A method for in-situ laser cutting and welding integrated preparation of tin-based solder joint

Abstract

This invention belongs to the field of electronic packaging interconnect technology and discloses a method for preparing tin-based solder joints by integrating in-situ laser cutting and welding. The method involves covering a rolled thin strip of solder sheet onto a C18150 fixture and fixing it with a horizontal clamping positioning plate. A PCB board coated with ROM0 type solder paste is pushed into the fixture cavity and fixed with a longitudinal limiting block. A 355nm ultraviolet picosecond laser is used for cutting, with auxiliary air blowing turned off, to cut a circular piece with a diameter of 1.8mm and a thickness of 200μm. The solder sheet falls vertically to the corresponding pad under gravity to form the structure to be packaged. Subsequently, a 915nm near-infrared semiconductor laser is switched in-situ, and welding is performed in three stages: heating, immersion heat preservation, and air cooling, to form a multi-element tin-based solder joint. This invention integrates laser cutting and welding, allowing the solder sheet to be placed in situ without the need for transfer and mounting, effectively solving the problems of high void ratio and low shear performance of traditional solder joints.

Description

Technical Field

[0001] This invention belongs to the field of electronic packaging interconnection technology, specifically relating to a method for preparing tin-based solder joints by integrating in-situ laser cutting and welding. Background Technology

[0002] As electronic products continue to evolve towards miniaturization, high density, fine pitch, and high integration, the interconnect dimensions and pad sizes between electronic components are gradually decreasing, while the electrical, thermal, and mechanical loads on solder joints are constantly increasing. As a crucial structure for achieving electrical, thermal, and mechanical connections between components and the substrate, the dimensional consistency, positional accuracy, shear strength, and service reliability of solder joints significantly impact the overall performance of electronic products. Defects such as internal voids, insufficient solder and pad wetting, and oxide inclusions can reduce solder joint strength, thereby affecting device reliability.

[0003] The forming quality of tin-based solder joints is influenced by a combination of factors, including solder alloy composition and melting characteristics, solder supply form, cleanliness of solder and pad surfaces, flux activation state, soldering heat input, and soldering method. Specifically, the melting temperature and wetting properties of the solder alloy affect the melting, flow, and spreading behavior of the solder; the size of the solder pads and the volume of solder paste affect the amount of solder used and the final solder joint size; oil, oxides, and particulate matter on the solder and pad surfaces can hinder interfacial wetting and reduce solder strength; furthermore, the control of parameters such as the heating rate and peak temperature during the soldering process affects the activation and volatilization of flux, the flow behavior of molten solder, the formation of internal voids in the solder joint, and the solidification structure of the solder joint.

[0004] Currently, the preparation of tin-based solder joints mainly adopts methods such as reflow soldering after solder paste screen printing (CN122400890B), transfer and reflow soldering after mechanically stamping solder into solder pads (CN120190529B), and laser soldering after manual transfer of pre-formed solder pads (CN116441714B). The solder paste screen printing and reflow process usually includes complicated steps such as alloy powder preparation, solder paste preparation, stencil printing, and overall reflow. Moreover, the accuracy of the solder supply position is easily affected by factors such as solder paste viscosity, squeegee pressure, demolding state, and solder paste collapse (Kim H, Mohammadhosseinzadeh M, Jung MK, et al. Stencil printing procedure using transient three-dimensional computational fluid dynamics[J]. Journal of Manufacturing Processes, 2025, 148:306-316.). While reflow soldering can improve the controllability of solder usage to some extent, solder pieces typically require complex processes such as separate preparation, collection, transfer, and positioning. These processes are prone to introducing surface contamination, oxidation, and positional deviations. For micro-solder pieces with small thickness and size, mechanical stamping or cutting can easily produce burrs, chipping, deformation, and dimensional deviations. Although laser welding can improve efficiency, issues such as solder piece contamination due to the separation of cutting and welding, as well as placement misalignment, can also affect the welding effect. All of these problems can lead to poor welding during the solder joint formation process, reduce the mechanical properties of the solder joint, and consequently affect its reliability. Summary of the Invention

[0005] The purpose of this invention is to solve the problems of high solder joint defects and weak shear strength in the preparation of tin-based solder joints in the prior art, and to provide a method for preparing tin-based solder joints by integrating in-situ laser cutting and welding, so as to reduce the void ratio of tin-based solder joints and improve the shear strength of solder joints.

