Short-pin special-shaped micro-texture gradient steel mesh process

Abstract

The invention discloses a short-pin special-shaped micro-texture gradient steel mesh process, which belongs to the technical field of solder paste printing, and comprises the following steps: carrying out three-dimensional modeling and data preparation in a CAD (Computer Aided Design) stage; the process comprises the following steps: firstly, accurately identifying the position of a bonding pad of an ultra-short pin element; then, a traditional rectangular opening is changed into a special-shaped inverted trapezoidal opening, the width of an opening in the scraper face is 0.9 time of the width of a pin, and the width of the opening in the PCB face is 1.1 times of the width of the pin; gradient thinning is carried out by taking the open hole as the center, so that the thickness of the steel sheet is gradually increased from the edge to the outside; and meanwhile, directive spiral micro grooves are machined in the inner walls of the open holes, hollow fractal buffer structures are designed between the adjacent open holes, gradient thinning is also conducted on the areas, the duty ratio is strictly controlled to be 30%, and therefore the solder paste release performance and the buffer effect are improved. According to the invention, the bottom solder paste filling rate of the ultra-short pin element can be improved; printing and backflow bridging between fine-pitch pins can be inhibited; and the stability and the consistency of the transfer rate of the solder paste are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of solder paste printing, and particularly to a short-pin special-shaped micro-texture gradient stencil process. Background Art

[0002] As Figure 1 shown, solder paste printing is the process of applying solder paste onto a printed circuit board (PCB). In SMT, solder paste printing is the first and most critical process in SMT processing, followed immediately by the component placement process. The equipment, tools, and materials used in the solder paste printing process are a printer, a stencil, a squeegee, and solder paste, respectively. The viscosity of the paste-like solder paste can hold electronic components in the specified positions on the printed circuit board for a certain period of time; therefore, the solder paste can be used as a temporary adhesive during component mounting and reflow processes. To obtain a final good welding quality, not only do the electronic components need to be in the correct positions, but also the amount of solder paste, i.e., the volume of the solder paste, needs to be appropriate.

[0003] Brief description of the working process of a solder paste printer: First, the printed circuit board needs to be placed into the printer, i.e., the printing equipment. The printing vision system aligns the stencil and the printed circuit board, and the squeegee pushes the solder paste through the openings of the stencil and prints it onto the printed circuit board. Then, the equipment separates the printed circuit board and the stencil, and this process is also often referred to as demolding. After printing a certain number of times at the bottom of the stencil, wiping is required to remove the residual solder paste on the bottom of the stencil and the inner walls of the openings. During the wiping process, the speed, frequency, and wiping mode need to be precisely controlled. When most printers select different wiping modes, solvents and vacuum suction are used as auxiliary means to remove the residual solder paste.

[0004] Based on this, a Chinese patent document with the publication number CN106714472A discloses a stencil process, in which openings are provided on the stencil. The openings include: a number of first openings corresponding to components with pads at the bottom, and the first openings are in a "field" shape; a number of second openings corresponding to socket components with pins, and the second openings are outwardly expanded; a number of third openings corresponding to IC components with a pin pitch of less than 0.5 mm, and a number of third openings are adjacent to form a row, and the two adjacent sides of each third opening are inwardly contracted, so that the interval between two adjacent third openings is expanded; a number of fourth openings corresponding to components with pads at the bottom and through holes for heat dissipation on the pads, and each fourth opening is in a "‖" shape. In the technical solution disclosed in this patent technical document, through various opening methods of the stencil process, the amount of solder paste can be increased or decreased to achieve stable quality and improve the yield and performance of the product.

[0005] However, the disclosed steel mesh process still has the technical problems of insufficient production efficiency and insufficient reliability. Specifically, in the tin paste printing process disclosed in the prior art, it takes about 15-45 seconds to complete the tin paste printing of each printed circuit board. The doctor blade pressure must be accurately controlled during the tin paste printing process. Generally, about 0.5 kg of force per inch of doctor blade is applied, i.e. calculated by dividing the total pressure applied by the total length of the doctor blade. The printed circuit board and the steel mesh cannot be misaligned during printing, otherwise it will cause the tin paste to deviate and result in poor printing, so the position of the printed circuit board must be accurately fixed before the tin paste printing starts. Mechanical or vacuum devices on the X and Y platforms of the equipment are generally used to adjust and fix the position of the printed circuit board. In the entire printing process, the longest process is that the doctor blade pushes the tin paste to roll through the entire steel mesh opening range with a certain pressure, and then separates the printed circuit board and the steel mesh to print the tin paste onto the printed circuit board.

[0006] More specifically, in the existing steel mesh process, the tin paste at the bottom of components with ultra-short pins, such as CSP or QFN, has the problem of insufficient filling; especially for fine-pitch pins, there is also the problem of tin paste bridging during printing and reflow; these problems not only affect the production efficiency of the steel mesh process, but also affect the formation of a good wetting angle and solder strength on the circuit board, making it difficult to stably adhere to the pads. SUMMARY

[0007] Therefore, it is necessary to provide a short pin-shaped micro-texture gradient steel mesh process to solve the technical problem of improving the tin paste filling efficiency of components with ultra-short pins.

