A surface mount reflow soldering method and apparatus

Abstract

This invention provides a surface mount reflow soldering method and apparatus. The method includes: pre-treating the solder paste state and performing reference positioning correction on the PCB pads to align the PCB pads with the target pad area; using an electroformed stencil with inverted conical openings and a surface nano-coating, printing the pre-treated solder paste onto the PCB pads under closed-loop monitoring; placing the printed PCB in a reflow soldering chamber, and strictly controlling the residual oxygen content in the chamber below a preset critical value through a closed-loop oxygen concentration control system; and performing thermal history control of the reflow soldering peak temperature, holding time, and cooling temperature based on the melting point characteristics of the solder paste to complete the soldering process. This improves printing transfer efficiency, reduces soldering oxidation problems, and ensures sufficient melting and wetting of the solder.

Description

Technical Field

[0001] This invention relates to the field of electronic assembly technology, and in particular to a surface mount reflow soldering method and apparatus. Background Technology

[0002] Surface Mount Technology (SMT) is a core process in the modern electronics assembly industry. In SMT processes, reflow soldering is a crucial step determining soldering quality, and its basic flow includes three main steps: solder paste printing, component placement, and reflow soldering.

[0003] As electronic products evolve towards miniaturization, high density, and multifunctionality, the spacing between solder pads is continuously decreasing. In ultra-fine pitch scenarios where the pad aperture diameter shrinks to 70μm and the spacing is less than 0.2mm, conventional solder pastes (such as T3-T5 types) cannot achieve effective release due to their coarse solder powder particles (typically 25-45μm). Therefore, finer powder solder pastes with even smaller particle diameters must be used. Among these, the seventh type of fine powder solder paste has solder powder particle diameters ranging from 1 to 12μm, which meets the requirements for ultra-fine pitch printing.

[0004] However, Type 7 fine powder solder paste has the following unique properties: First, its viscosity is high, typically around 290 Pa·s, far exceeding the approximately 100 Pa·s of conventional solder paste; second, its specific surface area is large, making it highly susceptible to oxidation at high temperatures. These characteristics pose significant challenges to the printing and soldering processes. In the printing process, traditional stencils (laser-cut straight-wall openings, no surface coating) combined with conventional printing parameters easily lead to poor solder paste release, hole clogging, and low transfer efficiency. In the reflow soldering process, traditional processes typically have relatively lax control over oxygen content (residual oxygen ≥100 ppm or even no control). For Type 7 fine powder solder paste, high residual oxygen levels result in severe solder oxidation, leading to increased spreading angle, insufficient rise height, and the formation of cold solder joints or poor solder joints. Furthermore, traditional processes lack precise matching of peak temperature and holding time, failing to tailor settings based on the melting point characteristics of fine powder solder paste, resulting in insufficient solder melting or overheating and embrittlement.

[0005] Therefore, how to establish a stable process method specifically for fine powder solder paste, adapted to ultra-fine pitch pads, and capable of precisely controlling the reflow soldering atmosphere has become an urgent technical problem to be solved in high-density SMT mass production. Summary of the Invention

[0006] This invention provides a surface mount reflow soldering method and apparatus, aiming to solve the problems of low printing transfer efficiency, severe soldering oxidation, and insufficient solder wetting that exist when using fine powder solder paste for reflow soldering in ultra-fine pitch scenarios.

[0007] In a first aspect, the present invention provides a surface mount reflow soldering method, comprising: The solder paste condition is pre-processed, and the PCB pads are calibrated to align the PCB pads with the target pad area. An electroformed steel mesh with inverted conical openings and a nano-coating on the surface is used to print pre-treated solder paste onto the PCB pads under closed-loop monitoring. The printed PCB is placed in the reflow soldering chamber, and the residual oxygen in the chamber is strictly controlled below the preset critical value through a closed-loop oxygen concentration control system. Based on the melting point characteristics of the solder paste, the thermal process control of reflow soldering peak temperature, holding time, and cooling temperature is performed to complete the soldering.

[0008] In one embodiment, the solder paste includes a seventh type of fine powder solder paste.

[0009] In one embodiment, the pretreatment of the solder paste state includes: The solder paste is placed in a constant temperature environment for a preset time to allow it to reheat. After the temperature recovery is complete, the solder paste is homogenized and stirred to re-disperse the solder powder particles and flux carrier evenly through shear force.

