A method for precise windowing and bonding of a flexible circuit board cover film for cardiac ultrasound examination

Abstract

This application discloses a method for precise windowing and bonding of a cover film on a flexible circuit board used in cardiac ultrasound examination. The method includes the following steps: providing a flexible circuit board with pad areas; attaching a low-viscosity masking film to the pad areas; temporarily covering the uncured cover film using its adhesive layer; laser-cutting the cover film to create windows corresponding to the pads, with the cutting depth stopping at the surface of the masking film; partially peeling off the cover film to expose the masking film; removing the masking film to expose the pads; re-attaching the cover film to align the windows with the pads; and finally, thermo-curing for permanent bonding. This method protects the pads from laser damage by pre-attaching a low-viscosity masking film to them, while the temporary adhesion of the cover film facilitates partial peeling and realignment, followed by thermo-curing. It solves the problem of window misalignment caused by the flexible stretching of the cover film on long flexible circuit boards, avoids pad damage, leaves no adhesive residue after masking film peeling, and improves window alignment accuracy and yield.

Description

Technical Field

[0001] This invention belongs to the field of flexible printed circuit board (FPC) manufacturing technology, and particularly relates to a method for precise windowing and bonding of a cover film for a flexible printed circuit board used in cardiac ultrasound examination. Background Technology

[0002] In intracardiac ultrasound (ICE) catheter technology, the clinical procedure typically involves inserting a catheter into the heart via a femoral vein puncture. A distally integrated ultrasound transducer array is then used to perform real-time imaging of the cardiac structures. To achieve independent drive and signal transmission for multiple elements, a long (up to tens of centimeters) flexible printed circuit board (FPC) with high wiring density is required inside the catheter as a connection medium to electrically connect the distal transducer units to the proximal signal processing system. The FPC is long and has very small pad spacing (typically less than 0.5 mm). The bonding and precise windowing of the cover film (CVL) of such long FPCs present unique challenges: traditional processes pre-window the cover film before bonding, but due to the softness of the cover film material, stretching or displacement easily occurs during bonding, leading to cumulative deviations between the window position and the actual pad position. These deviations become more significant with longer FPCs. This deviation directly affects the flip-chip soldering yield of the transducer chips and can even cause short circuits or open circuits due to the window edge covering the pads, ultimately affecting the clarity and diagnostic accuracy of the intracardiac ultrasound images. To improve alignment accuracy, some existing technologies attempt to perform laser windowing after the cover film is laminated, but this lacks temporary protection for the pads, and the laser can easily damage the underlying pads or circuitry. Other technologies use flexible ink as a temporary protective layer, but this involves complex processes such as printing, curing, and laser ablation, making both the materials and procedures cumbersome. Therefore, how to simplify the process and avoid pad damage while ensuring the accuracy of the windowing position has become a pressing technical problem in this field. Summary of the Invention

[0003] (a) Purpose of the invention To overcome the above shortcomings, the present invention aims to provide a method for precise windowing and bonding of the cover film of a flexible circuit board for cardiac ultrasound examination, so as to solve the technical problems in the existing process where the windowing position of the cover film of long-sized FPC is easily offset and the pads are easily damaged by laser.

[0004] (II) Technical Solution To achieve the above objectives, the technical solution provided in this application is as follows: A method for precisely ventilating and bonding a flexible circuit board cover film for cardiac ultrasound examination, characterized by comprising the following steps: A flexible circuit board is provided, the surface of which has pad areas; Apply a low-viscosity masking film to the pad area; A cover film is temporarily applied to a flexible circuit board and a low-viscosity masking film. The cover film is temporarily adhered to the flexible circuit board and the low-viscosity masking film by the adhesive layer on its bottom surface, and is not subjected to hot pressing curing. The cover film is cut by laser to create windows in the corresponding pad areas. The laser cutting depth stops at the surface of the low-viscosity masking film. Partially peel off the cover film corresponding to the pad area to expose the low-viscosity masking film; Remove the low-viscosity masking film to expose the pad area; The partially peeled-off cover film is reattached to the flexible circuit board, aligning the opening with the pad area. The re-attached cover film is then heat-pressed and cured to permanently adhere it to the flexible circuit board.