[0006] The objective of this invention is achieved through the following technical solution: a method for preparing tin-based solder joints by integrating in-situ laser cutting and welding. A thin strip of solder is directly applied to a C18150 fixture, and the strip is fixed using the fixture's horizontal clamping positioning plates. A PCB board coated with flux is pushed into the fixture cavity, and the PCB board is fixed using longitudinal limiting blocks at both ends of the cavity. Marking parameters, such as the cutting pattern for the corresponding circular solder strip, are input into the laser marking software based on the PCB board pad size and distribution, as well as global reference points, ensuring that the center of the circular solder strip and the center of the pad coincide in the height direction. Laser cutting is used, and the cut circular solder strip falls vertically to the corresponding pad at the bottom under gravity, forming a solder strip-flux-pad structure to be packaged. Then, the laser is used again, and the laser parameters are adjusted to complete local laser welding. After the solder joint cools to room temperature, the solder strip cutting tail is removed, and the PCB board is taken out for subsequent void testing, shear performance testing, and shear fracture characterization.

[0007] The thin strip welding sheet has a final rolling thickness of 200 μm, a width of 38 mm, and a length of 300 mm. The tin-based multi-element alloy composition corresponding to the thin strip welding sheet is SnCu0.5Bi5In15Co0.02Mn0.05. The melting point is determined by differential scanning calorimetry (DSC) testing of the tail material after reciprocating rolling.

[0008] Furthermore, the laser cutting and laser welding integrate an ultraviolet picosecond laser and a near-infrared semiconductor laser into one unit; in the cutting mode, the average power is 30W, the pulse frequency is 102kHz, the pulse width is 10ps, the focusing lens working distance is 125mm, and the cutting speed is 500mm / s; in the welding mode, the welding is divided into three stages in sequence: the heating stage, the immersion and heat preservation stage, and the air cooling stage.

[0009] Furthermore, the peak power, pulse frequency, spiral diameter, and scanning speed are both set to 25W and 100Hz for the heating and immersion holding stages, respectively. The heating stage raises the temperature from 25℃ to 230℃, with a light output duration of 0.42s, an average power of 10W, and a pulse width of 4ms. The immersion holding stage maintains the temperature at 230℃, with a light output duration of 3.5s, an average power of 1.5W, and a pulse width of 0.6ms. The air cooling stage stops laser output after reaching the peak temperature of 230℃ and completing the heat preservation, and then air cools to room temperature to form tin-based solder joints.

[0010] Furthermore, the fixture is made of C18150 chromium zirconium copper alloy, which has excellent heat dissipation performance, structural strength and good machinability. It includes a rectangular fixture body, a hollow cavity, four horizontal clamping and positioning plates set at the top of the front and rear ends of the fixture body, and two longitudinal limiting blocks set at the PCB inlet and outlet ends of the fixture. The horizontal clamping and positioning plates are used to position and clamp the thin strip tin-based solder sheet, and the longitudinal limiting blocks are used to limit the displacement of the PCB board along its length.

[0011] The four horizontal clamping and positioning plates are symmetrically distributed at the top four corners of the front and rear ends of the fixture. They have a waist-shaped structure and are used to position and clamp the welding plates placed on them in the horizontal direction, effectively preventing the welding plates from shifting or warping during processing.

[0012] The two longitudinal limiting blocks are respectively protruding from the side walls of the inlet and outlet ends of the fixture, and their inner end faces abut against the edge of the PCB board to form a rigid physical stop structure, thereby restricting the degree of freedom of the PCB board along its length direction (i.e., the front-to-back direction). Together with the lateral limiting at the left and right ends and the side guide of the fixture body, the positional stability of the PCB board during the pushing, processing and pulling out processes is ensured in all aspects.

[0013] Furthermore, the left and right ends of the fixture body are respectively provided with hollowed-out notches that communicate with the hollowed-out cavity. The hollowed-out notches form heat dissipation channels and dust emission channels. The bottom edge of the hollowed-out notches is higher than the bottom edge of the hollowed-out cavity, forming a lateral stop structure to limit the displacement of the PCB board along its width direction.

[0014] The front and rear ends of the fixture serve as the infeed and discharge ends of the PCB board, used for horizontally pushing and pulling the PCB board in and out. The left and right ends of the fixture are provided with hollowed-out notches, which have multiple functions: First, as heat dissipation channels, the high thermal conductivity of C18150 alloy is used to quickly dissipate the heat generated by laser processing; second, as dust exhaust channels, the smoke and exhaust gas generated during cutting or welding are directionally discharged; finally, the edge of the notch above the bottom of the cavity forms a lateral physical stop (lateral limit) on the width direction of the PCB board, which cooperates with the side guide of the fixture body to restrict the degree of freedom of the PCB board in the width direction.

[0015] Furthermore, the fixture has a length of 55mm, a width of 40mm, and a thickness of 9mm; the hollow cavity has a length of 45.3mm, a width of 35.3mm, and a height of 4mm; the dimensional accuracy of the fixture is controlled within 0.1mm; the surface roughness Ra of the inner surface of the hollow cavity and the upper surface of the fixture is not greater than 0.4μm, and the surface roughness Ra of the other surfaces is not greater than 0.8μm.

