Improved SMT patch process for thin strip-shaped PCB board

Abstract

This invention discloses an SMT (Surface Mount Technology) process to improve the warping of thin, strip-shaped PCBs. The steps include: material preparation, board separation, tray placement, and mounting. This invention places the board separation step earlier, making each individual area an independent substrate, thereby initially releasing stress and preventing stress accumulation that could lead to warping later. The mounting fixture clamps and positions the separated substrates, preventing warping during mounting and providing a basis for accurate chip placement. Instead of using a target positioning method for the entire PCB, optical recognition points are set on each individual area as mounting reference positions, allowing for better chip alignment.

Description

Technical Field

[0001] This invention belongs to the field of flexible circuit board manufacturing technology, specifically relating to an SMT assembly process for improving the warping of thin strip PCB boards. Background Technology

[0002] In the SMT (Surface Mount Technology) production process of printed circuit boards (PCBs), many products have a limited number of micro-connection points between PCS units due to structural design limitations, or the PCB itself is thin (e.g., 0.05mm–0.15mm). This makes it easy for problems such as board warping, twisting, and deformation to occur during processes such as solder paste printing, chip mounting, and reflow soldering. In particular, board warping causes AOI and pick-and-place machine optical recognition failures, resulting in a high downtime rate; the number of processes increases, efficiency decreases, and the scrap rate rises.

[0003] Traditional solutions typically employ the following methods, but each has its own drawbacks: 1) Applying high-temperature PI tape to the back of the PCB can leave adhesive residue, affecting conductivity and appearance; 2) Using integral cover plate pressing results in a complex cover plate structure, poor versatility, and high cost; 3) Adding extra support blocks or positioning pillars can cause these positioning structures to warp after being heated, leading to problems such as open solder joints, poor solder joints, floating components, and component misalignment.

[0004] Chinese patent CN106793565A discloses a welding process for FPC (Flexible Printed Circuit) electronic components prone to warping. This process targets FPCs (warping originates from the high-temperature deformation of flexible materials), rather than rigid PCBs. The warping is caused by an insufficient number of micro-connection points in the design layout, so a magnetic base plate and magnetic steel sheets are used to clamp the FPC components. However, the specific structure of the FPC components is not described here. For thin, strip-shaped products, the cumulative stress release from the rapid temperature changes during lamination, drilling, surface treatment, baking, and reflow soldering can lead to severe warping.

[0005] Therefore, it is necessary to improve the SMT (Surface Mount Technology) process to solve the above problems. Summary of the Invention

[0006] The main objective of this invention is to provide an SMT placement process that improves the warping of thin strip PCB boards. This process allows the stress of individual substrates to be gradually released through the board separation step. Based on the pressing and positioning of the individual substrates by the placement fixture, optical recognition points are used as the reference positions for placement, so that the chips can be placed more accurately.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: an SMT assembly process for improving warping of thin strip PCB boards, comprising the following steps: S1. Raw material preparation: manufacture a PCB board with a thickness of 0.05mm–0.15mm. The PCB board has multiple elongated single-piece areas arranged side by side. The aspect ratio of the single-piece area is at least 10:1. Adjacent single-piece areas are connected by multiple connection points. Each single-piece area has several product holes. S2, Separation: Cut all connection points to separate the single-piece area from the PCB board, resulting in multiple single-piece substrates; S3. Plating: A mounting fixture is provided, comprising a base plate, a support plate, and a cover plate. The support plate has several support plate precision positioning holes, and the cover plate has several cover plate precision positioning holes and several mounting through holes in its center. Multiple precision positioning pins are vertically arranged in the center of the base plate. The precision positioning holes of the support plate, the precision positioning holes of the cover plate, and the precision positioning pins are arranged in the same way. During plating, the base plate is first placed with the pin-side facing up. Then, using the correspondence between the precision positioning holes of the support plate and the precision positioning pins, the support plate is placed on top of the base plate. Next, using the correspondence between the product holes and the precision positioning pins, the individual substrates are placed one by one onto the support plate. Finally, using the correspondence between the precision positioning holes of the cover plate and the precision positioning pins, the cover plate is placed on top of all the individual substrates to obtain the first assembly. S4. Mounting: Remove the bottom plate from the lower part of the first assembly. The support plate and the cover plate clamp all the single substrates to form a second assembly. The second assembly is sent into the mounting machine with the cover plate on top. The chip is pre-mounted to the mounting position of the exposed single substrate through the mounting holes and soldered in place to obtain a third assembly. The chip and the single substrate form a PCB unit.

[0008] Specifically, the upper surface of the pallet is provided with multiple strip-shaped positioning grooves arranged side by side. The range of the strip-shaped positioning grooves matches the outer contour of the single substrate, and all the precision positioning holes of the pallet are located within the range of the strip-shaped positioning grooves.

[0009] Specifically, in the raw material preparation process, an optical identification point is provided at each end of the single-piece area, and an observation hole is provided on the cover plate to expose the optical identification point. The diameter of the optical identification point is smaller than the diameter of the observation hole. The mounting step uses the optical identification point as a reference to mount the chip onto the single-piece substrate.

[0010] Furthermore, the diameter of the optical recognition point is 0.1-0.5 mm smaller than the diameter of the observation hole, and the outline of the patch perforation is 0.1-0.5 mm larger than the outline of the chip.

[0011] Specifically, after the mounting step, there is a functional testing step: while keeping the third assembly intact, a functional test is performed to distinguish between qualified and defective products among all PCB individual products. Then, the third assembly is disassembled, and qualified products are kept while defective products are removed.

[0012] Specifically, the tray and the cover are made of a magnetic material that attracts each other from the surface.

[0013] Specifically, the tray has several heat dissipation holes located near the patch perforation.

[0014] Specifically, multiple coarse positioning posts are erected around the perimeter of the base plate. The height of the coarse positioning posts is higher than that of the fine positioning pin, and their diameter is larger than that of the fine positioning pin. The perimeter of the support plate is provided with support plate coarse positioning holes that match the coarse positioning posts, and the perimeter of the cover plate is provided with cover plate coarse positioning holes that match the coarse positioning posts.

