A method for optimizing the solder joint structure parameters of 2.5D packaged BGA

Abstract

This invention provides a method for optimizing the structural parameters of solder joints in 2.5D packaged BGAs, aiming to improve the reliability of 2.5D packaged BGA solder joints under random vibration loads. First, a finite element model of the 2.5D packaged device is established to analyze the stress distribution under random vibration conditions. Then, multiple orthogonal experiments are designed using the Taguchi method, and grey relational analysis is performed to optimize the stress of the upper and lower BGA solder joints under dual objectives, resulting in an optimized combination of solder joint structural parameters that improves the reliability of the 2.5D packaged BGA solder joints.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging, and more particularly to a method for optimizing the solder joint structure parameters of a 2.5D packaged BGA. Technical Background

[0002] In modern electronic products, with the continuous improvement of performance and integration, 2.5D packaging technology has gradually become a key solution for designing high-performance, multi-functional integrated circuits. However, with the continuous improvement of chip area, performance, and integration, the challenges of chip reliability under various complex environments such as thermal cycling loads, bending and torsional loads, vibration, and shock have also emerged. The U.S. Air Force has found that approximately 20% of electronic devices fail under vibration and shock environments. When electronic devices operate under vibration and shock environments, the vibration load is transmitted through the device casing to the PCB and then to the solder joints. In the interconnect structure composed of chip-solder joint-PCB, the solder joint, which connects the chip and PCB and plays a dual role of mechanical support and electrical interconnection, is the weakest link in the interconnect structure. The cumulative damage of vibration fatigue to the solder joint directly threatens the service life of electronic devices. Therefore, it is essential to study the stress of interconnect solder joints in 2.5D packaging structures under random vibration loads to improve the service reliability of solder joints and further improve the reliability of the 2.5D packaging structure.

[0003] The Taguchi method is a parameter optimization method based on experimental design. It analyzes the impact of various factors on product performance by rationally arranging experimental schemes, and uses orthogonal experiments and signal-to-noise ratio evaluation indicators to reduce the number of experiments and improve parameter optimization efficiency. Grey relational analysis is suitable for analyzing small samples and systems with incomplete information. It can comprehensively evaluate and rank different schemes based on the degree of correlation between each factor and evaluation indicator.

[0004] In summary, to reduce the stress on 2.5D packaged BGA solder joints under random vibration loads, it is necessary to optimize their structural parameters. Combining the Taguchi method with grey relational analysis can provide a reference for optimizing the structural parameters and designing the reliability of 2.5D packaged BGA solder joints. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for optimizing the structural parameters of solder joints in 2.5D packaged BGA. This method can achieve dual-objective synchronous optimization of the maximum stress of upper and lower layer BGA solder joints in 2.5D packaged devices under random vibration loads.

[0006] To solve the above-mentioned technical problems, the present invention is implemented by the following technical solution:

[0007] A method for optimizing the solder joint structure parameters of a 2.5D packaged BGA includes the following steps:

[0008] Step 1: Establish a finite element model of the 2.5D packaged BGA solder joints;

[0009] Step 2: Perform finite element stress simulation analysis under random vibration conditions to obtain the maximum stress of the upper and lower BGA solder joints of the 2.5D packaged BGA solder joint model under random vibration conditions.

[0010] Step 3: Select the structural parameters that affect the maximum stress of the weld joint to establish a factor level table, use the Taguchi method to establish an orthogonal experimental table and conduct 16 sets of experiments;

[0011] Step 4: Calculate the maximum stress value and corresponding signal-to-noise ratio of the weld joint in each test group, and analyze the influence of each factor on the stress of the weld joint;

[0012] Step 5: Use grey relational analysis to perform dual-objective optimization, obtain and verify the optimal combination of structural parameters to minimize the maximum stress value of the upper and lower BGA solder joints, thereby improving the reliability of the solder joints.

[0013] Step 1 includes the following specific methods:

[0014] The geometric model of the 2.5D packaged device was established using ANSYS software, which mainly includes the following parts: BGA chip, silicon adapter board, lower BGA solder joints and PCB board.