[0016] The PCB board has pads with one or more of the following surface treatments: organic solderable protective film (OSP), electroless nickel plating, or electroless tin deposition. The pads have a diameter of 850 μm, and the PCB board is 45 mm long, 35 mm wide, and 2 mm high. The entire PCB board consists of three PCB boards of equal width, which can be freely disassembled and assembled for soldering.

[0017] The cutting laser source is an ultraviolet picosecond laser with a pulse width on the order of picoseconds, utilizing its high peak power and small heat-affected zone to achieve precise cutting of tin-based solder sheets. The welding laser source is a near-infrared semiconductor laser source. By segmenting and setting parameters such as the output power and pulse width of the laser, the solder joint temperature sequentially passes through heating and holding stages, thereby completing the wetting and spreading of the solder sheet and the formation of the solder joint. The ultraviolet picosecond laser and the semiconductor laser are integrated into the same customized laser processing equipment. Through the light source switching function within the system, in conjunction with the C18150 fixture, continuous cutting and welding of solder sheets can be achieved at the same station.

[0018] Before welding, the temperature measurement system is calibrated: a thermocouple is pre-embedded under the weld nugget to heat it, and the temperature data of the F8L1 infrared thermometer and the T-type thermocouple are recorded simultaneously. This is used to calibrate the infrared temperature display. After calibration, the deviation between the infrared temperature measurement value and the actual thermocouple measurement value should not exceed ±3℃ in the temperature range below the welding peak temperature. During the adjustment of the calibrated laser welding parameters, the surface temperature of the weld joint is monitored using a corrected infrared thermometer to adjust the laser welding parameters. When the solder alloy composition, flux type, or weld nugget size changes, the laser cutting parameters and welding parameters are adjusted accordingly based on the melting temperature range of the corresponding solder and the activation temperature of the matching flux, and using the actual temperature of the weld joint measured by the calibrated infrared thermometer.

[0019] The present invention has the following significant advantages: (1) The process is simple and controllable, significantly shortening the solder joint preparation cycle. This invention integrates the laser cutting, gravity blanking, and local laser welding of thin strip tin-based solder sheets into the same C18150 fixture station for in-situ completion. The cut circular solder sheets automatically fall to the corresponding pads by their own gravity. The entire process does not require disassembly, transfer, or secondary positioning of the solder sheets after prefabrication, nor does it require solder paste printing or scraping. The process is simple, the operation is simple, and the parameters are easy to control precisely. In contrast, Comparative Examples 1 and 3 use "laser cutting of prefabricated solder sheets + manual transfer and mounting", Comparative Examples 2 and 4 use "mechanical cutting of prefabricated solder sheets + manual transfer and mounting", and Comparative Example 5 uses "solder paste stencil printing + drawer-type reflow soldering". All of these methods involve intermediate steps such as solder sheet prefabrication, manual transfer and mounting or printing, and independent reflow. The process is complicated, the cycle is long, and human error is difficult to avoid. Therefore, this invention can significantly shorten the solder joint preparation cycle.

[0020] (2) Significantly reduces solder joint void rate. This invention uses ultraviolet picosecond laser to precisely cut thin strip-shaped tin-based solder sheets in situ. Cutting, blanking, and welding are completed in the same fixture station, avoiding secondary contamination and positional deviation during the intermediate transfer of the solder sheet. The circular solder sheet formed by laser cutting has high dimensional accuracy, consistent contour, and good edge quality. The solder sheet can completely cover the pad. Combined with the constraint of the solder diffusion range by the solder resist layer at the edge of the pad, the flux and gas can be fully released during the welding process, thereby effectively suppressing void formation. This invention can control the solder joint void rate to 0.29±0.04%—regardless of whether the comparison uses laser cutting or mechanical cutting of prefabricated solder sheets, combined with manual transfer mounting or drawer-type reflow soldering, or uses solder paste stencil printing reflow, the solder joint void rate is several times or even several times higher than that of this invention.

[0021] (3) Significantly improves the shear strength of the solder joint. The solder of the present invention is supplied in one go with precisely sized circular solder pads. The volume and position of the solder are precisely controllable. The solder pads completely cover the solder pads, resulting in good consistency of the weld formation, sufficient wetting of the interface, and strong interface bonding. The shear strength of the resulting solder joint is significantly higher than that of the above-mentioned comparative examples, and the reliability of the solder joint connection is significantly enhanced.

