阅读说明：本技术 一种锡铋系合金焊点微观组织的定量表征方法 (Quantitative characterization method for microstructure of tin-bismuth alloy welding spot ) 是由 刘志权 王畅畅 高丽茵 孙蓉 于 2020-05-26 设计创作，主要内容包括：本发明公开了一种锡铋系合金焊点微观组织的定量表征方法,涉及金相检测方法以及微电子技术领域。该方法是以锡铋系焊点中富铋相的组织特征来定量描述整个锡铋焊点的微观组织特征或其演化过程,其中,使用平均片层厚度来表征所述不规则片层状富铋相的微观组织特征,使用与颗粒铋相等面积圆的直径大小来表征所述细碎状富铋相的微观组织特征。本发明方法可为焊料成分设计、性能优化和焊接工艺改良中提供组织参数,又可在焊点的温度循环、高温存储、电迁移等可靠性测试中进行微观组织的定量精细化表征,从而可寻找焊点失效时关键组织特征的门槛值,推进相关工业标准的树立。与定性描述微观组织方法相比,该方法量化准确、操作简单、实用性强、便于推广。(The invention discloses a quantitative characterization method for a microstructure of a tin-bismuth alloy welding spot, and relates to the technical fields of metallographic detection methods and microelectronics. The method quantitatively describes the microstructure characteristics of the whole tin-bismuth welding spot or the evolution process of the tin-bismuth welding spot by using the structure characteristics of a bismuth-rich phase in the tin-bismuth welding spot, wherein the microstructure characteristics of the irregular lamellar bismuth-rich phase are represented by using the average lamella thickness, and the microstructure characteristics of the fine fragmented bismuth-rich phase are represented by using the diameter of a circle with the same area as that of granular bismuth. The method can provide organization parameters for solder composition design, performance optimization and welding process improvement, and can also carry out quantitative and fine characterization on microstructures in reliability tests of temperature circulation, high-temperature storage, electromigration and the like of the welding spots, so that the threshold value of key organization characteristics can be searched when the welding spots fail, and the establishment of relevant industrial standards is promoted. Compared with the qualitative description of the microstructure, the method has the advantages of accurate quantification, simple operation, strong practicability and convenient popularization.)

1. A quantitative characterization method for microstructure of a tin-bismuth alloy welding spot is characterized by comprising the following steps: the method quantitatively describes the microstructure characteristics of the whole tin-bismuth welding spot or the evolution process of the microstructure of the tin-bismuth welding spot by using the structure characteristics of a bismuth-rich phase in the tin-bismuth alloy welding spot.

2. The quantitative characterization method of tin-bismuth alloy solder joint microstructure according to claim 1, characterized in that: the quantitative characterization method comprises the following steps:

(1) dividing bismuth-rich phases in the tin-bismuth series welding spots into two types, wherein the first type is an irregular lamellar bismuth-rich phase (namely a lamellar bismuth phase), and the second type is a fine-grained bismuth-rich phase (namely a granular bismuth phase) in a tin-rich phase matrix;

(2) quantitatively describing microstructure characteristics of the welding spots or the evolution process of the microstructure by adopting microstructure characteristics of the irregular lamellar bismuth-rich phase and/or the fine lamellar bismuth-rich phase; wherein: the microstructure characteristics of the irregular lamellar bismuth-rich phase are characterized using the average lamellar thickness, and the microstructure characteristics of the finely-divided bismuth-rich phase are characterized using the diameter size of the area circle equal to the granular bismuth.

3. The method for quantitatively characterizing the microstructure of a tin-bismuth alloy solder joint according to claim 2, wherein: the average ply thickness value is calculated as: and (3) intercepting the two-phase boundary (the phase boundary of the tin-rich phase and the bismuth-rich phase) for multiple times by using a fixed-length straight line, and taking the average value of multiple intercept as the average lamella thickness value.

4. The quantitative characterization method of the microstructure of the tin-bismuth alloy solder joint according to claim 3, wherein: the average ply thickness value is calculated according to equation (1):

in the formula (1), I is the average lamella thickness value, L_iThe length of each section after the fixed-length straight line is cut is shown, P is the number of nodes, and M is the magnification factor of the picture.