[0010] In one embodiment, the reference positioning correction of the PCB pads includes: Identify the actual coordinates of the geometric center of the empty PCB pads and compare them with the theoretical coordinates to obtain the translation offset and rotation angle deviation. Based on the translation offset and rotation angle deviation, the steel mesh fine-tuning mechanism is driven to perform sub-micron level compensation and correction.

[0011] In one embodiment, the step of printing the pre-treated solder paste onto the PCB pads under closed-loop monitoring includes: The diameter of the rolling cylinder formed by the solder paste on the stencil surface is monitored in real time, and the demolding morphology of the solder paste at the moment of detachment from the stencil is captured. The transfer efficiency ratio is obtained based on the diameter of the rolling cylinder, the demolding morphology, and the designed thickness of the steel mesh. When the transfer efficiency ratio is lower than a preset value, the system automatically increases the printing pressure or reduces the squeegee travel speed; when edge collapse or adhesion of the solder paste layer is detected, the system automatically reduces the printing pressure or increases the squeegee travel speed.

[0012] In one embodiment, before placing the printed PCB into the reflow oven, the process includes: determining whether the printed PCB is qualified.

[0013] In one embodiment, placing the printed PCB in a reflow soldering chamber and strictly controlling the residual oxygen level in the chamber below a preset critical value using a closed-loop oxygen concentration control system includes: The printed PCB is then transferred into the preheating section of the reflow soldering equipment. High-purity nitrogen is injected into the furnace cavity at a preset flow rate, and the residual oxygen level in the cavity is monitored in real time. When the residual oxygen level exceeds the preset upper limit, the nitrogen injection flow rate is automatically increased to stably maintain the residual oxygen level below the preset lower limit.

[0014] In one implementation, the thermal history control includes: The reflow peak temperature is set to an overheating margin of 20 to 30 degrees Celsius above the melting point of the solder paste; After reaching the reflow peak temperature, the PCB is held at the reflow peak temperature for a controlled holding time of 60 to 90 seconds. After the heat preservation phase, the PCB enters the cooling phase, where the system gradually reduces the board temperature to below 150 degrees Celsius at a controlled cooling rate of 2 to 5 degrees Celsius per second.

[0015] In a second aspect, the present invention provides a surface mount reflow soldering apparatus, comprising: The preprocessing module is used to preprocess the solder paste state and perform reference positioning correction on the PCB pads to align the PCB pads with the target pad area. A printing module is used to print pre-treated solder paste onto the PCB pads using an electroformed stencil with inverted conical openings and a surface nano-coating, under closed-loop monitoring. The control module is used to place the printed PCB into the reflow soldering chamber and strictly control the residual oxygen in the chamber below the preset critical value through a closed-loop oxygen concentration control system. The thermal history control module is used to perform thermal history control of reflow soldering peak temperature, holding time and cooling temperature based on the melting point characteristics of the solder paste in order to complete the soldering.

[0016] Thirdly, the present invention provides an electronic device including a processor and a memory, the memory storing computer-readable instructions that, when executed by the processor, perform the method as described in any of the preceding claims.

[0017] As can be seen from the above technical solutions, the present invention has the following advantages: This invention provides a surface mount reflow soldering method and apparatus. The method includes: pre-treating the solder paste state and performing reference positioning correction on the PCB pads to align the PCB pads with the target pad area; using an electroformed stencil with inverted conical openings and a surface nano-coating, printing the pre-treated solder paste onto the PCB pads under closed-loop monitoring; placing the printed PCB in a reflow oven, and strictly controlling the residual oxygen content in the oven below a preset critical value through a closed-loop oxygen concentration control system; and performing thermal history control of the reflow peak temperature, holding time, and cooling temperature based on the melting point characteristics of the solder paste to complete the soldering. By pre-treating the solder paste, the rheological properties of the solder paste can be restored to their optimal printing state. By employing a reference positioning and correction system, precise alignment between the stencil openings and PCB pads is ensured, preventing printing misalignment. The electroformed stencil, featuring inverted conical openings and a nano-coating, significantly reduces solder paste release resistance and improves transfer efficiency. A closed-loop oxygen concentration control system keeps residual oxygen levels below a preset critical value, effectively preventing high-temperature oxidation of the solder paste. Furthermore, customized thermal history management based on the solder paste's melting point characteristics ensures complete solder melting and wetting. These technical features work synergistically to achieve highly reliable reflow soldering in ultra-fine pitch scenarios. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a flowchart illustrating the steps of a surface mount reflow soldering method according to an embodiment of the present invention. Figure 2 This is a structural block diagram of a second embodiment of a surface mount reflow soldering apparatus of the present invention. Detailed Implementation

[0020] This invention provides a surface mount reflow soldering method and apparatus, aiming to solve problems such as low printing transfer efficiency, severe soldering oxidation, and insufficient solder wetting when using fine powder solder paste for reflow soldering in ultra-fine pitch scenarios.