[0008] A short pin-shaped micro-texture gradient steel mesh process, comprising the following steps: Three-dimensional modeling and data preparation of the steel mesh in the CAD stage include: S11. Pin area recognition: accurately positioning the pad positions of all ultra-short pin components in the steel mesh design file; S12. Special-shaped opening design: change the traditional rectangular opening to an inverted trapezoidal opening; The opening width of the top of the special-shaped opening, i.e. the doctor blade surface = pin width x 0.9; The opening width of the bottom of the special-shaped opening, i.e. the PCB surface = pin width x 1.1; S13. Gradient thinning design: non-uniform local thinning is performed around each inverted trapezoidal opening; the thinning thickness increases gradually from the opening edge to the outside; S14. Micro-texture and fractal buffer structure design: directional spiral micro-grooves are designed on the inner wall of the opening; between adjacent pin openings, a structure with a preset fractal geometric pattern is designed, and the structure area is gradient-thinned, with an area ratio control of 30%.

[0009] Specifically, in step S13, the core parameters of the inverted trapezoidal opening and the gradient thinning structure include: the inclination angle of the inverted trapezoid is 15°; the thinning gradient is controlled from 0.08mm at the opening edge to 0.05mm at the opening center; the thinning range is a ring-shaped area of 0.3mm beyond the opening boundary.

[0010] Specifically, in step S14, the preset fractal geometric pattern is a Koch snowflake, the iteration level of the Koch snowflake is 2, and the single unit size is the pitch of the corresponding pin multiplied by 0.3.

[0011] Further, after completing the CAD stage, the process flow of steel mesh manufacturing is as follows: S21. Laser precision machining: using an ultra-short pulse fiber laser, the special-shaped opening, fractal buffer structure carving and inner wall spiral micro-groove machining are completed at one time; S22. Electrochemical gradient etching: by controlling the etching liquid spraying pressure and time, the laser processed area is gradient thinned to realize the designed wedge structure; S23. Composite coating vapor deposition: using mask spraying technology, two steps are taken: a. Covering the opening bottom and lower section of the inner wall, spraying a hydrophilic nano-titanium oxide coating by physical vapor deposition, with a contact angle of 30°; b. Covering the upper surface of the steel mesh and the upper section of the inner wall of the opening, spraying a hydrophobic nano-fluoropolymer coating, with a contact angle > 150°; Low-temperature curing at 120°C for 1 hour to make the coating firmly adhere.

[0012] Specifically, in step S21, the inner wall spiral micro-groove has a depth of 5μm and a width of 10μm, the direction of the inner wall spiral micro-groove is directed to the root of the element pin; the direction of the inner wall spiral micro-groove is clockwise from top to bottom.

[0013] Specifically, the parameters of tin paste printing are optimized, including: Squeegee pressure: reduced to 4-4.5kg to avoid excessive pressure damaging the micro-texture flow guiding effect; Squeegee speed: using a squeegee speed of 20-30mm / s to ensure that the tin paste has sufficient time to fill the micro-groove and special-shaped opening; Demolding mode: two-step demolding is adopted: first, quickly separate to 0.2mm, then extremely slowly, i.e. 0.1mm / s to completely demold the tin paste, using the differential separation force generated by the gradient thinning structure to smoothly transfer the tin paste; Tin paste selection: using tin powder type 6 to better adapt to the micro-texture and special-shaped opening.

[0014] Further, integrate ultrasonic generator or laser module on the screen or squeegee system; for ultrasonic assistance: at the moment of squeegee printing, apply high-frequency micro-amplitude ultrasonic waves to the screen of the specific area to reduce the viscosity of the solder paste, so that the solder paste flows into every corner of the micro-texture and special-shaped opening like water; for laser instantaneous heating: before demolding, use a low-power infrared laser to irradiate the pad area to reduce the viscosity of the solder paste at that point, reduce the adhesion of the solder paste to the screen.

[0015] Specifically, the ultrasonic-assisted step includes: S31. Synchronous triggering: the PLC control system of the printing machine is linked with the ultrasonic generator; when the squeegee moves to the moment of covering the short pin element area, trigger the preset ultrasonic pulse, and the width of the pulse is 100-500 ms; S32. Energy application: the ultrasonic wave is transmitted to the solder paste through the screen; the cavitation effect generates micro-bubbles in the solder paste and breaks them, crushing the connection between the solder paste particles; the acoustic streaming effect generates strong micro-flow in the solder paste; S33. Effect: under the joint action of the above effects, the viscosity of the solder paste suddenly decreases, the fluidity sharply increases, and the solder paste is instantaneously “liquefied”, thereby flowing into the thinnest opening and micro-texture without resistance; S34. Stop: after the pulse ends, the viscosity of the solder paste instantaneously recovers, maintains the shaped form, and prevents collapse.

[0016] Specifically, the working process of the laser heat-assisted demolding is as follows: S41. Printing completion: the squeegee completes the printing stroke, and the solder paste has filled the screen opening; S42. Precise positioning: the vision system performs rapid photographing positioning to confirm the relative position of the PCB and the screen; S43. Laser irradiation: 100-200 ms before the printing platform starts to descend, i.e., before demolding starts, the galvanometer controls the laser beam to irradiate all the pads of the target short pin element in a high-speed point-shooting or rapid scanning mode; the irradiation time is extremely short and is controlled to be less than 50 ms / pad; S44. Local heating: the laser energy is absorbed by the PCB pad, so that the local temperature of the absorbed pad instantaneously rises by 10-20°C; this temperature rise is transmitted to the solder paste in contact with the pad, significantly reducing the viscosity of the solder paste at the point; S45. Smooth demolding: when demolding, because the solder paste at the pad has low viscosity and good fluidity, and the solder paste at the screen hole wall has high viscosity and poor adhesion, the solder paste is “cut off” cleanly and perfectly remains on the pad, effectively eliminating the sharpness and residue.