[0015] Specifically, each side of the tray has a tray notch, and the innermost side of the tray notch is located inside the edge of the cover plate.

[0016] Specifically, each side of the base plate has a base plate notch, and the innermost side of the base plate notch is located inside the edge of the support plate.

[0017] The beneficial effects of the technical solution of this invention are: 1. By advancing the board separation process, each area becomes an independent substrate, thereby initially releasing stress and preventing stress from accumulating until later and causing warping problems. 2. The mounting fixture clamps and positions the separate single substrates, preventing warping during the mounting process and providing a basis for accurate chip placement; 3. Instead of using a target positioning method on the entire PCB board, optical recognition points are set on a single area as the reference position for mounting, so that the chip can be mounted more accurately. Attached Figure Description

[0018] Figure 1 This diagram shows the structural changes during the board separation step in the SMT (Surface Mount Technology) assembly process. Figure 2 This is a top view of the three parts of the patch fixture; Figure 3 This is a top view of the first assembly; Figure 4 This is a partial schematic diagram of the stacked structure of the third assembly.

[0019] The diagram is marked as follows: 100-Patch fixture, 1-Base plate, 11-Precision positioning pin, 12-Coarse positioning pin, 13-Base plate notch, 2-Support plate, 21-Strip positioning groove, 22-Support plate precision positioning hole, 23-Support plate coarse positioning hole, 24-Heat dissipation hole, 25-Support plate notch, 3-Cover plate, 31-Cover plate precision positioning hole, 32-Patch perforation, 33-Cover plate coarse positioning hole, 34-Observation hole; 200 - PCB board, 201 - Single area, 202 - Product hole, 203 - Connection point, 204 - Optical recognition point, 205 - Target; 300 - PCB unit, 301 - single substrate, 3011 - mounting position, 302 - chip. Detailed Implementation

[0020] This invention relates to the field of SMT (Surface Mount Technology) assembly technology, specifically to an SMT assembly process for improving the warping of thin strip PCBs. It primarily targets thin strip PCBs with a thickness of 0.05mm–0.15mm and an aspect ratio of not less than 10:1, addressing technical challenges such as warping, low mounting accuracy, and poor soldering quality during SMT assembly. By optimizing the fixture structure and process steps, precise mounting and warping control of thin strip PCBs are achieved, improving product yield and production efficiency.

[0021] The process will be described in detail below with reference to specific embodiments. These embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0022] Example I. Detailed structural description of the patch fixture like Figure 2 As shown, the mounting fixture 100 used in this process has a three-layer collaborative structure, consisting of a base plate 1, a support plate 2, and a cover plate 3. These three layers work together precisely to achieve accurate positioning and stable clamping of the single substrate 301, providing reliable assurance for subsequent mounting and welding processes and effectively preventing warping or displacement of the thin strip-shaped substrate 301 during processing. The specific parameters, material selection, and functional design of each layer are as follows, and all component numbers correspond to each other: 1.1 Structure of base plate 1 As the foundational load-bearing component of the entire surface mount fixture 100, the flatness and rigidity of the base plate 1 directly determine the accuracy of the entire positioning system. Therefore, the base plate 1 is made of high-strength rigid material, preferably stainless steel or aluminum alloy, with its thickness strictly controlled within the range of 5–8 mm. This thickness range has been verified through multiple tests to effectively ensure the flatness and rigidity of the base plate 1 itself, preventing deformation due to insufficient weight, inadequate material strength, or uneven stress, which could lead to positioning deviations of the subsequent tray 2, cover plate 3, and single substrate 301. It also ensures ease of operation, preventing the base plate 1 from being too heavy and causing inconvenience for operators in placing and removing trays. After the tray 2, the PCB board 200, and the cover plate 3 are stacked, the base plate 1 has completed its function and must be removed, leaving only the thinner tray 2 and cover plate 3. Otherwise, the complete rigid board beneath the base plate 1 will block the heat dissipation path, causing warping problems; the fine positioning pins 11 and coarse positioning posts 12 will also protrude above the cover plate 3, affecting the placement of the chip 302.

[0023] Multiple precision positioning pins 11 are vertically installed in the middle of the base plate 1. These precision positioning pins 11 are the core components for achieving precise positioning of the single substrate 301, the support plate 2, and the cover plate 3. Their number, size, and position are completely consistent with the layout of the product hole 202 on the single substrate 301, the support plate precision positioning hole 22 on the support plate 2, and the cover plate precision positioning hole 31 on the cover plate 3, ensuring that the four can be precisely aligned and seamlessly fitted. The diameter of the precision positioning pins 11 is controlled between 0.7 and 1.1 mm, and is strictly 0.1 mm smaller than the diameter of the product hole 202. This dimensional difference is precisely calculated to ensure that the precision positioning pins 11 can be smoothly inserted into the product hole 202, avoiding difficulty in insertion or scratching of the product hole 202 due to excessive size. It also ensures that the precision positioning pins 11 fit tightly against the inner wall of the product hole 202, with the gap controlled within a reasonable range, thereby achieving precise positioning of the single substrate 301 and preventing horizontal offset during mounting and welding.

[0024] The height of the precision positioning pin 11 is set to 3–5 mm. It is made of corrosion-resistant, high-strength stainless steel, and its surface undergoes fine polishing. After polishing, the surface of the precision positioning pin 11 is smooth, burr-free, and scratch-free, with a roughness controlled below Ra0.8. This design effectively prevents the precision positioning pin 11 from scratching the inner wall of the product hole 202, the wall of the precision positioning hole 22 of the pallet, and the wall of the precision positioning hole 31 of the cover plate during insertion into the product hole 202, the precision positioning hole 22 of the pallet, and the precision positioning hole 31 of the cover plate, as well as during contact with the single-piece substrate 301. It also prevents scratching the circuitry and pads on the surface of the single-piece substrate 301, ensuring the stability of the product's appearance quality and positioning accuracy. The precision positioning pin 11 is fixed to the base plate 1 by welding. The weld joint is free of deformation and protrusions, ensuring that the precision positioning pin 11 is perpendicular to the surface of the base plate 1, with a perpendicularity deviation not exceeding 0.02 mm, further improving positioning accuracy.