[0021] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0022] Example 1 Please see Figure 1 , Figure 1 This is a flowchart illustrating the steps of a surface mount reflow soldering method according to an embodiment of the present invention; the method includes: Step S101: Preprocess the solder paste state and perform reference positioning correction on the PCB pads to align the PCB pads with the target pad area.

[0023] In an optional embodiment, the solder paste includes a seventh type of fine powder solder paste.

[0024] In a specific embodiment, the first step is to perform a pre-printing process to complete the state control of the seventh type of fine solder paste and the PCB reference positioning correction.

[0025] The state control of solder paste includes: Before officially starting the precision printing operation, the system first performs strict state recovery and rheological property stabilization treatment on the selected Type VII solder paste. Type VII solder paste, due to its fine tin powder alloy particle diameter distribution ranging from 1 micrometer to 12 micrometers and its nominal viscosity as high as 290 Pascal-second, has a decisive influence on the printing quality due to its rheological properties after low-temperature storage. Therefore, the refrigerated solder paste container must be removed and placed in a constant temperature environment for natural warming. Optionally, the warming time is strictly controlled between 2 and 4 hours to ensure that the overall temperature of the solder paste rises uniformly to room temperature, avoiding flux condensation and water absorption caused by temperature differences. After warming, a planetary centrifugal mixer is used to homogenize the solder paste. Optionally, the mixing speed is set between 100 rpm and 300 rpm. Through the action of shear force, the tin powder particles and flux carrier are redispersed uniformly, ultimately stabilizing the solder paste viscosity within a tolerance range of ±10% of 290 Pascal-second.

[0026] PCB reference positioning and correction includes: the system calls upon an automated optical inspection device to perform a high-precision reference positioning scan on the empty PCB pad array about to enter the printing station. An image matching algorithm accurately identifies the actual physical coordinates of the geometric center of each pad and compares them one by one with the theoretical coordinates in the standard design file. This calculates the overall translational offset and minute rotational angle deviations generated during PCB clamping. Based on the calculated overall offset, the control system automatically drives the stencil fine-tuning mechanism before printing begins to perform sub-micron level compensation and correction on the relative spatial position between the stencil and the PCB. This compensation is strictly limited to within 10% of the pad design diameter to prevent excessive offset from causing solder paste printing to deviate from the target pad area.

[0027] In step S102, an electroformed steel mesh with inverted conical openings and a surface nano-coating is used to print the pre-treated solder paste onto the PCB pads under closed-loop monitoring.

[0028] In a specific embodiment, an electroformed inverted conical stencil modified with a nano-coating is used to perform precision printing and deposition of Type 7 solder paste under a closed-loop monitoring mechanism. The electroformed stencil used is not a product of traditional laser cutting or chemical etching processes, but rather a monolithic microporous structure constructed by depositing high-purity nickel metal atoms layer by layer on a high-precision substrate surface using electroforming technology. This process gives the stencil openings a unique inverted conical three-dimensional geometric configuration. Optionally, the top opening diameter of the electroformed inverted conical stencil is precisely controlled to 70 micrometers to match the size of fine-pitch pads, while the bottom opening diameter gradually decreases to the range of 60 to 65 micrometers. This structured electroformed conical stencil can form a gradually converging flow channel from top to bottom during the solder paste filling process under pressure, significantly reducing the adhesion resistance between the solder paste and the hole walls when it is released from the stencil openings.

[0029] Furthermore, the stencil surface is pre-treated with a nanoscale low surface energy coating. This coating material adheres firmly to the nickel substrate surface through chemical bonding, reducing the sliding friction coefficient between the solder paste and the stencil to an extremely low level of less than 0.3. After the printing operation starts, the squeegee propels the solder paste forward at a preset travel speed, optionally between 50 N and 100 N of pressure and 20 mm / s to 50 mm / s. During this process, a machine vision module integrated into the printer monitors the diameter of the rolling cylinder formed by the solder paste on the stencil surface in real time, and captures the demolding morphology of the solder paste the moment it leaves the stencil using a high-speed camera. The control system simultaneously receives the actual height data of the printed solder paste layer from the three-dimensional optical inspection equipment and calculates its real-time ratio with the designed thickness of the stencil. If the transfer efficiency ratio falls below the preset value, it indicates that the solder paste filling or demolding is insufficient. The system will automatically decide to increase the printing pressure or reduce the squeegee travel speed to enhance the filling driving force, where the preset value is 80%. Conversely, if edge collapse of the solder paste layer or signs of adhesion between adjacent pads are detected, the system will automatically reduce the printing pressure or increase the squeegee speed to reduce the deposition of excessive solder paste.