[0022] (4) The volume and position of the solder are precisely controllable, and the consistency and repeatability of the solder joints are good. Compared with the solder paste printing process used in Comparative Example 5, the present invention directly replaces the solder paste with laser-cut circular solder pads, which fundamentally avoids the process fluctuations caused by factors such as solder paste printing thickness fluctuations, position deviations and collapses. Compared with the mechanical cutting of pre-made solder pads used in Comparative Examples 2 and 4, the present invention uses ultraviolet picosecond laser cutting, which has a small heat-affected zone, no obvious remelting at the edges, high dimensional accuracy and contour consistency of the solder pads, low edge burr coverage, and fewer defects such as edge chipping and deformation, thereby significantly improving the consistency and repeatability of solder joint formation. Attached Figure Description

[0023] Figure 1 This is a graph showing the melting and solidification characteristics of the alloy. Figure 2 This is a three-dimensional structural diagram of the C18150 fixture used in the method of the present invention. In the figure: 1- Fixture body, 2- Hollowed-out cavity, 3- Horizontal clamping and positioning piece, 4- Longitudinal limiting block, 5- Hollowed-out notch; Figure 3 for Figure 2 The front view; Figure 4 for Figure 2 Side view; Figure 5 for Figure 2 Top view; Figure 6 The above is a statistical chart of the solder joint void rate for Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4 and Comparative Example 5. Figure 7 The shear strength statistics are for Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4 and Comparative Example 5. Figure 8 This is a graph showing the shear performance of the weld joint in Example 1; Figure 9 , Figure 10 , Figure 11 , Figure 12 and Figure 13 The shear performance curves of the weld joints are shown for comparative examples 1 to 5. Figure 14 This is a shear fracture diagram of the weld joint in Example 1; Figure 15 , Figure 16 , Figure 17 , Figure 18 and Figure 19 The images shown are the shear fracture surfaces of the weld joints in Comparative Examples 1 to 5. Detailed Implementation

[0024] The present invention will be further described in detail below through embodiments, but the scope of protection of the present invention is not limited to the contents described in the embodiments.

[0025] The method for preparing tin-based solder joints by in-situ laser cutting and welding as described in this invention uses a C18150 fixture to position the thin strip solder sheet and the PCB board. The C18150 fixture is made of C18150 chromium zirconium copper alloy, which has good thermal conductivity, structural strength and machinability. It can quickly conduct and disperse local heat during laser cutting and local welding, reducing the risk of local overheating of the fixture and the PCB board.

[0026] In this invention, the C18150 fixture includes a rectangular fixture body 1, a central hollow cavity 2, four horizontal clamping and positioning plates 3 located at the top four corners of the front and rear ends of the fixture, and two longitudinal limiting blocks 4 located at the inlet and outlet ends of the PCB board. The front and rear ends of the fixture serve as the inlet and outlet ends of the PCB board, respectively, for pushing and pulling the PCB board in the horizontal direction. The left and right ends of the fixture are provided with hollow notches 5. These hollow notches 5 serve as heat dissipation channels to dissipate heat generated during laser processing and as dust and exhaust gas discharge channels to directionally discharge fumes generated during cutting or welding. At the same time, the edge of the hollow notch 5, which is higher than the bottom of the cavity, forms a lateral physical stop structure in the width direction of the PCB board, which cooperates with the side guide of the fixture body 1 to restrict the degree of freedom of the PCB board in the width direction.

[0027] The four horizontal clamping and positioning plates 3 are symmetrically distributed at the top four corners of the front and rear ends of the fixture, forming a waist-shaped structure. They are used to horizontally position and clamp the thin strip of solder sheet covering the fixture, preventing displacement or warping of the thin strip of solder sheet during laser cutting and subsequent processing. The two longitudinal limiting blocks 4 are respectively protruding from the side walls of the fixture's inlet and outlet ends, with their inner end faces abutting against the edge of the PCB board, forming a rigid physical stop structure to restrict the degree of freedom of the PCB board along its length. Through the cooperation of the above-mentioned horizontal clamping and positioning plates, longitudinal limiting blocks, lateral limiting, and side guide structures, the thin strip of solder sheet and the PCB board maintain a stable relative positional relationship during processing.

[0028] In this invention, the C18150 fixture has a length of 55mm, a width of 40mm, a thickness of 9mm, an inner cavity length of 45.3mm, an inner cavity width of 35.3mm, and an inner cavity height of 4mm, with dimensional accuracy controlled within 0.1mm. The surface roughness Ra of the inner and upper surfaces of the C18150 fixture does not exceed 0.4μm, and the surface roughness Ra of the remaining surfaces does not exceed 0.8μm, in order to reduce frictional resistance during the PCB board pushing, positioning, and removal processes, and to reduce contamination or scratches to the PCB board pads caused by burrs, particles, or metal debris.

[0029] The PCB board used in this invention has OSP, ENIG or ImSn pads, with OSP pads being the most common. The pad diameter is 850μm. The PCB board dimensions are 45mm in length, 35mm in width and 2mm in height. The entire PCB board is composed of 3 PCB boards of equal width. Each PCB board of equal width has 5 circular pads of the same size. It can be disassembled and reassembled according to testing needs.

[0030] The thin strip used in this invention is a tin-based multi-element alloy strip with a thickness of 200 μm, a width of 38 mm, and a length of 300 mm. The corresponding tin-based multi-element alloy strip composition is SnCu0.5Bi5In15Co0.02Mn0.05. The alloy melting point was obtained by DSC testing of the tail material after reciprocating rolling, and was 196.05℃. The melting and solidification curves are detailed in [reference needed]. Figure 1 .

