Method for preparing high-reliability low-temperature solderable core-shell composite solder

Abstract

The application discloses a preparation method of a high-reliability core-shell composite solder capable of low-temperature welding, and belongs to the technical field of welding materials of electronic components. The core-shell composite solder is made of Sn-based solder powder by means of rolling plating, and a series of Sn-Bi low-temperature alloy plating layers are plated on the surface of the solder powder. The core-shell composite solder powder is produced in batches by changing the type and size of the commonly used Sn-based solder in the core to plate a series of Sn-x Bi low-temperature solder alloy plating layers on the shell, so as to produce core-shell composite solder powders of different sizes and types and composite BGA solder balls. The composite solder powder is suitable for the preparation of solder paste capable of low-temperature reflow and high-temperature service, and the composite BGA solder ball can be directly used for solder balls capable of low-temperature reflow and high-temperature service, so that the types and usability of the composite solder at the present stage are greatly improved, and the welding reliability is improved. Compared with the alloy solder prepared by the traditional melting and casting method, the method has the advantages of simple process, low cost and easy operation.

Description

Technical Field

[0001] This invention belongs to the technical field of soldering materials for electronic components, and relates to a method for preparing a high-reliability, multi-scale (20nm~1000μm) core-shell composite solder powder (Sn-based solder (20nm~100μm)@Sn-xBi, x: 21%-57%, mass fraction) and a standard-diameter BGA composite solder ball (Sn-based solder (100μm~1000μm)@Sn-xBi, x: 21%-57%, mass fraction) that can be soldered at low temperatures. This invention is the first to apply this preparation method to achieve the goal of meeting the requirements of low-temperature reflow (140℃~200℃) high-temperature service. Background Technology

[0002] As electronic products evolve towards being "lighter, thinner, shorter, smaller, and more multifunctional," advanced packaging technologies are developing towards miniaturization, high density, and finer pitch. Consequently, issues such as insufficient conductivity and heat dissipation of solder balls, and susceptibility to packaging defects, are becoming increasingly prominent. A good solution is to use core-shell composite solders with a core made of a high-strength, high-conductivity, and high-melting-point material. During reflow soldering, the core, due to its high melting point, will not be molten, thus ensuring a larger flip-chip clearance. This facilitates capillary effects and direct die-casting bottom filling processes, minimizing void formation. The reflow solderability of solder balls requires selective surface treatment of the leadframe to prevent solder overflow or excessive extension, which could lead to gap collapse. When using core-shell solder balls, selective surface treatment of the leadframe to prevent excessive extension is unnecessary because the amount and composition of solder in the shell are limited and controllable. Furthermore, if the shell of the core-shell solder is a near-eutectic Sn-Bi low-temperature lead-free alloy, this structure can not only avoid the large thermal damage, heat consumption, heat dissipation difficulties, and warping caused by stress due to the mismatch of thermal expansion coefficients during reflow, but also take into account the good elastic modulus and wetting properties of traditional Sn-Pb solder. Moreover, it is green and non-toxic, which not only meets the current soldering needs of some heat-sensitive components and temperature-sensitive components, but also is harmless to the environment.

[0003] Therefore, it is necessary to develop a composite solder that can balance low-temperature reflow and mechanical properties. One study used jet fracture method (Dai R, Zhang JF, Zhang SG, et al. Liquid immiscibility and core-shell morphology formation in ternary Al-Bi-Sn alloys[J]. Materials Characterization, 2013, 81(Complete):49-55.) to prepare Sn-Bi core-shell composite solders. The main process flow of this method is as follows: weighing the metal according to the required proportion - melting it at high temperature in a vacuum induction furnace (argon protection required) - thermocouple correction of the material emissivity - blowing the molten metal into a glycerol furnace through the internal and external pressure difference - dispersing the liquid into droplets during the descent - removing the alloy particles after natural cooling. This method is complex, requires a vacuum and argon protection environment throughout, and the thickness and shape of the outer shell structure are difficult to control. Furthermore, it cannot prepare a very small (nanoscale) and uniform core-shell structure.