[0030] Step S103: Place the printed PCB in the reflow oven and strictly control the residual oxygen content in the oven below the preset critical value using a closed-loop oxygen concentration control system.

[0031] In a specific embodiment, before placing the printed PCB into the reflow soldering chamber, the quality of the printed PCB is determined. The qualified PCB is placed in a nitrogen-protected reflow soldering chamber, where a closed-loop oxygen concentration control system strictly suppresses the residual oxygen content below the critical threshold of 100 ppm. After the printing process is completed and online inspection confirms its qualification, the PCB bearing the Type 7 solder paste deposition pattern is automatically conveyed into the preheating section of the reflow soldering equipment. This step addresses the inherent material characteristics of Type 7 fine powder solder paste—large specific surface area and easy oxidation at high temperatures—by designing a precise low-oxygen atmosphere protection soldering environment control strategy. Inside the heating chamber of the reflow soldering equipment, an independent nitrogen supply and distribution system continuously operates, injecting high-purity nitrogen into the chamber at a preset flow rate of 10 liters / minute to 30 liters / minute through gas nozzles located at key positions within the chamber, forcibly diluting and displacing the original air atmosphere in the chamber.

[0032] Meanwhile, a zirconia oxygen concentration sensor installed in the center of the heating zone continuously monitors the real-time residual oxygen content in the chamber at a sampling frequency of no less than 1 Hz. The monitoring data is transmitted in real time to the closed-loop control unit, which incorporates an intelligent PID control algorithm with hysteresis characteristics. Once the sensor reading exceeds the preset upper limit warning line of 100 ppm, the control system immediately outputs a command to automatically increase the opening of the nitrogen mass flow controller, increasing the nitrogen injection rate until the oxygen concentration in the chamber is rapidly reduced and stabilized below the ideal safe range of 50 ppm. When the oxygen concentration reading falls back below 50 ppm and maintains a stable trend, the control system reduces the nitrogen supply flow rate in a timely manner to balance the process protection effect with the nitrogen consumption operating cost.

[0033] Step S104: Based on the melting point characteristics of the solder paste, perform thermal process control of reflow soldering peak temperature, holding time and cooling temperature to complete the soldering.

[0034] In a specific embodiment, the thermal history control of reflow soldering peak temperature and holding time is precisely executed based on the melting point characteristics of Type 7 solder paste, ensuring sufficient melting, wetting, spreading, and controlled cooling and solidification of the solder. Under a protective atmosphere with effectively controlled residual oxygen, the PCB sequentially passes through the preheating zone, isothermal zone, and reflow peak zone of the reflow soldering equipment. This step precisely controls the soldering thermal history according to the datasheet recommended curves corresponding to the Type 7 solder paste alloy composition and is specifically optimized for the soldering requirements of fine-pitch, small pads.

[0035] Based on the inherent melting point range of 200°C to 220°C for the seventh type of solder paste alloy components, the control system precisely sets the reflow peak temperature to an overheat margin of 20°C to 30°C above this melting point. This ensures that the solder powder particles can completely break through the constraints of their surface oxide film and undergo sufficient phase transformation. After reaching the preset peak temperature, the PCB is held at this high temperature range for a controlled duration of 60 to 90 seconds. This time window provides ample opportunity for the molten solder to undergo rheological relaxation and surface diffusion, allowing it to effectively wet and climb to cover all solderable surfaces of the copper pads under the synergistic effect of the flux active ingredients. After the holding phase, the PCB immediately enters the cooling phase, where the system gradually reduces the board temperature to below the solidus temperature of 150°C at a controlled cooling rate of 2°C to 5°C per second. This slow-descent cooling strategy effectively avoids the thermal stress concentration caused by excessive temperature difference between the inside and outside of the solder joint, thereby inhibiting the initiation and propagation of microcracks inside the solder joint, and ultimately forming a bright, full and dense metallurgical bond between the solder pad and the component lead.

