Solder welding method

Abstract

The present invention discloses a solder welding method: The method involves using different electromagnetic torques to enhance the movement of liquid metal at the solder joint during different stages of the cooling and solidification process, effectively eliminating or reducing the accumulation of debris and bubbles within the ingot during the initial stages. This method suppresses elemental segregation during the early stages of solidification, reduces the presence of columnar crystals, and increases the area of ​​equiaxed crystals. This method not only reduces porosity during welding under a protective atmosphere but also further improves the mechanical properties of the solder joint by altering the crystal structure within the solder joint. This reduces the material cost of the protective atmosphere and the equipment cost of the vacuum system.

Description

Technical Field

[0001] The invention relates to the field of soldering of electronic products, and in particular to a soldering method for enhancing the strength of soldering points by changing the crystal phase structure inside the soldering points. Background Art

[0002] In recent years, competition in electronic products has been fierce. As products become more and more high-end, people are paying more and more attention to product quality. The reliability of electronic component assembly welding plays an absolutely important role as the basic assembly of electronic products. People have been constantly exploring and researching how to improve the reliability of electronic welding.

[0003] Improving solder joint quality is one of the key points in improving welding reliability. To improve solder joint quality, a shielding gas is often required during welding of high-end electronic products. Inert gas and nitrogen are commonly used shielding gases. Some even vacuum the process during shielding gas welding to reduce residual gas in the solder joint, thereby reducing voids and improving welding quality.

[0004] The primary function of a shielding gas during welding is to remove oxygen from the soldering process, improve solderability, prevent reoxidation and reoxidation of the solder joint, and create excellent wetting conditions for the solder, thereby improving solderability. When shielding gas is introduced into the soldering equipment cavity, it automatically drives air out of the soldering area by utilizing the different specific gravities of the shielding gas and air. During the soldering process, the PCB board continuously introduces oxygen into the soldering area. Therefore, shielding gas must be continuously injected into the soldering area to continuously exhaust oxygen to the outlet, creating an oxygen-free environment in the soldering area. The primary purpose of shielding gas in welding is to utilize the inert properties of high-purity shielding gas to isolate the soldering area from oxygen in the air, inhibiting the adverse effects of oxidation. This protects the flux's activity during the soldering process, enhances solder wettability and fluidity, and improves solderability at the soldering interface, thereby improving soldering quality.

[0005] Vacuuming: During the welding process, the flux components and moisture or oxides in the solder react chemically when heated to produce gases that are entrained in the solder and cannot be discharged by themselves. Therefore, an artificial vacuum environment is created during the welding process to discharge the entrained gases from the solder joints.

[0006] In order to improve the quality of solder joints, traditional welding processes such as wave soldering or reflow soldering generally use high-purity shielding gas during the soldering process to protect the process, and this needs to be continuously supplied. Due to the high cost of shielding gas materials, the production cost of electronic products increases.

[0007] The vacuum system cannot be added to wave soldering, but if it is added to the reflow soldering equipment, the cost of the soldering equipment will increase sharply. In addition, tubular components such as electrolytic capacitors are easily damaged in a vacuum environment, causing permanent failure of the product. In addition, non-gaseous debris contained in the solder joints cannot be discharged.

[0008] The above existing technologies, whether using shielding gas welding or vacuum welding, can only protect the weld point, but cannot improve the welding quality problems caused by the weld itself. For example: 1. The element segregation during the solidification process of the solder leads to a localized decrease in toughness; 2. The formation of columnar crystal structure during the casting process leads to a decrease in plasticity and joint strength. Summary of the Invention

[0009] In view of this, the present invention provides a solder welding method, which at least to a certain extent solves one of the technical problems existing in the prior art.