[0017] Specifically, the process flow of integrating the ultrasonic module with the equipment PLC and MES system is as follows: S51. Perception: SPI system detects that the filling volume or height of certain components is consistently low; S52. Decision: MES system automatically judges and loads the "ultrasonic auxiliary" process parameters for the production program of this PCB batch; S53. Execution: The ultrasonic module is automatically enabled in the specified area during printing; S54. Re-perception: SPI detection results show that the defect rate has decreased; S55. Optimization learning: AI model records this successful parameter combination for subsequent production of similar products.

[0018] In summary, the present application discloses a short pin special-shaped micro-texture gradient steel mesh process, which is realized by three-dimensional modeling and data preparation in the CAD stage. The process first accurately identifies the pad position of the ultra-short pin component; then changes the traditional rectangular opening to a special-shaped inverted trapezoidal opening, the opening width of the scraper face is 0.9 times the pin width, and the PCB face is 1.1 times; and gradient thinning is implemented with the opening as the center, so that the thickness of the steel sheet increases from the edge outward. At the same time, directional spiral micro-grooves are processed on the inner wall of the opening, and a hollow fractal buffer structure is designed between adjacent openings, which is also gradient-thinned and the duty cycle is strictly controlled at 30%, so as to improve the solder paste release performance and buffer effect. The short pin special-shaped micro-texture gradient steel mesh process of the present application is based on the systematic integration of five technical means of special-shaped opening, gradient thinning, micro-texture, composite coating and fractal buffer structure, which can improve the bottom solder paste filling rate of ultra-short pin components such as QFN, CSP and other components; It can also inhibit the printing and reflow bridging between fine pitch pins; improve the stability and consistency of solder paste transfer rate, so that it can maintain a high Cpk value in the actual production process; thereby, finally forming a reliable and strong solder joint; so as to improve the technical problem of insufficient solder paste filling efficiency of components with ultra-short pins. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The system framework schematic diagram of the steel mesh process disclosed by the prior art; Figure 2 The process flow schematic diagram of part of the short pin special-shaped micro-texture gradient steel mesh process of the present application; Figure 3 The process flow schematic diagram of another part of the short pin special-shaped micro-texture gradient steel mesh process of the present application; Figure 4 The structure schematic diagram of the inverted trapezoidal opening and gradient thinning structure applied in the short pin special-shaped micro-texture gradient steel mesh process of the present application; Figure 5 The structure schematic diagram of the spiral micro-groove and double-material coating applied in the short pin special-shaped micro-texture gradient steel mesh process of the present application; Figure 6 Structure diagram of fractal buffer structure applied to short pin special-shaped micro-textured gradient steel mesh process of the application. DETAILED DESCRIPTION

[0020] In order to make the above objectives, characteristics and advantages of the present application more apparent, concrete embodiments of the present application will be described in detail below with reference to the accompanying drawings. In the following description, a large number of specific details are set forth in order to provide a comprehensive understanding of the present application. However, the present application can be implemented in many other different ways than those described herein, and those skilled in the art can make similar improvements without departing from the spirit of the present application, so the present application is not limited to the specific embodiments disclosed below.

[0021] In the description of the present application, it should be understood that the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation of the present application.

[0022] In addition, the terms "first", "second" are only for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, for example, two, three, etc., unless otherwise explicitly specified and limited.

[0023] In the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connection", "fixing" and the like should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrated; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the internal communication of two elements or the interaction relationship between two elements, unless otherwise explicitly limited. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0024] In the present invention, unless otherwise explicitly specified and limited, a first feature is "on" or "under" a second feature can be that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, the first feature is "over", "above" and "on top of" the second feature can be that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is horizontally higher than the second feature. The first feature is "under", "below" and "underneath" the second feature can be that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is horizontally lower than the second feature.

[0025] It should be noted that when an element is referred to as being "fixed to" or "set to" another element, it can be directly on the other element or there can be an intermediate element. When an element is considered to be "connected" to another element, it can be directly connected to the other element or there can be an intermediate element. The terms "vertical", "horizontal", "up", "down", "left", "right" and similar expressions used herein are for illustrative purposes only and are not intended to be the only implementation.