[0036] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention uses an electroformed steel mesh with inverted conical openings. The openings have an inverted conical structure with a large top and a small bottom, which creates a "trumpet mouth" effect when the solder paste is demolded, effectively reducing demolding resistance. Combined with a surface nano-coating, the sliding friction coefficient is reduced to below 0.3, further reducing solder paste adhesion. At the same time, a closed-loop monitoring mechanism detects the transfer efficiency in real time and dynamically adjusts the printing parameters to ensure that the transfer efficiency is stably maintained above 80%, effectively solving the problem of difficult printing of fine powder and high viscosity solder paste.

[0037] (2) This invention employs a closed-loop oxygen concentration control system. Through real-time monitoring and PID adjustment, the residual oxygen content in the reflow soldering chamber is strictly controlled to below 100 ppm and can be stably maintained within the ideal range of 50 ppm. For the Type VII fine powder solder paste, which has a large specific surface area and is easily oxidized, the low-oxygen atmosphere significantly inhibits the formation of oxide film on the solder surface, enabling the solder wetting angle to be controlled below 90° and the creepage ratio to reach more than 50%, fundamentally avoiding defects such as cold solder joints and poor solder joints.

[0038] (3) Based on the melting point characteristics of fine powder solder paste, the present invention precisely sets the reflow peak temperature to 20-30°C above the melting point and sets a holding time of 60-90 seconds to ensure that the solder is fully melted, wetted and spread. At the same time, a slow cooling at a rate of 2-5°C / second effectively suppresses the generation of internal thermal stress and microcracks in the solder joint and improves the long-term reliability of the solder joint.

[0039] (4) This invention forms a complete process closed loop, from solder paste pretreatment, reference positioning, printing closed-loop monitoring, atmosphere closed-loop control to precise control of thermal history. Each link cooperates with each other and is interconnected, which together ensures the stability and consistency of fine powder solder paste reflow soldering in ultra-fine pitch scenarios, and provides a highly reliable technical solution for high-density SMT mass production.

[0040] (5) This invention is not only applicable to Type 7 fine powder solder paste, but can also be extended to other types of fine powder solder paste (such as Type 6, Type 8, etc.). At the same time, the method and apparatus of this invention can be integrated into existing SMT production lines, which can be achieved by upgrading the control software and replacing key hardware (stencil, nitrogen control system, etc.), and has good prospects for industrial application.

[0041] Example 2 Please see Figure 2 , Figure 2 This is a structural block diagram of a second embodiment of a surface mount reflow soldering apparatus of the present invention. The apparatus includes: The preprocessing module 201 is used to preprocess the solder paste state and perform reference positioning correction on the PCB pads to align the PCB pads with the target pad area; optionally, the solder paste includes a seventh type of fine powder solder paste.

[0042] The printing module 202 is used to print pre-treated solder paste onto the PCB pads using an electroformed stencil with inverted conical openings and a surface nano-coating, under closed-loop monitoring.

[0043] The control module 203 is used to place the printed PCB in the reflow soldering chamber and strictly control the residual oxygen in the chamber below the preset critical value through a closed-loop oxygen concentration control system.

[0044] The thermal history control module 204 is used to perform thermal history control of reflow soldering peak temperature, holding time and cooling temperature according to the melting point characteristics of the solder paste in order to complete the soldering.

[0045] In one specific embodiment, the pretreatment module 201 is further used for the state control of the solder paste, including, before the formal start of the precision printing operation, the system first performs strict state recovery and rheological property stabilization treatment on the selected Type 7 solder paste. Type 7 solder paste, due to its fine tin powder alloy particle diameter distribution ranging from 1 micrometer to 12 micrometers and its nominal viscosity as high as 290 Pascal-second, has a decisive influence on the printing quality due to its rheological properties after low-temperature storage. Therefore, the refrigerated solder paste container must be removed and placed in a constant temperature environment for natural warming treatment. Optionally, the warming time is strictly controlled between 2 and 4 hours to ensure that the overall temperature of the solder paste rises uniformly to room temperature, avoiding flux condensation and water absorption caused by temperature differences. After warming, a planetary centrifugal mixer is used to homogenize the solder paste. Optionally, the mixing speed is set in the range of 100 rpm to 300 rpm. The solder powder particles and flux carrier are redispersed uniformly by shear force, and the viscosity of the solder paste is finally stabilized within a tolerance range of ±10% of 290 Pascals per second.