[0010] To achieve the above-mentioned purpose, the present invention provides a solder welding method, which enhances the strength of the solder welding point by adding an electromagnetic field around the solder joint, and specifically comprises the following steps: in the impurity removal and exhaust stage, when the solder joint is melt-welded with the welded component at high temperature and begins to cool down, an electromagnetic field is added around the solder joint, and the electromagnetic field is used to strengthen the movement of the liquid metal of the solder joint, so that impurities and bubbles are pushed to the surface of the liquid metal and float to the top to be removed, or randomly solidified on the surface and distributed in a dispersed state; in the segregation inhibition stage, when the temperature of the liquid metal solder joint is further reduced and the proportion of the third metal element component in the liquid metal solder joint is greater than the solubility of the parent metal, the metal liquid at the solder joint forms a convection movement by changing the magnetic pole direction of the electromagnetic field, and the shear force generated by the convection is suppressed. Under the action, the tips of the segregated are continuously cut off and broken; in the stage of forming and increasing the equiaxed crystal area, when the temperature continues to drop to the point where the liquid metal solder joint begins to solidify, a large number of crystal nuclei are generated on the surface of the solder joint to form a dense fine crystal area, and the heat dissipation crystals in the vertical direction will grow inward along the heat dissipation direction to form a columnar crystal area; as the columnar crystal area grows, the temperature difference of the remaining liquid metal becomes smaller, the heat dissipation direction is not obvious, and it is in a uniform cooling state, and the growth speed of the crystal nuclei in different directions tends to be the same, forming an equiaxed crystal area; in this solidification process, the magnetic pole direction of the electromagnetic field is changed again, so that the flowing liquid metal has a dynamic breaking and melting effect on the front end of the columnar crystal, resulting in a large number of crystal dendrite fragments to serve as crystal nuclei; at the same time, the magnetic field is used to promote the strong flow of metal, and the two-phase area expands rapidly to form a larger equiaxed crystal area.

[0011] Preferably, in the impurity removal and exhaust stage, the welding spot is in a completely liquefied state when it is melted and welded with the welded component at high temperature and begins to cool down.

[0012] Preferably, in the stage of suppressing segregation, when the temperature of the liquid metal solder joint is further reduced and the proportion of the third metal element in the liquid metal solder joint is greater than the solubility of the parent metal, suction is generated due to solidification, solid-state phase change and element density difference to form segregation, and the liquid metal solder joint rich in the third metal element is sucked to form segregation.

[0013] Preferably, in the stage of forming and increasing the equiaxed crystal region, when the temperature continues to drop until the liquid metal solder joint begins to solidify, the liquid phase of the high-temperature liquid metal solder joint is instantly cooled, and a large number of crystal nuclei are formed on the surface to form a dense fine crystal region; while the fine crystal region is formed, the surface temperature rises and the cooling rate decreases. Since the speed of forming crystal nuclei by releasing latent heat in the fine crystal region decreases, the heat dissipating crystals in the vertical direction will grow inward along the heat dissipation direction to form a columnar crystal region.

[0014] Preferably, during the impurity removal and exhaust stage, the electromagnetic field strengthens the molten metal to move from bottom to top, in rotation, or from inside to outside, or a combination thereof.

[0015] Preferably, the changing of the magnetic pole direction of the electromagnetic field in the stage of suppressing segregation is one or a combination of setting an electromagnetic field to strengthen the spiral and rotational motion of the molten metal.

[0016] Preferably, the direction of the magnetic poles of the electromagnetic field is changed again during the stage of forming and increasing the equiaxed crystal region, so that the electromagnetic field is set to strengthen the metal liquid to perform one or a combination of spiral and rotational motions.

[0017] Preferably, in the impurity removal and exhaust stage, the electromagnetic field is used to strengthen the molten metal to move from bottom to top; in the stage of suppressing segregation, the magnetic pole direction of the electromagnetic field is changed by setting the electromagnetic field to strengthen the molten metal to move in a spiral motion; in the stage of forming and increasing the equiaxed crystal region, the magnetic pole direction of the electromagnetic field is changed again by setting the electromagnetic field to strengthen the molten metal to move in a spiral motion.

[0018] Preferably, the electromagnetic field is composed of a single or multiple electromagnetic fields. When there are two electromagnetic fields, they are respectively arranged on both sides of the welding point in a symmetrical shape.