5. The quantitative characterization method of the microstructure of the tin-bismuth alloy solder joint according to claim 4, wherein: the calculation of the average sheet thickness value is performed according to the following steps (A) to (D);

(A) obtaining a microstructure photo of the solder marked with the magnification factor and the scale, and binarizing the microstructure photo to improve the contrast between the tin-rich phase and the bismuth-rich phase;

(B) intersecting a fixed-length straight line with the microstructure photo, and calculating the number of intersection points (the number of nodes) of the fixed-length straight line and a two-phase boundary (a tin-rich phase and a bismuth-rich phase boundary);

(C) substituting the measurement result into a formula (1) to calculate the thickness of the bismuth-rich phase lamella;

(D) and (4) repeating the steps (B) to (C) for multiple times to obtain multiple measurement values, and taking the average value of the measurement results to obtain the average lamella thickness value of the bismuth-rich phase.

6. The method for quantitatively characterizing the microstructure of a tin-bismuth alloy solder joint according to claim 2, wherein: when the microstructure characteristics of the particulate bismuth phase are characterized by the diameter of a circle of equal area to the particulate bismuth, the diameter of the circle of equal area is calculated according to the formula (2):

in the formula (2), d is the diameter of the circle with equal area, A₀Pixel area of a circle of equal area, A_pThe actual area represented by a single pixel point.

7. The quantitative characterization method of the microstructure of the tin-bismuth alloy solder joint according to claim 6, wherein: the diameter of the circle with equal area is calculated according to the following steps (a) to (d);

(a) obtaining a microstructure photo marked with a magnification factor and a welding spot of a scale, and binarizing the microstructure photo to improve the contrast between a tin-rich phase and a bismuth-rich phase;

(b) calculating the pixel area of a certain particle bismuth phase in a tin-rich phase on the tissue photo;

(c) substituting the formula (2) into the fine bismuth powder to calculate the diameter of a small circle with the same area as the fine bismuth powder;

(d) repeating the steps (b) to (c) for a plurality of times to obtain all the fine bismuth with the same area circle diameter.

8. The quantitative characterization method of tin-bismuth alloy solder joint microstructure according to claim 1, characterized in that: the tin-bismuth system welding spot is an interconnection welding spot formed after tin-bismuth system welding flux is welded with a metal welding pad, and the content of bismuth element in the tin-bismuth system welding flux is 40-80 wt.%.

Technical Field

The invention relates to the technical field of metallographic detection and microelectronic interconnection, in particular to a quantitative characterization method for a microstructure of a tin-bismuth alloy welding spot.

Background

The tin-bismuth series solder has the advantages of low welding temperature, low price, good comprehensive service performance and the like. In order to control the cost and reduce the damage to electronic components in the welding process, the tin-bismuth series solder is popularized and used in the production of personal portable electronic equipment such as mobile phones, computers and the like. However, the heat emitted by the electronic device during use may cause the microstructure of the solder joint to gradually change, and the service performance of the solder joint may also change accordingly.

Fig. 1 is a typical example photograph of a microstructure of a tin-bismuth based solder. The typical microstructure of the tin-bismuth alloy is an irregular lamellar structure formed by overlapping a tin-rich phase and a bismuth-rich phase, the tin-rich phase in a welding spot or a fine-grained bismuth-rich phase precipitated in a cooling process can exist according to different bismuth element contents, the microstructure characteristics are complex, and in the using process of electronic equipment, the tin-rich phase in the welding spot can further perform an alloying reaction with a metal substrate, while the bismuth element does not participate in the reaction, so that the complexity of the microstructure evolution of the welding spot is further increased. The microstructure of the welding spot is directly linked with the service performance of the welding spot, and with the increase of the market of portable electronic products, the requirements on quantitative representation of the microstructure evolution of the tin-bismuth series welding flux and the influence of the microstructure evolution on the welding spot performance are urgent, and the method has important economic value and production significance.

At present, the characterization of the microstructure of the tin-bismuth welding spot still remains in the stage of shooting a microstructure photo of the welding spot through a metallographic microscope or a scanning electron microscope, comparing the microstructure photo and performing qualitative structural feature description. The characterization means cannot quantitatively analyze microstructure characteristics of the welding spot and accurately describe microstructure evolution in the service process of the welding spot, the obtained experimental data are difficult to transversely compare, the full and effective utilization cannot be realized, manpower and material resources are wasted, and the research and development work of the tin-bismuth solder is greatly hindered. Therefore, a method for quantitatively characterizing the microstructure of the tin-bismuth alloy with wide application range and simple and easy operation is urgently found.