[0004] Based on this, a method of preparing core-shell composite solder powder suitable for low-temperature soldering and high-temperature service using barrel plating and a highly stable Sn-Bi alloy plating solution is proposed. The prepared core-shell composite solder powder and BGA composite solder balls can be mass-produced with uniform shape and controllable thickness. Furthermore, core-shell composite solder powder with small core size (nanoscale) can be prepared. Summary of the Invention

[0005] To achieve the above objectives, this invention provides a core-shell composite solder powder suitable for low-temperature welding and high-temperature service. It is a Sn-Bi composite solder powder with a melting point of 140–200°C and high reliability, enabling low-temperature welding and high-temperature service. This invention also provides a method for preparing this composite solder. Furthermore, this invention can also prepare core-shell standard-diameter BGA (100μm–1000μm) composite solder balls capable of low-temperature welding and high-temperature service.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a high-reliability core-shell composite solder that can be soldered at low temperatures, comprising the following steps:

[0008] Step 1: Ultrasonic cleaning of Sn-based solder balls to remove surface contaminants, impurities and other deposits. The size of the Sn-based solder balls is between 20nm and 100μm.

[0009] Step 2: Place the Sn-based solder balls obtained in Step 1 into the Sn-Bi alloy plating solution and ultrasonically clean for 5 to 30 minutes.

[0010] Step 3: Place the Sn-based solder balls and Sn-Bi alloy plating solution obtained in Step 2 into a barrel plating equipment for electroplating to obtain a Sn-based solder@Sn-Bi core-shell composite solder.

[0011] Step 4: The Sn-based solder @Sn-Bi core-shell composite solder obtained in Step 3 is removed by vacuum filtration, washed with deionized water, and dried.

[0012] Furthermore, in step 1, the Sn-based solder ball is one of the following:

[0013] ① Pure Sn solder; ② Sn-x Cu solder, where x is 0.1-3% Cu by mass; ③ Sn-y Ag solder, where y is 0.5-5% Ag by mass; ④ Sn-a Ag-b Cu solder, where a is 0.1-3% Ag by mass and b is 0.5-5% Cu by mass; ⑤ Sn-z Zn solder, where z is 0.1-12% Zn by mass.

[0014] Furthermore, in steps 2 and 3, the divalent tin ion metal main salt in the Sn-Bi alloy plating solution uses gelatin as the main complexing agent, the trivalent bismuth ion metal main salt uses disodium ethylenediaminetetraacetate as the complexing agent, and the plating solution additive uses catechol.

[0015] The Sn-Bi alloy plating solution is carried out in a methanesulfonate system, and the main components and proportions are as follows:

[0016] Sn 2+ The molar ratio of gelatin is 1:259 to 900;

[0017] Bi 3+ The molar ratio of EDTA to 2Na is 0.2 to 5:1.

[0018] The concentration of catechol in the Sn-Bi alloy plating bath is 0.5–2 g·L⁻¹. -1 ;

[0019] Deionized water;

[0020] The pH value of the Sn-Bi alloy plating solution is 1 to 4.

[0021] Furthermore, the Sn-Bi alloy plating solution also includes polyethylene glycol as an auxiliary complexing agent for the trivalent bismuth ion metal main salt, Bi 3+ The molar ratio of polyethylene glycol is 0.01 to 30:1.

[0022] Furthermore, in step 4, the barrel plating process parameters are as follows: the plating is carried out in a barrel plating equipment at room temperature, the plating time is 600–1800 s, and the current density is 1–4 A / dm³. 2 The process employs unidirectional square wave pulsed DC electroplating with a duty cycle of 10%–50% and a frequency of 100–500 Hz. The anode is a platinum sheet with a purity greater than 99%, the cathode is Sn-based brazing filler metal, and the barrel plating speed is 0.5–10 rpm.

[0023] Furthermore, according to the preparation method, the characteristic is that, in step 1, replacing the solder with Sn-based solder of 100μm to 1000μm can prepare high-reliability standard-diameter BGA composite solder balls.