[0005] By pre-applying a low-viscosity masking film to the pad area, and then temporarily covering the flexible circuit board and the masking film with an adhesive layer on the bottom, without thermosetting, the masking film is stably fixed by the initial tack of the adhesive layer at room temperature. This avoids displacement during subsequent operations while retaining the feasibility of partial removal. During laser windowing, the cutting depth stops at the surface of the masking film, reliably protecting the pads from heat damage. Subsequently, the masking film is partially removed, and the low-viscosity masking film is removed. Since the masking film is only temporarily adhered, the removal process is easy and does not damage the film. After re-application and alignment, the masking film is thermoset and cured to permanently adhere to the flexible circuit board. This method completely solves the windowing misalignment problem caused by the flexible stretching of the masking film in traditional processes, while avoiding damage to the pads caused by direct laser windowing after pressing. The low-viscosity masking film leaves no adhesive residue after peeling, ensuring clean and solderable pads. The entire process is logically clear and is particularly suitable for the mass production of long flexible circuit boards, significantly improving windowing alignment accuracy and yield.

[0006] In some embodiments, the position of the laser window is adjusted to compensate for the thickness of the low-viscosity shielding film before laser cutting.

[0007] When the low-viscosity masking film is applied to the pad area, its physical thickness causes the overlay film to bulge locally at that location, forming a slight slope. If the window is cut according to the theoretical planar position during laser cutting, when the masking film is subsequently removed and the overlay film returns to a flat state from its bulging state, the original cut window position will shift laterally relative to the pad. By pre-compromising the laser window position based on the masking film thickness, this offset caused by the height difference of the bulge can be counteracted, ensuring that the final flattened window accurately falls directly above the pad. This compensation adjustment method directly addresses the geometric root cause of the offset, effectively ensuring the alignment accuracy between the window and the pad and avoiding systematic deviations caused by negligible thickness.

[0008] In some embodiments, the compensation adjustment includes: calculating a position compensation value based on the thickness of the low-viscosity shielding film and the angle formed between the low-viscosity shielding film and the covering film; the formula for calculating the position compensation value is: compensation value = -T × tanθ, where T is the vertical thickness of the low-viscosity shielding film, θ is the angle, and the negative sign indicates that the compensation direction is opposite to the thickness measurement direction.

[0009] By introducing the tangent relationship between thickness and angle, the geometric parameters of the convex slope of the covering film are transformed into precise compensation values. Here, the thickness T reflects the height of the convexity, and the angle θ reflects the steepness of the slope. The tangent of their product directly corresponds to the amount of horizontal drift of the window after the covering film is removed.

[0010] In some embodiments, the compensation adjustment further includes: finding a pre-tested angle based on the material of the cover film, wherein cover films of different materials form different angles with low-viscosity shielding films of the same thickness.

[0011] Different materials used for the cover film have varying degrees of curvature, resulting in differences in the actual bonding angle between the cover film and the shielding film. By establishing a correspondence between materials and angles through pre-testing, the accurate angle can be quickly obtained during production simply by referring to a table based on the cover film material, avoiding re-measurement each time and improving production efficiency.

[0012] In some embodiments, after reattaching the partially peeled cover film to the flexible circuit board, the method further includes: rolling the reattached cover film portion to make it fit tightly against the flexible circuit board.

[0013] The rolling process can remove residual air between the cover film and the flexible circuit board, enhance the adhesion of the cover film, avoid bubbles or warping in subsequent processes, and make the window edges smoothly conform to the area around the pads.

[0014] In some embodiments, the low-viscosity shielding film is a slightly tacky polyimide film or polyester film, one side of which is coated with self-adhesive to form a low-viscosity adhesive layer.

[0015] The polyimide or polyester film exhibits excellent temperature resistance, enabling it to withstand the instantaneous heat effects of laser processing without melting or carbonizing. The micro-adhesive properties ensure easy application and removal of the masking film without leaving adhesive residue on the pads, thus maintaining pad solderability.

[0016] In some embodiments, during laser cutting, alignment marks on a flexible circuit board are used to locate the opening position.

