Fluent-based hot air reflow welding simulation analysis method

Abstract

The invention provides a Fluent-based hot air reflow soldering simulation analysis method, which comprises the following steps of: establishing a fluid calculation model comprising a circuit board product and a temperature zone cavity, simulating a flow field by adopting a Realizable model of a k-epsilon equation, tracking heat change by starting an energy equation, customizing cavity boundary conditions by cooperatively using a UDF (Unified Description Framework), and simulating the flow field by taking production process parameters as a reference. Relative fluid movement and heat change in the hot air backflow cavity are simulated; the starting position and the ending position of a cavity nozzle are searched through a self-defined function, so that hot air boundary conditions are loaded to the nozzle in a delayed activation mode, and relative movement similar to a product and a cavity body temperature area is achieved; a Couple solution scheme and energy second-order windward format discretization are used, the heat calculation precision is ensured, and meanwhile good convergence is achieved; meanwhile, self-adaptive boundary condition loading can be carried out according to different products and cavity sizes, and the hot air reflow welding process is rapidly and truly simulated.

Description

Technical Field

[0001] The present invention relates to the technical field of SMT soldering thermal simulation, and in particular to a hot air reflow soldering simulation analysis method based on Fluent. Background Art

[0002] Hot air reflow soldering is a widely used soldering technique in the field of SMT soldering technology. Thermal simulation of hot air reflow soldering and the resulting process improvements are important research areas. Fluent, a mainstream analysis software in Computational Fluid Dynamics (CFD), is used to simulate the momentum and heat processes of the thermal convection physical phenomena in this field to obtain accurate soldering temperature curves. Summary of the Invention

[0003] The technical problems to be solved are:

[0004] The present invention aims to solve the problems existing in existing hot air reflow soldering simulation and analysis methods, such as insufficient simulation accuracy, poor convergence, poor adaptability of boundary conditions, and difficulty in truly reflecting the relative movement process between the product and the cavity temperature zone. It provides a hot air reflow soldering simulation and analysis method based on Fluent to achieve fast, accurate and realistic simulation of the hot air reflow soldering process.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] To achieve the above objectives, the present invention provides a hot air reflow soldering simulation analysis method based on Fluent, the specific steps of which are as follows:

[0007] Model creation: Establish the electronic board product and temperature zone cavity models, and create a fluid calculation domain where fluid and solid coexist. The fluid calculation domain includes the electronic board product calculation domain and the temperature zone cavity calculation domain.

[0008] Computational domain meshing: The Fluent Meshing module is used to add boundary layer meshes to the solid computational domain to ensure the accuracy of heat changes on the solid wall. The Poly-Hexcore (mosaic method) algorithm is used to divide the volume mesh, realizing hexahedral and polyhedral mesh division into regions to increase fluid calculation accuracy and reduce the number of meshes.

[0009] Solution model settings: Use the Realizable model of the k-epsilon equation to simulate the flow field, and enable the energy equation to track product heat changes.

[0010] UDF writing and loading: Use custom written functions to load into Fluent's callable customization in the form of UDF.

[0011] Boundary conditions and initial condition settings: Use the custom function written in UDF to load the momentum velocity setting and heat temperature setting into the nozzle to give the initial temperature of the product measured data.

[0012] Solution and discretization settings: The Couple solution and second-order energy upwind discretization are used to ensure solution accuracy.

[0013] Report visualization definition: Set the temperature measurement points on the corresponding product according to the experimental data, and require Fluent to report and draw the temperature-time curve. If needed, add a cross-sectional temperature cloud map to view the temperature distribution in the simulated fluid domain.

[0014] Solution and result post-processing: Set the solution duration, time step, and number of iterations per step as required to perform a numerical solution. After obtaining the results, use the "Results" module to generate animations or extract the temperature-time curve for reference.

[0015] In the above steps, the starting and ending positions of the cavity nozzle are found through the UDF custom function, so that the hot air boundary conditions are loaded on the nozzle in the form of delayed activation, achieving relative movement similar to that between the product and the cavity temperature zone; at the same time, adaptive boundary condition loading can be made for different products and cavity sizes.