[0046] In one specific embodiment, the preprocessing module 201 is further used for PCB reference positioning correction. This includes calling an automatic optical inspection device to perform a high-precision reference positioning scan on the empty PCB pad array that is about to enter the printing station. An image matching algorithm is used to accurately identify the actual physical coordinates of the geometric center of each pad, and these coordinates are compared one by one with the theoretical coordinates in the standard design file to calculate the overall translational offset and minute rotational angle deviation of the PCB during clamping. Based on the calculated overall offset, the control system automatically drives the stencil fine-tuning mechanism before printing begins to perform sub-micron level compensation correction on the relative spatial position between the stencil and the PCB. This compensation amount is strictly limited to within 10% of the pad design diameter to prevent excessive offset from causing the solder paste printing to deviate from the target pad area.

[0047] In one specific embodiment, the printing module 202 employs an electroformed inverted conical stencil modified with a nano-coating to perform precision printing deposition of Type 7 solder paste under a closed-loop monitoring mechanism. The electroformed stencil used is not a product of traditional laser cutting or chemical etching processes, but rather an integral microporous structure constructed by depositing high-purity nickel metal atoms layer by layer on a high-precision substrate surface using electroforming technology. This process gives the stencil openings a unique inverted conical three-dimensional geometric configuration. Optionally, the top opening diameter of the electroformed inverted conical stencil is precisely controlled to 70 micrometers to match the size of fine-pitch pads, while the bottom opening diameter gradually decreases to the range of 60 to 65 micrometers. This structured electroformed conical stencil can form a gradually converging flow channel from top to bottom during the solder paste filling process under pressure, significantly reducing the adhesion resistance between the solder paste and the hole walls when it is released from the stencil openings.

[0048] In one specific embodiment, the control module 203 is further used to determine whether the printed PCB is qualified before placing it into the reflow soldering chamber. The qualified PCB is placed in a nitrogen-protected reflow soldering chamber, and the residual oxygen content is strictly suppressed below the critical threshold of 100 ppm through a closed-loop oxygen concentration control system. After the printing process is completed and online detection determines it is qualified, the PCB carrying the Type 7 solder paste deposition pattern is automatically conveyed into the preheating section of the reflow soldering equipment. This step addresses the inherent material characteristics of Type 7 fine powder solder paste, such as its large specific surface area and susceptibility to oxidation at high temperatures, by designing a precise low-oxygen atmosphere protection soldering environment control strategy. Inside the heating chamber of the reflow soldering equipment, an independent nitrogen supply and distribution system continuously operates, injecting high-purity nitrogen into the furnace chamber at a preset flow rate of 10 liters / minute to 30 liters / minute through gas nozzles located at key positions in the chamber, forcibly diluting and displacing the original air atmosphere in the chamber. Meanwhile, a zirconia oxygen concentration sensor installed in the center of the heating zone continuously monitors the real-time residual oxygen content in the chamber at a sampling frequency of no less than 1 Hz. The monitoring data is transmitted in real time to the closed-loop control unit, which incorporates an intelligent PID control algorithm with hysteresis characteristics. Once the sensor reading exceeds the preset upper limit warning line of 100 ppm, the control system immediately outputs a command to automatically increase the opening of the nitrogen mass flow controller, increasing the nitrogen injection rate until the oxygen concentration in the chamber is rapidly reduced and stabilized below the ideal safe range of 50 ppm. When the oxygen concentration reading falls back below 50 ppm and maintains a stable trend, the control system reduces the nitrogen supply flow rate in a timely manner to balance the process protection effect with the nitrogen consumption operating cost.

[0049] In one specific embodiment, the thermal history control module 204 precisely executes thermal history control of the reflow soldering peak temperature and holding time based on the melting point characteristics of Type 7 solder paste, completing the full melting, wetting and spreading of the solder, and controlled cooling and solidification. Under a protective atmosphere where residual oxygen content is effectively controlled, the PCB sequentially passes through the preheating zone, constant temperature zone, and reflow peak zone of the reflow soldering equipment. This step precisely controls the soldering thermal history according to the recommended curve in the datasheet corresponding to the Type 7 solder paste alloy composition, and is specifically optimized for the soldering requirements of fine-pitch micro pads. Based on the inherent melting point range of 200°C to 220°C of the Type 7 solder paste alloy composition, the control system precisely sets the reflow peak temperature to an overheating margin of 20°C to 30°C above this melting point to ensure that the solder powder particles can completely break through the constraints of their surface oxide film and undergo sufficient phase transformation. After reaching the preset peak temperature, the PCB is held at this high temperature for a controlled duration of 60 to 90 seconds. This time window provides ample opportunity for the molten solder to undergo rheological relaxation and surface diffusion, allowing it to effectively wet and climb across the entire solderable surface of the copper pads under the synergistic effect of the flux's active ingredients. After the holding phase, the PCB immediately enters the cooling phase, where the system gradually reduces the board temperature to below the solidus temperature of 150 degrees Celsius at a controlled cooling rate of 2 to 5 degrees Celsius per second. This gradual cooling strategy effectively avoids thermal stress concentration caused by excessive temperature differences between the inside and outside of the solder joint, thereby suppressing the initiation and propagation of microcracks within the solder joint, ultimately forming a bright, full, and dense metallurgically bonded solder joint between the pads and component leads.