[0026] Please refer to Figures 2-3 The short pin special-shaped micro-textured gradient steel mesh process of the present invention comprises the following steps: Step one, three-dimensional modeling of steel mesh and data preparation, namely the CAD stage 1. Pin area identification: In the steel mesh design file, accurately locate the solder pad positions of all ultra-short pin elements; 2. Special-shaped hole design: Change the traditional rectangular hole to "inverted trapezoidal" or "horn-shaped"; The top of the special-shaped hole, i.e. the opening width of the scraper surface = pin width x 0.9, thereby playing a gathering role; The bottom of the special-shaped hole, i.e. the opening width of the PCB surface = pin width x 1.1, thereby facilitating demolding and providing more solder paste; 3. Gradient thinning design: Perform non-uniform local thinning with each inverted trapezoidal hole as the center; the thinning thickness increases gradually from the hole edge to the outside, for example: the hole edge is thinned by 0.05mm, and the periphery is thinned by 0.08mm; thereby forming a "wedge-shaped" or "bowl-shaped" structure; please refer to Figure 4 In the inverted trapezoidal hole and gradient thinning structure disclosed in the present invention, the core parameters include: the inclination angle of the inverted trapezoidal hole is 15°; the thinning gradient can be controlled to be from 0.08mm at the hole edge to 0.05mm at the hole center; the thinning range is a ring-shaped area of 0.3mm beyond the hole boundary; 4. Fractal buffer structure design: as Figure 6As shown, between adjacent pin holes, a Koch Snowflake or other fractal geometric pattern is designed to be hollow; the structure area is also gradient-thinned, with a duty cycle controlled at 30% to ensure structural strength; as shown Figure 6 As shown, the iteration level of the Koch Snowflake is 2, and the size of a single unit is pin pitch x 0.3; the structure function is to absorb thermal expansion stress; Step two, high-precision steel mesh manufacturing 1. Laser precision machining: use an ultra-short pulse fiber laser to complete the machining of special-shaped holes, fractal buffer structure carving, and inner wall spiral micro-groove in one go; as shown Figure 5 As shown, the inner wall spiral micro-groove has a depth of 5 μm and a width of 10 μm; the direction of the groove must be accurately pointed to the pin root; moreover, the spiral groove direction rotates clockwise from top to bottom 2. Electrochemical gradient etching: by controlling the etching liquid spray pressure and time, the laser-processed area is gradient-thinned to realize the designed wedge structure; 3. Composite coating vapor deposition: adopt mask spraying technology and perform in two steps: a. Cover the bottom of the hole and the lower section of the inner wall, and spray a hydrophilic nano-titanium oxide coating by physical vapor deposition, with a contact angle of 30°; b. Cover the upper surface of the steel mesh and the upper section of the inner wall of the hole, and spray a hydrophobic nano-fluoropolymer coating, with a contact angle > 150°; Low-temperature curing is performed at 120°C for 1 hour to make the coating firmly adhere; the structure of the double coating is as shown Figure 5 ; Step three, printing parameter optimization Squeegee pressure: appropriately reduce to 4-4.5 kg to avoid excessive pressure damaging the micro-texture flow guiding effect; Squeegee speed: adopt medium-low speed, such as 20-30 mm / s, to ensure that the solder paste has sufficient time to fill the micro-groove and special-shaped hole; Demolding mode: adopt two-step demolding: first, quickly separate to 0.2 mm, and then extremely slowly, such as 0.1 mm / s, to completely separate, use the differential separation force generated by the gradient-thinned structure to make the solder paste transfer smoothly; Solder paste selection: use type 6, such as 15-25 μm or finer tin powder, to better adapt to the micro-texture and special-shaped hole; Step four, verification and quality control Offline verification: 3D microscope: detect micro-texture, coating coverage, and gradient-thinning profile; Contact angle measuring instrument: verify the performance of the hydrophilic and hydrophobic areas respectively; Online monitoring: monitor key indicators, such as solder paste volume, height / area ratio, and shape consistency; Key areas to focus on: whether the solder paste at the base of the pins is sufficient and whether there is a risk of bridging.

[0027] Specifically, as mentioned above, in order to solve the technical problems of passive current limiting and insufficient bottom filling in the application of ultra-short lead components, the present invention adopts an inverted trapezoidal irregular opening process, which works in conjunction with microtexture and hydrophilic coating to actively guide solder paste to gather at the root of the component lead.

[0028] Furthermore, in order to solve the technical problems of high demolding resistance and unstable transfer rate in the existing stencil process, the short-lead irregular micro-textured gradient stencil process of the present invention adopts a gradient thinning process, which can generate differential separation force in the stencil process, and work together with the composite coating to ensure that the solder paste preferentially detaches from the stencil.

[0029] Furthermore, in order to solve the technical problem of non-directional solder paste flow in existing stencil processes, the short-lead irregular micro-textured gradient stencil process of this invention adopts a spiral micro-groove process, which can generate directional capillary force and become a "micro-guide rail" to guide the solder paste flow.

[0030] Furthermore, in order to solve the technical problem of uncontrollable solder paste behavior in existing stencil processes, the present invention employs a hydrophilic-hydrophobic composite coating process for short-lead irregular microtextured gradient stencils. This coating can prevent diffusion by being hydrophobic at the top of the stencil and promote wetting by being hydrophilic at the bottom, thus precisely controlling the final shape of the solder paste.

[0031] Furthermore, in order to solve the technical problem of bridging caused by reflow soldering thermal stress in existing stencil processes, the short-lead irregular microtextured gradient stencil process of this invention adopts a fractal buffer structure; it can absorb the lateral expansion stress in the reflow soldering process, and combined with the above-mentioned technical means, it suppresses bridging throughout the entire process from printing to reflow.