[0025] Meanwhile, multiple coarse positioning posts 12 are erected around the perimeter of the base plate 1. The number of coarse positioning posts 12 is 4-6, evenly distributed at the four corners and center of the base plate 1, preferably 6 (one at each corner and one at the center of each long side). This distribution method enables omnidirectional coarse positioning of the tray 2 and cover plate 3, preventing significant misalignment during assembly and assisting the fine positioning pin 11 in achieving precise positioning. The height of the coarse positioning posts 12 is higher than that of the fine positioning pin 11, typically 1-2 mm higher, and their diameter is larger than that of the fine positioning pin 11, typically 2-3 mm. They are also made of stainless steel with a polished surface. The main function of the coarse positioning posts 12 is to achieve coarse positioning of the tray 2 and cover plate 3, assisting the fine positioning pin 11 in precise positioning, preventing misalignment during assembly, and providing guidance for the assembly of the tray 2 and cover plate 3, thus improving tray placement efficiency.

[0026] In addition, each side of the base plate 1 has a base plate notch 13. The innermost side of the base plate notch 13 is located inside the edge of the tray 2. The core purpose of this design is to facilitate the operator to quickly and smoothly separate the base plate 1 after tray placement. The operator can pinch the edge of the tray 2 through the base plate notch 13 to pull the second assembly (the whole formed by the tray 2 and the cover plate 3 clamping all the single substrates 301) from the base plate 1, laying the foundation for subsequent mounting processes. The lower surface of the base plate 1 is also provided with anti-slip texture to prevent the base plate 1 from sliding during tray placement, further improving positioning stability.

[0027] 1.2 Structure of tray 2 The tray 2, serving as the support and positioning component for the single substrate 301, needs to achieve precise fit with the base plate 1 and magnetic adsorption with the cover plate 3. Therefore, the tray 2 is made of magnetic material to ensure a tight fit with the cover plate 3, effectively clamping the single substrate 301 and preventing loosening or warping of the single substrate 301 during the mounting process. The thickness of the tray 2 is controlled at 2–3 mm to ensure its own structural strength, prevent deformation under stress, and also ensure the magnetic adsorption effect. At the same time, it avoids excessive thickness, which would make the entire mounting fixture 100 too large and inconvenient to operate.

[0028] The tray 2 has several tray positioning holes 22. The number, size, and position of the tray positioning holes 22 are completely consistent with the positioning pins 11 on the base plate 1. The diameter of the tray positioning holes 22 is 0.05–0.1 mm larger than the diameter of the positioning pins 11. This gap design facilitates the tray 2 to be quickly fitted onto the positioning pins 11, improving the tray placement efficiency, while also ensuring positioning accuracy and preventing the tray 2 from shifting due to excessive gap, thus ensuring precise alignment between the tray 2 and the base plate 1. The walls of the tray positioning holes 22 are smoothed to avoid scratching the surface of the positioning pins 11.

[0029] The upper surface of the tray 2 has multiple strip-shaped positioning grooves 21 arranged side by side. The strip-shaped positioning grooves 21 are the direct supporting components of the single substrate 301, and their number is exactly the same as the number of single substrates 301, ensuring that each single substrate 301 can be placed in a corresponding strip-shaped positioning groove 21. The range of the strip-shaped positioning grooves 21 is completely matched with the outer contour of the single substrate 301, including its length, width, and rounded corners. The rounded corner size is consistent with the rounded corners of the single substrate 301 to avoid sharp edges scratching the single substrate 301. The depth of the strip positioning groove 21 is 0.03–0.05 mm, which is slightly less than the thickness of the single substrate 301 (the thickness of the single substrate 301 is consistent with that of the PCB board 200, which is 0.05 mm–0.15 mm). This depth design ensures that after the single substrate 301 is placed in the strip positioning groove 21, its upper surface can be flush with or slightly higher than the upper surface of the tray 2 (the difference does not exceed 0.02 mm). This facilitates the tight fit between the cover plate 3 and the tray 2, achieving stable clamping of the single substrate 301. At the same time, it avoids the strip positioning groove 21 being too deep, which would make it difficult to remove the single substrate 301, or too shallow, which would not be able to play an effective positioning and anti-warping role.

[0030] All the precision positioning holes 22 on the pallet are located within the range of the strip positioning groove 21, and correspond one-to-one with the positions of the product holes 202 on the single substrate 301. This ensures that the precision positioning pin 11 can pass through the precision positioning holes 22 on the pallet and the product holes 202 on the single substrate 301 in sequence, achieving dual positioning of the pallet 2 and the single substrate 301, further improving positioning accuracy and preventing the single substrate 301 from shifting within the strip positioning groove 21. The inner wall of the strip positioning groove 21 is finely polished, with a smooth surface free of burrs, to avoid scratching the surface lines and pads of the single substrate 301.

[0031] In addition, each side of the tray 2 has a tray notch 25, the shape of which is the same as the base plate notch 13 (both are rectangular or arc-shaped), and the size is slightly smaller than the base plate notch 13 (the reduction range is 0.5–1 mm). The innermost side of the tray notch 25 is located inside the edge of the cover plate 3. The purpose of this design is to facilitate the operator to quickly separate the tray 2 and the cover plate 3 through the tray notch 25 when disassembling the third assembly after mounting, avoiding contact with the PCB component 300 during disassembly, preventing the chip 302 from falling off and the substrate 301 from warping, while improving disassembly efficiency and reducing the labor intensity of the operator.

