A controllable preparation method of Sn-based twin crystal solder joint

Abstract

The application relates to the technical field of microelectronic packaging, in particular to a controllable preparation method of Sn-based twin crystal solder joints. 2 ‑10 4 K / s cooling rate, and twin crystal organization is induced in the solder joint. The Sn-based solder paste is alloyed with indium to reduce the beta-Sn layer spacing energy, and the superfast cooling is used to increase the supercooling degree, so that the high-density twin crystal organization is induced in the solder joint, and the twin crystal generation rate can reach 90%. The problem of insufficient anti-electromigration capacity of the Sn-based solder joint caused by the anisotropy of beta-Sn is effectively solved, and the reliability of the microelectronic packaging solder joint is remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of microelectronic packaging technology, and in particular to a controllable method for preparing Sn-based twin solder joints. Background Technology

[0002] Sn-Ag-Cu (SAC) lead-free solders, with their excellent wettability, good mechanical properties, and mature processing technology, are gradually replacing traditional Sn-Pb solders and becoming the mainstream interconnect material in the microelectronics packaging field. However, in the microstructure of SAC alloys, the β-Sn solid solution, as a continuous matrix phase, has a body-centered tetragonal crystal structure with significant differences in its lattice constant, leading to obvious anisotropy in the mechanical, thermal, and electrical properties of the material. With the trend towards solder joint miniaturization, the limited number of β-Sn grains amplifies the adverse effects of anisotropy. When the c-axis of β-Sn is parallel to the current direction, elements such as Cu and Ni diffuse rapidly along the c-axis, easily causing rapid dissolution of intermetallic compounds and premature failure of the barrier layer; while when the c-axis is perpendicular to the current direction, the failure process is relatively slow. Therefore, controlling the grain orientation to mitigate the effects of anisotropy is of great significance for improving solder joint reliability.

[0003] Studies have shown that twin boundaries possess low interfacial energy and good structural stability, which can hinder dislocation movement during deformation and coordinate strain release, thereby significantly mitigating the adverse effects of β-Sn anisotropy. Research has found that adding trace amounts of indium (In) to SAC solder paste, combined with a high cooling rate, can significantly increase the probability of twin formation, thus enhancing the electromigration resistance of solder joints. However, the cooling zones of existing reflow soldering equipment generally lack rapid cooling capabilities. Traditional reflow ovens, limited by design parameters, struggle to achieve ideal cooling rates, which not only restricts twin formation but may also lead to uneven microstructure of solder joints, exacerbating uneven internal stress distribution and reducing electromigration resistance and service life.

[0004] Therefore, it is urgent to develop a controllable preparation method for Sn-based twin solder joints to solve the above-mentioned technical problems and achieve controllable preparation of twin structures in Sn-based solder joints. Summary of the Invention

[0005] The purpose of this invention is to provide a controllable preparation method for Sn-based twinned solder joints. By optimizing the solder paste formulation and controlling the cooling process, the microstructure of the solder joints can be adjusted to improve the twinning ratio of solder balls / solder joints, thereby solving the problems existing in the prior art.

[0006] To achieve the above objectives, the present invention provides the following solution: The present invention provides a controllable preparation method for Sn-based twin solder joints, comprising the following steps: Prepare Sn-based lead-free solder paste, which contains 2, 5, 10, or 13% indium by mass. Sn-based lead-free solder paste is applied to Cu pads to form a sample to be soldered, or Sn-based lead-free solder paste is screen-printed into individual solder balls; The sample is sent into the heating zone of the reflow oven and heated and soldered according to the preset reflow temperature profile, so that the Sn-based lead-free solder paste melts to form solder joints. The completed weld joints are then sent to the cooling zone of the reflow oven at 10°C. 2 -10 4 Rapid cooling at a rate of K / s induces the formation of twinned structures within the solder joint.

[0007] Preferably, in the step of preparing Sn-based lead-free solder paste, the Sn-based lead-free solder paste is SAC305 series solder paste or its derivative.

