VOF-based welding spot cavity numerical simulation method for vacuum vapor welding

Abstract

The invention provides a VOF-based numerical simulation method for a welding spot cavity of a vacuum vapor welding, which comprises the following steps: creating a BGA welding spot two-dimensional model and presetting a plurality of bubbles, simulating the flowing of a welding flux by adopting a laminar flow model, and simulating the interface of the welding flux and the bubbles by adopting a multiphase flow VOF model. A multi-area method quadrilateral mesh is adopted, and a UDF user-defined program is matched to simulate a flow field when bubbles escape from the solder; and meanwhile, by improving boundary condition setting, the calculation time is shortened, the calculation efficiency is improved, and the process that bubbles escape from the BGA welding spots in the welding process is rapidly and truly simulated.

Description

Technical Field

[0001] This invention relates to the field of reliability simulation of electronic packaging products, and in particular to a numerical simulation method for voids in vacuum vapor phase solder joints based on VOF. Background Technology

[0002] In the field of electronic packaging, soldering is a core step in ensuring the reliability of electrical and mechanical connections. However, the formation of voids within solder joints is unavoidable during the soldering process. These voids are typically caused by flux evaporation residues and gases adsorbed on the substrate or component lead surfaces, which fail to escape completely before the molten solder solidifies. The presence of voids significantly reduces the effective connection area of ​​the solder joint, leading to problems such as increased thermal resistance, uneven current density, and decreased mechanical strength. In severe cases, it can cause premature solder joint failure, thus affecting the long-term reliability and lifespan of electronic products. Therefore, accurately predicting, controlling, and minimizing the occurrence of solder joint voids is crucial for improving the quality of electronic packaging.

[0003] Existing techniques for controlling weld void ratio mainly rely on physical experiments and experience, typically requiring post-weld X-ray irradiation to obtain void ratio results. While this method yields relatively accurate void ratio results, it is resource-intensive and requires significant manpower and resources. Furthermore, due to equipment limitations, most technicians cannot observe the void formation process, resulting in a lack of clarity regarding the escape mechanism of bubbles within the weld joint. With the development of numerical simulation technology, simulating the bubble escape process using Fluent simulation software has become possible. Therefore, a numerical simulation method for void ratio in vacuum vapor phase welding based on VOF is needed to address the aforementioned issues. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a numerical simulation method for void formation in vacuum vapor phase welding based on VOF that can truly reflect the formation of voids in weld joints.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] A numerical simulation method for voids in vacuum vapor phase welding joints based on VOF includes the following steps:

[0007] Model creation: Create a two-dimensional model of the vacuum vapor phase welding BGA solder joint and pre-place a bubble model within the solder joint;

[0008] Mesh generation: Select the multi-region method in the ANSYS mesh module to mesh the model, and verify the mesh independence and numerical method stability after mesh generation. If the verification is successful, output the final computational domain mesh.

[0009] Solution model settings: A laminar flow model is used to simulate the flow field, a multiphase flow VOF model is used to simulate the interface between solder and bubbles, and a UDF user-defined program is used to simulate the vacuum environment;

[0010] Boundary conditions and initial conditions settings: Boundary conditions and initial conditions are set according to the welding situation. Boundary conditions include the inlet and outlet boundaries of the calculation area and the wall boundary of the calculation domain. Initial conditions include reference pressure, gravitational acceleration, fluid viscosity, and density.

[0011] Discretization method settings: The basic governing equations are discretized using the finite volume method, the velocity-pressure coupled solver is solved using the PISO algorithm, and the sub-relaxation factor is set according to the mesh quality;

[0012] Solution report and visualization: Set the parameters for the solution report output and the content of the visualization window output;

[0013] Solution and post-processing: Start the calculation and solution, monitor and output the phase cloud diagram of the weld joint.

[0014] The beneficial technical effects of this invention are as follows:

[0015] 1. The above-mentioned numerical simulation method for voids in vacuum vapor phase welding joints based on VOF significantly reduces numerical divergence through the interface reconstruction algorithm and surface tension model in the VOF method, realizes accurate tracking of the interface between molten solder and gas in vacuum vapor phase welding joints, and clearly depicts the dynamic evolution process of voids.