[0032] Meanwhile, the tray 2 has several heat dissipation holes 24 near the mounting holes 32. The core function of the heat dissipation holes 24 is to dissipate the heat generated during the mounting and soldering process in a timely manner, preventing heat accumulation that could cause deformation or warping of the tray 2, cover plate 3, or single substrate 301. This is especially true for thin strip-shaped single substrates 301, which are prone to warping when heated. The heat dissipation holes 24 effectively alleviate this problem and ensure mounting quality. The heat dissipation holes 24 are circular, with a diameter of 0.8–1.2 mm. There are 2–4 holes corresponding to each strip positioning slot 21, evenly distributed around the mounting holes 32 to ensure uniform heat dissipation. The layout of the heat dissipation holes 24 must strictly avoid the mounting area of ​​the single substrate 301 and the product hole 202 to avoid affecting mounting quality and positioning accuracy. At the same time, the heat dissipation holes 24 should not be too dense, which could reduce the structural strength of the tray 2.

[0033] The pallet 2 is also provided with coarse positioning holes 23 around its perimeter, matching the coarse positioning pins 12. The number and position of the coarse positioning holes 23 are exactly the same as those of the coarse positioning pins 12. The diameter of the coarse positioning holes 23 is 0.05–0.1 mm larger than that of the coarse positioning pins 12, facilitating the smooth passage of the coarse positioning pins 12 and achieving coarse positioning of the pallet 2. In conjunction with the fine positioning pins 11 and the fine positioning holes 22, the positioning accuracy and stability of the pallet 2 are further improved, preventing the pallet 2 from shifting on the base plate 1. The walls of the coarse positioning holes 23 are also smoothed to ensure smooth engagement with the coarse positioning pins 12.

[0034] 1.3 Structure of cover plate 3 As a clamping component for the single substrate 301, the cover plate 3 needs to achieve tight adhesion with the tray 2 while providing operating space for the mounting process. Therefore, the cover plate 3 and the tray 2 are made of the same magnetic material (preferably neodymium iron boron magnets) to ensure that they can attract each other surface-to-surface with uniform adhesion force, achieving stable clamping of the single substrate 301. The clamping force is controlled at 0.5–1N, which ensures that the single substrate 301 does not loosen or warp, while also preventing damage to the single substrate 301 due to excessive clamping force. The thickness of the cover plate 3 is 2–3mm, consistent with the thickness of the tray 2, and its dimensions are perfectly matched with the tray 2 and the base plate 1, ensuring that the cover plate 3 can stably cover the tray 2 and be precisely aligned with the tray 2 without offset or misalignment.

[0035] The cover plate 3 has several precision positioning holes 31 and several patch through holes 32 in its center. The number, size, and position of the precision positioning holes 31 are completely consistent with the precision positioning pins 11 on the base plate 1 and the precision positioning holes 22 on the support plate 2. The diameter of the precision positioning holes 31 is 0.05–0.1 mm larger than the diameter of the precision positioning pins 11, and the gap between them is consistent with the precision positioning holes 22 on the support plate. This facilitates the quick fitting of the cover plate 3 onto the precision positioning pins 11, achieving precise alignment between the cover plate 3 and the support plate 2 and the base plate 1. This ensures that the cover plate 3 can evenly cover all the single substrates 301 and achieve stable clamping. The hole walls of the precision positioning holes 31 are finely processed to avoid scratching the precision positioning pins 11.

[0036] The through-holes 32 are crucial channels for chip 302 mounting. Their number is exactly the same as the number of mounting positions 3011 on the substrate 301, with one through-hole 32 corresponding to each mounting position 3011. The outline of the through-hole 32 is 0.1–0.5 mm larger than the outline of the chip 302, preferably 0.3 mm. This expansion dimension has been verified through multiple tests to ensure that the nozzle of the mounting machine can smoothly pass through the through-holes 32 to accurately mount the chip 302 to the mounting position 3011 on the substrate 301. It also avoids the through-holes being too large, causing chip 302 misalignment, or too small, preventing the nozzle from operating properly and touching the edge of the cover plate 3, thus affecting mounting accuracy and efficiency. The edges of the through-holes 32 are chamfered to prevent sharp corners from scratching the nozzle or the chip 302.

[0037] The cover plate 3 is also provided with observation holes 34 for exposing the optical recognition points 204. The number of observation holes 34 is exactly the same as the number of optical recognition points 204. The optical recognition points 204 are located at both ends of the single-piece area 201 of the PCB board 200. Therefore, each single-piece area 201 has two observation holes 34 at the corresponding position of the cover plate 3, corresponding to the optical recognition points 204 at both ends of the single-piece area 201. The observation holes 34 adopt a circular design. The diameter of the optical recognition point 204 is 0.1–0.5 mm smaller than the diameter of the observation hole 34, preferably 0.3 mm. This size design ensures that the optical recognition point 204 can be fully exposed, which is convenient for the optical recognition system of the placement machine to accurately identify the chip 302 based on the optical recognition point 204, further improving the placement accuracy. At the same time, it avoids the observation holes 34 being too large, which would reduce the contact area between the cover plate 3 and the single-piece substrate 301, affecting the clamping effect and preventing the single-piece substrate 301 from warping or shifting during the placement process.

[0038] The periphery of the cover plate 3 is also provided with cover plate coarse positioning holes 33 that match the coarse positioning pins 12. The number and position of the cover plate coarse positioning holes 33 are completely consistent with the coarse positioning pins 12 and the coarse positioning holes 23 of the support plate. The diameter of the cover plate coarse positioning holes 33 is 0.05–0.1 mm larger than the diameter of the coarse positioning pins 12, and the gap with the coarse positioning holes 23 of the support plate is consistent. It can perfectly cooperate with the coarse positioning holes 23 of the support plate and the coarse positioning pins 12 to achieve coarse positioning of the cover plate 3. It assists the cover plate fine positioning holes 31 in cooperating with the fine positioning pins 11 to further improve the positioning accuracy of the cover plate 3, ensure that the cover plate 3 and the support plate 2 are accurately aligned, and the clamping force is evenly distributed on the single substrate 301.