[0008] Preferably, in the step of sending the sample into the heating zone of the reflow oven, the preset reflow temperature profile includes a peak temperature of 250-270°C and a holding time of 45-90 seconds.

[0009] Preferably, in the step of sending the welded joints into the cooling zone of the reflow oven, rapid cooling is achieved by introducing a cooling medium into the cooling zone.

[0010] Preferably, the cooling medium is selected from one or more of water, liquid nitrogen, or dry ice.

[0011] Preferably, in the step of sending the welded joints into the cooling zone of the reflow oven, the cooling rate is dynamically adjusted.

[0012] Preferably, the reference items for adjusting the cooling rate include the indium content in the Sn-based lead-free solder paste and the size of the solder joint; the indium content is positively correlated with the cooling rate; and the solder joint size is inversely correlated with the cooling rate.

[0013] Preferably, the heating zone includes a first heating zone, a second heating zone, a third heating zone, a fourth heating zone, a fifth heating zone, and a sixth heating zone that are gradually heated towards the cooling zone. The solder joint enters the cooling zone after passing through the sixth heating zone.

[0014] Preferably, the cooling zone of the reflow oven is equipped with cooling pipes and a cooling medium storage tank. The cooling pipes are used to circulate and import the cooling medium, and the cooling medium storage tank is used to store and switch different types of cooling media.

[0015] Preferably, the cooling zone of the reflow oven is equipped with a temperature sensing device and a control system. The temperature sensing device is used to monitor the temperature of the solder joint in real time, and the control system dynamically adjusts the flow rate of the cooling medium according to the deviation between the monitored temperature and the preset cooling rate.

[0016] Compared with the prior art, the present invention has the following advantages and technical effects: The present invention provides a controllable preparation method for Sn-based twin solder joints. First, Sn-based lead-free solder paste containing 2, 5, 10, or 13% indium by mass is prepared. The alloying effect of indium reduces the stacking fault energy of β-Sn, creating thermodynamic conditions for twin nucleation. Then, the solder paste is coated onto Cu pads to form samples to be soldered or screen-printed into individual solder balls, which are then sent to the heating zone of a reflow oven for heating and soldering according to a preset reflow temperature profile to melt the solder paste and form solder joints. Finally, the soldered joints are sent to the cooling zone of the reflow oven at 10°C. 2— 10 4 Rapid cooling at a high cooling rate of K / s represents an order-of-magnitude leap compared to traditional reflow soldering equipment. Through indium alloying, the stacking fault energy and undercooling are synergistically controlled, inducing the formation of a high-density twin structure within the solder joint. This equipment achieves integrated heating and cooling, inducing twin structure formation in In-containing solder paste by controlling the cooling rate, achieving a twin boundary ratio of up to 90% in the solder ball / joint. Furthermore, this approach is universally applicable, adaptable to different solder paste systems by adjusting the cooling medium, effectively addressing the insufficient electromigration resistance of Sn-based solder joints due to β-Sn anisotropy in existing technologies. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly described below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. In the drawings: Figure 1 This is a schematic diagram of the furnace body structure of the present invention; Figure 2 This is a perspective view of the furnace body structure of the present invention; Figure 3 This is an enlarged view of the cooling area of ​​the present invention; Figure 4 Partial air cooling (10) prepared for this invention 2 EBSD characterization results of solder balls (SAC305-2 wt.%In) K / s; Figure 5 Partial water cooling (10) prepared for this invention 3 EBSD characterization results of solder balls (SAC305-2 wt.%In) K / s; Figure 6 The partial liquid nitrogen refrigeration (10) prepared for this invention 4 EBSD characterization results of solder balls (SAC305-2 wt.%In) K / s; In the diagram: 1. Furnace body; 2. Conveyor belt; 3. Control system; 4. First heating zone; 5. Second heating zone; 6. Third heating zone; 7. Fourth heating zone; 8. Fifth heating zone; 9. Sixth heating zone; 10. Cooling zone; 11. Extended cooling zone; 12. Heating assembly; 13. Cooling pipe; 14. Temperature sensing device; 15. Cooling medium storage tank; 16. Cooling medium transfer box; 17. Outer baffle assembly; 18. Inner baffle assembly; 19. Baffle plate. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] Reference Figures 1 to 6 As shown, this embodiment provides a controllable preparation method for Sn-based twin solder joints, including the following steps: Prepare Sn-based lead-free solder paste, which contains 2, 5, 10, or 13% indium by mass. Sn-based lead-free solder paste is applied to Cu pads to form a sample to be soldered, or Sn-based lead-free solder paste is screen-printed into individual solder balls; The sample is sent into the heating zone of the reflow oven and heated and soldered according to the preset reflow temperature profile, so that the Sn-based lead-free solder paste melts to form solder joints. The completed weld joints are then sent to the cooling zone 10 of the reflow oven, where they are cooled at 10°C. 2— 10 4 Rapid cooling at a rate of K / s induces the formation of twinned structures within the solder joint.

