Adhesive-based FPC substrates consist of copper foil, adhesive, and insulating film. The adhesive is sandwiched between the copper foil and the insulating film, functioning to firmly bond these two components. For instance, in a common three-layer adhesive-based FPC substrate, the middle layer is adhesive, with copper foil and insulating film layered on top and bottom, respectively. This structure ensures the copper foil adheres securely to the insulating film, providing a foundation for subsequent circuit fabrication.
Adhesive-free FPC substrates are primarily formed by directly laminating copper foil and insulating film without an intermediate adhesive layer. They achieve tight bonding through specialized processes such as hot pressing. This simplified structure eliminates the adhesive layer, enabling unique performance characteristics tailored to specific application requirements.
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Adhesive-Based Substrates: The flexibility of adhesive-based substrates is partially determined by the adhesive’s properties. While adhesives with good flexibility can enhance the overall flexibility of the substrate, their presence may introduce bending hysteresis. For example, during frequent bending of FPCs, micro-deformation accumulation in the adhesive can gradually reduce the bonding strength between the copper foil and insulating film, potentially leading to delamination over time.
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Adhesive-Free Substrates: Adhesive-free substrates exhibit superior flexibility due to the absence of an adhesive layer. The direct bonding between copper foil and insulating film allows for better synchronous deformation during bending, enabling them to withstand higher-frequency bending and smaller bending radii. A is their application in foldable smartphones, where adhesive-free FPCs reliably endure repeated screen folding, minimizing the risk of circuit damage caused by bending.
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Adhesive-Based Substrates: The dielectric properties of the adhesive significantly impact the overall electrical performance of adhesive-based substrates. A high dielectric constant in the adhesive may increase signal delay and attenuation during transmission. For instance, in FPCs used for high-speed signal transmission, the adhesive can absorb high-frequency signals, compromising signal integrity. Additionally, poor insulation resistance of the adhesive raises the risk of short circuits between circuits.
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Adhesive-Free Substrates: Without an adhesive layer, adhesive-free substrates offer more stable electrical performance. Their insulation resistance and dielectric constant are primarily determined by the insulating film, providing a cleaner signal transmission environment. This makes them ideal for high-frequency and high-speed signal applications, as they reduce signal interference and distortion effectively.
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Adhesive-Based Substrates: The thermal stability of adhesive-based substrates is governed by the adhesive. At elevated temperatures, the adhesive may soften or flow. For example, during FPC soldering, insufficient high-temperature resistance of the adhesive can weaken the bond between copper foil and insulating film, potentially causing copper foil displacement. Moreover, mismatched thermal expansion coefficients between the adhesive, copper foil, and insulating film can generate internal stress during temperature cycling, reducing the FPC’s service life.
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Adhesive-Free Substrates: The thermal performance of adhesive-free substrates depends on the copper foil and insulating film. Without the thermal expansion and stability issues associated with adhesives, these substrates maintain better dimensional stability under temperature variations. They retain their physical and electrical properties more effectively in high-temperature environments, making them suitable for applications such as FPCs near engine control units in automotive electronics.
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Adhesive-Based Substrates: The thickness accuracy of adhesive-based substrates is affected by the adhesive layer, which is challenging to control uniformly. This can lead to thickness deviations, limiting their suitability for ultra-thin FPCs where precise thickness control is critical.
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Adhesive-Free Substrates: Adhesive-free substrates offer superior thickness and dimensional accuracy. Their thickness, determined primarily by the copper foil and insulating film, can be precisely controlled through advanced lamination processes. This precision supports the fabrication of high-accuracy circuits, meeting stringent dimensional requirements.
Processing adhesive-based substrates requires careful consideration of the adhesive curing process. During circuit patterning, etchants and other chemical reagents may affect the adhesive; for example, etchants can penetrate the adhesive layer, degrading its performance. Additionally, parameters such as temperature, pressure, and time must be optimized during lamination to ensure strong bonding between the copper foil and insulating film.
The key processing step for adhesive-free substrates is the precise control of temperature, pressure, and time during the lamination of copper foil and insulating film to achieve robust bonding. Etching and other patterning processes are more manageable due to the absence of adhesive interference. However, bonding adhesive-free substrates to other components often requires specialized techniques, as they lack an inherent adhesive layer.
Adhesive-based substrates are widely used in general electronic devices with moderate performance requirements due to their lower cost. Examples include FPCs in consumer electronics such as electronic toys and basic calculators, where they meet fundamental circuit connection and signal transmission needs.
Adhesive-free substrates are primarily employed in high-end electronic devices demanding exceptional flexibility, electrical performance, and thermal stability. Applications include aerospace electronics, advanced medical equipment, and cutting-edge communication devices. In these scenarios, adhesive-free substrates ensure reliable operation and accurate signal transmission, critical for device performance.
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