[0019] The soldering method of the present invention has the following beneficial effects:

[0020] During welding, different electromagnetic torques are used to enhance the movement of the liquid metal at the weld during the different stages of the cooling and solidification process. This effectively eliminates or reduces the accumulation of debris and bubbles within the ingot during the initial stages. This approach suppresses elemental segregation during the early stages of solidification, reduces the presence of columnar crystals, and increases the area of ​​equiaxed crystals, effectively improving the reliability of the weld. This not only reduces porosity during welding under a protective atmosphere but also further improves the mechanical properties of the weld by altering the crystal structure within the weld. This also reduces the material costs of the protective atmosphere and the equipment costs of the vacuum system. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the present invention for eliminating bubbles and inclusions in liquid metal by using a magnetic field;

[0022] Figure 2 Schematic diagram of the present invention suppressing liquid metal solder joint segregation by magnetic field;

[0023] Figure 3 Schematic diagram of reducing columnar crystal structure in liquid metal solder joints by using a magnetic field in the present invention;

[0024] Figure 4 This is a photograph of the segregation of the third metal element Bi observed using a metallographic microscope on a sample obtained using an embodiment of the present invention;

[0025] Figure 5 This is a photograph of the segregation of the third metal element Bi observed using a metallographic microscope using the sample obtained in Comparative Example 1;

[0026] Figure 6 This is a photograph of the segregation of the third metal element Bi observed using a metallographic microscope using the sample obtained in Comparative Example 2;

[0027] Figure 7 This is a photograph showing the distribution of axial crystal structure and columnar crystal structure of a sample obtained using an embodiment of the present invention observed using a metallographic microscope;

[0028] Figure 8 This is a photograph of the distribution of axial crystal structure and columnar crystal structure observed using a metallographic microscope using the sample obtained in Comparative Example 1;

[0029] Figure 9 The following is a photograph of the distribution of axial crystal structure and columnar crystal structure observed using a metallographic microscope using the sample obtained in Comparative Example 2. DETAILED DESCRIPTION

[0030] In order to improve the internal problems of the solder joint, the present invention conceives of using different electromagnetic torques to strengthen the flow movement of the liquid metal in the solder joint at different stages of the cooling and solidification process of the liquid metal solder joint, so as to change the crystal phase structure inside the solder joint and thus improve the mechanical properties of the solder joint.

[0031] The first stage is to remove impurities and exhaust. During the welding process, when the solder joint is connected to the welded component at high temperature and begins to cool down, an electromagnetic field is added around the solder joint. The electromagnetic force induced by the electromagnetic field in the liquid metal strengthens the movement of the liquid metal from bottom to top. The difference in conductivity between impurities and liquid metal is used to separate the two, and the impurities and bubbles are pushed to the surface of the liquid metal and float to the top for removal, or randomly solidified to the surface and dispersed, thereby eliminating or reducing the accumulation of impurities and bubbles in the solder joint.

[0032] In the second stage, segregation is suppressed and the temperature of the full liquid metal solder joint is further reduced. When the temperature of the full liquid metal solder joint is reduced to a certain temperature, a semi-solidified metal solder joint is formed. When the proportion of the third metal element in the semi-solidified metal solder joint is greater than the solubility of the parent metal, suction is generated due to solidification, solid-state phase change and element density difference to form segregation. Due to the differences in molar volume, electron density, electronegativity, etc. between the solvent and solute of the semi-solidified metal solder joint, the interaction between the elements causes the suction of the liquid metal rich in the third metal element to form segregation. Due to the generation of segregation, the alloy solder exhibits the physical properties of a single metal, which reduces its physical properties and leads to a reduction in welding strength and toughness. By changing the magnetic pole direction of the electromagnetic field, the liquid metal in the semi-solidified metal solder joint forms convection motion, and the metal convection motion is conducive to the uniformity of liquid phase composition and temperature. Due to the increase in convection, the tips of the segregated particles are continuously cut off and broken under the shear force generated by convection, thereby inhibiting the growth of the crystals. At the same time, the number of new crystal nuclei in the melt is also increased. The more crystal nuclei there are under the same volume, the finer the grains are, which plays a role in refining the grains and inhibiting segregation.