[0021] This invention provides a controllable preparation method for Sn-based twin solder joints. First, Sn-based lead-free solder paste containing 2, 5, 10, or 13% indium by mass is prepared. The alloying effect of indium reduces the stacking fault energy of β-Sn, creating thermodynamic conditions for twin nucleation. Then, the solder paste is coated onto Cu pads to form samples to be soldered, or individual solder balls are created through screen printing. These samples are then placed in the reflow oven heating zone and heated according to a preset reflow temperature profile to melt the solder paste and form solder joints. Finally, the soldered joints are placed in the cooling zone 10 of the reflow oven at 10°C. 2 -10 4 Rapid cooling at a rate of K / s represents a leap in speed compared to traditional reflow soldering equipment. Through indium alloying, the stacking fault energy and undercooling are synergistically controlled, inducing explosive nucleation within the solder joint to form a high-density twin structure. This equipment achieves integrated heating and cooling, not only allowing for controlled cooling rates within a certain range but, more importantly, inducing twin formation in In-containing solder paste, achieving a twin boundary ratio of up to 90% in the solder ball / joint. Furthermore, this approach is universally applicable, adaptable to different solder paste systems by adjusting the cooling medium, effectively addressing the insufficient electromigration resistance of Sn-based solder joints due to β-Sn anisotropy in existing technologies.

[0022] To further optimize the solution, in the step of preparing Sn-based lead-free solder paste, the Sn-based lead-free solder paste is SAC305-based solder paste or its derivatives. This embodiment uses the SAC305 system or similar materials derived from it as the base alloy for adding indium (In). SAC305 is currently the most mature and widely used lead-free solder system in microelectronic packaging. Using it as the base alloy has good process compatibility and stability, lowers the technical industrialization threshold, and facilitates direct replacement of solder paste materials in existing production lines.

[0023] In one embodiment of the present invention, SAC305 (Sn-3.0Ag-0.5Cu) is used as the base alloy system for illustrative purposes. However, those skilled in the art should understand that other Sn-based lead-free solder systems (such as SAC105, SAC405, Sn-Cu, Sn-Ag, etc.) can also achieve controllable twinning structures after adding 2, 5, 10, or 13% In and using an ultra-fast cooling process, and all fall within the protection scope of this application.

[0024] To further optimize the design, in the step of sending the sample into the heating zone of the reflow oven, the preset reflow temperature profile includes a peak temperature of 250-270℃ and a holding time of 45-90 seconds. The highest temperature reached by the solder paste melting is 250 to 270℃, and it is maintained within this temperature range for 45 to 90 seconds to ensure that the solder fully wets and completes the metallurgical reaction. This ensures that the solder paste completely melts to form a dense solder joint, while avoiding excessive growth of intermetallic compounds due to excessively high temperatures or long holding times. This provides a uniform and stable melt state as a prerequisite for twin nucleation in the subsequent rapid cooling stage.

