2. Technical Field
Pengding is involved in Flexible Circuit Boards (FPCB), Chip-on-Film (COF) modules, and electronic devices that include flexible circuit boards, particularly flexible printed circuit boards used for COF in various electronic devices.
3. Background Technical Issues
As electronic products trend towards thinner, smaller, and lighter designs, there is a need for high-density installation of semiconductor chips in narrow areas. The COF method, which utilizes flexible substrates, can be applied to flat panel displays and flexible displays, especially suitable for wearable electronic devices, achieving fine pitch high-resolution displays. However, the main issue with existing COF flexible circuit boards is that cracks may occur when the circuit patterns on the flexible substrate are repeatedly bent or bonded in a bent state, potentially leading to damage due to the tension generated by bending.
4. Invention Content (Core Solution)
The purpose of this invention is to provide a flexible circuit board, COF module, and electronic device that can solve the aforementioned background technical issues. The core solution lies in improving the structure of the flexible circuit board, particularly by setting a special protective layer structure on the wiring pattern layer and plating layer in the bending area.
The main innovations are reflected in the following aspects:
Flexible Circuit Board Structure: Includes a substrate (with bending and non-bending areas), a wiring pattern layer (set on the substrate), a plating layer (set on the wiring pattern layer with an open area corresponding to the bending area), and a protective layer.
Relationship between Protective Layer and Plating Layer and Wiring Pattern Layer: The protective layer directly contacts one surface of the wiring pattern layer exposed at the open area and the side of the plating layer.
Protective Layer Thickness: The thickness of the protective layer is greater than that of the plating layer.
Double-layer Plating Structure: The plating layer may include a first plating layer and a second plating layer set on top of it. The width of the open area of the second plating layer can be greater than that of the first plating layer.
Overlapping Area of Protective Layer and Plating Layer: The protective layer contacts the first plating layer on one side of the open area (first overlapping area) and on the other side (second overlapping area). The widths of the first and second overlapping areas can differ, and each width is greater than the thickness of the first plating layer. Claim 8 further specifies that this width can be greater than the sum of the thicknesses of the first and second plating layers. Claim 11 specifically states that this width can be greater than 400μm.
Shape of Protective Layer: The protective layer can be single-layer or multi-layer (such as upper and lower protective layers). A single-layer protective layer can directly contact the wiring pattern layer, first plating layer, and second plating layer. In a multi-layer protective layer (e.g., double-sided flexible circuit board), the protective layer on the tension-receiving side (usually the thicker layer) can contact the wiring pattern layer to prevent cracking caused by bending. The lower protective layer can be partially embedded between the first and second plating layers.
Application in Electronic Devices: Includes the flexible circuit board, a display panel connected to one end of the flexible circuit board, and a printed circuit board connected to the other end.
5. Key Technical Features in Claims (Refined)
According to the claims, the most important technical features include:
The flexible circuit board includes a substrate, a first wiring pattern layer, a first plating layer (with an open area), and a first protective layer.
The first protective layer contacts the first wiring pattern layer, the first plating layer, and a portion of the substrate.
The first protective layer includes two overlapping areas that contact the first plating layer on both sides of the open area.
The widths of the two overlapping areas differ, and each width is greater than the thickness of the first plating layer. (Core feature of Claim 1)
The flexible circuit board may include a second plating layer set on the first plating layer. (Claim 2)
The upper surface of the first protective layer is higher than the upper surfaces of the first and second plating layers. (Claim 3)
The flexible circuit board may include a second wiring pattern layer, a third plating layer, and a second protective layer set on the other side of the substrate. (Claims 4, 6)
The thickness of the first wiring pattern layer is greater than the thicknesses of the first and second plating layers. (Claim 7)
The width of the overlapping area can be greater than the sum of the thicknesses of the first and second plating layers. (Claim 8)
The substrate has bending and non-bending areas, with the bending area being smaller than the second open area. (Claim 9)
The first protective layer overlaps with the first plating layer in both the non-bending and bending areas. (Claim 10)
The width of the overlapping area can reach over 400μm. (Claim 11)
The widths of the two overlapping areas can differ. (Claim 12)
The thickness range of the first and second protective layers (for double-sided circuit boards) can be from 5μm to 20μm. (Claim 13)
At least one protective layer has a thickness of 1μm to 20μm. (Claim 14)
In the open area, the thickness of the first protective layer is greater than that of the second protective layer. (Claim 15 – regarding the thickness difference of protective layers on both sides of the double-sided circuit board, favoring a thicker protective layer on the tension side)
The flexible circuit board encapsulates chips and electronic devices that contain the structural features of the aforementioned flexible circuit board. (Claims 16, 17)
6. Important Details and Facts in Specific Implementations
Substrate: Flexible substrates, such as polyimide (PI), PET, PEN, and other polymer materials. The thickness range is typically from 12μm to 125μm, preferably less than 50μm, and more preferably from 20μm to 40μm.