[0050] In an optional implementation, the preprocessing module 201 includes: A reheating unit is used to place the solder paste in a constant temperature environment for a preset time for reheating treatment; The stirring unit is used to homogenize the solder paste after the temperature recovery is completed, so as to re-disperse the solder powder particles and flux carrier evenly through the action of shear force.

[0051] In an optional implementation, the preprocessing module 201 is used for: The data acquisition and processing unit is used to identify the actual coordinates of the geometric center of the empty PCB pads and compare them with the theoretical coordinates to obtain the translation offset and rotation angle deviation. The correction unit is used to drive the steel mesh fine-tuning mechanism to perform sub-micron level compensation correction based on the translation offset and rotation angle deviation.

[0052] In an optional implementation, the printing module 202 is used for: The monitoring unit is used to monitor the diameter of the rolling cylinder formed by the solder paste on the stencil surface in real time, and to capture the demolding morphology of the solder paste the moment it leaves the stencil. The calculation unit is used to obtain the transfer efficiency ratio based on the diameter of the rolling cylinder, the demolding morphology, and the design thickness of the steel mesh; The drive unit is used to automatically increase the printing pressure or reduce the squeegee travel speed when the transfer efficiency ratio is lower than a preset value; and to automatically reduce the printing pressure or increase the squeegee travel speed when edge collapse or adhesion of the solder paste layer is detected.

[0053] In an optional implementation, the control module 203 includes: The conveyor unit is used to transfer the printed PCB into the preheating section of the reflow soldering equipment. The gas injection unit is used to inject high-purity nitrogen into the furnace cavity at a preset flow rate and monitor the residual oxygen content in the cavity in real time. The flow rate adjustment unit is used to automatically increase the nitrogen injection flow rate when the residual oxygen content exceeds the preset upper limit, so as to stably maintain the residual oxygen content below the preset lower limit.

[0054] In an optional implementation, the thermal history control module 204 includes: A temperature setting unit is used to set the reflow peak temperature to an overheating margin of 20 to 30 degrees Celsius above the melting point of the solder paste. A temperature holding unit is used to maintain the PCB within the reflow peak temperature for a controlled holding time of 60 to 90 seconds after the reflow peak temperature is reached. The temperature cooling unit is used to gradually reduce the temperature of the PCB to below 150 degrees Celsius after the heat preservation stage ends and the PCB enters the cooling zone. The system reduces the temperature of the board to below 150 degrees Celsius at a controlled cooling rate of 2 to 5 degrees Celsius per second.

[0055] This embodiment ensures precise alignment between the stencil openings and PCB pads through the reference positioning correction of the preprocessing module 201, avoiding printing misalignment. The electroformed stencil with inverted conical openings and a nano-coating surface in the printing module 202 significantly reduces solder paste demolding resistance and improves transfer efficiency. The control module 203 keeps residual oxygen below a preset critical value, effectively preventing high-temperature oxidation of the solder paste. The thermal history management module 204, based on the solder paste melting point characteristics, ensures sufficient melting and wetting of the solder. These technical features work synergistically to achieve highly reliable reflow soldering in ultra-fine pitch scenarios.

[0056] Example 3 This invention also provides an electronic device, including a memory and a processor. The memory stores a computer program, which, when executed by the processor, causes the processor to perform the steps of a tension and alignment coordination control method for an ultra-large bag making machine according to any embodiment.

[0057] Example 4 This invention also provides a computer storage medium storing a computer program, which, when executed by the processor, implements the steps of a tension and alignment coordination control method for an ultra-large bag making machine according to any embodiment.

[0058] Example 5 This invention also provides a computer program product storing a computer program, which, when executed by the processor, implements the steps of a tension and alignment coordination control method for an ultra-large bag making machine according to any embodiment.

[0059] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0060] In the several embodiments provided in this application, it should be understood that the methods, apparatuses, electronic devices, and storage media disclosed in this invention can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative. For instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the displayed or discussed mutual couplings, direct couplings, or communication connections may be through some interfaces; indirect couplings or communication connections between devices or units may be electrical, mechanical, or other forms.