[0039] II. Detailed Steps of SMT (Surface Mount Technology) Assembly Process like Figure 1 , Figure 3 and Figure 4 As shown, this improved SMT placement process for thin strip PCBs aims to suppress warping and improve placement accuracy. Based on the structural characteristics of the placement fixture 100, the process is structured in steps to ensure precise parameters and high operability at each stage. The specific steps are as follows, with each component labeled with a corresponding number: 2.1 Step S1: Raw material preparation Raw material preparation is fundamental to ensuring the smooth progress of subsequent processes and improving product quality. The core of this step is to manufacture a complete PCB board (200) that meets the requirements. The specific operations are as follows: A PCB board 200 with a thickness of 0.05mm–0.15mm is manufactured. The PCB board 200 is made of FR-4 substrate and undergoes an anti-oxidation treatment to ensure good solderability. Multiple elongated single-piece areas 201 are arranged side-by-side on the PCB board 200. Each single-piece area 201 serves as the prototype for the subsequent single-piece substrate 301, with an aspect ratio of at least 10:1. The specific dimensions can be flexibly adjusted according to actual product requirements. Adjacent single-piece areas 201 are connected by multiple connection points 203. The number of connection points 203 is generally 3–5 between each adjacent single-piece area 201, distributed along the length of the single-piece area 201. The width of the connection points 203 is 0.5–1mm, and the thickness is the same as that of the PCB board 200. Since ordinary PCB boards are relatively small, they are often manufactured as a whole board first and then segmented. During the manufacturing stage, the PCB board 200 uses its surrounding targets 205 as positioning references. Connection point 203 is a connection location formed during the manufacturing process of the PCB board 200, ensuring the structural integrity of the PCB board 200. Connection point 203 enables the single-piece areas 201 on both sides to exert force, ensuring the high flatness of the PCB board 200 in the previous manufacturing steps, but this also brings the problem of stress not being released.

[0040] Each individual substrate area 201 has several product holes 202. The number, size, and position of the product holes 202 correspond exactly to the precision positioning pins 11 of the placement fixture 100, which are used for the precise positioning of the subsequent individual substrates 301. The diameter of the product holes 202 is 0.1 mm larger than that of the precision positioning pins 11 to ensure that the precision positioning pins 11 can be inserted smoothly. At the same time, an optical recognition point 204 is provided at each end of each individual substrate area 201. The optical recognition point 204 is generally circular with a diameter of 0.8–1.2 mm and has a metallic luster, so as to ensure that the optical recognition system of the placement machine can clearly recognize it. The position of the optical recognition point 204 corresponds exactly to the observation hole 34 on the cover plate 3, ensuring that the observation hole 34 can fully expose the optical recognition point 204. Although the PCB board 200 will have target 205 structures set in the surrounding non-product areas during the manufacturing process, the non-product areas will be cut off in advance and can no longer be used as welding positioning references because the board separation step is followed immediately in the new process. Therefore, it is necessary to pre-set optical recognition points 204 within the range of the single area 201.

[0041] In addition, each individual area 201 is equipped with several mounting positions 3011. The number and size of the mounting positions 3011 are perfectly matched with the chip 302. The surface of the mounting positions 3011 is treated with solder pads, and the size of the solder pads is slightly larger than the pin size of the chip 302 to ensure a firm solder joint. After the raw materials are prepared, the entire PCB board 200 needs to be visually inspected to remove defective products with scratches, broken circuits, misaligned product holes 202, or blurred optical identification points 204, thus ensuring the quality of the raw materials.

[0042] 2.2 Step S2: Plate Separation The purpose of the depaneling step is to disconnect the connection points 203 between individual sections 201 and remove the useless parts around the periphery of the entire PCB board 200, so as to initially release the material stress. The specific operation is as follows: A depaneling machine is used to depanel the PCB board 200. The blade thickness of the depaneling machine is controlled at 0.1–0.2 mm, and the depaneling speed is 5–10 mm / s. During depaneling, the PCB board 200 is placed stably on the worktable of the depaneling machine and accurately positioned to ensure that the blade of the depaneling machine is aligned with the connection point 203 between adjacent single-piece areas 201. All connection points 203 are completely cut off, so that the single-piece area 201 is separated from the PCB board 200 to obtain multiple independent single-piece substrates 301.

[0043] During the separation process, the separation force and speed must be strictly controlled to avoid excessive force causing warping or circuit damage to the individual substrate 301, or excessive speed causing unevenness or burrs. After separation, each individual substrate 301 is removed and placed in a dedicated storage box lined with soft cushioning material (such as sponge) to prevent collisions and scratches between the individual substrates 301. Simultaneously, each individual substrate 301 undergoes a visual inspection, rejecting defective products with warping, circuit damage, blocked product holes 202, or damaged optical identification points 204, ensuring that the individual substrates 301 meet the requirements for subsequent tray placement and mounting.

[0044] 2.3 Step S3: Plating The tray placement step is crucial for achieving precise positioning of the single substrate 301 and preventing warping. It requires the cooperation of the base plate 1, support plate 2, and cover plate 3 of the mounting fixture 100 to precisely fix the single substrate 301, resulting in the first assembly. The specific operation is as follows, and must be strictly performed in sequence: Step 1, Preparation before placement: Clean the base plate 1, tray 2, and cover plate 3 of the placement fixture 100. Wipe the surface with a lint-free cloth dampened with anhydrous ethanol to remove dust, oil, and other impurities to avoid affecting the positioning accuracy or placement quality. Check whether the precision positioning pins 11 and coarse positioning pins 12 are intact and free from deformation or burrs. Check whether the precision positioning holes 22 and 23 of the tray, the precision positioning holes 31 and 33 of the cover plate, the placement through hole 32, and the observation hole 34 are blocked or have burrs. Ensure that all components are intact and clean.

[0045] The second step is to place the base plate 1: Place the base plate 1 with the pin side (the side with the fine positioning pin 11 and the coarse positioning pin 12) facing up on the tray placement worktable, adjust the position of the base plate 1 to ensure that the base plate 1 is stable and does not wobble, and use the positioning blocks of the worktable to fix the base plate 1 to prevent the base plate 1 from shifting during the tray placement process.