[0025] Further optimizing the solution, in the step of sending the welded joints into the cooling zone 10 of the reflow oven, rapid cooling is achieved by introducing a cooling medium into the cooling zone 10. Specifically, this invention achieves ultra-rapid cooling by introducing a cooling medium into the cooling zone 10 of the reflow oven. The intense heat exchange between the medium and the high-temperature weld joint removes heat, overcoming the bottleneck of traditional reflow ovens that rely on natural convection or air cooling. This makes the cooling rate controllable and adjustable, achieving a cooling rate of 10... 3 —10 4 K / s ultrafast cooling provides a practical engineering implementation path.

[0026] To further optimize the solution, the cooling medium is selected from one or more of water, liquid nitrogen, or dry ice. The cooling medium can be water-cooled, refrigerant-cooled, or liquid nitrogen-cooled. The cooling medium can be adjusted according to requirements to control the cooling rate, while also achieving ultra-fast cooling. Different media provide different cooling capabilities—water is suitable for medium rates, while liquid nitrogen achieves extremely fast cooling. This multi-media adaptability makes the method universal, flexibly matching different solder paste compositions, solder joint sizes, and mass production efficiency requirements.

[0027] The solution is further optimized by dynamically adjusting the cooling rate during the step of sending the welded joints into the cooling zone 10 of the reflow oven. Instead of using a fixed cooling rate during the weld joint cooling process, the cooling rate is dynamically adjusted in real-time or by setting a preset value based on actual conditions. This dynamic adjustment mechanism allows for more precise process control, avoiding quality fluctuations caused by a one-size-fits-all approach to solidification cooling. Cooling parameters can be optimized according to different operating conditions, ensuring twin formation while also considering production efficiency and yield.

[0028] Further optimization of the scheme involves adjusting the cooling rate based on factors including the indium content in the Sn-based lead-free solder paste and the solder joint size. Indium content is positively correlated with cooling rate, while solder joint size is inversely correlated. The dynamic adjustment of the cooling rate is related to indium content and / or solder joint size. Higher indium content significantly reduces stacking fault energy; the higher the indium content, the higher the cooling rate. Smaller solder joint sizes also result in higher cooling rates, providing a basis for selecting process parameters and improving operability and predictability.

[0029] The design is further optimized so that the heating zone includes a first heating zone 4, a second heating zone 5, a third heating zone 6, a fourth heating zone 7, a fifth heating zone 8, and a sixth heating zone 9, which are heated in a gradient towards the cooling zone 10. The weld joint passes through the sixth heating zone 9 and enters the cooling zone 10. The reflow oven includes a furnace body 1, which contains the first heating zone 4, the second heating zone 5, the third heating zone 6, the fourth heating zone 7, the fifth heating zone 8, the sixth heating zone 9, and the cooling zone 10. The furnace body 1 has an inlet end and an outlet end at opposite ends. The workpiece enters from the inlet end of the furnace body 1 and undergoes gradient heating sequentially through the first heating zone 4, the second heating zone 5, the third heating zone 6, the fourth heating zone 7, the fifth heating zone 8, and the sixth heating zone 9. This gradient heating effectively avoids welding defects caused by excessive temperature gradients and ensures the stability and consistency of the welding process.

[0030] In one embodiment of the present invention, a conveyor belt 2 extending from the inlet to the outlet is fixedly installed inside the furnace body 1. The conveyor belt passes through the heating zone and the cooling zone 10, extending the cooling zone 11 to ensure that the sample moves continuously within the furnace along a predetermined path. This allows the material to be welded to be transported from the inlet through the first heating zone 4, the second heating zone 5, the third heating zone 6, the fourth heating zone 7, the fifth heating zone 8, and the sixth heating zone 9 for welding. After welding, the material is conveyed by the conveyor belt 2 to the cooling zone 10 for cooling. After cooling, it continues to be conveyed to the outlet. This continuous transport design has the following advantages: First, it improves production efficiency and enables large-scale continuous production; second, it ensures the stability of the welding process, avoiding welding defects caused by excessively long or short residence times of the sample within the furnace body 1; finally, the speed of the conveyor belt 2 can be adjusted according to the specific requirements of the welding process to meet the welding requirements of different samples. In practical applications, after welding is completed in the heating zone, the sample is immediately sent by the conveyor belt 2 into the cooling zone 10 for rapid cooling. The entire process is continuous and efficient, greatly improving production efficiency and welding quality.