[0032] Therefore, the short-lead irregular microtextured gradient stencil process of this invention is based on the systematic integration of five technical means: irregular opening, gradient thinning, microtexturing, composite coating, and fractal buffer structure. It can improve the bottom solder paste filling rate of ultra-short lead components, such as QFN and CSP components; it can also suppress printing and reflow bridging between fine-pitch leads; improve the stability and consistency of solder paste transfer rate, so that it can maintain a high Cpk value in actual production process; thus, it ultimately forms a solder joint with high reliability and sufficient strength; thereby improving the technical problem of insufficient solder paste filling efficiency of components with ultra-short leads.

[0033] Further, in order to prove that the short pin special-shaped micro-textured gradient steel mesh process of the present application is significantly superior to the traditional laser steel mesh and ordinary stepped steel mesh in the above technical effects, the present application will adopt the "same plate comparison" principle, design three different pad areas on the same test plate, and use the corresponding steel mesh for printing: Area A is control group 1: using traditional laser steel mesh, that is, its opening is rectangular, no thinning, no coating process; Area B is control group 2: using ordinary stepped steel mesh, that is, only four edges are uniformly thinned.

[0034] Area C is the experimental group: using the short pin special-shaped micro-textured gradient steel mesh of the short pin special-shaped micro-textured gradient steel mesh process of the present application.

[0035] Thus, it can be ensured that all printing parameters such as squeegee, speed, pressure, and solder paste are completely consistent, and the interference of external variables is excluded.

[0036] Further, based on the foregoing control principle, the verification process and detection method disclosed by the present application comprises the following steps: First stage: printing quality evaluation, that is, using the method of SPI+microscopic observation 1.3D SPI, data collection of solder paste detector: 100% SPI detection is performed on the three areas, and the solder paste data of each pin is collected.

[0037] Key measurement indicators: volume: the solder paste volume of the experimental group / C should be closer to the theoretical value, and the standard deviation is smaller. Height: the solder paste of the experimental group / C should present a more uniform "hump" shape, and the height of the pin root is higher. Area: observe whether the solder paste forming is clear, and whether there is a sharp tip or collapse.

[0038] Data analysis: calculate the Cp / Cpk of each area, that is, the process capability index, and the Cpk value of the new process should be significantly higher than that of the control group, and the target is >1.67.

[0039] 2. Microscopic observation: using ultra-high depth-of-field optical microscope or 3D digital microscope to observe the morphology of the solder paste after printing.

[0040] Focus on: whether the solder paste accurately covers the pad, especially the pin root. Whether the solder paste profile is affected by micro-texture and presents directional flow traces. Whether there is a risk of solder paste residue or bridging between adjacent pins.

[0041] Second stage: reflow soldering quality evaluation, that is, X-Ray+section analysis 1. X-Ray, 2D / 3D X-ray detection: this is the most critical and most intuitive non-destructive testing method for evaluating underfill and bridging.

[0042] Evaluation criteria, refer to IPC-A-610: Bottom fill: observe the solder fill at the end of the pin. Experimental group / C should show continuous, full solder fill without "shadow" or voids.

[0043] Bridge: count the number of bridge defects in three areas. Experimental group / C should have a number of bridges close to zero.

[0044] Solder ball: check for harmful solder balls around the component.

[0045] 2. Section analysis: select typical solder joints from each area and make metallographic sections.

[0046] Evaluation criteria: Wetting angle: measure and compare. Experimental group / C should form a good wetting angle, usually 25° < θ < 55° is optimal.

[0047] Interface IMC, i.e. intermetallic compound: observe whether the IMC layer is continuous, uniform, and the thickness is moderate, usually 1-4 μm is optimal; this indicates good soldering reaction.

[0048] Fill level: accurately measure the solder climbing height on the side of the pin, and the new process should achieve more complete tin climbing.

[0049] Void rate: calculate the area ratio of internal voids in the solder joint, which should be less than 25%, or can be optimized according to the product grade requirements.

[0050] Third stage: mechanical and reliability testing, i.e. push test + thermal cycling test 1. Push test: Use a high-precision push tester, such as DAGE4000, to test the component.

[0051] Record data: record the peak force at the time of each component failure.

[0052] Failure mode analysis: observe the failure cross section.

[0053] Ideal state: component body fracture or internal solder fracture, indicating that the soldering strength is higher than the component itself.

[0054] Unqualified state: solder pad and PCB peeling or IMC layer fracture, indicating weak soldering interface bonding force.

[0055] Expected results: experimental group / C has higher average push force value and better failure mode.

[0056] 2. Thermal cycling test, i.e. Thermal Cycling Test, TCT: Put the welded test board into the high-low temperature test box for accelerated life test, such as -40°C~+125°C, cycle 500-1000 times.

[0057] After the test, X-Ray and section analysis are performed again to observe whether there is crack propagation, IMC layer overgrowth or cavity enlargement in the solder joint.

[0058] Expected result: The solder joint reliability of the experimental group / C is higher, and the failure cycle number is more.

[0059] All the above data are arranged into a comparison table and a chart to form a final verification report, as shown in Table 1 below.

[0060] Table 1: Verification result summary and report

[0061] According to the results in Table 1, through the above comprehensive verification, the short pin special-shaped micro-texture gradient steel mesh process of the present application has shown significant advantages in all key indicators. It not only solves the problem of bottom filling of ultra-short pins, but also effectively suppresses bridging, improves welding strength and long-term reliability, and the technical effect is fully proved.