[0061] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0062] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0063] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a readable storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present invention. The aforementioned readable storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0064] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A surface mount reflow soldering method, characterized in that, include: The solder paste condition is pre-processed, and the PCB pads are calibrated to align the PCB pads with the target pad area. An electroformed steel mesh with inverted conical openings and a nano-coating on the surface is used to print pre-treated solder paste onto the PCB pads under closed-loop monitoring. The printed PCB is placed in the reflow soldering chamber, and the residual oxygen in the chamber is strictly controlled below the preset critical value through a closed-loop oxygen concentration control system. Based on the melting point characteristics of the solder paste, the thermal process control of reflow soldering peak temperature, holding time, and cooling temperature is performed to complete the soldering.

2. The surface mount reflow soldering method according to claim 1, characterized in that, The solder paste includes the seventh type of fine powder solder paste.

3. The surface mount reflow soldering method according to claim 1, characterized in that, The pretreatment of the solder paste state includes: The solder paste is placed in a constant temperature environment for a preset time to allow it to reheat. After the temperature recovery is complete, the solder paste is homogenized and stirred to re-disperse the solder powder particles and flux carrier evenly through shear force.

4. The surface mount reflow soldering method according to claim 1, characterized in that, The reference positioning and correction of the PCB pads includes: Identify the actual coordinates of the geometric center of the empty PCB pads and compare them with the theoretical coordinates to obtain the translation offset and rotation angle deviation. Based on the translation offset and rotation angle deviation, the steel mesh fine-tuning mechanism is driven to perform sub-micron level compensation and correction.

5. The surface mount reflow soldering method according to claim 1, characterized in that, The step of printing the pre-treated solder paste onto the PCB pads under closed-loop monitoring includes: The diameter of the rolling cylinder formed by the solder paste on the stencil surface is monitored in real time, and the demolding morphology of the solder paste at the moment of detachment from the stencil is captured. The transfer efficiency ratio is obtained based on the diameter of the rolling cylinder, the demolding morphology, and the designed thickness of the steel mesh. When the transfer efficiency ratio is lower than a preset value, the system automatically increases the printing pressure or reduces the squeegee travel speed; when edge collapse or adhesion of the solder paste layer is detected, the system automatically reduces the printing pressure or increases the squeegee travel speed.

6. The surface mount reflow soldering method according to claim 1, characterized in that, Before placing the printed PCB into the reflow oven, the process includes: determining whether the printed PCB is qualified.

7. The surface mount reflow soldering method according to claim 6, characterized in that, The step of placing the printed PCB in a reflow soldering chamber and strictly controlling the residual oxygen level in the chamber below a preset critical value using a closed-loop oxygen concentration control system includes: The printed PCB is then transferred into the preheating section of the reflow soldering equipment. High-purity nitrogen is injected into the furnace cavity at a preset flow rate, and the residual oxygen level in the cavity is monitored in real time. When the residual oxygen level exceeds the preset upper limit, the nitrogen injection flow rate is automatically increased to stably maintain the residual oxygen level below the preset lower limit.

8. The surface mount reflow soldering method according to claim 1, characterized in that, The thermal history control includes; The reflow peak temperature is set to an overheating margin of 20 to 30 degrees Celsius above the melting point of the solder paste; After reaching the reflow peak temperature, the PCB is held at the reflow peak temperature for a controlled holding time of 60 to 90 seconds. After the heat preservation phase, the PCB enters the cooling phase, where the system gradually reduces the board temperature to below 150 degrees Celsius at a controlled cooling rate of 2 to 5 degrees Celsius per second.

9. A surface mount reflow soldering apparatus, characterized in that, include: The preprocessing module is used to preprocess the solder paste state and perform reference positioning correction on the PCB pads to align the PCB pads with the target pad area. A printing module is used to print pre-treated solder paste onto the PCB pads using an electroformed stencil with inverted conical openings and a surface nano-coating, under closed-loop monitoring. The control module is used to place the printed PCB into the reflow soldering chamber and strictly control the residual oxygen in the chamber below the preset critical value through a closed-loop oxygen concentration control system. The thermal history control module is used to perform thermal history control of reflow soldering peak temperature, holding time and cooling temperature based on the melting point characteristics of the solder paste in order to complete the soldering.

10. An electronic device, characterized in that, It includes a processor and a memory, the memory storing computer-readable instructions that, when executed by the processor, perform the method as described in any one of claims 1-8.

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