[0031] In one embodiment of the present invention, each heating zone is equipped with a heating element 12 and a temperature sensing device 14. The heating element 12 is an infrared radiation heating tube, which heats through heat convection and heat radiation, providing efficient and uniform heat energy to ensure that the solder paste quickly heats up to the melting temperature in a short time. The temperature sensing device 14 is a K-type thermocouple, used to monitor the temperature of each zone in real time, and to achieve precise temperature control through the control system 3. This is prior art and will not be described in detail here.

[0032] Further optimizing the design, the cooling zone 10 of the reflow oven is equipped with cooling pipes 13 and a cooling medium storage tank 15. The cooling pipes 13 are used for the circulation and export of the cooling medium, while the cooling medium storage tank 15 is used to store and switch between different types of cooling media. Located in the middle to rear section of the oven body 1, the reflow oven cooling zone 10 is a crucial area for rapid cooling of the solder joints. This zone includes cooling nozzles and cooling pipes 13. The cooling nozzles can evenly spray the cooling medium onto the solder joint surface for rapid cooling. The two ends of the cooling pipes 13 are connected to the cooling medium storage tank 15, and a circulation system is used to import, export, and recycle the cooling medium. A cooling medium transfer box 16 is also provided, which improves cooling efficiency and reduces energy consumption, enabling a 10... 3— 10 4 The cooling rate of K / s is higher than that of traditional equipment, which creates conditions for the fine control of the microstructure of the solder joint.

[0033] In one embodiment of the present invention, multiple cooling medium storage tanks 15 enable the equipment to quickly switch between different cooling media during the same production process, thereby improving process flexibility and equipment utilization.

[0034] In one embodiment of the present invention, the cooling pipe 13 can circulate various cooling media, such as water, liquid nitrogen, or other condensers. Different cooling media have different cooling rates and thermal conductivity properties, and can be selected according to the specific requirements of the welding process. For example, water has a high specific heat capacity, suitable for medium cooling rate requirements; liquid nitrogen has an extremely low boiling point, enabling extremely fast cooling rates, suitable for applications with extremely high cooling rate requirements; the recycling design of the cooling pipe 13 not only improves the efficiency of cooling media utilization but also reduces equipment operating costs; at the same time, the compatibility of the cooling pipe 13 with various cooling media further improves the flexibility and efficiency of the cooling system, enabling it to meet the diverse cooling rate requirements of different welding processes.

[0035] Further optimization of the design involves equipping the cooling zone 10 of the reflow oven with a temperature sensor 14 and a control system 3. The temperature sensor 14 monitors the solder joint temperature in real time, while the control system 3 dynamically adjusts the flow rate of the cooling medium based on the deviation between the monitored temperature and the preset cooling rate. This closed-loop feedback control achieves precision and automation in the cooling process, eliminating errors caused by manual operation or open-loop control. It ensures that each solder joint solidifies according to the preset cooling curve, significantly improving the consistency and reliability of mass production.

[0036] In one embodiment of the present invention, reference is made to... Figure 1Both ends of the furnace body 1 are provided with an outer partition assembly 17. The outer partition assembly 17 includes an inner partition assembly 18 fixedly connected to the inner wall of the furnace body 1, and a barrier plate 19 is vertically arranged. The outer surface of the outer partition assembly 17 is in close contact with the inner wall of the furnace body 1.

[0037] When the material is about to move from the inlet to the first heating zone 4, the baffle plate 19 near the inlet end is activated, and a gap is formed between the bottom surface of the baffle plate 19 and the surface of the conveyor belt 2. After the material enters the heating zone through the gap, the baffle plate 19 abuts against the surface of the conveyor belt 2 again. When the welding material approaches the cooling zone 10, the baffle plate 19 corresponding to the cooling zone 10 is activated in the same way and closes again after the material enters the cooling zone 10. After the material completes a series of heating and cooling processes, the baffle plate 19 at the outlet end is lifted, and the material is conveyed to the outlet by the conveyor belt 2.

