: using lead-free etching solution and water-based adhesive to reduce environmental pollution during the production process.
Transparent PET material flexible FPC, with its combination advantages of "transparency, flexibility, and low cost", is becoming an important branch in the field of flexible electronics, especially in consumer electronics and wearable devices. It is expected to gradually replace traditional non transparent FPC and promote product form innovation.