[0024] A high-reliability composite solder that can be soldered at low temperatures is prepared by the above-mentioned method. It is made by uniformly depositing a layer of near-eutectic Sn-Bi-based low-temperature alloy on the surface of Sn-based solder through a roll plating process.

[0025] The application of a high-reliability composite solder that can be soldered at low temperatures, as a soldering material in the field of electronic component technology.

[0026] The beneficial effects of this invention are as follows: The Sn-based solder@near-eutectic Sn-Bi composite solder provided by this invention can prepare core-shell composite solders of various shapes, sizes, thicknesses, and compositions by adjusting the composition and size of the core Sn-based solder and the composition and thickness of the shell near-eutectic series low-temperature Sn-Bi alloy. The prepared composite solder has the advantage of a high core melting point: because the melting point is higher than that of solder, the core can remain solid during reflow soldering, which can reduce the voids generated during the bottom filling process of die casting. The core-shell composite solder powder and standard-diameter BGA composite solder balls prepared by this method can achieve the purpose of low-temperature reflow and high-temperature service, and are suitable for the preparation of solder paste and the soldering of temperature-sensitive devices, and are suitable for industrial mass production. Detailed Implementation

[0027] The specific embodiments of the present invention will be further described below in conjunction with the technical solution.

[0028] It should be noted that the solders used in this invention are all commercially available solders.

[0029] Example 1:

[0030] A method for preparing a high-reliability core-shell composite solder that can be soldered at low temperatures includes the following steps:

[0031] Step 1: Place the pure Sn solder with a size of 20nm in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0032] Step 2, according to the molar ratio of stannous methanesulfonate to gelatin of 1:500, bismuth methanesulfonate... 和 The molar ratio of EDTA-2Na is 1:5, the molar ratio of bismuth methanesulfonate to polyethylene glycol is 100:1, and the concentration of catechol is 0.5 g·L⁻¹. -1 Adjust the pH of the solution to 3 to prepare the Sn-Bi alloy plating solution. Place the pure Sn solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 10 minutes.

[0033] Step 3: Place the pure Sn solder obtained in Step 2 into a new Sn-Bi alloy plating bath at room temperature, using 4A / dm 2 The current density was 100 Hz, a unidirectional square wave DC pulse current with a duty cycle of 10% was applied, a platinum sheet with a purity greater than 99% was used as the anode, the rotation speed was 0.5 rpm, and the barrel plating time was 10 min.

[0034] Step 4: Remove the core-shell composite brazing filler metal obtained in Step 3 by vacuum filtration, and dry it to obtain a core-shell composite brazing filler metal powder with a pure Sn core and a Sn75Bi25 shell, suitable for low-temperature welding and high-temperature service.

[0035] Example 2:

[0036] A method for preparing a high-reliability composite solder that can be soldered at low temperatures includes the following steps:

[0037] Step 1: Place the 100μm SAC305 solder in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0038] Step 2: Add catechol at a molar ratio of stannous methanesulfonate to gelatin of 1:700, bismuth methanesulfonate to EDTA-2Na of 5:1, and bismuth methanesulfonate to polyethylene glycol of 30:1, with a concentration of 2 g / L. -1 Adjust the pH of the solution to 4 to prepare the Sn-Bi alloy plating solution. Place the SAC305 solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 30 minutes.

[0039] Step 3: Place the SAC305 solder after step 2 into a new Sn-Bi alloy plating bath at room temperature, using 2A / dm². 2 The current density was 200 Hz, a unidirectional square wave pulse current with a 30% duty cycle was applied, a platinum sheet with a purity greater than 99% was used as the anode, the rotation speed was 5 rpm, and the barrel plating time was 30 min.

[0040] Step 4: Remove the core-shell composite brazing alloy obtained in Step 3 by vacuum filtration, and dry it to obtain a core-shell composite brazing alloy powder with a core of SAC305 and a shell of Sn42.26Bi57.74, suitable for low-temperature welding and high-temperature service.