Wiring Pattern Layer: Conductive materials, preferably copper (Cu). The thickness range is from 1μm to 20μm, preferably from 5μm to 20μm, and more preferably from 5μm to 15μm.
Plating Layer (Surface Treatment Layer): The main purpose is to prevent oxidation of the wiring pattern layer and improve conductivity. It can include any of tin (Sn), Ni/Au alloy, gold (Au), Electroless Nickel Immersion Gold (ENIG), Ni/Pd alloy, and organic solderability preservative (OSP).
When the plating layer is tin (Sn), a two-layer structure (first plating layer and second plating layer) can be used to prevent whisker formation and short circuits in the wiring pattern.
The first and second plating layers can be formed from tin (Sn) but in different steps.
Heat treatment (such as curing) may cause diffusion of copper (Cu) and tin (Sn), forming an alloy. The copper content of the first plating layer in direct contact with the copper wiring may be higher than that of the second plating layer. This alloy structure helps prevent issues related to electrochemical migration resistance and short circuits caused by metal growth.
The thickness of the first plating layer may be less than that of the second plating layer. The total plating thickness is preferably below 1.1μm.
Protective Layer: The main purpose is to cover and protect the wiring pattern layer and plating layer in the bending area, alleviating bending stress. It can be formed from polyimide, polyamide-imide, epoxy resin, or acrylic resin. The thickness is usually greater than that of the plating layer.
The side of the protective layer (the part in contact with the plating layer) can be an inclined surface or a vertical surface, with the inclined surface (especially sharp angles) increasing the contact area to prevent detachment. The edge of the side can have curvature.
Single-sided Flexible Circuit Board (Figures 1-4): The protective layer covers the bending area and part of the non-bending area, with a width greater than the bending area or open area. The protective layer contacts the wiring layer, first plating layer, and second plating layer. One surface of the protective layer (the lower surface) can contact the wiring pattern layer, helping to prevent cracking on the tension side.
Double-sided Flexible Circuit Board (Figures 5-8, 21-26): The wiring pattern layer and protective layer are set on both sides of the substrate. Typically, one protective layer is used to withstand tension, while the other withstands pressure.
The protective layer on the tension-receiving side (e.g., the first protective layer 410 in Figure 5) can contact the wiring pattern layer, and its thickness (T3) is greater than that of the protective layer on the pressure-receiving side (the second protective layer 420 in Figure 5, thickness T4).
The protective layer in the double-sided flexible circuit board can also be a multi-layer structure (e.g., upper and lower protective layers). The lower protective layer can be partially embedded between the plating layers. The upper protective layer can cover the lower protective layer and part of the surrounding plating layer, with a width greater than the lower protective layer, and its upper surface higher than the lower protective layer, helping to disperse stress, prevent penetration, and improve adhesion.
Bending Area (BA): The part of the substrate that performs bending. In electronic devices, it is usually located between the display panel and the printed circuit board, in a C shape or other curved form.
Non-bending Area (NBA): The part outside the bending area, usually plate-shaped, used for connecting chips, display panels, and printed circuit boards.
Open Area (OA): The opening in the plating layer corresponding to the bending area, allowing the protective layer to contact the wiring pattern layer and the side of the plating layer. The double-layer plating has a first open area (first plating layer) and a second open area (second plating layer), with the second open area typically wider than the first open area.
Overlapping Area (CA): The area where the protective layer contacts and overlaps with the plating layer (especially the first plating layer or the lower protective layer with the plating layer), usually located around the open area, helping to improve adhesion and disperse stress. The width is a key parameter (e.g., greater than 400μm).