[0062] Further, in the process flow disclosed in the foregoing short pin special-shaped micro-texture gradient steel mesh process of the present application, there may be the following technical problems: only relying on capillary phenomenon and coating, the control force for ultra-high density, such as <0.2mmpitch pin, may reach the physical limit. Based on this, the present application further discloses an improved scheme of integrated multi-physical field assisted printing, the core technical means of which is: integrating an ultrasonic generator or a laser module on the steel mesh or the doctor blade system; the working principles corresponding to the two are: for ultrasonic assistance: at the moment of doctor blade printing, high-frequency micro-amplitude ultrasonic waves, such as >40kHz ultrasonic waves, are applied to the steel mesh in a specific area. The "cavitation effect" and "acoustic streaming effect" generated by the ultrasonic waves can greatly reduce the viscosity of the solder paste, making it flow into every corner of the micro-texture and special-shaped opening like water, which is especially suitable for solving the bottom filling problem; for laser instantaneous heating: before demolding, a low-power infrared laser is used to irradiate the pad area. Local micro-heating, such as heating by 10-20°C, can instantaneously reduce the viscosity of the solder paste at that point, reduce the adhesion to the steel mesh, realize "thermal assisted demolding", and perfectly solve the problem of pin pulling and residue under extremely fine pitch.

[0063] Specifically, the core of the above-mentioned improvement scheme is: no longer rely on the static geometry and surface characteristics of the steel mesh to passively affect the solder paste, but actively introduce controllable external energy field, accurately change the physical state of the solder paste such as viscosity, fluidity at the key moment of printing, so as to realize the ultimate control of the behavior of the solder paste. Therefore, the physical limit can be broken through to provide technical reserves for the next generation of chip packaging, such as chip with pitch below 0.1mm. And significantly improve the yield of the first printing, reduce the wiping frequency.

[0064] Specifically, for the scheme of ultrasonic-assisted printing, the core technical problem to be solved is that the solder paste filling of ultra-high density pins such as <0.2mm pitch pins is not sufficient and has voids; the working principle is: using the cavitation effect and acoustic streaming effect of ultrasonic waves, the apparent viscosity of the solder paste is instantaneously greatly reduced, so that it flows like water and fully fills every corner of the micro-texture and irregular opening.

[0065] In order to realize this improvement scheme, the specific implementation of the system composition and integration scheme of ultrasonic-assisted printing is shown in Table 2 as follows: Table 2: System composition and integration method of ultrasonic-assisted printing

[0066] Specifically, the filling capacity of the aforementioned ultrasonic-assisted printing scheme is extremely strong, and almost any defect caused by insufficient filling can be eliminated; it is suitable for various solder paste formulations; its specific working process is as follows: 1. Synchronous triggering: the PLC control system of the printing machine is linked with the ultrasonic generator; when the doctor blade moves to the area covering the short pin element, an extremely short ultrasonic pulse is triggered, such as the pulse width can be: 100-500ms; 2. Energy application: ultrasonic waves are transmitted to the solder paste through the steel mesh; cavitation effect generates micro-bubbles in the solder paste and breaks them, crushing the connection between solder paste particles; acoustic streaming effect generates strong micro-flow in the solder paste; 3. Effect: under the joint action of the above-mentioned effects, the viscosity of the solder paste drops sharply, the fluidity increases dramatically, and the solder paste is instantaneously "liquefied", so that it flows into the thinnest opening and micro-texture without resistance; 4. Stop: after the pulse ends, the viscosity of the solder paste instantaneously recovers, maintains the shape of the formed solder paste, and prevents collapse.

[0067] Further, for the aforementioned improvement scheme of laser heat-assisted demolding, the core technical problem to be solved is: demolding tip in ultra-fine pitch printing, solder paste residue blocking holes; the working principle is: before demolding, use laser to heat the pad area in a small area, locally reduce the viscosity of the solder paste, reduce its adhesion to the steel mesh hole wall, so that the solder paste is more "willing" to stay on the pad rather than being taken away by the steel mesh.

[0068] To achieve this improvement scheme, a laser heat-assisted demolding system composition and integration scheme is implemented as shown in the following Table 3: Table 3: Laser heat-assisted demolding system composition and integration method

[0069] Specifically, the aforementioned laser heat-assisted demolding scheme can specifically solve the demolding problem without affecting other areas; the energy consumption is extremely low; it is non-contact and non-polluting; and the specific work flow is as follows: 1. Printing is completed: the squeegee completes the printing stroke, and the solder paste has filled the steel mesh openings; 2. Precise positioning: the vision system takes a quick photo to position and confirm the relative position of the PCB and the steel mesh; 3. Laser irradiation: at the beginning of the printing platform descending, such as the first 100-200 ms of demolding, the galvanometer controls the laser beam to irradiate all the pads of the target short pin component in a high-speed point shooting or fast scanning mode. The irradiation time is extremely short, controlled at <50 ms / pad; 4. Local heating: the laser energy is absorbed by the PCB pad, i.e., copper, causing its local temperature to rise instantaneously by 10-20℃; this temperature rise is transmitted to the solder paste in contact with it, significantly reducing its viscosity; 5. Smooth demolding: during demolding, the solder paste at the pad has low viscosity and good flowability, while the solder paste at the steel mesh hole wall has high viscosity and poor adhesion, and the solder paste is cleanly "cut off" and perfectly retained on the pad, effectively eliminating the sharpness and residue.