[0017] The alignment mark and the circuit pattern are in the same coordinate system. Using it for laser positioning can make the window pattern and the pads fit precisely, further eliminating the cumulative errors that may occur during bonding and transmission.

[0018] In some embodiments, the process of applying the covering film is performed using a roll-to-roll method.

[0019] Roll-to-roll processing enables continuous production, making it particularly suitable for the mass production of long, flexible circuit boards. The cover film, when taut, provides uniform coverage, reducing manual intervention and improving overall production speed.

[0020] In some embodiments, the edge of the low-viscosity shielding film extends beyond the window edge by a distance of 0.3 mm to 2 mm.

[0021] By controlling the distance between the edge of the shielding film and the edge of the window opening to be between 0.3mm and 2mm, sufficient process tolerance is provided to prevent the cut window from exceeding the protection range of the shielding film due to minor errors in laser positioning or misalignment of the cover film protrusions. At the same time, an excessively large shielding film area does not increase the difficulty of application or waste material. If the excess is less than 0.3mm, the safety margin is insufficient, and there is still a risk of damaging the solder pads. If the excess is greater than 2mm, the edge of the shielding film is too far from the solder pads, making manual or automatic removal difficult after partially peeling off the cover film, and potentially increasing unnecessary material costs. This range represents a reasonable balance between the precision of conventional FPC production and operational convenience, effectively balancing protection and process efficiency. Attached Figure Description

[0022] Figure 1 This is a flowchart of the method for window-opening and bonding the cover film of the flexible circuit board of the present invention. Figure 2 This is a cross-sectional view of the flexible circuit board of the present invention after it has been covered by a low-viscosity shielding film and a cover film. Figure 3 yes Figure 2 A magnified view of part A in the middle.

[0023] Figure label: 1. Covering film; 2. Low viscosity shielding film; 3. Flexible circuit board. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. It should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0025] The method of this invention is particularly applicable to the fabrication of flexible circuit boards for intracardiac ultrasound (ICE) catheters. In ICE examination systems, the distal ultrasound transducer array typically consists of dozens to hundreds of tiny piezoelectric elements or capacitive micromechanical ultrasound transducer (CMUT) units. These transducer units need to be precisely soldered onto the pads of a long strip of flexible circuit board (FPC) to achieve independent electrical signal excitation and reception. The typical length of this flexible circuit board can reach 200 mm to 500 mm, while its width is only 1 mm to 5 mm, and the pad width and spacing can be as small as less than 0.1 mm. During the examination, the catheter is inserted into the heart via a vascular access, and the distally integrated ultrasound transducer array is used to perform real-time imaging of the cardiac chamber structures.

[0026] This invention provides a method for precise windowing and bonding of a flexible circuit board cover film for cardiac ultrasound examination, which will be described in detail below with specific implementation steps.

[0027] Step 1: Provide a flexible circuit board 3, the surface of which is pre-formed with pad areas. These pads are the parts that need to be exposed for soldering connections later, and are typically made of copper foil and have undergone surface treatment.

[0028] Step 2: Apply a layer of low-viscosity masking film 2 to each pad area. This masking film has slight adhesion, allowing it to be temporarily fixed to the pad surface but easily removed later. Specifically, the area of ​​this low-viscosity masking film 2 needs to be larger than the area to be laser-cut; that is, the edge of the masking film should extend beyond the preset window boundary by a certain distance, typically about 0.5mm to 1mm on each side. The advantages of this are: ensuring the laser-cut window falls completely within the protection range of the masking film, preventing damage to the pad even with minor positioning errors; and facilitating removal of the extended edge when partially peeling off the covering film 1 later. As an alternative, the extension can be controlled between 0.3mm and 2mm, depending on the actual processing precision. When applying the masking film, ensure it completely covers the pad area, is symmetrically centered, and that the edges slightly extend beyond the pad and window boundaries.