[0033] The third stage is to reduce columnar crystal structure: When the temperature continues to drop and the semi-solidified metal solder joint forms a metal solder joint, the liquid phase of the high-temperature liquid metal solder joint is instantly cooled, forming a large number of crystal nuclei on the surface, forming a dense fine-grained area. As the fine-grained area forms, the surface temperature rises and the cooling rate decreases. As the fine-grained area releases latent heat, the speed of crystal nucleation decreases. Due to the vertical heat dissipation, the crystals will grow inward along the heat dissipation direction to form columnar crystal areas. As the columnar crystal area grows, the temperature difference of the remaining liquid metal body becomes smaller, the heat dissipation direction is not obvious, and it is in a uniform cooling state. The growth rate of crystal nuclei in different directions tends to be the same, forming an equiaxed crystal area. The properties of columnar crystals are directional, so the plasticity is poor. If the columnar crystal ratio is high, it is easy to cause the solder joint to crack and reduce the solder joint's ability to withstand physical impact. During this solidification process, the direction of the magnetic poles of the electromagnetic field is changed again, and the flowing liquid metal can dynamically break and melt the front end of the dendrite, thereby creating a large number of dendrite fragments to serve as crystal nuclei. At the same time, the strong flow can greatly accelerate the heat transfer of the molten metal, causing the superheat to disappear quickly, the two-phase region to expand rapidly, and the formation of a larger equiaxed crystal region.

[0034] In order to make the purpose and technical solution of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0035] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0036] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0037] In the present invention, unless otherwise specified, directional words such as "up, down, left, right" etc. are usually used with reference to the directions shown in the drawings, or with reference to the vertical, perpendicular or gravity directions of the components themselves; similarly, for ease of understanding and description, "inside and outside" refer to the inside and outside relative to the outline of each component itself, but the above directional words are not used to limit the present invention.

[0038] like Figure 1 FIG. 1 is a schematic diagram illustrating the magnetic field movement effect of eliminating bubbles and inclusions in the liquid metal during the impurity removal and exhaust phase of the solder welding method of the present invention. During this phase, as the solder and the component being welded are melted and connected and begin to cool, the cooled, fully liquid metal solder joint 100 contains residual bubbles or inclusions 101. An electromagnetic field 102 is provided around the cooled, fully liquid metal solder joint 100, where N represents the North Pole and S represents the South Pole. When electromagnetic field 102 is energized, it generates an electromagnetic field that strengthens the upward movement of the liquid metal within the fully liquid metal solder joint 100, pushing the remaining bubbles or inclusions 101 within the solder joint toward the top of the fully liquid metal solder joint 100. These bubbles or inclusions 101 float to the top and are removed, or are dispersed on the surface of the fully liquid metal solder joint 100, thereby eliminating or reducing the accumulation of inclusions and bubbles within the fully liquid metal solder joint 100.

[0039] Preferably, the liquid metal solder joints in the impurity removal and exhaust stage are in a state of cooling but not yet solidifying, and are in a state of full liquid metal;

[0040] Preferably, the electromagnetic field consists of a single or multiple components;

[0041] Preferably, the electromagnetic fields are two symmetrically arranged on both sides of the welding point;

[0042] Preferably, the electromagnetic field strengthens the liquid metal weld to move from bottom to top, rotate, or move from inside to outside, or a combination thereof. The electromagnetic force induced by the electromagnetic field in the liquid metal strengthens the directional flow movement of the liquid metal, and the difference in conductivity between the impurities and the liquid metal is utilized to achieve separation of the two. The impurities and bubbles are pushed to the surface of the liquid metal and float to the top for removal, or are randomly solidified to the surface and distributed in a dispersed state, thereby eliminating or reducing the accumulation of impurities and bubbles in the weld.