[0016] 2. The aforementioned numerical simulation method for voids in vacuum vapor phase welding based on VOF can quantitatively analyze the impact of atmospheric pressure process variables on the void ratio and accurately locate the critical conditions for bubble capture. This reduces physical trial-and-error costs and accelerates the development cycle of high-reliability weld joint processes. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating the numerical simulation method for voids in vacuum vapor phase welding based on VOF according to the present invention.

[0018] Figure 2 Two-dimensional phase cloud diagram of BGA solder joint. Detailed Implementation

[0019] To enable those skilled in the art to more clearly understand the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0020] This invention provides a numerical simulation method for voids in vacuum vapor phase welding welds based on VOF.

[0021] like Figure 1As shown, in one embodiment of the present invention, the numerical simulation method for gas-liquid two-phase flow in weld joint voids based on VOF includes steps S1-S7:

[0022] S1. Model Creation: Create a 2D model of the reflow soldering BGA solder joint, and create a bubble model within the solder joint. The specific steps are as follows:

[0023] Based on the actual BGA soldering situation, the model data dimensions of the BGA solder are determined. In this embodiment, a two-dimensional model of the solder joint is established based on a BGA solder ball. To reduce computational load, the fluid domain around the solder joint is ignored, and only the liquid solder is retained. The quality attributes of the solder joint model are determined according to the data provided by the solder paste manufacturer; this embodiment uses tin-lead solder. The top and bottom of the solder joint model are flat, simulating the state of the solder joint when in contact with the chip and PCB in reality. The left and right sides of the solder joint model are arcs, simulating the shape of the solder ball.

[0024] S2. Computational Domain Mesh Generation: Based on the computational domain created in step 1, the model is meshed using the multi-region method in the ANSYS mesh module. The mesh uses quadrilateral meshes, with each mesh element measuring 0.005mm x 0.005mm. The total number of nodes and elements in the model is 42593 and 42180, respectively.

[0025] After mesh generation, the mesh independence and numerical method stability are verified. Calculations are performed by dividing the mesh into different densities, and the results are compared to see if they are similar. If the results are similar, the verification is successful, and the final computational domain mesh is output for solver calculation.

[0026] S3. Solution Model Setup: Use a laminar flow model to simulate the flow of highly viscous solder, and a VOF model to track the gas-liquid interface.

[0027] ρ=αρ l +[1-α]ρ g

[0028] μ=αμ l +(1-α)μ g

[0029] Where, ρ g -Gas density, kg / m³ 3 ;ρ l -Liquid phase density, kg / m³ 3 μ g -Air viscosity, Pa·s; μ l - Liquid phase viscosity, Pa·s.

[0030] The momentum equation is solved over a unified computational domain and by a shared velocity field between the two phases. The solution to this equation is related to the viscosity density calculated above.

[0031]

[0032] in It is a velocity vector. It is a pressure gradient. It is gravitational acceleration. It is a source project caused by surface tension.

[0033] By using UDF, the outlets on both sides of the model are set as pressure outlets. Except for the vacuuming condition, the outlets are set to standard atmospheric pressure in all other conditions.

[0034] S4. Boundary conditions and initial conditions settings: Set boundary conditions and initial conditions according to the operating conditions. Boundary conditions include the inlet and outlet boundaries of the calculation area and the wall boundary of the calculation domain. Initial conditions include reference pressure, gravitational acceleration, fluid viscosity, and density.

[0035] In the specific implementation process, the initial state is set at the gas-liquid free interface of the computational domain VOF model;

[0036] S5. Discretization Settings: The basic governing equations are discretized using the finite volume method. The velocity-pressure coupled solver uses the implicit operator splitting method for pressure, and the sub-relaxation factor is set according to the mesh quality. A sub-relaxation factor of 1.0 or close to 1.0 is recommended for all equations; if the mesh is highly distorted, PISO tilt correction is recommended, with the sum of the momentum and pressure sub-relaxation factors set to 1.0.