Disclosure of Invention

The invention aims to provide a quantitative characterization method for the microstructure of a tin-bismuth alloy welding spot, which can optimize the component performance of tin-bismuth alloy welding flux and evaluate the service reliability of the welding spot in actual production.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows:

a quantitative characterization method for the microstructure of a tin-bismuth alloy welding spot is characterized in that a bismuth-rich phase which does not participate in interface reaction in the tin-bismuth welding spot is used as a key tissue, and the microstructure and the evolution process of the whole tin-bismuth welding spot are quantitatively described through the characteristics of the key tissue. The quantitative characterization method comprises the following steps:

(1) dividing bismuth-rich phases in the tin-bismuth series welding spots into two types, wherein the first type is an irregular lamellar bismuth-rich phase (namely a lamellar bismuth phase), and the second type is a fine-grained bismuth-rich phase (namely a granular bismuth phase) in a tin-rich phase matrix;

(2) quantitatively describing microstructure characteristics of the welding spots or the evolution process of the microstructure by adopting microstructure characteristics of the irregular lamellar bismuth-rich phase and/or the fine lamellar bismuth-rich phase; wherein: the microstructure characteristics of the irregular lamellar bismuth-rich phase are characterized using the average lamellar thickness, and the microstructure characteristics of the finely-divided bismuth-rich phase are characterized using the diameter size of the area circle equal to the granular bismuth.

The average ply thickness value is calculated as: and intercepting the tin-rich phase and the bismuth-rich phase for multiple times by using a fixed-length straight line, and taking the average value of multiple intercept as the average lamella thickness value. The average ply thickness value is calculated according to equation (1):

in the formula (1), I is the average lamella thickness value, L_iThe length of each section after the fixed-length straight line is cut is shown, P is the number of nodes, and M is the magnification factor of the picture.

The calculation of the average sheet thickness value is performed according to the following steps (A) to (D);

(A) obtaining a microstructure photo of the solder marked with the magnification factor and the scale, and binarizing the microstructure photo to improve the contrast between the tin-rich phase and the bismuth-rich phase;

(B) intersecting a fixed-length straight line with the microstructure photo, and calculating the number of intersection points (the number of nodes) of the fixed-length straight line and a two-phase boundary (a tin-rich phase and a bismuth-rich phase boundary);

(C) substituting the equation (1) to calculate the thickness of the phase-rich layer of the current measurement;

(D) and (4) repeating the steps (B) to (C) for multiple times to obtain multiple measurement values, and taking the average value of the measurement results to obtain the average lamella thickness value of the bismuth-rich phase.

When the microstructure characteristics of the particulate bismuth phase are characterized using the diameter of a circle of equal area to the particulate bismuth, the diameter of the circle of equal area is calculated according to equation (2):

in the formula (2), d is the diameter of the circle with equal area, A₀Pixel area of a circle of equal area, A_pThe actual area represented by a single pixel point.

The diameter of the circle with equal area is calculated according to the following steps (a) to (e);

(a) obtaining a microstructure photo marked with a magnification factor and a welding spot of a scale, and binarizing the microstructure photo to improve the contrast between a tin-rich phase and a bismuth-rich phase;

(b) calculating the pixel area of a certain particle bismuth phase in a tin-rich phase on the tissue photo;

(c) substituting the formula (2) into the fine bismuth powder to calculate the diameter of a small circle with the same area as the fine bismuth powder;

(d) repeating the steps (b) to (c) for a plurality of times to obtain all the fine bismuth with the same area circle diameter.

The tin-bismuth system welding spot is an interconnection welding spot formed after tin-bismuth system welding flux is welded with a metal welding pad, and the content of bismuth element in the tin-bismuth system welding flux is 40-80 wt.%.

The design mechanism of the invention is as follows:

the invention can realize quantitative characterization of the microscopic structure of the tin-bismuth series solder by extracting two key structure characteristics of the lamellar thickness of the bismuth-rich phase and the size of the particle bismuth-rich phase in the tin-rich phase from the structure of the tin-bismuth series solder. Under the condition of known alloy components, the method can be used for quantitatively representing the welding spot tissues after various experimental conditions such as reflow soldering, high-temperature storage, temperature circulation and the like, analyzing and monitoring the evolution dynamics of the welding spot tissues and comparing the change of the tissue characteristics under each experimental condition. In addition, as the coarsening and growth of the bismuth-rich phase in the service process of the electronic equipment are main reasons of the performance degradation and the welding spot failure of the tin-bismuth series solder, the microstructure characteristic value measured by the method can be utilized to explore and establish the microstructure threshold value of the welding spot failure, and a foundation is laid for enterprises and industrial standards for making related products.

Compared with the traditional method for qualitatively describing the microstructure of the tin-bismuth welding spot, the method has the beneficial effects that:

1. the microstructure of the tin-bismuth welding spot can be quantitatively measured, so that the structural characteristics of the tin-bismuth welding spot can be more accurately represented, and the structural change of the tin-bismuth welding spot can be more accurately detected, so that the traditional qualitative description method is replaced.