[0031] In the specific preparation process, the thin strip of solder sheet, after reciprocating rolling and plasma cleaning, is directly covered onto the C18150 fixture and fixed by the horizontal clamping positioning plate at the top of the fixture. Subsequently, the PCB board coated with halogen-free medium-activity resin-type no-clean ROM0 type flux paste is pushed into the fixture cavity from the feed end of the fixture, so that the PCB board enters the corresponding position under the thin strip of solder sheet, and is positioned by the longitudinal limiting blocks at both ends of the cavity, the lateral limiting blocks at both ends of the left and right, and the side guide structure of the fixture body.

[0032] After positioning the thin strip solder sheet and PCB board, the cutting pattern and processing parameters of the corresponding circular solder sheet are input or imported into the laser marking software according to the PCB board pad size, pad distribution, and global reference points, so that the center of the circular solder sheet to be cut coincides with the center of the corresponding PCB pad in the height direction. The cutting pattern is a circular outline with a diameter of 1.8mm, used to form a micro tin-based circular solder sheet.

[0033] In this invention, an ultraviolet picosecond laser is used as the cutting laser source. The laser wavelength of the ultraviolet picosecond laser is 355nm, and the average power is 30W. The pulse frequency is 102kHz, the pulse width is 10ps, the focusing lens working distance is 125mm, and the cutting speed is 500mm / s. During the cutting process, the auxiliary air blowing is turned off to avoid the airflow disturbing the circular solder sheet after cutting, preventing the solder sheet from shifting, flipping, or scattering, and ensuring that the solder sheet can fall freely vertically 2mm to the corresponding pad under the action of gravity. By combining the short ultraviolet wavelength, picosecond pulse width, and high beam quality, the laser energy is concentrated on the predetermined cutting contour area of ​​the thin strip tin-based solder sheet, reducing the heat-affected zone and the degree of edge remelting, thereby achieving precision cutting of a miniature tin-based circular solder sheet with a diameter of 1.8mm and a thickness of 200±10μm, according to a density of 7.8g / cm³. 3 The calculated mass of the disc is approximately 3.97 mg.

[0034] After laser cutting, the circular tin-based solder sheet falls vertically from the thin strip position to the corresponding PCB pad surface under its own gravity, and comes into contact with the ROM0 type flux pre-coated on the pad. Approximately 0.44 mg of flux is taken according to a metal-to-flux ratio of approximately 9:1, forming a solder sheet-flux-pad structure to be packaged. Since the pad diameter is 850 μm and the solder sheet diameter is 1.8 mm (the solder sheet diameter is larger than the pad diameter), the solder sheet can completely cover the pad surface. Simultaneously, a solder resist layer is provided at the pad edge to constrain the solder diffusion range, which is beneficial for stable solder joint formation. Because solder sheet cutting, blanking, and pad positioning are all completed within the same fixture station, secondary contamination and positional deviations during intermediate transfer of the solder sheet are reduced.

[0035] Before welding, the temperature measurement system is calibrated: a thermocouple is pre-embedded under the weld nugget to heat it, and the temperature data of the F8L1 infrared thermometer and the T-type thermocouple are recorded simultaneously; a correspondence is established between the infrared temperature measurement value and the actual thermocouple value; the temperature compensation coefficient is corrected based on this correspondence. After calibration, the deviation between the infrared temperature measurement value and the actual thermocouple value is no greater than ±3℃ in the temperature range below the melting point of the solder. During the adjustment of the calibrated laser welding parameters, the corrected infrared thermometer is used to monitor the surface temperature of the weld joint to adjust the laser welding parameters.

[0036] Next, a near-infrared semiconductor laser with a wavelength of 915nm was used. Its operating mode is adjustable for continuous output, pulsed output, or modulated output. In accordance with the principle that the welding temperature of tin-based solder should be 30°C above the melting point, the peak welding temperature of the alloy was set to 230°C. During welding, the laser beam scans along a spiral trajectory with the center of the solder sheet as the scanning center. The welding process is divided into three stages: heating, wetting and holding, and air cooling. The peak power is 25W, the pulse frequency is 100Hz, the spiral diameter is 1.2mm, and the scanning speed is 30mm / s. In the heating stage, the temperature is raised from 25°C to 230°C, with a light output duration of 0.42s, an average power of 10W, and a pulse width of 4ms. In the wetting and holding stage, the temperature is maintained at 230°C, with a light output duration of 3.5s, an average power of 1.5W, and a pulse width of 0.6ms. After reaching the peak temperature of 230°C and completing the holding stage, the laser output is stopped, and the solder is air-cooled to room temperature, thus forming a tin-based solder joint. Spiral scanning is used in both of the above stages to avoid fixed-point irradiation. The above parameters correspond to the initial setting parameters of the tin-based solder and the solder pad size. When the solder alloy composition, flux type or solder pad size changes, the laser cutting parameters and welding parameters are adjusted accordingly based on the melting temperature range of the corresponding solder and the matching flux activation temperature, and based on the actual temperature of the solder joint measured by the calibrated infrared thermometer.