[0041] Example 3:

[0042] A method for preparing a high-reliability composite solder that can be soldered at low temperatures includes the following steps:

[0043] Step 1: Place the 50μm Sn0.7Cu solder in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0044] Step 2: The molar ratio of stannous methanesulfonate to gelatin is 1:300, the molar ratio of bismuth methanesulfonate to EDTA-2Na is 3:1, the molar ratio of bismuth methanesulfonate to polyethylene glycol is 10:1, and the concentration of the additive catechol is 1 g·L⁻¹. -1 Adjust the pH of the solution to 2 to prepare the Sn-Bi alloy plating solution. Place the Sn0.7Cu solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 20 minutes.

[0045] Step 3: Place the Sn0.7Cu solder from Step 2 into a new Sn-Bi alloy plating bath at room temperature, 4 A / dm 2 The current density was 300 Hz, a unidirectional square wave pulse current with a duty cycle of 20% was applied, a tin sheet with a purity greater than 99% was used as the anode, the rotation speed was 10 rpm, and the barrel plating time was 15 min.

[0046] Step 4: The core-shell composite brazing filler metal obtained in Step 3 is removed by vacuum filtration and dried to obtain a core-shell composite brazing filler metal powder with a core of Sn0.7Cu and a shell of Sn60Bi40, suitable for low-temperature welding and high-temperature service.

[0047] Example 4:

[0048] A method for preparing a high-reliability composite solder that can be soldered at low temperatures includes the following steps:

[0049] Step 1: Place the Sn3.5Ag solder with a thickness of 80μm in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0050] Step 2: The molar ratio of tin methanesulfonate to gelatin is 1:688, the molar ratio of methanesulfonic acid to EDTA-2Na is 2:1, the molar ratio of bismuth methanesulfonate to polyethylene glycol is 5:1, and the concentration of the additive catechol is 1 g·L⁻¹. -1Adjust the pH of the solution to 2 to prepare the Sn-Bi alloy plating solution. Place the Sn3.5Ag solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 15 minutes.

[0051] Step 3: Place the Sn3.5Ag solder from Step 2 into a new Sn-Bi alloy plating bath at room temperature, 3A / dm 2 The current density was 500 Hz, a unidirectional square wave pulse current with a duty cycle of 40% was applied, a platinum sheet with a purity greater than 99% was used as the anode, the rotation speed was 1 rpm, and the barrel plating time was 15 min.

[0052] Step 4: The core-shell composite brazing filler metal obtained in Step 3 is removed by vacuum filtration and dried to obtain a core-shell composite brazing filler metal powder with a core of Sn3.5Ag and a shell of Sn70Bi30, suitable for low-temperature welding and high-temperature service.

[0053] Example 5:

[0054] A method for preparing a high-reliability composite solder that can be soldered at low temperatures includes the following steps:

[0055] Step 1: Place the Sn9Zn solder with a thickness of 80μm in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0056] Step 2: The molar ratio of tin methanesulfonate to gelatin is 1:650, the molar ratio of bismuth methanesulfonate to EDTA-2Na is 4:1, the molar ratio of bismuth methanesulfonate to polyethylene glycol is 1:1, and the concentration of the additive catechol is 1 g·L⁻¹. -1 Adjust the pH of the solution to 1 to prepare the Sn-Bi alloy plating solution. Place the Sn9Zn solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 15 minutes.

[0057] Step 3: Place the Sn9Zn solder from Step 2 into a new Sn-Bi alloy plating bath at room temperature, 3A / dm 2 The current density is 100 Hz, a unidirectional square wave pulse current with a duty cycle of 10% is applied, a platinum sheet with a purity greater than 99% is used as the anode, the rotation speed is 1 rpm, and the barrel plating time is 20 minutes.

[0058] Step 4: The core-shell composite brazing alloy obtained in Step 3 is removed by vacuum filtration and dried to obtain a core-shell composite brazing alloy powder with a core of Sn9Zn and a shell of Sn49.1Bi51.9, suitable for low-temperature welding and high-temperature service.