[0046] The third step is to place the tray 2: First, align the coarse positioning hole 23 of the tray with the coarse positioning pin 12, allowing the coarse positioning pin 12 to pass smoothly through the coarse positioning hole 23. At this point, there is only a small deviation between the fine positioning hole 22 and the fine positioning pin 11. Therefore, using their correspondence, slowly slide the tray 2 onto the top of the base plate 1, ensuring that the tray 2 is placed stably on the base plate 1, with the lower surface of the tray 2 tightly fitted to the upper surface of the base plate 1 without gaps or offset. During placement, avoid excessive force that could deform the fine positioning pin 11 or the coarse positioning pin 12, or scratch the tray 2.

[0047] The fourth step is to place the single substrate 301: Utilizing the correspondence between the product holes 202 on the single substrate 301 and the precision positioning pins 11 on the base plate 1, the single substrates 301 are placed one by one into the strip positioning grooves 21 of the tray 2. This ensures that the product holes 202 of each single substrate 301 fit onto the corresponding precision positioning pins 11, and that the outer contour of the single substrate 301 is completely fitted with the strip positioning grooves 21 without any offset or looseness. The precision positioning pins 11 provide temporary horizontal position correction for the single substrates 301 after initial stress relief (mainly in the area near the product holes 202), while the strip positioning grooves 21 also provide temporary correction for the outer contour of the single substrates 301, preparing for the mounting of the chip 302. During placement, operators must wear dust-free gloves to avoid touching the mounting area, optical identification point 204, and product hole 202 of the single substrate 301 with their fingers to prevent contamination or scratches. At the same time, ensure that the upper surface of the single substrate 301 is flush with or slightly higher than the upper surface of the tray 2. If it does not meet the requirements, it needs to be readjusted.

[0048] Step 5, placing the cover plate 3: First, align the coarse positioning hole 33 of the cover plate with the coarse positioning post 12, allowing the coarse positioning post 12 to pass smoothly through the coarse positioning hole 33. At this point, there is only a small deviation between the fine positioning hole 31 and the fine positioning pin 11 of the cover plate. Therefore, using their correspondence, slowly place the cover plate 3 above all the single substrates 301. The lower surface of the cover plate 3 is tightly attached to the upper surface of the single substrate 301. Under the action of magnetic force, the cover plate 3 and the support plate 2 are tightly attracted, achieving stable clamping of the single substrates 301. Each single substrate 301 will be flattened to avoid vertical warping. With the strip positioning groove 21 on the surface of the support plate 2, the contour of the flattened single substrate 301 can also maintain the state required for mounting, especially the optical recognition point 204 can be properly exposed from the observation hole 34. Considering that the optical identification point 204 also has a certain degree of error, the size of the observation hole 34 will be appropriately enlarged (0.1-0.5mm) relative to the size of the optical identification point 204, and the size of the surface mount through hole 32 will also be appropriately enlarged (0.1-0.5mm) relative to the designed chip mounting position 3011. During placement, it is necessary to ensure that the cover plate 3 and the support plate 2 are precisely aligned to avoid uneven stress and warping of the substrate 301 caused by the cover plate 3 shifting. At the same time, it is necessary to check whether the observation hole 34 corresponds precisely to the optical identification point 204 of the substrate 301 to ensure that the optical identification point 204 can be fully exposed.

[0049] Step 6: Inspect the first assembly: After the tray is set up, the first assembly is obtained, consisting of base plate 1, support plate 2, single substrate 301, and cover plate 3. A comprehensive inspection of the first assembly is conducted to confirm that support plate 2 and cover plate 3 are not misaligned, single substrate 301 is not warped or misaligned, precision positioning pin 11 and coarse positioning post 12 are not deformed, and observation hole 34 and patch perforation hole 32 are not blocked. Any unqualified first assemblies must be disassembled and re-set up to ensure the quality of the tray setting.

[0050] 2.4 Step S4: Mounting The mounting step is the core step to achieve precise bonding and soldering between the chip 302 and the monolithic substrate 301. It requires pre-mounting and soldering of the chip 302 while ensuring the monolithic substrate 301 is free of warping, to obtain the third assembly. The specific operations are as follows: Step 1: Remove base plate 1. The operator, wearing clean gloves, pinches the edges of the support plate 2 and cover plate 3 through the notch 13 on base plate 1 and slowly and steadily removes them from base plate 1. During the removal process, the magnetic force automatically ensures that the support plate 2 and cover plate 3 maintain a clamping state on all individual substrates 301. After removing base plate 1, support plate 2, cover plate 3, and individual substrates 301 form the second assembly. Although the precision positioning pin 11 no longer plays a positioning role for the product hole 202 at this time, the groove wall of the strip positioning groove 21 can still maintain the contour shape of the individual substrates 301, keeping the individual substrates 301 with appropriate flatness.

[0051] The second step is to prepare the placement machine: Start the placement machine and adjust its parameters, including selecting the nozzle model (choose a matching nozzle based on the size of the chip 302), placing speed (controlled at 10–15 pieces / minute), placing pressure (controlled at 0.3–0.5N), and optical recognition parameters (adjusting the recognition accuracy to ensure clear recognition of the optical recognition point 204). Simultaneously, place the chip 302 into the placement machine's tray and precisely position the tray to ensure the nozzle can accurately pick up the chip 302.

[0052] The third step is to feed in the second assembly: With the cover plate 3 on top and the tray 2 on the bottom, smoothly feed the second assembly onto the worktable of the placement machine. Use the positioning device of the worktable to fix the second assembly in place, ensuring that the second assembly does not shake or shift. During the feeding process, avoid touching the edges of the second assembly to prevent the tray 2 from separating from the cover plate 3 and the single substrate 301 from shifting.

[0053] Step 4, chip pre-placement and soldering: After the placement machine is started, the optical recognition system is used to identify the optical recognition points 204 at both ends of the single substrate 301 (identified through the observation holes 34 of the cover plate 3). Using the optical recognition points 204 as a reference, the position of the nozzle is adjusted so that the nozzle passes through the placement through holes 32 of the cover plate 3, picks up the chip 302 from the tray, and accurately pre-places it onto the placement position 3011 of the single substrate 301. After pre-placement, the position of the chip 302 is checked to ensure that the chip 302 is accurately aligned with the placement position 3011.