[0037] After laser welding, the tailings formed by cutting the thin strip of solder are removed, and the PCB board is taken out. The resulting tin-based solder joints can be used for subsequent void ratio testing, shear strength testing, and shear displacement testing to evaluate the solder joint formation quality and connection reliability.

[0038] The technical effects of the present invention were analyzed by performing post-solder void, shear performance tests, and shear fracture characterization on tin-based solder joints obtained in the following embodiments and comparative examples. The void testing equipment was a UNICOMPAX9600M fully automatic X-ray inspection machine, and the test conditions were a tube voltage of 100kV, a tube current of 150μA, and an average number of stacked frames of 10. The shear performance testing and fracture characterization equipment was a Try-precisionABT2000C multi-functional fully automatic push-pull force tester, with a shear rate of 0.1mm / s. Example 1

[0039] Take a thin strip of solder with an alloy composition of SnCu0.5Bi5In15Co0.02Mn0.05, a width of 38mm, a length of 300mm, and a thickness of 200μm. Cover the upper surface of the C18150 fixture with this strip. Use a waist-shaped horizontal clamping and positioning piece to fix the strip of solder. Then, apply 0.44mg of ROM0 type flux paste to the OSP pads. Push the coated PCB board into the fixture cavity and use a longitudinal limiting block to fix the PCB board. Place the fixture... Below the ultraviolet picosecond laser, based on the PCB board pad positions and global reference points, a circular solder pad cutting outline with a diameter of 1.8mm is input into the marking system. The cutting pulse frequency is set to 102kHz, the pulse width to 10ps, the focusing lens working distance to 125mm, and the cutting speed to 500mm / s. The auxiliary air blowing is turned off. After cutting each solder pad, the solder pad falls vertically 2mm to the corresponding pad at the bottom under the action of gravity, forming a solder pad-flux paste-pad structure to be packaged. The welding process was then switched to a near-infrared semiconductor laser with a wavelength of 915nm. The laser beam scanned along a spiral trajectory with the center of the solder pad as the scanning center, and the welding process consisted of three stages: heating, wetting and holding, and air cooling. The peak power of the first two stages was set to 25W, the pulse frequency to 100Hz, the spiral diameter to 1.2mm, and the scanning speed to 30mm / s. In the heating stage, the temperature was increased from 25℃ to 230℃, the light output duration was 0.42s, the average power was 10W, and the pulse width was 4ms. In the wetting and holding stage, the temperature was maintained at 230℃, the light output duration was 3.5s, the average power was 1.5W, and the pulse width was 0.6ms. In the air cooling stage, after reaching the peak temperature of 230℃ and completing the holding period, the laser output was stopped, and the surface was air cooled to room temperature before the PCB was removed. The solder joints on the PCB were then subjected to void testing, shear performance testing, and shear fracture characterization. The alloy welds prepared by this method had an average void ratio of 0.29% and an average shear strength of 41.12 MPa. No voids were observed on the fracture surface after shearing. Figure 14 .

[0040] Comparative Example 1 The same alloy strip solder sheet as in Example 1 was used. A single ultraviolet picosecond laser was used to cut circular solder sheets using the same cutting parameters as in Example 1. After manual sorting, the solder sheets were manually mounted. The PCB board and solder paste were the same as in Example 1, and the mounting structure was also the same. The mounted structure was then manually transferred to a near-infrared semiconductor laser for soldering, using the same soldering parameters as in Example 1. Finally, void tests, shear performance tests, and shear fracture characterization were performed on the solder joints on the PCB. The average void rate of the alloy solder joints prepared in this comparative example was 0.54%, representing an 86.21% increase compared to Example 1; the average shear strength was 34.65 MPa, a decrease of 15.73%. Visible void traces were observed on the fracture surface after shearing. Figure 15 .

[0041] Comparative Example 2 The same alloy strip solder sheet as in Example 1 was used. Circular solder sheets were mechanically cut using a slicing machine with the same cutting parameters as in Example 1. After manual sorting, the solder sheets were manually mounted. The PCB board and solder paste were the same as in Example 1, and the mounting structure was also the same. The mounted structure was then manually transferred to a near-infrared semiconductor laser for soldering, with the same soldering parameters as in Example 1. Finally, void tests, shear performance tests, and shear fracture characterization were performed on the solder joints on the PCB. The average void rate of the alloy solder joints prepared in this comparative example was 0.82%, representing an increase of 182.76% compared to Example 1; the average shear strength was 30.30 MPa, a decrease of 26.31%. Visible void traces were observed on the fracture surface after shearing. Figure 16 .