[0059] Example 6:

[0060] A method for preparing a high-reliability core-shell composite solder that can be soldered at low temperatures includes the following steps:

[0061] Step 1: Place the pure Sn solder with a size of 150μm in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0062] Step 2: The molar ratio of stannous methanesulfonate to gelatin is 1:500, the molar ratio of bismuth methanesulfonate to EDTA-2Na is 1:5, the molar ratio of bismuth methanesulfonate to polyethylene glycol is 100:1, and the concentration of the additive catechol is 0.5 g·L⁻¹. -1 Adjust the pH of the solution to 3 to prepare the Sn-Bi alloy plating solution. Place the pure Sn solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 10 minutes.

[0063] Step 3: Place the pure Sn solder obtained in Step 2 into a new Sn-Bi alloy plating bath at room temperature, using 4A / dm 2 The current density was 100 Hz, a unidirectional square wave DC pulse current with a duty cycle of 10% was applied, a platinum sheet with a purity greater than 99% was used as the anode, the rotation speed was 0.5 rpm, and the barrel plating time was 10 min.

[0064] Step 4: Remove the core-shell composite solder obtained in Step 3 by vacuum filtration, and dry it to obtain a core-shell composite BGA solder ball with a pure Sn core and a Sn75Bi25 shell, suitable for low-temperature soldering and high-temperature service.

[0065] Example 7:

[0066] A method for preparing a high-reliability composite solder that can be soldered at low temperatures includes the following steps:

[0067] Step 1: Place the 300μm SAC305 solder in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0068] Step 2: The molar ratio of stannous methanesulfonate to gelatin is 1:700, the molar ratio of bismuth methanesulfonate to EDTA-2Na is 5:1, the molar ratio of bismuth methanesulfonate to polyethylene glycol is 30:1, and the concentration of the additive catechol is 2 g·L⁻¹. -1 Adjust the pH of the solution to 4 to prepare the Sn-Bi alloy plating solution. Place the SAC305 solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 30 minutes.

[0069] Step 3: Place the SAC305 solder after step 2 into a new Sn-Bi alloy plating bath at room temperature, using 2A / dm². 2The current density was 200 Hz, a unidirectional square wave pulse current with a 30% duty cycle was applied, a platinum sheet with a purity greater than 99% was used as the anode, the rotation speed was 5 rpm, and the barrel plating time was 30 min.

[0070] Step 4: Remove the core-shell composite solder obtained in Step 3 by vacuum filtration, and dry it to obtain a core-shell composite BGA solder ball with a core of SAC305 and a shell of Sn42.26Bi57.74, suitable for low-temperature soldering and high-temperature service.

[0071] Example 8:

[0072] A method for preparing a high-reliability composite solder that can be soldered at low temperatures includes the following steps:

[0073] Step 1: Place the 500μm Sn0.7Cu solder in an ultrasonic cleaner to remove surface contaminants, impurities and other attachments, and dry it for later use.

[0074] Step 2: The molar ratio of stannous methanesulfonate to gelatin is 1:300, the molar ratio of bismuth methanesulfonate to EDTA-2Na is 3:1, the molar ratio of bismuth methanesulfonate to polyethylene glycol is 10:1, and the concentration of the additive catechol is 1 g·L⁻¹. -1 Adjust the pH of the solution to 2 to prepare the Sn-Bi alloy plating solution. Place the Sn0.7Cu solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 20 minutes.

[0075] Step 3: Place the Sn0.7Cu solder from Step 2 into a new Sn-Bi alloy plating bath at room temperature, 4 A / dm 2 The current density was 300 Hz, a unidirectional square wave pulse current with a duty cycle of 20% was applied, a tin sheet with a purity greater than 99% was used as the anode, the rotation speed was 10 rpm, and the barrel plating time was 15 min.

[0076] Step 4: Remove the core-shell composite solder obtained in Step 3 by vacuum filtration, and dry it to obtain a core-shell composite BGA solder ball with a Sn0.7Cu core and a Sn60Bi40 shell, suitable for low-temperature soldering and high-temperature service.