[0016] The beneficial technical effects of the present invention are:

[0017] High simulation accuracy: The Realizable model of the k-epsilon equation is used to simulate the flow field, and the energy equation is enabled to track heat changes. The Couple solution and the second-order upwind discretization of energy are combined to ensure the accuracy of heat calculation. In the meshing process, a boundary layer mesh is added to the solid computational domain, and the Poly-Hexcore algorithm is used to divide the volume mesh. This algorithm combines the Cartesian-based octree algorithm with the polyhedron mesh generation algorithm, improving the calculation accuracy of heat changes in the fluid and solid wall, reducing the number of meshes in the computational domain, and lowering the computational cost.

[0018] Good convergence: The use of the Couple solution ensures good convergence while ensuring computational accuracy, reducing fluctuations and instability during the solution process.

[0019] True reflection of the relative movement process: Through the UDF custom function, the hot air boundary condition is loaded on the nozzle in the form of delayed activation, achieving relative movement similar to that between the product and the cavity temperature zone, and more realistically simulating the hot air reflow soldering scenario in actual production.

[0020] Strong adaptability: Adaptive boundary condition loading can be made for different products and cavity sizes, which improves the versatility and flexibility of the method and can be quickly applied to various simulation needs.

[0021] Good visualization effect: By setting temperature measurement points, drawing temperature-time curves, and adding cross-sectional temperature cloud maps, the simulation results can be displayed intuitively, facilitating the analysis and evaluation of the hot air reflow process. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the process of the hot air reflow soldering simulation analysis method based on Fluent of the present invention;

[0023] Figure 2 Schematic diagram of the single temperature zone model structure used in the example;

[0024] Figure 3 Schematic diagram of the dual-temperature zone model structure used in the example. DETAILED DESCRIPTION

[0025] In order to enable those skilled in the art to more clearly understand the technical solutions and beneficial effects of the present invention, the present invention is further described in detail below with reference to specific embodiments.

[0026] This embodiment 1: The hot air reflow soldering process of a certain type A electronic board in a single temperature zone furnace is simulated. Figure 2 , using the hot air reflow soldering simulation analysis method based on Fluent described in the present invention, see Figure 1 , the specific steps are as follows:

[0027] S1. Model creation:

[0028] During the specific implementation, a three-dimensional model is established based on the actual size of the electronic board and the parameters of the reflow oven chamber:

[0029] Temperature zone cavity model: A single temperature zone cavity is used with dimensions set to length L1 × width W1 × height H1. Hexagonal array nozzles are evenly distributed on the upper and lower walls, the left and right sides are insulated walls, and the front and rear ends are open outlets.

[0030] Electronic board product model: Simplify the components on the mainboard (retain key heating elements such as chips, resistors, and capacitors), build a solid model and suspend it in the center of the cavity, with a distance of about H1 / 2 from the upper and lower nozzles.

[0031] Fluid computational domain: This includes the temperature zone cavity computational domain (fluid domain) and the electronic board computational domain (solid domain). Together, they form the fluid-solid coupling computational domain. The fluid domain encompasses the entire cavity and the space around the electronic board.

[0032] S2. Computational domain meshing:

[0033] The Fluent Meshing module is used for mesh generation, and the specific settings are as follows:

[0034] Boundary layer mesh: Add 5 layers of boundary layer mesh on the surface of the electronic board to ensure the accuracy of thermal gradient calculation at the solid wall.

[0035] Volume Meshing: The Poly-Hexcore algorithm was used to generate hexahedral meshes in the cavity fluid domain (large area) and polyhedral meshes around the electronic board and components (complex structure area). The final total number of meshes was approximately 1.8 million, and the mesh quality indicators (orthogonality > 0.2, distortion ratio < 0.8) met the calculation requirements.

[0036] Grid verification: The independence verification was performed by dividing the grid into three densities: 900,000, 1.8 million, and 5 million. The temperature curves of key temperature measurement points were compared. When the number of grids increased from 900,000 to 5 million, the deviation rate was less than 5%. Therefore, a 1.8 million grid was selected as the final calculation grid.

[0037] S3. Solution model settings:

[0038] Flow field model: The Realizable model of the k-epsilon equation was selected, and the turbulence intensity was set to 5% to simulate the turbulent flow of hot air.

[0039] Energy equation: Enables the energy equation, considering convective heat transfer between the fluid and the solid, and heat conduction within the solid, while ignoring radiation heat transfer (because the cavity is a closed space and the temperature difference is small).

[0040] S4. UDF writing and loading:

[0041] Write a user-defined function (UDF) to implement the following functions and load it into Fluent:

[0042] Nozzle position identification: By traversing the surface area of ​​the upper and lower walls of the cavity, the starting (first row at the front) and ending (last row at the back) positions of the nozzles are automatically identified, and the coordinate parameters of the nozzles at both ends are recorded. The begin_f_loop(f,t) function is used to traverse the surface area and search.