[0043] like Figure 2FIG. 1 is a schematic diagram of the magnetic field movement effect of suppressing the segregation of liquid metal welding points by the magnetic field in the segregation suppression stage of the solder welding method of the present invention. FIG. At this stage, the temperature of the metal solder joint 100 in the fully liquid phase further decreases. When the temperature of the metal solder solution further decreases to a certain temperature to form a semi-solid metal solder joint 200, and the proportion of the third metal element 201 in the semi-solid metal solder joint 200 is greater than the solubility of the semi-solid metal solder joint 200, suction is generated due to solidification, solid-state phase transformation, and element density differences to form segregation. Due to the differences in molar volume, electron density, electronegativity, etc. between the solvent and solute in the liquid metal, the elements interact with each other and absorb the semi-solid metal solder joint 200 rich in the third metal element 201, causing segregation. The generation of segregation causes the alloy solder to exhibit the physical properties of a single metal, resulting in reduced physical properties and reduced welding strength and toughness. By changing the magnetic pole direction of the electromagnetic field 202, convection motion is generated inside the semi-solid metal solder joint 200. Convection motion inside the semi-solid metal solder joint 200 is conducive to the uniformity of liquid phase composition and temperature. Due to the increase in convection, the segregated tips 203 are continuously cut off and broken under the shear force generated by the convection, thereby inhibiting the growth of the crystal and increasing the number of new crystal nuclei in the melt. The more crystal nuclei there are under the same volume, the finer the grains are, which plays a role in refining the grains and inhibiting segregation.

[0044] Preferably, the magnetic pole direction of the electromagnetic field 202 is changed in the stage of suppressing the segregation, by setting the electromagnetic field to strengthen the spiral motion, rotational motion or a combination of the liquid metal, so that the liquid metal body forms a convection motion. Due to the increase in convection, the segregated tips 203 are continuously cut off and broken by the shear force generated by the convection, thereby suppressing the growth of the crystal, and also increasing the number of new crystal nuclei in the melt.

[0045] like Figure 3Figure 1 shows a schematic diagram of the magnetic field effect of reducing the magnetic field motion of columnar crystals in a metal solder joint during the formation and addition of an equiaxed crystal region in the soldering method of the present invention. During this stage, the semi-solidified metal solder joint begins to solidify, forming a metal solder joint 300. Due to the instantaneous cooling of the high-temperature liquid metal solder joint, a large number of crystal nuclei are generated on the solder joint surface, forming a dense, fine-grained region 305. As fine-grained region 305 forms, the surface temperature rises and the cooling rate decreases. Because fine-grained region 305 releases latent heat, the rate of crystal nucleation decreases, causing vertical heat dissipation crystals to grow inward along the heat dissipation direction, forming columnar crystal regions 301. As columnar crystal regions 301 grow, the temperature difference of the remaining liquid metal decreases, the heat dissipation direction becomes less distinct, and the metal is cooled uniformly. The crystal nuclei grow at the same rate in different directions, forming an equiaxed crystal region 304. During this solidification process, the direction of the magnetic poles of the electromagnetic field 302 is changed again, and the flowing liquid metal can dynamically break and dissolve the front end 303 of the dendrite, thereby creating a large number of dendrite fragments to serve as crystal nuclei. At the same time, the magnetic field promotes the strong flow of metal, which can greatly accelerate the heat transfer of the metal liquid and quickly eliminate the overheating. The two-phase region quickly expands to form a larger equiaxed crystal region 304, and finally form a high-strength solder joint.

[0046] The performance of columnar crystals is directional, so the plasticity is poor. If the ratio of columnar crystals is high, it is easy to cause the solder joints to crack and reduce the solder joints' ability to withstand physical impact.

[0047] Preferably, the direction of the magnetic pole of the electromagnetic field 302 is changed again in the stage of forming and increasing the equiaxed crystal region, by setting the electromagnetic field to strengthen the metal welding point to perform a spiral motion, a rotational motion or a combination thereof, so as to produce a dynamic breaking effect and a melting effect on the front end 303 of the dendrite through the flowing liquid metal, and promote the strong flow of metal through the magnetic field, which can greatly accelerate the heat transfer of the metal liquid and quickly eliminate the overheating.

[0048] The following describes the soldering of multilayer ceramic capacitors (MLCCs) (0805 package components) using low-temperature solder paste Sn42Bi58. Different soldering methods are used in different embodiments, and the performance of the obtained samples is compared experimentally.