[0077] Example 9:

[0078] A method for preparing a high-reliability composite solder that can be soldered at low temperatures includes the following steps:

[0079] Step 1: Place the Sn3.5Ag solder with a thickness of 700μm in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0080] Step 2: The molar ratio of tin methanesulfonate to gelatin is 1:688, the molar ratio of bismuth methanesulfonate to EDTA-2Na is 2:1, the molar ratio of bismuth methanesulfonate to polyethylene glycol is 5:1, and the concentration of the additive catechol is 1 g·L⁻¹. -1 Adjust the pH of the solution to 2 to prepare the Sn-Bi alloy plating solution. Place the Sn3.5Ag solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 15 minutes.

[0081] Step 3: Place the Sn3.5Ag solder from Step 2 into a new Sn-Bi alloy plating bath at room temperature, 3A / dm 2 The current density was 500 Hz, a unidirectional square wave pulse current with a duty cycle of 40% was applied, a platinum sheet with a purity greater than 99% was used as the anode, the rotation speed was 1 rpm, and the barrel plating time was 15 min.

[0082] Step 4: Remove the core-shell composite solder obtained in Step 3 by vacuum filtration, and dry it to obtain a core-shell composite BGA solder ball with a Sn3.5Ag core and a Sn70Bi30 shell, suitable for low-temperature soldering and high-temperature service.

[0083] Example 10:

[0084] A method for preparing a high-reliability composite solder that can be soldered at low temperatures includes the following steps:

[0085] Step 1: Place the Sn9Zn solder with a thickness of 1000μm in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0086] Step 2: The molar ratio of tin methanesulfonate to gelatin is 1:650, the molar ratio of bismuth methanesulfonate to EDTA-2Na is 4:1, the molar ratio of bismuth methanesulfonate to polyethylene glycol is 1:1, and the concentration of the additive catechol is 1 g·L⁻¹. -1 Adjust the pH of the solution to 1 to prepare the Sn-Bi alloy plating solution. Place the Sn9Zn solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 15 minutes.

[0087] Step 3: Place the Sn9Zn solder from Step 2 into a new Sn-Bi alloy plating bath at room temperature, 3A / dm 2 The current density is 100 Hz, a unidirectional square wave pulse current with a duty cycle of 10% is applied, a platinum sheet with a purity greater than 99% is used as the anode, the rotation speed is 1 rpm, and the barrel plating time is 20 minutes.

[0088] Step 4: Remove the core-shell composite solder obtained in Step 3 by vacuum filtration, and dry it to obtain a core-shell composite BGA solder ball with a Sn9Zn core and a Sn49.1Bi51.9 shell, suitable for low-temperature soldering and high-temperature service.

[0089] Comparative Example 1:

[0090] Step 1: Place the 100μm SAC305 solder in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0091] Step 2: The molar ratio of stannous methanesulfonate to gelatin is 1:700, the molar ratio of bismuth methanesulfonate to EDTA-2Na is 5:1, and the concentration of the additive catechol is 2 g·L⁻¹. -1 Adjust the pH of the solution to 4 to prepare the Sn-Bi alloy plating solution. Place the SAC305 solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 30 minutes.

[0092] Step 3: Place the SAC305 solder after step 2 into a new Sn-Bi alloy plating bath at room temperature, using 2A / dm². 2 The current density was 200 Hz, a unidirectional square wave pulse current with a 30% duty cycle was applied, a platinum sheet with a purity greater than 99% was used as the anode, the rotation speed was 5 rpm, and the barrel plating time was 30 min.

[0093] Step 4: Remove the core-shell composite brazing filler metal obtained in Step 3 by vacuum filtration, and dry it to obtain a core-shell composite brazing filler metal powder with a core of SAC305 and a shell of Sn42.13Bi57.87, suitable for low-temperature welding and high-temperature service.

[0094] This comparative example is the same as Example 2, intended to illustrate that the polyethylene glycol in the provided electroplating solution formulation is Bi. 3+ The auxiliary complexing agent can be omitted, and it has little impact on the morphology and composition of the Sn-Bi shell coating, further demonstrating that the provided plating solution formula is simple and highly stable.