[0038] Specific example 1: 9.8g, 9.5g, 9g, and 8.7g of Sn-3.0Ag-0.5Cu alloy powder and 0.2g, 0.5g, 1g, and 1.3g of In powder were weighed and placed together in 50mL of anhydrous ethanol. After mechanical stirring for 30min, the solution was filtered off. The mixed powder was placed in a vacuum drying oven and dried at 40℃ for 12h to obtain 10g of mixed powder with good micro-dispersion and macro-density. Weigh 1.0g of KL-558 lead-free, halogen-free, non-conductive solder paste and mechanically mix it with the dried mixed powder for at least 30 minutes to prepare Sn-based lead-free composite solder paste. Reflow the paste at 260℃ for 1 minute and then air-cool it to form solder balls of approximately 300μm. After embedding the solder balls using a cold mounting solution and conductive powder, polish the surface with 3000# sandpaper. Rough polishing of the sample surface is then performed using 0.3μm Al2O3 polishing agent produced by BUEHLER at a mechanical polishing machine speed of 100r / min until no obvious scratches are visible. The sample is then placed in a suspension for fine polishing until no scratches are visible. Microstructure observation is performed using a polarizing microscope. EBSD samples were ion-polished using a Leica ion polisher after mechanical polishing to remove surface stress marks, and then characterized using a SU8230 scanning electron microscope.

[0039] Specific example 2: 9.8g, 9.5g, 9g, and 8.7g of Sn-3.0Ag-0.5Cu alloy powder and 0.2g, 0.5g, 1g, and 1.3g of In powder were weighed and placed together in 50mL of anhydrous ethanol. After mechanical stirring for 30min, the solution was filtered off. The mixed powder was placed in a vacuum drying oven and dried at 40℃ for 12h to obtain 10g of mixed powder with good micro-dispersion and macro-density. Weigh 1.0g of KL-558 lead-free, halogen-free, non-conductive solder paste and mechanically mix it with the dried mixed powder for at least 30 minutes to prepare Sn-based lead-free composite solder paste. Reflow the paste at 260℃ for 1 minute, then water-cool it to form solder balls of approximately 300μm. After embedding the solder balls using a cold mounting solution and conductive powder, polish the surface with 3000# sandpaper. Rough polishing of the sample surface is performed using 0.3μm Al2O3 polishing agent produced by BUEHLER at a mechanical polishing machine speed of 100r / min until no obvious scratches are visible. Then, the sample is placed in a suspension for fine polishing until no scratches are visible. Microstructure observation is performed using a polarizing microscope. EBSD samples were ion-polished using a Leica ion polisher after mechanical polishing to remove surface stress marks, and then characterized using a SU8230 scanning electron microscope.

[0040] Specific example 3: 9.8g, 9.5g, 9g, and 8.7g of Sn-3.0Ag-0.5Cu alloy powder and 0.2g, 0.5g, 1g, and 1.3g of In powder were weighed and placed together in 50mL of anhydrous ethanol. After mechanical stirring for 30min, the solution was filtered off. The mixed powder was placed in a vacuum drying oven and dried at 40℃ for 12h to obtain 10g of mixed powder with good micro-dispersion and macro-density. Weigh 1.0g of KL-558 lead-free, halogen-free, non-conductive solder paste and mechanically mix it with the dried mixed powder for at least 30 minutes to prepare Sn-based lead-free composite solder paste. Reflow the paste at 260℃ for 1 minute, then cool it with liquid nitrogen to form solder balls of approximately 300μm. After embedding the solder balls using a cold mounting solution and conductive powder, polish the surface with 3000# sandpaper. Rough polishing of the sample surface is performed using 0.3μm Al2O3 polishing agent produced by BUEHLER at a mechanical polishing machine speed of 100 rpm until no obvious scratches are visible. The sample is then placed in a suspension for fine polishing until no scratches are visible. Microstructure observation is performed using a polarizing microscope. EBSD samples were ion polished using a Leica ion polisher after mechanical polishing to remove surface stress marks, and then characterized using a SU8230 scanning electron microscope.