[0042] Comparative Example 3 The same alloy strip solder sheet as in Example 1 was used. A single ultraviolet picosecond laser was used to cut circular solder sheets with the same cutting parameters as in Example 1. After manual sorting, the solder sheets were manually mounted. The PCB board and flux were the same as in Example 1, and the mounting structure was also the same. The mounted structure was then manually transferred to a desktop drawer-type reflow oven for soldering. The temperature was first raised from 25°C to 120°C over 50 seconds; then from 120°C to 170°C over 80 seconds to achieve PCB board homogenization and flux activation; subsequently, the temperature was raised from 170°C to 230°C over 90 seconds to melt the solder powder and wet the pads; heating was stopped after reaching the peak temperature of 230°C, and the PCB board was removed from the reflow oven heating zone and air-cooled to room temperature, thus forming tin-based solder joints. Finally, void tests, shear performance tests, and shear fracture characterization were performed on the solder joints on the PCB. The alloy weld joints prepared in this comparative example had an average void ratio of 1.92%, representing a 562.07% increase in voids compared to Example 1; the average shear strength was 26.63 MPa, a decrease of 35.24%, and voids were visible to the naked eye at the fracture surface after shearing. Figure 17 .

[0043] Comparative Example 4 The same alloy strip solder sheet as in Example 1 was used to cut circular solder sheets using a slicing machine with the same cutting parameters as in Example 1. After manual sorting, the solder sheets were manually mounted. The PCB board and solder paste were the same as in Example 1, and the mounting structure was the same as in Example 1. Then, the mounted structure was manually transferred to a desktop drawer-type reflow oven for soldering, using the same soldering method as Comparative Example 3. Finally, void tests, shear performance tests, and shear fracture characterization were performed on the solder joints on the PCB. The average void rate of the alloy solder joints prepared in this comparative example was 2.02%, which was 596.55% higher than that in Example 1; the average shear strength was 23.17 MPa, which was 43.65% lower. Obvious voids were visible to the naked eye after shearing. Figure 18 .

[0044] Comparative Example 5 The alloy composition of Example 1 was used to prepare T4 tin powder with a particle size of 20-38 μm by centrifugal atomization. Then, the same flux as in Example 1 was mixed to form solder paste. A stencil with an opening diameter of 1.8 mm and a thickness of 0.4 mm was used to print the solder paste onto the PCB pads, ensuring coverage of the corresponding pad areas. After printing, the PCB was placed in a desktop reflow oven for overall reflow soldering. The soldering method was the same as in Comparative Example 3. Finally, void tests, shear performance tests, and shear fracture characterization were performed on the solder joints on the PCB. The average void rate of the alloy solder joints prepared in this comparative example was 2.07%, representing a 613.79% increase compared to Example 1; the average shear strength was 20.14 MPa, a 51.02% decrease. Significant voids were visible to the naked eye at the fracture surface after shearing. Figure 19 .

[0045] The melting point test of the alloy was conducted using a STA449F5 synchronous thermal analyzer. The test sample consisted of 40 mg of weld scrap. The temperature was raised to 250 °C under Ar atmosphere and then cooled to room temperature under Ar atmosphere. The heating and cooling rates were both 10 K / min.

[0046] The laser cutting and welding in the above embodiments uses the MSK-ALC-60S laser equipment, which integrates an ultraviolet picosecond laser and a near-infrared semiconductor laser.

[0047] The slicing machine used in the above comparative examples is an ME type servo slicing machine, the laser cutting machine is a UVC type ultraviolet laser cutting machine, the laser welding machine is a WD type near-infrared laser welding machine, and the reflow soldering machine is a T200C drawer-type reflow oven.

[0048] The above embodiments and comparative examples involved testing the void ratio, shear performance, and fracture surface characterization of the weld joints. The weld joint void ratio was tested using a UNICOMPAX9600M fully automatic X-ray inspection machine, which can directly output the weld joint void ratio. Shear performance testing and fracture surface image capture were performed using a Try-precisionABT2000C multi-functional fully automatic push-pull force tester.

[0049] Table 1. Statistical results of weld void rate

[0050] Table 2 Statistical results of weld joint shear strength

[0051] Figure 6The statistics of solder joint void rates for Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4 and Comparative Example 5 are shown. Figure 7 The shear strength statistics of Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4 and Comparative Example 5 are shown.

[0052] As can be seen from the data in the above embodiments and comparative examples, the method of the present invention effectively solves the problems of high void ratio and weak shear performance that exist in traditional solder joint preparation methods, which involve mechanical cutting or laser cutting of prefabricated solder sheets, combined with manual transfer and mounting, and using laser welding, drawer-type reflow soldering, and stencil printing, respectively. Compared with the above-mentioned traditional solder joint preparation methods, the void ratio of the same alloy solder joint prepared by the in-situ laser cutting and welding integrated by the present invention is only 0.29%, while the void ratio of the traditional method increases by 86.21%~613.79% compared with the present invention. At the same time, the average shear strength of the solder joint of the present invention reaches 41.12 MPa, which is 15.73%~51.02% higher than that of the traditional method. The solder joint prepared by the present invention has significantly improved performance and can fully meet the high requirements of electronic packaging.