[0029] Step 3: Apply the cover film 1 over the flexible circuit board 3 and the already attached low-viscosity masking film 2. Cover film 1 is an insulating film used to protect the circuitry, typically made of polyimide with a thermosetting adhesive layer on its underside. It is important to note that in this step, cover film 1 only utilizes the initial tack of its adhesive layer at room temperature for temporary adhesion (what is known in the industry as "false adhesion"), and no thermosetting curing is performed on cover film 1. This temporary adhesion is sufficient to keep cover film 1 stable during subsequent laser cutting and partial removal operations, preventing it from shifting, while allowing for easy partial removal later. Manual or automated lamination equipment can be used during application to ensure that there are no large wrinkles or air bubbles between cover film 1 and the board surface.

[0030] Step 4: Use a laser device to cut the cover film 1, with the cut position corresponding precisely to the pad area, thus creating the required window. It is important to note that the laser cutting depth needs to be precisely controlled, stopping exactly on the surface of the low-viscosity masking film 2, so as not to damage the pads underneath. The laser power and pulse frequency can be adjusted according to the thickness of the cover film 1; for example, a medium-power ultraviolet laser can be used for a cover film 1 with a thickness of approximately 25 micrometers.

[0031] Step 5: Partially peel off the portion of the cover film 1 corresponding to the solder pad area to expose the previously covered low-viscosity masking film 2. Since the cover film 1 was only temporarily adhered in Step 3, the adhesive layer was not heat-cured, and the surface of the low-viscosity masking film 2 has a certain release effect, the adhesion between the cover film 1 and the masking film is very weak. This portion of the cover film 1 can be easily peeled off without tearing or excessive deformation. The peeling process should be gentle to avoid affecting the temporary adhesion of other areas of the cover film 1.

[0032] Step 6: Remove the low-viscosity masking film 2 manually or using automated tools. Because the masking film is designed for low viscosity, it will not leave any adhesive residue on the pads when peeled off, and the pad area will be fully exposed.

[0033] Step 7: Reattach the previously partially peeled-off cover film 1 back onto the flexible circuit board 3, ensuring the opening is precisely aligned with the pad area using visual alignment or mechanical positioning. After reattaching, gently press to utilize the remaining initial tack of the adhesive layer for temporary fixation.

[0034] Step 8: Perform thermosetting and curing of the entire cover film 1. Specifically, the entire flexible circuit board 3, along with the re-attached cover film 1, is fed into a thermosetting device. Appropriate temperature and pressure are applied to fully heat-crosslink the thermosetting adhesive layer on the bottom surface of the cover film 1, thereby permanently and firmly adhering the cover film 1 to the surface of the flexible circuit board 3. This step covers all areas of the cover film 1, including areas that have never been peeled off and areas that have been re-attached, ensuring a uniform and strong bond between the entire cover film 1 and the flexible circuit board 3.

[0035] In this way, the entire process protects the pads from laser damage, ensures high-precision alignment between the window and the pads, and finally achieves permanent and reliable bonding strength through overall thermo-press curing, which is especially suitable for mass production of long flexible circuit boards 3.

[0036] Based on the above method, the present invention also provides a preferred embodiment. Before laser cutting (i.e., before step 4), the position of the laser window is compensated and adjusted according to the thickness of the low-viscosity shielding film 2. Because the low-viscosity shielding film 2 itself has a certain physical thickness, when it is attached to the pad area, the cover film 1 above it will locally bulge, forming a small slope. If the laser still cuts according to the theoretical plane position at this time, when the shielding film is removed and the cover film 1 returns to the flat state from the bulging state, the position of the previously cut window will be laterally offset relative to the pad. By pre-compensating the laser window position according to the thickness of the shielding film, this offset caused by the height difference of the bulge can be offset, thereby ensuring that the final flat window falls accurately above the pad. This compensation adjustment can be directly integrated into the control software of the laser equipment. The operator only needs to input the thickness value of the shielding film, and the equipment will automatically calculate and adjust the cutting coordinates, which is very convenient.