[0041] The above are non-limiting embodiments of the Sn-based twinned solder and its preparation method according to the present invention. The described embodiments are only some embodiments of the present invention and do not include all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Table 1 shows the composition, cooling method, and subsequent use method of the Sn-based lead-free alloy solder.

[0042] Table 1. Data Comparison of Three Specific Examples In the description of this invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0043] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A controllable preparation method for Sn-based twinned solder joints, characterized in that, Includes the following steps: Prepare Sn-based lead-free solder paste, which contains 2, 5, 10, or 13% indium by mass. Sn-based lead-free solder paste is applied to Cu pads to form a sample to be soldered, or Sn-based lead-free solder paste is screen-printed into individual solder balls; The sample is sent into the heating zone of the reflow oven and heated and soldered according to the preset reflow temperature profile, so that the Sn-based lead-free solder paste melts to form solder joints. The completed solder joint is sent into the cooling zone (10) of the reflow soldering furnace at 10 2 -10 4 K / s cooling rate for rapid cooling, inducing the formation of twin crystal structure inside the solder joint.

2. The controllable preparation method of Sn-based twin solder joints according to claim 1, characterized in that: In the step of preparing Sn-based lead-free solder paste, the Sn-based lead-free solder paste is SAC305 series solder paste or its derivative.

3. The controllable preparation method of Sn-based twin solder joints according to claim 1, characterized in that: In the step of sending the sample into the heating zone of the reflow oven, the preset reflow temperature profile includes a peak temperature of 250-270℃ and a holding time of 45-90 seconds.

4. The controllable preparation method of Sn-based twin solder joints according to claim 1, characterized in that: In the step of sending the welded joints into the cooling zone (10) of the reflow oven, rapid cooling is achieved by introducing a cooling medium into the cooling zone (10).

5. The controllable preparation method of Sn-based twin solder joints according to claim 4, characterized in that: The cooling medium is selected from one or more of water, liquid nitrogen, or dry ice.

6. The controllable preparation method of Sn-based twin solder joints according to claim 1, characterized in that: In the step of sending the completed weld joints into the cooling zone (10) of the reflow oven, the cooling rate is dynamically adjusted.

7. The controllable preparation method of Sn-based twin solder joints according to claim 6, characterized in that: The reference factors for adjusting the cooling rate include the indium content in the Sn-based lead-free solder paste and the size of the solder joint; the indium content is positively correlated with the cooling rate; the solder joint size is inversely correlated with the cooling rate.

8. The controllable preparation method of Sn-based twin solder joints according to claim 1, characterized in that: The heating zone includes a first heating zone (4), a second heating zone (5), a third heating zone (6), a fourth heating zone (7), a fifth heating zone (8), and a sixth heating zone (9) that are heated in a gradient towards the cooling zone (10). The solder joint enters the cooling zone (10) after passing through the sixth heating zone (9).

9. The controllable preparation method of Sn-based twin solder joints according to claim 1, characterized in that: The cooling zone (10) of the reflow oven is equipped with a cooling pipe (13) and a cooling medium storage tank (15). The cooling pipe (13) is used to realize the circulation of cooling medium in and out, and the cooling medium storage tank (15) is used to store and switch different types of cooling medium.

10. The controllable preparation method of Sn-based twin solder joints according to claim 1, characterized in that: The cooling zone (10) of the reflow oven is equipped with a temperature sensing device (14) and a control system (3). The temperature sensing device (14) is used to monitor the temperature of the solder joint in real time, and the control system (3) dynamically adjusts the flow rate of the cooling medium according to the deviation between the monitored temperature and the preset cooling rate.

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