[0054] After pre-mounting, the second assembly is sent to the soldering area of ​​the mounting machine and soldered using a reflow soldering process. The soldering temperature is controlled at 220–260℃, and the soldering time is 3–5 minutes. During the soldering process, the heat dissipation holes 24 on the tray 2 dissipate the heat generated during soldering in a timely manner, preventing heat accumulation that could cause deformation or warping of the tray 2, cover plate 3, or single substrate 301. After soldering, the chip 302 is firmly bonded to the single substrate 301, forming a PCB single product 300. At this point, the second assembly becomes a third assembly consisting of the tray 2, cover plate 3, and PCB single product 300.

[0055] Step 5: Remove the third assembly: After soldering, remove the third assembly from the placement machine and place it on the cooling table to cool naturally. After cooling to room temperature (25±5℃), proceed with subsequent processing to avoid PCB individual item 300 deformation and chip 302 falling off due to disassembly under high temperature.

[0056] 2.5 Step S5: Functional Testing and Disassembly To ensure product quality, a functional testing step is added after the mounting step. While keeping the third assembly intact, functional tests are performed on individual PCB components (300 units) to distinguish between qualified and defective products. The specific procedures are as follows: The third assembly, cooled to room temperature, is placed on a functional test bench. Test probes are used to contact the pads of PCB component 300 through the surface mount through-hole 32 of cover plate 3 to conduct comprehensive tests on the electrical performance and signal transmission performance of PCB component 300. The test standards are set according to product requirements. The test results of each PCB component 300 are recorded during the test. Qualified products and defective products are distinguished among all PCB components 300, and the positions of defective products are marked on cover plate 3 for subsequent screening.

[0057] After functional testing, the third assembly is disassembled: Operators wearing cleanroom gloves pinch the edges of tray 2 and cover 3 through the tray notch 25, slowly separating tray 2 and cover 3. During separation, excessive force should be avoided to prevent damage to PCB component 300 or chip 302 from falling off. After separation, PCB components 300 are removed one by one. Qualified components are kept and placed in a dedicated storage box; defective components are removed and processed separately to avoid confusion with qualified components.

[0058] After disassembly, the tray 2 and cover plate 3 are cleaned to remove solder residue, dust and other impurities from the surface. Each component is checked for integrity. Intact components can be reused, and damaged components are replaced in time to ensure the normal operation of subsequent processes.

[0059] III. Optimization of Process Parameters To further improve the process effect, suppress warping of thin strip PCB boards, and optimize mounting accuracy and product yield, the key parameters of this process have been optimized as follows: 3.1 Parameters of optical identification point 204 and observation hole 34 The diameter of the optical identification point 204 is preferably 1.0 mm, and the diameter of the observation hole is preferably 1.3 mm, with a difference of 0.3 mm. This ensures that the optical identification point 204 is fully exposed for easy identification by the placement machine, while also guaranteeing the contact area between the cover plate and the single substrate, thus improving the clamping effect. If the diameter of the optical identification point 204 is too small, it may lead to unclear identification; if the difference is too large, it will reduce the clamping area of ​​the cover plate and affect stability. To improve the representativeness of the two optical identification points 204 on the same single substrate 301, these two optical identification points 204 need to avoid areas sensitive to stress deformation, and the distance between them should be appropriately far. Therefore, they are set at both ends (in the length direction) of the single substrate 301.

[0060] 3.2 Parameters for Surface Mount Through-Hole 32 The outline of the surface mount via 32 is 0.3mm larger than that of the chip 302. The surface mount via 32 provides a slightly larger range for the placement of the chip 302. With the position guidance provided by the optical recognition point 204, the nozzle can fine-tune the position of the chip 302, so that it can be placed more accurately at the target position, avoiding chip 302 placement misalignment and improving placement accuracy.

[0061] 3.3 Welding Parameters The welding temperature is optimized to 245±5℃, the preheating temperature is 145±5℃, the holding time is 30±5 seconds, and the cooling rate is controlled at 5–10℃ / minute. These parameter settings can reduce thermal stress concentration, prevent the PCB 300 from warping during welding and cooling, and ensure the quality of the solder joints.

[0062] 3.4 Magnetic clamping force parameters: The magnetic attraction between the tray 2 and the cover plate 3 is controlled at 5–10N, which can not only achieve stable clamping of the single substrate 301 to avoid displacement and warping during mounting and welding, but also avoid deformation and damage of the single substrate 301 due to excessive clamping force.

[0063] IV. Verification of Process Effect To verify the effectiveness of this SMT assembly process in improving the warpage of thin strip PCBs, a thin strip PCB with a thickness of 0.1mm, a length of 15mm, and a width of 1.5mm (aspect ratio 10:1) was selected. SMT assembly was performed using both this process and a traditional process, with 1000 units produced in each group. The product yield, PCB warpage rate, and mounting accuracy were compared between the two processes. The verification results are as follows: 4.1 Product Qualification Rate: The product qualification rate using this process is 99.2%, with defects mainly including chip damage (0.3%), soldering defects (0.4%), and other defects (0.1%). The product qualification rate using the traditional process is 88.5%, with defects mainly including PCB warping (7.2%), placement misalignment (2.1%), soldering defects (1.8%), and other defects (0.4%). It is evident that this process significantly improves the product qualification rate, primarily because it effectively suppresses PCB warping and reduces placement misalignment and soldering defects.

[0064] 4.2 PCB Warpage Rate: PCBs manufactured using this process have a warpage rate of 0.3%, with warpage controlled within 0.01mm, meeting product design requirements. PCBs manufactured using traditional processes have a warpage rate of 7.5%, with some PCBs exhibiting warpage exceeding 0.03mm, failing to meet subsequent assembly requirements. This process effectively suppresses PCB warpage through stable clamping with specialized fixtures and optimized process parameters.