[0037] Preferably, the specific calculation method for the above compensation adjustment is as follows: The position compensation value is calculated based on the thickness of the low-viscosity shielding film 2 and the angle formed between the low-viscosity shielding film 2 and the covering film 1. The formula for calculating the position compensation value is: Compensation value = -T × tanθ, where T represents the vertical thickness of the low-viscosity shielding film 2, usually in micrometers or millimeters, and θ represents the angle formed between the low-viscosity shielding film 2 and the covering film 1, which is the slope angle caused by the bulge of the covering film 1. The negative sign indicates that the compensation direction is opposite to the thickness measurement direction; that is, if the shielding film causes the covering film 1 to bulge upwards, then the compensation should move in the opposite horizontal direction. The physical meaning of this formula is clear: the larger the thickness T, the higher the slope, and the greater the offset; the larger the angle θ, the steeper the slope, and the greater the offset for the same thickness. Through this simple tangent relationship, geometric parameters can be converted into precise compensation values, facilitating automatic execution by the laser equipment.

[0038] Building upon this, the present invention provides another improvement. Since the flexibility and surface properties of different materials for the cover film 1 vary, even when using a low-viscosity shielding film 2 of the same thickness, the included angle θ formed by the cover film 1 and the shielding film 2 will differ depending on the material. Therefore, tests can be conducted in advance on commonly used cover film 1 materials (e.g., different types of polyimide cover film 1) to establish a database of correspondences between material types and included angle θ. During actual production, it is only necessary to look up the pre-tested and determined included angle value from this database based on the currently used cover film 1 material, and then substitute it into the compensation formula. This eliminates the need to remeasure the included angle every time a batch of materials is changed, greatly improving the efficiency of production line changes and making the calculation of the compensation value more accurate and reliable.

[0039] It is worth noting that after completing the step of re-attaching the cover film 1 (i.e., step 7) and before the hot-pressing curing in step 8, the present invention may also include a rolling process. Specifically, after the partially peeled cover film 1 is re-attached to the flexible circuit board 3 and the opening is aligned with the pad, a roller is used to roll the re-attached portion of the cover film 1. The rolling operation can apply uniform pressure, expel any air that may remain between the cover film 1 and the flexible circuit board 3, and at the same time ensure that the adhesive layer at the bottom of the cover film 1 is in full contact with the board surface, thereby enhancing the flatness of the temporary adhesion and creating better contact conditions for subsequent hot-pressing curing. However, it should be noted that the final permanent bonding still depends on the overall hot-pressing curing in step 8, and the rolling is only an auxiliary preferred pre-pressing method.

[0040] Specifically, the low-viscosity masking film 2 adopts a single-layer structure. This masking film is a slightly tacky polyimide or polyester film with an adhesive coating on one side, forming a low-viscosity adhesive layer. In use, the adhesive-coated side faces the pad area, providing moderate temporary fixing force; the other side of the film (facing the cover film 1) utilizes the low surface energy of the polyimide or polyester material itself, providing a certain degree of natural release. When the laser cuts the cover film 1 and touches the masking film, the polyimide or polyester film itself can absorb or reflect most of the remaining laser energy, preventing the laser from penetrating further down to the pad surface, thus providing reliable physical protection for the pads. This design is because during laser windowing and partial removal of the cover film 1, if the contact surface between the cover film 1 and the masking film is too tacky, it can lead to difficulty in removal or even tearing of the cover film 1. By selecting polyimide or polyester film with low surface energy as the substrate, the adhesion between the cover film 1 and the masking film can be effectively reduced, allowing the cover film 1 to be easily separated. Simultaneously, the self-adhesive coating on the bottom surface ensures the stable positioning of the masking film on the pads. Furthermore, the film surface prevents trace amounts of molten material generated during laser cutting from directly adhering to the masking film, facilitating subsequent overall peeling. In this way, the single-layer structure balances adhesion stability, laser masking capability, and ease of film removal, further simplifying material structure and process operations, and reducing costs.

[0041] Furthermore, during the laser cutting process (i.e., step 4), this invention also utilizes pre-fabricated alignment marks on the flexible circuit board 3 to locate the window opening position. These alignment marks (commonly referred to as Mark points) and the circuit pattern on the flexible circuit board 3 are formed in the same photolithographic coordinate system, thus their positional relationship is very precise. By identifying these alignment marks, the laser equipment can accurately map the window opening pattern onto the actual position of the pad, thereby eliminating the overall offset caused by factors such as board deformation and transmission positioning errors. Combined with the aforementioned thickness compensation adjustment, dual guarantees of coarse positioning and fine compensation can be achieved, optimizing the window opening alignment accuracy.