[0070] Further, the two aforementioned improvement schemes can also be combined with each other, integrated with the equipment PLC and MES system, and form an intelligent closed-loop control system, and the specific steps are as follows: 1. Perception: the SPI system detects that the filling volume or height of a certain type of component is continuously low; 2. Decision: the MES system automatically judges and loads the "ultrasonic assisted" process parameters for the production program of this PCB batch; 3. Execution: during printing, the ultrasonic module is automatically enabled in the specified area; 4. Re-perception: the SPI detection result shows that the defect rate is reduced; 5. Optimization learning: the AI model records the successful parameter combination for subsequent production of similar products.

[0071] Therefore, by introducing the two energy fields of ultrasonic waves and laser, the short pin special-shaped micro-texture gradient steel mesh process of the present application can actively, dynamically and locally change the physical properties of the solder paste, thereby breaking through the physical limit of the traditional pure mechanical steel mesh process. This not only solves the current most difficult process problem, but also provides a specific implementable technical solution for future electronic packaging towards micron level and chip level.

[0072] Further, the steps of the short pin special-shaped micro-texture gradient steel mesh process of the present application are summarized as shown in Table 4 below: Table 4: Detailed description of each step of the present application In summary, the short pin special-shaped micro-texture gradient steel mesh process of the present application is provided with a short pin special-shaped micro-texture gradient steel mesh process disclosed by the present application, which is realized by three-dimensional modeling and data preparation in the CAD stage. The process first accurately identifies the pad position of the ultra-short pin component; then changes the traditional rectangular opening to a special-shaped inverted trapezoidal opening, the opening width of the scraper face is 0.9 times the pin width, and the PCB face is 1.1 times; and gradient thinning is implemented with the opening as the center, so that the thickness of the steel sheet increases from the edge outward. At the same time, a directional spiral micro-groove is processed on the inner wall of the opening, and a hollow fractal buffer structure is designed between adjacent openings, which is also gradient-thinned and the duty cycle is strictly controlled at 30%, thereby improving the solder paste release performance and buffer effect. The short pin special-shaped micro-texture gradient steel mesh process of the present application is based on the systematic integration of five technical means of special-shaped opening, gradient thinning, micro-texture, composite coating and fractal buffer structure, which can improve the bottom solder paste filling rate of ultra-short pin components such as QFN, CSP and other components; can also inhibit the printing and reflow bridging between fine pitch pins; can improve the stability and consistency of the solder paste transfer rate, so that it can maintain a high Cpk value in the actual production process; thereby, finally forming a reliable and strong solder joint; thereby improving the technical problem of insufficient solder paste filling efficiency of components with ultra-short pins. Therefore, the short pin special-shaped micro-texture gradient steel mesh process of the present application solves the technical problem of how to improve the solder paste filling efficiency of components with ultra-short pins.

[0073] The technical features of the above-described embodiments can be combined in any manner. In order to make the description concise, not all possible combinations of the technical features in the above-described embodiments are described, however, as long as the combinations of the technical features do not exist contradictions, they should be considered as the scope of the present disclosure.

[0074] The above embodiments only express several implementation manners of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, several modifications and improvements can be made, which are within the scope of protection of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A process for fabricating short-lead irregularly shaped microtextured gradient steel mesh, characterized in that, It includes the following steps: The 3D modeling and data preparation of the steel mesh in the CAD stage includes: S11. Pin Area Identification: Precisely locate the pad positions of all ultra-short pin components in the stencil design file; S12. Irregular opening design: The traditional rectangular opening is changed to an inverted trapezoidal opening; The top of the irregular opening, i.e. the opening width of the scraper face, = pin width × 0.9; The bottom of the irregular opening, i.e., the opening width on the PCB surface, is equal to the pin width × 1.1; S13. Gradient thinning design: Non-uniform local thinning is performed with each inverted trapezoidal opening as the center; the thinning thickness gradually increases from the edge of the opening to the outside. S14. Microtexture and fractal buffer structure design: Design directional spiral microgrooves on the inner wall of the opening; design a structure with a hollowed-out pre-defined fractal geometric pattern between adjacent pin openings, and make the structure area gradient thinned, with the duty cycle controlled at 30%.

2. The short-lead irregular microtextured gradient steel mesh process according to claim 1, characterized in that: In step S13, the core parameters of the inverted trapezoidal opening and gradient thinning structure include: the inclination angle of the inverted trapezoid is 15°; the thinning gradient control is to control the gradient from 0.08mm at the edge of the opening to 0.05mm at the center of the opening; and the thinning range is an annular area extending 0.3mm beyond the boundary of the opening.

3. The short-lead irregular microtextured gradient steel mesh process according to claim 1, characterized in that: In step S14, the preset fractal geometric pattern is a Koch snowflake, the Koch snowflake has an iteration level of 2 and its single unit size is the spacing of the corresponding pins multiplied by 0.

3.