2. According to the invention, the coarsening degree of the bismuth-rich phase of the tin-bismuth welding spot in the use process can be effectively explained by measuring the thickness of the bismuth-rich phase of the sheet layer, and then the welding spot failure threshold value is set, so that the reliability of the welding spot can be conveniently evaluated.

3. According to the method, the size change and the size distribution of the particle bismuth phase inside the tin-rich phase can be effectively analyzed by measuring the diameter of the circle with the same area as the finely-divided bismuth phase, so that the growth rule of the particle bismuth phase in the aging or thermal cycle process is presumed, and the rule between the service performance and the microstructure evolution of the solder is more accurately analyzed.

4. The method provided by the invention has the advantages of simple and easy measurement mode, no limitation of observation mode of microstructure, and strong operability in actual production.

Drawings

FIG. 1 is a photograph of a solder joint structure and a microstructure of a typical Sn-Bi solder; wherein: (a) a picture of a welding spot structure; (b) typical tin bismuth based solder microstructures.

FIG. 2 is a photograph of the microstructure before and after intersection of a fixed length straight line with the tin-bismuth phase boundary; wherein: (a) before intersecting; (b) after the intersection.

FIG. 3 is a photograph of the microstructure before and after the conversion of the bismuth phase of the particles into a circle of equal area; wherein: (a) before conversion; (b) after the scaling.

FIG. 4 is a comparison of the mean lamellar thickness values of the bismuth-rich phase under different reflux conditions, as measured by the method of the present invention.

FIG. 5 is the change of the mean lamellar thickness of the bismuth-rich phase after different high-temperature storage times measured by the method of the invention.

FIG. 6 is a histogram of the size distribution of the particulate bismuth phase in the solder joints before and after 500 cycles of temperature cycling measured by the method of the present invention; wherein: (a) before temperature circulation; (b) after temperature cycling.

Detailed Description

The invention relates to a method for quantitatively representing a microscopic structure of a tin-bismuth series welding spot, which quantitatively represents the microscopic structure of tin-bismuth series welding flux through the average lamella thickness of a lamellar bismuth-rich phase and the equal area circle diameter of a particle bismuth phase, and the actual operation process of the invention is described in detail in the following by combining with the attached drawings.

The content of bismuth element in the tin-bismuth system solder is 40-80 wt.%, and the tin-bismuth system solder and the metal bonding pad are welded to form a tin-bismuth system interconnection welding spot. The invention aims to quantitatively measure the microstructure of a tin-bismuth system solder alloy welding spot, and the specific implementation steps are as follows:

1. and obtaining a microstructure photo marked with a magnification and a welding spot of the scale by a metallographic microscope or a scanning electron microscope, wherein the magnification is M.

2. And carrying out binarization treatment on the tissue photo to improve the contrast between a tin-rich phase and a bismuth-rich phase in the photo.

3. Using a fixed-length straight line with the length less than the width of the picture to intersect with the picture of the microstructure, and measuring the cut length L of each section of the fixed-length straight line_iAnd calculating the number P of intersection points of the fixed-length straight line and the two-phase boundary (the tin-rich phase bismuth-rich phase boundary). See figure 2 for details.

4. Will change the variable L_iP, M substituting the formula (1) to calculate the average sheet thickness value measured this time, wherein the specific formula is as follows:

in equation (1): i is the average sheet thickness, Li is the length of each segment after the fixed-length straight line is cut (I is 1, 2 … … n, n is the number of segments), P is the number of nodes (the number of intersections between the fixed-length straight line and the phase boundary), and M is the photographic magnification.

5. Repeating steps 3-4 for multiple times, obtaining multiple I measured values and averaging the measured values

The average lamella thickness value of the bismuth-rich phase is obtained.

6. Then selecting a certain particle bismuth-rich phase in the picture obtained after the binarization treatment to obtain the pixel area A of the picture₀。

7. And calculating the diameter of a circle with the same size as the area of the bismuth phase through the pixel area of the bismuth phase to finish the size calibration of the bismuth phase. The specific calculation formula is formula (2):

in the formula (2), d is the diameter of the circle with equal area, A₀Pixel area of a circle of equal area, A_pThe actual area represented by a single pixel point.

8. And (5) repeating the steps 6-7 to obtain the circle diameters with the same area of all the particle bismuth phases, completing quantitative characterization of the particle bismuth phases, and showing the microstructure photos before and after the calibration is completed in figure 3.