[0095] Comparative Example 2:

[0096] Step 1: Place the 100μm SAC305 solder in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0097] Step 2: The molar ratio of stannous methanesulfonate to gelatin is 1:200, the molar ratio of bismuth methanesulfonate to EDTA-2Na is 6:1, the molar ratio of bismuth methanesulfonate to polyethylene glycol is 40:1, and the concentration of the additive catechol is 0.1 g·L⁻¹. -1Adjust the pH of the solution to 4 to prepare the Sn-Bi alloy plating solution. Place the SAC305 solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 30 minutes.

[0098] Step 3: Place the SAC305 solder after step 2 into a new Sn-Bi alloy plating bath at room temperature, using 2A / dm². 2 The current density was 200 Hz, a unidirectional square wave pulse current with a 30% duty cycle was applied, a platinum sheet with a purity greater than 99% was used as the anode, the rotation speed was 5 rpm, and the barrel plating time was 30 min.

[0099] Step 4: The core-shell composite solder obtained in Step 3 is removed by vacuum filtration and dried to obtain a core-shell composite solder powder with a core of SAC305 and a shell of Sn11Bi89.

[0100] This comparative example is the same as Example 2 above, and is intended to illustrate the use of Sn as the primary salt in the provided electroplating solution formulation. 2+ If the range of the corresponding complexing agent gelatin molar ratio is not specified, it is impossible to obtain a Sn-Bi shell alloy coating with a Bi content of 21-58 wt% by barrel plating.

[0101] Comparative Example 3:

[0102] Step 1: Place the 100μm SAC305 solder in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0103] Step 2: The molar ratio of stannous methanesulfonate to gelatin is 1:1000, the molar ratio of bismuth methanesulfonate to EDTA-2Na is 0.1:1, the molar ratio of bismuth methanesulfonate to polyethylene glycol is 0.001:1, and the concentration of hydroquinone additive is 3 g·L⁻¹. -1 Adjust the pH of the solution to 4 to prepare the Sn-Bi alloy plating solution. Place the SAC305 solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 30 minutes.

[0104] Step 3: Place the SAC305 solder after step 2 into a new Sn-Bi alloy plating bath at room temperature, using 2A / dm². 2 The current density was 200 Hz, a unidirectional square wave pulse current with a 30% duty cycle was applied, a platinum sheet with a purity greater than 99% was used as the anode, the rotation speed was 5 rpm, and the barrel plating time was 30 min.

[0105] Step 4: The core-shell composite solder obtained in Step 3 is removed by vacuum filtration and dried to obtain a core-shell composite solder powder with a core of SAC305 and a shell of Sn3Bi97.

[0106] This comparative example is the comparative example of Example 2 above, and is intended to illustrate the effect of exceeding the amount of the main salt Bi in the provided electroplating solution formulation. 3+ If the molar ratio of the corresponding complexing agent EDTA-2Na is within a certain range, it is impossible to obtain a Sn-Bi shell alloy coating with a Bi content of 21-58 wt% by barrel plating.

[0107] Comparative Example 4:

[0108] Step 1: Place the 100μm SAC305 solder in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0109] Step 2: The molar ratio of stannous methanesulfonate to gelatin is 1:700, the molar ratio of bismuth methanesulfonate to EDTA is 5:1, the molar ratio of bismuth methanesulfonate to polyethylene glycol is 30:1, and the concentration of the additive catechol is 2 g·L⁻¹. -1 Adjust the pH of the solution to 4 to prepare the Sn-Bi alloy plating solution. Place the SAC305 solder from step 1 into the prepared Sn-Bi alloy plating solution and ultrasonically clean for 30 minutes.

[0110] Step 3: Place the SAC305 solder after step 2 into a new Sn-Bi alloy plating bath at room temperature, using 2A / dm². 2 The current density was 200 Hz, a unidirectional square wave pulse current with a 30% duty cycle was applied, a platinum sheet with a purity greater than 99% was used as the anode, the rotation speed was 5 rpm, and the barrel plating time was 30 min.

[0111] Step 4: The core-shell composite solder obtained in Step 3 is removed by vacuum filtration and dried to obtain a core-shell composite solder powder with a core of SAC305 and a shell of Sn95Bi5.