4. The short-lead irregular microtextured gradient steel mesh process according to claim 1, characterized in that: After completing the CAD stage, the process flow for steel mesh manufacturing is as follows: S21. Laser Precision Machining: Using an ultra-short pulse fiber laser, the machining of irregular openings, fractal buffer structures, and inner wall spiral microgrooves can be completed in one go. S22. Electrochemical gradient etching: By controlling the jet pressure and time of the etching solution, the area after laser processing is thinned in a gradient to achieve the designed wedge structure; S23. Composite coating vapor deposition: Performed in two steps using mask spraying technology: a. Cover the bottom of the opening and the lower section of the inner wall by spraying a hydrophilic nano-titanium oxide coating with a contact angle of 30° using physical vapor deposition; b. Spray a hydrophobic nano-fluoropolymer coating onto the upper surface of the steel mesh and the upper section of the inner wall of the opening, controlling the contact angle to be >150°; Perform low-temperature curing, curing at 120°C for 1 hour to ensure firm adhesion of the coating.

5. The short-lead irregular microtextured gradient steel mesh process according to claim 4, characterized in that: In step S21, the inner wall spiral microgroove has a depth of 5μm and a width of 10μm, and the direction of the inner wall spiral microgroove is towards the root of the component pin; the direction of the inner wall spiral microgroove is clockwise rotation from top to bottom.

6. The short-lead irregular microtextured gradient steel mesh process according to claim 5, characterized in that: Optimize the parameters for solder paste printing, including: Scraper pressure: Reduce to 4-4.5 kg to avoid excessive pressure from damaging the flow-guiding effect of the microtexture; Scraper speed: Use a scraper speed of 20-30mm / s to ensure that the solder paste has sufficient time to fill micro trenches and irregular openings; Demolding mode: Two-step demolding is adopted: first, the solder paste is quickly demolded to 0.2mm, and then at an extremely slow speed of 0.1mm / s to completely demold the solder paste. The differential separation force generated by the gradient thinning structure allows the solder paste to be transferred smoothly. Solder paste selection: Use grade 6 solder powder for better adaptation to microtextures and irregular openings.

7. The short-lead irregular microtextured gradient steel mesh process according to claim 1, characterized in that: Integrate ultrasonic generators or laser modules into the stencil or squeegee system; for ultrasonic assistance: apply high-frequency micro-amplitude ultrasonic waves to a specific area of ​​the stencil at the moment of squeegee printing to reduce the viscosity of the solder paste, allowing the solder paste to flow instantly and fully into every corner of the microtexture and irregular openings like water; for instantaneous laser heating: irradiate the pad area with a low-power infrared laser beam before demolding to reduce the viscosity of the solder paste at that point and reduce the adhesion of the solder paste to the stencil.

8. The short-lead irregular microtextured gradient steel mesh process according to claim 7, characterized in that: The steps involved in ultrasound-assisted procedures include: S31. Synchronous Trigger: The PLC control system of the printing press is linked with the ultrasonic generator; when the doctor blade moves to the area covering the short-pin component, a preset ultrasonic pulse is triggered, and the width of the pulse is 100-500ms. S32. Energy application: Ultrasonic waves are transmitted to the solder paste through the stencil; the cavitation effect generates tiny bubbles inside the solder paste and causes them to burst, breaking the bonds between the solder paste particles; the acoustic flow effect generates strong micro-flow inside the solder paste. S33. Effect: Under the combined effect of the above, the viscosity of the solder paste drops sharply and its fluidity increases dramatically. It is instantly "liquefied" and flows into the finest openings and microtextures without resistance. S34. Stop: After the pulse ends, the solder paste viscosity instantly returns to normal, maintaining the formed shape and preventing collapse.

9. The short-lead irregular microtextured gradient steel mesh process according to claim 8, characterized in that: The workflow of the laser-assisted heat release process is as follows: S41. Printing complete: The squeegee has completed the printing stroke, and the solder paste has filled the stencil openings; S42. Precise Positioning: The vision system performs rapid image positioning to confirm the relative position of the PCB and the stencil; S43. Laser Irradiation: 100-200ms before the printing platform begins to descend, i.e., before demolding begins, the galvanometer controls the laser beam to irradiate all pads of the target short-pin component in a high-speed spot firing or fast scanning mode; the irradiation time is extremely short, controlled to be less than 50ms / pad. S44. Localized heating: The laser energy is absorbed by the PCB pads, causing the local temperature of the pads to rise instantaneously by 10-20°C; this temperature rise is transferred to the solder paste in contact with it, significantly reducing the viscosity of the solder paste at that location. S45. Smooth Demolding: During demolding, the solder paste at the pads has low viscosity and good fluidity, while the solder paste at the stencil wall has high viscosity and poor adhesion. The solder paste is cleanly "cut" and perfectly retained on the pads, effectively eliminating spikes and residues.

10. The short-lead irregular microtextured gradient steel mesh process according to claim 9, characterized in that: The process flow for integrating the ultrasonic module with the equipment's PLC and MES system is as follows: S51. Sensing: The SPI system detects that the fill volume or height of a certain type of component is consistently low; S52. Decision: The MES system automatically determines and loads the "ultrasonic-assisted" process parameters into the production program for this PCB batch; S53. Execution: During printing, the ultrasonic module is automatically activated in a specific area; S54. Re-sensing: SPI detection results show a decrease in defect rate; S55. Optimization Learning: The AI ​​model records the successful parameter combinations for use in the production of similar products in the future.

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