[0053] Unless otherwise stated, all percentages mentioned in this invention are mass percentages.

[0054] The cutting, welding, and testing equipment described in this invention are all existing technology equipment.

Claims

1. A method for preparing tin-based solder joints by integrating in-situ laser cutting and welding, characterized in that, Includes the following steps: Step 1: Cover the rolled thin strip of tin-based solder sheet onto the C18150 fixture, and use the horizontal clamping and positioning piece on the fixture to position and clamp the thin strip of tin-based solder sheet; Step 2: Push the PCB board coated with ROM0 type solder paste into the hollow cavity of the fixture, and fix the PCB board using the longitudinal limiting blocks at both ends of the cavity; Step 3: Based on the size and distribution of the PCB board pads and the global reference points, input the marking parameters of the cutting pattern of the corresponding circular pads into the laser marking software; Step 4: Using laser cutting mode, the fixture is placed under a 355nm ultraviolet picosecond laser, the auxiliary blowing is turned off, and a circular solder pad with a diameter of 1.8mm and a thickness of 200μm is cut. The circular solder pad falls vertically 2mm under the action of gravity to the corresponding pad at the bottom, forming a solder pad-flux paste-pad structure to be packaged. Step 5: Keeping the C18150 fixture in the same position, switch to a near-infrared semiconductor laser with a wavelength of 915nm in situ, and use laser welding mode to weld to form tin-based solder joints.

2. The method for preparing tin-based solder joints by in-situ laser cutting and welding as described in claim 1, characterized in that, The composition of the tin-based solder joint, by mass percentage, is SnCu0.5Bi5In15Co0.02Mn0.05, and the alloy melting point is 196.05℃.

3. The method for preparing tin-based solder joints by in-situ laser cutting and welding as described in claim 1 or 2, characterized in that, The laser cutting and laser welding integrate an ultraviolet picosecond laser and a near-infrared semiconductor laser. In the cutting mode, the average power is 30W, the pulse frequency is 102kHz, the pulse width is 10ps, the focusing lens working distance is 125mm, and the cutting speed is 500mm / s. In the welding mode, the welding is divided into three stages: heating stage, wetting and heat preservation stage, and air cooling stage.

4. The method for preparing tin-based solder joints by in-situ laser cutting and welding as described in claim 3, characterized in that, The peak power, pulse frequency, spiral diameter, and scanning speed were both set to 25W and 100Hz for both the heating and immersion holding stages. The heating stage increased the temperature from 25℃ to 230℃, with a light output duration of 0.42s, an average power of 10W, and a pulse width of 4ms. The immersion holding stage maintained the temperature at 230℃, with a light output duration of 3.5s, an average power of 1.5W, and a pulse width of 0.6ms. The air cooling stage stopped laser output after reaching the peak temperature of 230℃ and completing the holding stage, and the laser was air-cooled to room temperature to form tin-based solder joints.

5. The method for preparing tin-based solder joints by in-situ laser cutting and welding as described in claim 1, characterized in that, The fixture is made of C18150 chromium zirconium copper alloy and includes a rectangular fixture body (1), a hollow cavity (2), four horizontal clamping and positioning plates (3) set at the top of the front and rear ends of the fixture body (1), and two longitudinal limiting blocks (4) set at the PCB inlet and outlet ends of the fixture; the horizontal clamping and positioning plates (3) are used to position and clamp the thin strip tin-based solder sheet, and the longitudinal limiting blocks (4) are used to limit the displacement of the PCB board along its length direction.

6. The method for preparing tin-based solder joints by in-situ laser cutting and welding as described in claim 5, characterized in that, The main body (1) of the fixture has hollowed-out notches (5) at its left and right ends respectively, which are connected to the hollowed-out cavity (2). The hollowed-out notches (5) form heat dissipation channels and dust emission channels. The bottom edge of the hollowed-out notches (5) is higher than the bottom edge of the hollowed-out cavity (2), forming a lateral blocking structure to limit the displacement of the PCB board along its width direction.

7. A method for preparing tin-based solder joints by in-situ laser cutting and welding as described in claim 5 or 6, characterized in that, The fixture has a length of 55mm, a width of 40mm, and a thickness of 9mm. The hollow cavity has a length of 45.3mm, a width of 35.3mm, and a height of 4mm. The dimensional accuracy of the fixture is controlled within 0.1mm. The surface roughness Ra of the inner surface of the hollow cavity and the upper surface of the fixture is not greater than 0.4μm, and the surface roughness Ra of the other surfaces is not greater than 0.8μm.

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