[0043] For adaptive position recognition, the following formula is used:

[0044]

[0045] The two functions PRF_GRLOW1(real) and PRF_GRHIGH1(real) are used to obtain the maximum values ​​of parallel calculations of all threads and determine the minimum initial value and maximum terminal value of the nozzle position.

[0046] Delayed activation logic: defines the correspondence between the nozzle activation time and the "movement" of the electronic board (simulating the electronic board passing through the cavity at the actual belt speed), that is, the front nozzle is activated first, and the boundary conditions are delayed to the rear nozzle in turn over time (delay time = Total length of nozzle / Belt speed ), using the DEFINE_PROFILE macro definition, the boundary loading function is as follows:

[0047]

[0048] Adaptive adjustment: Automatically calculate the delay time interval based on the input cavity length and number of nozzles to adapt to cavity models of different sizes.

[0049] S5. Boundary conditions and initial conditions settings:

[0050] Initial conditions: The initial temperature of the entire calculation domain is set to 45°C (the measured initial temperature of the process), and the reference pressure is set to standard atmospheric pressure (101325 Pa).

[0051] Boundary conditions: The nozzle is loaded with wind speed (speed value in each temperature zone) and temperature (set according to the process curve) through UDF and activated according to the delay logic; the cavity wall is set as an adiabatic boundary (heat flux density 0); the material properties of the solid domain are set according to the actual conditions; the fluid domain uses a protective atmosphere gas Nitrogen is used as the main fluid and is assumed to be an incompressible ideal gas.

[0052] S6. Solution and discretization settings:

[0053] Solver: Adopts Coupled solution to solve flow field and energy equations simultaneously.

[0054] Discretization format: The energy equation uses a second-order upwind format, and the momentum and k-epsilon equations use a second-order central difference format to ensure the accuracy of heat transfer calculations.

[0055] Convergence criterion: Set the residual of the energy equation to <1e-6 and the residual of other equations to <1e-3.

[0056] S7. Report visualization definition:

[0057] Temperature measurement point setting: Select 5 experimental temperature measurement points on the electronic board and define them as temperature monitoring points.

[0058] Visual output: Set to output the temperature-time curve of each temperature measurement point, as well as the cross-sectional temperature cloud diagram along the length of the cavity, and record data once at each time step.

[0059] S8. Solution and result post-processing:

[0060] Solution parameters: total time 288 seconds (corresponding to the time the electronic board is in the cavity), time step 2 seconds, and 2 iterations per step.

[0061] Post-processing: Generate temperature-time curves, compare simulation results with experimental temperature measurement data (deviation rate <5%), and verify model accuracy; generate cross-sectional temperature cloud animations to intuitively display the hot air temperature distribution inside the cavity and the heating process of the electronic board; extract the temperature curve at the solder joint to analyze whether it meets the reflow process requirements.

[0062] Through the above steps, Example 1 realistically simulates the temperature changes of electronic boards during hot air reflow soldering, accurately reflecting the heat exchange process between the hot air from the nozzle and the electronic boards. Furthermore, the "relative movement" simulation achieved through UDF effectively reproduces the dynamic heating scenario of the electronic boards in the temperature zone during actual production. This method can quickly adapt to electronic boards and cavities of different sizes, providing a reliable simulation basis for optimizing reflow process parameters.

[0063] This embodiment 2: The S1 model, S2 meshing and S4 UDF definition of the steps in embodiment 1 are modified:

[0064] The S1 model is modified, refer to Figure 3 , the size of a certain model B electronic board is too long, so it is changed to a dual-temperature zone cavity model, and the cavity parameters are changed to length L2×width W2×height H2.

[0065] The mesh subdivision of S2 is modified because the cavity size and the electronic board size are changed due to the modification of the S1 model. The mesh is modified accordingly, the number of meshes is increased, and the mosaic algorithm is used to generate a fusion hexahedron-polyhedron body mesh.

[0066] The UDF definition of S4 is modified, the temperature zone cavity size is changed, and the number of nozzles is changed. Referring to the adaptive adjustment formula given in Example 1, the initial and end positions of the cavity nozzles can be adaptively found, and the boundary loading of the 'relative movement' simulation process can be given.