[0049] (1) The welding method of each embodiment is as follows:

[0050] In the embodiment of the present invention, welding is performed using the solder welding solution of the present invention that uses a magnetic field to enhance the strength of the solder welding spot;

[0051] Comparative Example 1: Direct soldering using conventional reflow soldering;

[0052] Comparative Example 2: Direct soldering using vacuum reflow soldering.

[0053] (2) The experimental operation of the performance test of the samples obtained in each embodiment is as follows:

[0054] Third-party metal element Bi segregation: Observed using a metallographic microscope, the white tissue elements observed are Bi elements, and the dispersion of the white tissue elements reflects the segregation of Bi elements in each sample;

[0055] Distribution of axial crystal structure and columnar crystal structure: The distribution of axial crystal structure and columnar crystal structure can be directly observed by metallographic microscope;

[0056] The percentage of air voids in solder joints: Observation is performed using 3D-X-ray inspection equipment. X-rays are transmitted through the weld area and the absorption of X-rays is detected to determine the air voids in the weld area. The percentage of air voids in the solder joints of each experimental sample can then be calculated.

[0057] Solder joint strength test: A thrust test is a destructive test. The specific method is to use a thrust gauge parallel to the top of the soldered component and push the middle position of the two electrodes of the chip capacitor at one time until the chip capacitor falls off. The solder joint strength of each experimental sample can be obtained;

[0058] Component peel strength test: The peel strength test is a destructive experiment. The specific method is to use a tensile gauge parallel to the bottom of the soldered component to pull the middle position of the two electrodes of the chip capacitor vertically toward the top until the chip capacitor falls off. The peel strength of each experimental sample component can be obtained.

[0059] Figure 4 This is a photo of the segregation of the third metal element Bi observed using a metallographic microscope on a sample obtained using an embodiment of the present invention. Figure 5 This is a photo of the segregation of the third metal element Bi observed using a metallographic microscope using the sample obtained in Comparative Example 1. Figure 6 The sample obtained in the comparative example 2 is observed with a metallographic microscope to observe the segregation of the third metal element Bi. Figure 4 It can be clearly seen that the white tissue elements are diffusely distributed and do not form obvious clusters. Figure 5 What we see is that the white tissue elements are obviously concentrated. Figure 6 The white tissue element segregation phenomenon seen in 4 and Figure 5 It can be seen that the solution of enhancing the strength of solder joints by the magnetic field in this embodiment can effectively reduce the element segregation phenomenon.

[0060] Figure 7 The following are photos of the distribution of axial crystal structure and columnar crystal structure observed by metallographic microscope using the sample obtained by the embodiment of the present invention. Figure 8 The following are photos of the distribution of axial crystal structure and columnar crystal structure observed by metallographic microscope using the sample obtained in Comparative Example 1: Figure 9The sample obtained in Comparative Example 2 is observed using a metallographic microscope to observe the distribution of axial crystal structure and columnar crystal structure. Figure 7 It can be clearly seen that the axial crystal structure and columnar crystal structure almost do not appear. Figure 8 The axial crystal structure and columnar crystal structure in the quartz crystal show obvious columnar crystal structure. Figure 9 The axial crystal structure and columnar crystal structure in the Figure 8 It is almost the same, which shows that the solution of enhancing the strength of the solder joint by the magnetic field in this embodiment can reduce the columnar crystal structure.

[0061] The air void ratio, solder joint strength and component peeling strength of the samples obtained in the above three embodiments are shown in Table 1 below:

[0062] Table 1 Comparative table of sample performance obtained in various embodiments

[0063]

[0064] As shown in Table 1 above, it can be seen that the soldering method of the present invention can reduce the proportion of air voids in the solder joint from 12-25% to 8%; enhance the solder joint strength to above 7 MPa, reaching 42 MPa; and improve the component peel strength of the solder joint by 40%, from 441 gf in the vacuum reflow direct soldering solution to 627 gf.