[0065] 4.3 Placement Accuracy: PCBs manufactured using this process achieve a chip placement accuracy of ±0.01mm, fully meeting design requirements. PCBs manufactured using traditional processes have a chip placement accuracy of ±0.03mm, with some products exhibiting placement misalignment, affecting soldering quality and product performance. This process improves placement accuracy through optical identification point 204 positioning and a dual positioning structure.

[0066] 4.4 Production Efficiency: Using this process, the ease of operation in tray placement, mounting, and testing is improved, resulting in a production efficiency of 1200 units / hour. In contrast, traditional processes require frequent PCB adjustments and handling of warped products, resulting in a production efficiency of only 800 units / hour. This process significantly improves production efficiency and reduces production costs.

[0067] The verification results show that this improved SMT assembly process for thin strip PCBs can effectively suppress warping deformation of thin strip PCBs during SMT assembly, improve mounting accuracy and soldering quality, significantly increase product qualification rate and production efficiency, solve many technical defects in traditional processes, and is suitable for SMT assembly production of thin strip PCBs with a thickness of 0.05mm–0.15mm and an aspect ratio of at least 10:1. It has good practicality and promotion value.

[0068] This improved SMT assembly process for thin, strip-shaped PCBs achieves warpage control through the following three aspects: 1. By advancing the board separation step, the single area 201 is made into a single substrate 301, thereby initially releasing stress and avoiding stress accumulation until later, which could cause warping problems. 2. The mounting fixture 100 clamps and positions the separate single substrates 301, preventing warping during the mounting process and providing a basis for the accurate placement of the chip 302. 3. The target positioning method of the entire PCB board 200 is abandoned. Instead, an optical recognition point 204 is set on a single area 201 as the reference position for mounting, so that the chip 302 can be better positioned.

[0069] In addition, by optimizing details such as optical recognition reference, heat dissipation structure, and fixture notch design, the mounting accuracy, soldering quality, and ease of operation are further improved, comprehensively suppressing the warping and deformation of thin strip PCB boards and ensuring that product performance meets design requirements.

[0070] The above descriptions are merely some embodiments of the present invention. Those skilled in the art can make various modifications and improvements without departing from the inventive concept of the present invention, and these all fall within the scope of protection of the present invention.

Claims

1. A process for improving the SMT pick and place process of thin profile strip PCBs, the process comprising the steps of: include: ​ S1. Raw material preparation: manufacture a PCB board with a thickness of 0.05mm–0.15mm. The PCB board has multiple elongated single-piece areas arranged side by side. The aspect ratio of the single-piece area is at least 10:

1. Adjacent single-piece areas are connected by multiple connection points. Each single-piece area has several product holes. S2, Separation: Cut all connection points to separate the single-piece area from the PCB board, resulting in multiple single-piece substrates; S3. Plating: A mounting fixture is provided, comprising a base plate, a support plate, and a cover plate. The support plate has several support plate precision positioning holes, and the cover plate has several cover plate precision positioning holes and several mounting through holes in its center. Multiple precision positioning pins are vertically arranged in the center of the base plate. The precision positioning holes of the support plate, the precision positioning holes of the cover plate, and the precision positioning pins are arranged in the same way. During plating, the base plate is first placed with the pin-side facing up. Then, using the correspondence between the precision positioning holes of the support plate and the precision positioning pins, the support plate is placed on top of the base plate. Next, using the correspondence between the product holes and the precision positioning pins, the individual substrates are placed one by one onto the support plate. Finally, using the correspondence between the precision positioning holes of the cover plate and the precision positioning pins, the cover plate is placed on top of all the individual substrates to obtain the first assembly. S4. Mounting: Remove the bottom plate from the lower part of the first assembly. The support plate and the cover plate clamp all the single substrates to form a second assembly. The second assembly is sent into the mounting machine with the cover plate on top. The chip is pre-mounted to the mounting position of the exposed single substrate through the mounting holes and soldered in place to obtain a third assembly. The chip and the single substrate form a PCB unit.

2. The SMT placement process according to claim 1, characterized in that: The upper surface of the tray has multiple strip-shaped positioning grooves arranged side by side. The range of the strip-shaped positioning grooves matches the outer contour of the single substrate. All the precision positioning holes of the tray are located within the range of the strip-shaped positioning grooves.

3. The SMT placement process according to claim 1, characterized in that: In the raw material preparation process, an optical identification point is provided at each end of the single-piece area, and an observation hole is provided on the cover plate to expose the optical identification point. The diameter of the optical identification point is smaller than the diameter of the observation hole. The mounting step uses the optical identification point as a reference to mount the chip onto the single-piece substrate.

4. The SMT placement process according to claim 3, characterized in that: The diameter of the optical recognition point is 0.1-0.5 mm smaller than the diameter of the observation hole, and the outline of the patch hole is 0.1-0.5 mm larger than the outline of the chip.

5. The SMT placement process according to claim 1, characterized in that: The mounting step is followed by a functional testing step: while keeping the third assembly intact, a functional test is performed to distinguish between qualified and defective products among all PCB individual products. Then, the third assembly is disassembled, and qualified products are kept while defective products are removed.

6. The SMT placement process according to claim 1, characterized in that: The tray and the cover are made of a magnetic material that attracts each other from the surface.

7. The SMT placement process according to claim 1, characterized in that: The tray has several heat dissipation holes located near the patch perforation.

8. The SMT placement process according to claim 1, characterized in that: The base plate is also provided with a plurality of coarse positioning posts erected around its perimeter. The height of the coarse positioning posts is higher than that of the fine positioning pins, and their diameter is larger than that of the fine positioning pins. The perimeter of the support plate is provided with support plate coarse positioning holes that match the coarse positioning posts, and the perimeter of the cover plate is provided with cover plate coarse positioning holes that match the coarse positioning posts.

9. The SMT placement process according to claim 1, characterized in that: Each of the opposite sides of the tray has a tray notch, and the innermost side of the tray notch is located inside the edge of the cover plate.

10. The SMT placement process according to claim 1, characterized in that: Each side of the base plate has a base plate notch, and the innermost side of the base plate notch is located inside the edge of the support plate.

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