Claims

1. The hot air reflow soldering simulation analysis method based on Fluent is characterized by: The following steps are included: S1. Model creation: Establish a circuit board product and temperature zone cavity model, and create a fluid calculation domain where fluid and solid coexist. The fluid calculation domain includes the electronic board product calculation domain and the temperature zone cavity calculation domain; S2. Computational domain meshing: Using the Fluent Meshing module, boundary layer meshes are added to the solid computational domain to ensure the accuracy of heat changes on the solid wall. The Poly-Hexcore (mosaic method) algorithm is used to divide the volume mesh, achieving hexahedral and polyhedral mesh division into regions, thereby increasing fluid computation accuracy and reducing the number of meshes. S3. Solution model setting: Use the Realizable model of the k-epsilon equation to simulate the process, and enable the energy equation to track product heat changes; S4. UDF writing and loading: Use custom written functions to load into Fluent's callable customization in the form of UDF; S5. Boundary conditions and initial conditions setting: Use the custom function written in UDF to load the momentum velocity setting and heat temperature setting into the nozzle and give the initial setting; S6, solution and discretization settings: adopt the Couple solution and the second-order energy upwind discretization to ensure the solution accuracy; S7. Report visualization definition: Set the temperature measurement points on the corresponding product according to the experimental data, and require Fluent to report and draw the temperature-time curve. If needed, you can add a cross-sectional temperature cloud map to view the temperature distribution of the simulated fluid domain; S8, solution and result post-processing: set the solution time, time step and number of iterations per step according to the requirements, and perform numerical solution; After obtaining the results, you can use the "Results" module to generate animations or extract the temperature-time curve for reference.

2. The hot air reflow soldering simulation analysis method based on Fluent according to claim 1, characterized in that: In step S1, a rectangular parallelepiped with a length of L, a width of W, and a height of H, with hexagonal array nozzles on the upper and lower sides, the left and right sides as walls, and the front and back sides as outlets, is used as the calculation domain of the temperature zone cavity. The cavity can be adjusted to a dual-temperature zone cavity according to the actual size of the electronic board product. In step S1 , an electronic board model with simplified components in actual size is suspended in the center of the cavity as a calculation domain for the electronic board product, and the model size of the electronic board is adjusted according to actual process requirements.

3. The hot air reflow soldering simulation analysis method based on Fluent according to claim 1, characterized in that: In step S2, the computational domain where fluid and solid coexist is meshed using the Poly-Hexcore algorithm. Based on a fusion algorithm of the octree algorithm and the polyhedron algorithm, a hexahedral mesh is used for the fluid domain with a relatively large area, and a polyhedral mesh is used for the solid domain with a relatively small area. This rationalizes the distribution of the mesh division, improves the calculation accuracy, and reduces the calculation cost.

4. The hot air reflow soldering simulation analysis method based on Fluent according to claim 1, characterized in that: In step S3, the physical field of hot air reflow soldering belongs to the thermal convection field and also to the field of turbulence calculation. The k-epsilon Realizable model is used as the fluid domain for turbulence field calculation, and the energy equation is enabled to calculate the heat change of thermal convection, which is consistent with the application of the physical field.

5. The hot air reflow soldering simulation analysis method based on Fluent according to claim 1, characterized in that: In step S4, the positions of the start and end nozzles are found according to the size of the cavity and the distribution of the nozzles. Begin_f_loop(f,t) is used in DEFINE_PROFILE to traverse the surface area for searching. The formula is as follows:

6. Use PRF_GRLOW1(real) and PRF_GRHIGH1(real) to obtain the overall minimum and maximum values ​​of the parallel thread operations, so that the start and end position values ​​are uniformly applied to all threads, and written with the temperature zone parameters of the actual process, and linear one-dimensional delayed activation is used to load the hot air temperature and nozzle wind speed.

7. The hot air reflow soldering simulation analysis method based on Fluent according to claim 1, characterized in that: In step S5, a linear one-dimensional variable boundary custom function is applied to the momentum and heat of the nozzle, using standard initialization to give experimentally measured initial temperature, gauge pressure and other parameters; In step S6, according to the physical field definition and accuracy requirements, the Couple solution and second-order heat discretization are used.

8. The hot air reflow soldering simulation analysis method based on Fluent according to claim 1, characterized in that: In steps S7 and S8, temperature measurement points and necessary interface physical parameter cloud maps are set up in the calculation domain, and corresponding visualization results can be obtained after solving.

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