[0065] The solder welding method of the present invention applies an electromagnetic field during the cooling and solidification process of the metal solder joint, utilizing different electromagnetic torques to strengthen the movement of the liquid metal in the liquid metal solder joint, thereby effectively eliminating or reducing the accumulation of debris and bubbles in the ingot in the initial stage. The method of suppressing element segregation in the early stage of solidification, reducing the columnar crystal structure, and increasing the area of ​​equiaxed crystal structure can effectively improve the reliability of the solder joint. Not only can the problem of porosity be reduced by welding under a protective atmosphere, but the mechanical properties of the solder joint can also be further improved by changing the crystal phase structure inside the metal solder joint. Furthermore, the material cost of the atmosphere protection and the equipment cost of the vacuum system are reduced.

[0066] The above are merely embodiments of the present invention. It should be particularly pointed out that the above embodiments should not be regarded as limitations of the present invention. For ordinary technicians in this technical field, several improvements and modifications can be made without departing from the spirit and scope of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A soldering method, wherein the strength of a soldering joint is enhanced by adding an electromagnetic field around the soldering joint, comprising the following steps: Decontamination and exhaust stage: When the solder joint is melted and connected to the component being welded at high temperature and begins to cool down, the solder joint is in the form of full liquid metal. An electromagnetic field is added around the solder joint, which strengthens the movement of the liquid metal from bottom to top, pushing impurities and bubbles to the surface of the liquid metal and floating to the top for removal, or randomly solidifying on the surface and dispersing them. Inhibiting segregation stage: When the temperature of the liquid metal solder joint is further reduced to form a semi-solidified state, and the proportion of the third metal element in the liquid metal solder joint is greater than the solubility of the parent metal, the metal liquid in the solder joint is caused to form a convection motion that combines spiral motion and rotational motion by changing the magnetic pole direction of the electromagnetic field. Under the action of the shear force generated by the convection, the segregated tips are continuously cut off and broken, thereby suppressing segregation and refining the grains; The stage of forming and increasing the equiaxed crystal region; the temperature continues to decrease, and when the solder joint begins to solidify from a semi-solid form, a large number of crystal nuclei are generated on the surface of the solder joint to form a dense fine-grained area, and the heat dissipation crystals in the vertical direction will grow inward along the heat dissipation direction to form a columnar crystal area; as the columnar crystal area grows, the temperature difference of the remaining liquid metal becomes smaller, the heat dissipation direction is not obvious, and it is in a uniform cooling state. The growth rate of the crystal nuclei in different directions tends to be the same, forming an equiaxed crystal area; during this solidification process, the magnetic pole direction of the electromagnetic field is changed again, so that the remaining liquid metal of the solder joint flows in a combination of spiral motion and rotational motion, and the dynamic breaking and melting effect on the front end of the columnar crystal causes a large number of crystal dendrite fragments to serve as crystal nuclei; at the same time, the magnetic field is used to promote the strong flow of metal, and the two-phase area expands rapidly to form a larger equiaxed crystal area.

2. The soldering method according to claim 1, wherein during the impurity removal and exhaust stage, the solder joint is in a completely liquefied state when it is melted and connected to the soldered component at high temperature and begins to cool down.

3. The solder welding method according to claim 1, in the stage of suppressing segregation, when the temperature of the liquid metal solder joint is further reduced and the proportion of the third metal element in the liquid metal solder joint is greater than the solubility of the parent metal, suction is generated due to solidification, solid-state phase change and element density difference to form segregation, and the liquid metal solder joint rich in the third metal element is sucked to form segregation.

4. The solder welding method according to claim 1, in the stage of forming and increasing the equiaxed crystal area, when the temperature continues to drop to the point where the liquid metal solder joint begins to solidify, a large number of crystal nuclei are formed on the surface due to the instantaneous cooling of the liquid phase of the high-temperature liquid metal solder joint; while the fine crystal area is formed, the surface temperature rises and the cooling rate decreases, and since the speed of forming crystal nuclei by releasing latent heat in the fine crystal area decreases, the heat dissipating crystals in the vertical direction will grow inward along the heat dissipation direction to form columnar crystal areas.

5. The soldering method according to claim 1, wherein the electromagnetic field is composed of a single or multiple electromagnetic fields, and when there are two electromagnetic fields, they are respectively arranged on both sides of the soldering point in a symmetrical shape.

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