[0042] Finally, in mass production, the process of applying the cover film 1 (i.e., step 3) can be performed using a roll-to-roll method. Roll-to-roll involves continuously unwinding the roll of cover film 1 and temporarily adhering it to the same roll of flexible circuit board 3 under tension (simple adhesion). Then, it is wound up for subsequent processes such as laser windowing, partial peeling, removal of the masking film, re-adhesion, and thermosetting curing. This method is particularly suitable for the mass production of long flexible circuit boards 3 because it enables continuous operation and avoids random deviations caused by manual operation or positioning fixtures in sheet bonding. Simultaneously, the roll-to-roll process significantly reduces loading and unloading auxiliary time, improving the overall production cycle time and automation level. During this process, the cover film 1 covers uniformly under constant tension, ensuring the consistency of the temporary adhesion.

[0043] It should be understood that the specific embodiments described above are merely illustrative or explanatory of the principles of the invention and do not constitute a limitation thereof. Therefore, any modifications, equivalent substitutions, improvements, etc., made without departing from the spirit and scope of the invention should be included within the protection scope of the invention. Furthermore, the appended claims are intended to cover all variations and modifications falling within the scope and boundaries of the appended claims, or equivalent forms of such scope and boundaries.

Claims

1. A method for precise windowing and bonding of a flexible circuit board cover film for cardiac ultrasound examination, characterized in that, Includes the following steps: A flexible circuit board (3) is provided, the surface of which is provided with pad areas; A low-viscosity masking film (2) is attached to the pad area. A cover film (1) is temporarily covered on the flexible circuit board (3) and the low viscosity shielding film (2). The cover film (1) is temporarily adhered to the flexible circuit board (3) and the low viscosity shielding film (2) by the adhesive layer on its bottom surface, and is not subjected to hot pressing curing. The cover film (1) is cut by laser to open a window at the position corresponding to the pad area. The cutting depth of the laser stops at the surface of the low viscosity shielding film (2). The cover film (1) is partially peeled off corresponding to the pad area to expose the low viscosity masking film (2). Remove the low-viscosity shielding film (2) to expose the pad area; The partially peeled-off cover film (1) is reattached to the flexible circuit board (3) so that the opening position is aligned with the pad area; The reattached cover film (1) is heat-pressed and cured to permanently adhere to the flexible circuit board (3).

2. The method according to claim 1, characterized in that, Before laser cutting, the position of the laser window is adjusted to compensate for the thickness of the low-viscosity shielding film (2).

3. The method according to claim 2, characterized in that, The compensation adjustment includes: calculating the position compensation value based on the thickness of the low viscosity shielding film (2) and the angle formed between the low viscosity shielding film (2) and the covering film (1); the calculation formula for the position compensation value is: compensation value = -T × tanθ, where T is the vertical thickness of the low viscosity shielding film (2), θ is the angle, and the negative sign indicates that the compensation direction is opposite to the thickness measurement direction.

4. The method according to claim 3, characterized in that, The compensation adjustment further includes: finding a pre-tested angle based on the material of the covering film (1), wherein the angle formed by the covering film (1) of different materials and the low viscosity shielding film (2) of the same thickness is different.

5. The method according to claim 1, characterized in that, After reattaching the partially peeled cover film (1) to the flexible circuit board (3), the method further includes: rolling the reattached cover film (1) portion to make it fit tightly against the flexible circuit board (3).

6. The method according to claim 1, characterized in that, The low-viscosity shielding film (2) is a slightly viscous polyimide film or polyester film, one side of which is coated with self-adhesive to form a low-viscosity adhesive layer.

7. The method according to claim 1, characterized in that, During laser cutting, the window opening position is located using the alignment marks on the flexible circuit board (3).

8. The method according to claim 1, characterized in that, The process of covering the covering film (1) is carried out in a roll-to-roll manner.

9. The method according to claim 1, characterized in that, The edge of the low-viscosity shielding film (2) extends beyond the window edge by 0.3 mm to 2 mm.

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