[0015] Step 2 includes the following specific methods:

[0016] Random vibration analysis was performed on the established finite element model using the indirect coupling method. First, a thermal cycling load was applied according to the standard, and then a random vibration load was applied. The maximum stress values ​​of the upper and lower BGA solder joints were obtained through finite element simulation calculations.

[0017] Step 4 includes the following specific methods:

[0018] Based on the optimization objective of reducing the maximum stress value of BGA solder joints, the signal-to-noise ratio corresponding to the maximum stress of upper and lower layer BGA solder joints in each group of experiments was calculated using the small characteristic signal-to-noise ratio formula, as follows:

[0019]

[0020] In the formula: y i Let be the maximum stress value of the weld joint in the i-th test group;

[0021] Step 5 includes the following specific methods:

[0022] Step 5-1: First, perform dimensionless processing on the maximum stress value of the upper and lower BGA solder joints according to the following formula:

[0023]

[0024] In the formula: x ky represents the maximum stress value of the weld joint in the k-th test group. k The values ​​are after dimensionless processing;

[0025] Step 5-2: Transform the dimensionless data into matrix Y;

[0026] Step 5-3, use matrix Y as the comparison sequence, denoted as Z for the reference scheme:

[0027]

[0028] Step 5-4: Calculate the correlation coefficient using the following formula, and use the magnitude of the correlation coefficient to measure the degree of fit between the comparison sequence and the reference sequence;

[0029]

[0030] In the formula, i represents the number of experimental groups, j represents the factor to be optimized, and the resolution coefficient ρ = 0.5. After calculation, the grey relational coefficient matrix of 16 experimental groups is obtained, denoted as Q;

[0031] Step 5-5: Calculate the grey relational degree using the following formula;

[0032]

[0033] In the formula, i = 1, ..., n, the grey relational degree of multiple sets of data can be calculated using the above formula.

[0034] The method of the present invention can perform dual-objective optimization of the maximum stress of the upper and lower BGA solder joints of 2.5D packaged devices under random vibration loads, effectively reducing the stress level of the solder joints and improving the reliability of 2.5D packaged solder joints under complex environmental loads. Attached Figure Description

[0035] Figure 1 This is an overall flowchart of a method for optimizing the solder joint structure parameters of a 2.5D packaged BGA according to the present invention;

[0036] Figure 2 Geometric model diagram of finite element analysis of solder joints for 2.5D packaged BGA;

[0037] Figure 3 This is a graph showing the loading curve of random vibration.

[0038] Figure 4 Random vibration stress distribution cloud map of the solder joints of the upper BGA of a 2.5D package;

[0039] Figure 5 Random vibration stress distribution cloud map of the solder joints of the lower layer BGA in a 2.5D package;

[0040] Figure 6Stress distribution cloud map of key solder joints on the upper layer of the 2.5D package BGA;

[0041] Figure 7 Stress distribution cloud map of key solder joints in the lower layer BGA of a 2.5D package. Detailed Implementation

[0042] This invention relates to a method for optimizing the structural parameters of solder joints in a 2.5D packaged BGA, used for dual-objective optimization of the maximum stress of solder joints on the upper and lower layers of a 2.5D packaged BGA, comprising the following steps:

[0043] Step 1, the three-dimensional finite element simulation model for the stress analysis of solder joints in the 2.5D packaged ball grid array established in this paper is as follows: Figure 2 As shown, the model includes two chips, one silicon adapter board, one PCB board, ball grid array solder joints for interconnecting the chips and the silicon adapter board, and ball grid array solder joints for interconnecting the silicon adapter board and the PCB board. The chip in the model is a Toshiba TC358775XBG chip, measuring 6mm × 6mm × 0.7mm. This chip contains 64 BGA solder joints arranged in an 8×8 full array. The solder joint diameter is 0.3mm, the solder joint height is 0.20mm, the pad diameter is 0.25mm, and the solder joint spacing is 0.65mm. The middle layer silicon adapter board measures 25mm × 25mm × 0.2mm. The BGA solder joints under the adapter board are arranged in a partial array, totaling 168 solder joints. The solder ball diameter is 0.55mm, the solder joint height is 0.40mm, the pad diameter is 0.45mm, and the solder joint spacing is 1mm. The bottom PCB board measures 120mm × 80mm × 1.6mm. The finite element model has 385,494 finite element nodes and 1,606,909 elements after meshing.