Through Hole (V): Holes that pass through the substrate, filled with conductive materials (such as copper), used to connect the wiring pattern layers on both sides of the substrate.
Lead Pattern Section and Test Pattern Section: The conductive pattern sections not covered by the protective layer, used for connecting chips, display panels, or printed circuit boards (lead patterns) or for product defect inspection (test patterns). Lead patterns can be divided into inner leads (near the chip) and outer leads (away from the chip).
7. Manufacturing Methods
The patent describes several methods for manufacturing double-sided flexible printed circuit boards, with core steps including:
Forming wiring pattern layers on both sides of the substrate.
Forming a first plating layer (and third plating layer) with open areas through sacrificial layers (dry film masks, photoresist layers, PET masks) or direct electroplating (Spurt Jar plating).
Setting the protective layer (lower protective layer), filling the open areas, and covering part of the plating layer’s side and upper surface.
Forming a second plating layer (and fourth plating layer) in the areas not covered by the protective layer.
(For multi-layer protective layer structures) forming an upper protective layer on the lower protective layer and surrounding plating layers.
8. COF Modules and Electronic Devices
COF Module: Composed of a flexible circuit board and a driving chip mounted in its non-bending area. The driving chip is electrically connected to the flexible circuit board through the lead pattern section.
Electronic Devices: Include the flexible circuit board (or COF module), display panel, and printed circuit board. The COF module is typically located between the display panel and the printed circuit board, electrically connected through ACF (Anisotropic Conductive Film) or other means. The bending area of the flexible circuit board allows it to bend and connect between the display panel and the printed circuit board.
The bending part of the flexible circuit board can be equal to, wider than, or narrower than its designed bending area when installed in the electronic device.
By etching the substrate surface between the wiring pattern layers, the seed layer can be removed to prevent short circuits between adjacent patterns. Therefore, the substrate thickness in areas with conductive patterns may be greater than in areas without patterns.
This flexible circuit board and COF module can be applied to various electronic devices such as flexible touch windows, wearable touch devices, and curved displays (e.g., TVs, monitors, laptops).
By placing the display panel on the surface opposite to that of the driving chip, design flexibility can be improved.
9. Advantages/Problems Solved by the Invention
Improved Reliability: By covering the wiring pattern layer and plating layer in the bending area with a protective layer, cracks and damage caused by bending are prevented. The protective layer directly contacts the side of the plating layer and the wiring pattern layer, enhancing adhesion and preventing detachment. The multi-layer protective layer structure and the width difference between the upper and lower protective layers help disperse stress. The lower protective layer embedded between the plating layers also helps buffer stress.
Prevention of Oxidation and Short Circuits: The plating layer (especially tin) prevents oxidation of the wiring pattern. The double-layer tin plating structure and the contact structure of the protective layer with the plating layer help prevent metal whisker formation and electrochemical migration, reducing short circuit defects.
Increased Processing Efficiency: The integrally formed protective layer can improve efficiency. The symmetrical protective layer shape in double-sided flexible circuit boards can also enhance processing efficiency.
Achieving High-Density Installation and High-Resolution Display: This invention improves the reliability of flexible circuit boards, allowing semiconductor chips to be installed at higher densities in narrow areas, thus supporting high-resolution display requirements.
Prevention of Appearance Defects and Reliability Degradation: Prevents the formation of Sn particles.
10. Conclusion
Pengding provides a flexible circuit board structure for COF modules, effectively alleviating stress generated by bending by setting a protective layer with specific overlapping area and thickness relationships on the wiring pattern layer and multi-layer plating in the bending area, preventing cracking, detachment, and short circuits of the wiring pattern layer and plating layer, significantly improving the reliability of flexible circuit boards and their applications in electronic devices. It particularly emphasizes the importance of the width of the overlapping area between the protective layer and the plating layer (especially the first plating layer) and the thickness of the protective layer being greater than that of the plating layer, as well as the role of the thickness difference of protective layers on both sides of the double-sided circuit board in bearing tension/compression stress. This improved flexible circuit board structure provides a more reliable foundation for achieving thinner, smaller, lighter electronic devices with high-resolution displays.
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