[0037] S6. Solution Report and Visualization: Set the parameters for the solution report output. The parameters output by the solution report include the pressure at the outlet and the pressure value inside the weld joint. The visualization window outputs the pressure cloud map of the weld joint and the gas-liquid distribution map of the computational domain.

[0038] S7. Solving and Post-processing: Start the calculation and solve the problem, monitor and output the pressure contour map and phase diagram.

[0039] The present invention provides a numerical simulation method for voids in vacuum vapor phase welding solder joints based on VOF. This method creates a two-dimensional model of the BGA solder joint and pre-sets multiple bubbles. It uses a laminar flow model to simulate solder flow, a multiphase flow VOF model to simulate the interface between solder and bubbles, and a quadrilateral mesh using a multi-region method combined with a user-defined function (UDF) to simulate the flow field when bubbles escape from the solder. At the same time, by improving boundary condition settings and shortening the calculation time, the method improves computational efficiency and quickly and realistically simulates the process of bubbles escaping from the BGA solder joint during the welding process.

[0040] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Those skilled in the art can make various equivalent changes and improvements based on the above embodiments, and all equivalent variations or modifications made within the scope of the claims should fall within the protection scope of the present invention.

Claims

1. A numerical simulation method for voids in vacuum vapor phase welding welds based on VOF, characterized in that, Includes the following steps: S1: Model Establishment: Establish a two-dimensional model of the reflow soldering BGA solder joint and pre-place air bubbles within the solder joint; S2: Mesh generation: Select the multi-region method in the ANSYS mesh module to generate a mesh for the model, and verify the mesh independence and numerical method stability after the mesh generation is completed. When the verification is successful, output the final computational domain mesh. S3: Solution model settings: The laminar flow model is used to simulate the flow field, the multiphase flow VOF model is used to simulate the interface between solder and bubbles, and the UDF user-defined program is used to simulate the vacuum environment; S4: Boundary and initial conditions settings: Set boundary and initial conditions according to the welding situation. Boundary conditions include the inlet and outlet boundaries of the calculation area and the wall boundary of the calculation domain. Initial conditions include reference pressure, gravitational acceleration, fluid viscosity, and density. S5: Discretization method settings: The basic control equations are discretized using the finite volume method, the velocity-pressure coupled solver uses the PISO algorithm, and the sub-relaxation factor is set according to the mesh quality; S6: Solution Report and Visualization: Set the parameters for the solution report output and the content of the visualization window output; S7: Solving and Post-processing: Start the calculation and solve, monitor and output the phase cloud diagram of the weld joint.

2. The numerical simulation method for voids in vacuum vapor phase welding based on VOF as described in claim 1, characterized in that, In step 1, the BGA solder joint shape, which is flat at the top and bottom and has the same diameter arc on the left and right, is used as the calculation domain; multiple small air bubbles are pre-placed within the solder joint as voids generated during the soldering process.

3. The numerical simulation method for voids in vacuum vapor phase welding based on VOF as described in claim 1, characterized in that, In step 2, the mesh is divided using a multi-region method, with each mesh cell measuring 0.005mm*0.005mm. The total number of nodes and cells in the model is 42593 and 42180, respectively.

4. The numerical simulation method for voids in vacuum vapor phase welding based on VOF as described in claim 1, characterized in that, In step 3, a laminar flow model is used to simulate the flow of solder with high viscosity; a VOF model is used to track the gas-liquid interface. in It is a velocity vector. It is a pressure gradient. It is gravitational acceleration. It is a source project caused by surface tension.

5. The numerical simulation method for voids in vacuum vapor phase welding based on VOF as described in claim 1, characterized in that, In step 4, the initial pressure inside the solder is given as atmospheric pressure. Then, the outlets on both sides of the model are set as pressure outlets through UDF. Except for the vacuuming condition, the outlet pressure is set to standard atmospheric pressure in all other conditions.

6. The numerical simulation method for voids in vacuum vapor phase welding welds based on VOF as described in claim 1, characterized in that, In step 6, the parameters output by the solution report include the pressure at the outlet and the pressure value inside the weld joint; the content output by the visualization window includes the pressure cloud map of the weld joint and the gas-liquid distribution cloud map of the computational domain.

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