[0044] Step 2: Fix the through holes at the four corners of the PCB bottom surface. Apply load to the model according to the standard, and its PSD curve is as follows. Figure 3 As shown. The maximum stress at the upper and lower BGA solder joints of the 2.5D package after applying random vibration load to the model is as follows: Figure 4 and Figure 5 As shown.

[0045] Step 3: Select three structural parameters as influencing factors: upper BGA solder joint height, lower BGA solder joint height, and silicon interposer thickness. Each factor has four levels, as shown in Table 1. Design 16 orthogonal experimental tables using the Taguchi method.

[0046] Table 1 Factor Level Table

[0047]

[0048] Step 4: Establish the corresponding finite element model according to the orthogonal test table, obtain the maximum stress value of the upper and lower BGA solder joints of the model, and use the small characteristic signal-to-noise ratio formula to calculate the corresponding signal-to-noise ratio, the value of which is shown in Table 2.

[0049] Table 2 Orthogonal Experiment Table

[0050]

[0051] In the Taguchi method, a higher signal-to-noise ratio (SNR) means that the system performs better at a specific level of that parameter. The response of each level of influencing factor is the average value of the influencing factor at the corresponding level. The average SNR of the three factors affecting the maximum stress of the upper and lower weld joints at different levels is shown in Table 3.

[0052] Table 3 Average signal-to-noise ratio of maximum stress at upper and lower layer BGA solder joints

[0053]

[0054] According to Table 3, under random vibration load, the order of the range of the upper BGA solder joint height, the lower BGA solder joint height, and the maximum stress of the upper BGA solder joint corresponding to the silicon adapter board is: upper BGA solder joint height > lower BGA solder joint height > silicon adapter board. The order of the range of the maximum stress of the lower BGA solder joint is: silicon adapter board > lower BGA solder joint height > upper BGA solder joint height.

[0055] Step 5: First, the maximum stress values ​​of the upper and lower BGA solder joints of the 2.5D packaged device are dimensionless. The calculation formula is as follows:

[0056]

[0057] x in the formula k Table 2 shows the maximum stress values ​​of the upper and lower weld joints, where yk is the data obtained after dimensionless processing, and k = 1, 2, ..., 16. The dimensionless data is transformed into the matrix Y shown below:

[0058]

[0059] The maximum value after dimensionless processing is used as the ideal reference scheme to determine the reference sequence and the comparison sequence. The reference scheme is as follows:

[0060]

[0061] The maximum stress values ​​of the upper and lower layer BGA solder joints in the 16 sets of Table 2 are used as comparison series. qi,j represents the correlation coefficient between the i-th comparison series and the j-th index in the reference series Z. The magnitude of the correlation coefficient is used to measure the degree of fit between the comparison series and the reference series. The calculation formula is as follows:

[0062]

[0063] In the formula, i represents 1, 2, 3, ..., 15, 16, j = 1, 2, and the resolution coefficient ρ = 0.5. After calculation, the grey relational coefficient matrix for the 16 groups of experiments is obtained, denoted as Q:

[0064]

[0065] When performing bi-objective optimization on the maximum stress of the upper and lower BGA solder joints of a 2.5D packaged device, the weighting coefficients for the maximum stress of the upper BGA and the maximum stress of the lower BGA solder joints are defined as λ1 and λ2, respectively. The formula for calculating the grey relational degree is then:

[0066]

[0067] Where i = 1, 2, ..., 16. By performing grey relational analysis on 16 sets of experimental data, the grey relational degree of the maximum stress at the upper and lower BGA solder joints can be obtained, that is, the comprehensive correlation between each experimental scheme and the two stress objectives. Based on this calculation result, the optimization analysis results of multiple factors and dual objectives can be obtained.

[0068] The weights of the upper and lower BGA solder joints of the 2.5D packaged device are set as λ1 and λ2, respectively, with a value of 0.5. Based on the formula, the grey relational degree is calculated, and the grey relational coefficients and bi-objective function grey relational degrees of the upper and lower solder joints are shown in Table 4.

[0069] Table 4. Grey Relational Coefficient and Grey Relational Degree of Bi-objective Function

[0070]

[0071] When determining the optimal combination of structural parameters, it is necessary to first calculate the average correlation value corresponding to different levels of each factor. Based on the correlation data of different parameter combinations in Table 4, the average correlation values ​​of each factor at each level are obtained after calculation and processing, as shown in Table 5.

[0072] Table 5. Average Grey Relational Degree of Each Factor at Each Level

[0073]

[0074] The optimal combination, as shown in Table 5, is: upper BGA solder joint height of 0.22 mm, lower BGA solder joint height of 0.42 mm, and silicon interposer height of 0.1 mm. To verify this optimal combination, a finite element model was established with upper BGA solder joint heights of 0.22 mm, lower BGA solder joint heights of 0.42 mm, and silicon interposer height of 0.1 mm. After applying random vibration loads to the 2.5D package structure model, the stress distribution of the key solder joints in the optimal combination is as follows: Figure 6 and Figure 7 As shown. According to Figure 6 and Figure 7 The stress distribution cloud diagrams of the upper and lower BGA solder joints show that, after optimizing the horizontal combination, the maximum stress value of the upper BGA solder joint in the 2.5D package is 3.56 MPa, a reduction of 0.92 MPa compared to the initial design without optimization; the maximum stress value of the lower BGA solder joint is 11.8 MPa, a reduction of 2.4 MPa compared to the initial design without optimization. This result demonstrates that the dual-objective optimization method based on Taguchi experimental design and grey relational analysis can effectively reduce the maximum stress of the upper and lower BGA solder joints. The reduction in the maximum stress value of the solder joints under random vibration loading helps to reduce cumulative vibration fatigue damage and delay the initiation of microcracks, thereby improving the overall reliability of the solder joint structure.

Claims

1. A method for optimizing the solder joint structure parameters of a 2.5D packaged BGA, characterized in that, Includes the following steps: Step 1: Establish a finite element model of the 2.5D packaged BGA solder joints; Step 2: Perform finite element stress simulation analysis under random vibration conditions to obtain the maximum stress of the upper and lower BGA solder joints of the 2.5D packaged BGA solder joint model under random vibration conditions. Step 3: Select the structural parameters that affect the maximum stress at the weld joint, establish an orthogonal experimental table using the Taguchi method, and conduct multiple sets of experiments; Step 4: Calculate the maximum stress value and corresponding signal-to-noise ratio of the weld joint in each group of tests, and analyze the significance of the influence of each factor on the stress of the weld joint. Step 5: Use grey relational analysis to perform dual-objective optimization, obtain and verify the optimal combination of structural parameters to minimize the maximum stress value of the upper and lower BGA solder joints, thereby improving the reliability of the solder joints.

2. The method for optimizing the solder joint structure parameters of a 2.5D packaged BGA as described in claim 1, characterized in that, In step 1, the 2.5D packaged BGA solder joint structure includes: BGA chip, silicon adapter board, lower BGA solder joint, and PCB board.

3. The method for optimizing the solder joint structure parameters of a 2.5D packaged BGA as described in claim 1, characterized in that, In step 2, random vibration loads were applied using the US military standard MIL-STD NAVMAT P9492. The maximum stress at the upper and lower BGA solder joints was obtained using Ansys software.

4. The method for optimizing the solder joint structure parameters of a 2.5D packaged BGA as described in claim 1, characterized in that, In step 3, the structural parameters affecting the maximum stress of the solder joints include the height of the upper BGA solder joint, the height of the lower BGA solder joint, and the thickness of the silicon adapter board, with each factor having multiple horizontal values.

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