[0112] This comparative example is the comparative example of Example 2 above, and is intended to illustrate if Bi... 3+ If the complexing agent of the salt is replaced with EDTA, which is similar to EDTA-2Na, the complexing effect cannot be good, and it is impossible to obtain a Sn-Bi shell alloy coating with a Bi content of 21-58 wt% by barrel plating.

[0113] Comparative Example 5:

[0114] Step 1: Place the 100μm SAC305 solder in an ultrasonic cleaner to remove surface contaminants, impurities and other adhering substances, and dry it for later use.

[0115] Step 2: Place the cleaned SAC305 solder into a Sn-Bi alloy plating bath. The plating bath is prepared according to the following molar ratios: stannous methanesulfonate to gelatin 1:700, bismuth methanesulfonate to EDTA-2Na 5:1, bismuth methanesulfonate to polyethylene glycol 30:1, and catechol additive concentration 2 g / L. -1 The solution was prepared by adjusting the pH to 4. At room temperature, 2 A / dm³ was used. 2 The current density was 200 Hz, a unidirectional square wave pulse current with a 30% duty cycle was applied, a platinum sheet with a purity greater than 99% was used as the anode, the rotation speed was 5 rpm, and the barrel plating time was 30 min.

[0116] Step 3: The core-shell composite brazing filler metal obtained in Step 3 is removed by vacuum filtration and dried to obtain a large amount of SAC305 clustered together, with a Sn42.26Bi57.74 outer shell and uneven plating, forming a blocky composite brazing filler metal.

[0117] This comparative example is a comparative example of Example 2 above, intended to highlight the importance of step 2 in the provided technical steps. Step 2 disperses the small-sized Sn-based solder powder in the core, acting as a dispersant. Without step 2, it is impossible to obtain uniform individual Sn-based solder@Sn-Bi (21-58, wt%) composite solder powder through barrel plating. Similarly, replacing the Sn-based solder size in step 1 with micro-solder balls (100μm-1000μm) will also prevent the acquisition of uniform individual Sn-based solder@Sn-Bi (21-58, wt%) composite BGA solder balls.

Claims

1. A method for preparing a high-reliability core-shell composite brazing filler metal capable of low-temperature soldering, characterized in that, The steps are as follows: Step 1: Ultrasonic cleaning of Sn-based solder balls to remove surface deposits. The size of the Sn-based solder balls is between 20 nm and 100 μm. The Sn-based solder balls are one of the following: ① Pure Sn solder; ② Sn-x Cu solder, where x is the mass percentage of Cu of 0.1-3%; ③ Sn-y Ag solder, where y is the mass percentage of Ag of 0.5-5%; ④ Sn-a Ag-b Cu solder, where a is the mass percentage of Ag of 0.1-3% and b is the mass percentage of Cu of 0.5-5%; ⑤ Sn-z Zn solder, where z is the mass percentage of Zn of 0.1-12%. Step 2: Place the Sn-based solder balls obtained in Step 1 into the Sn-Bi alloy plating solution and ultrasonically clean for 5 to 30 minutes. In the Sn-Bi alloy plating solution, gelatin is used as the main complexing agent for divalent tin ion metal salt, disodium ethylenediaminetetraacetate is used as the complexing agent for trivalent bismuth ion metal salt, and catechol is used as the plating solution additive. The Sn-Bi alloy plating solution is carried out in a methanesulfonate system, and the main components and proportions are as follows: Sn 2+ The molar ratio of gelatin is 1:259~900; Bi 3+ The molar ratio of EDTA to 2Na is 0.2 to 5 to 1. The concentration of catechol in Sn-Bi alloy plating bath is 0.5 ~ 2 g·L. -1 ; Deionized water; The pH value of the Sn-Bi alloy plating solution is 1 ~ 4; Step 3: Place the Sn-based solder balls and Sn-Bi alloy plating solution obtained in Step 2 into a barrel plating equipment for electroplating to obtain a Sn-based solder@Sn-Bi core-shell structure composite solder. Step 4: The Sn-based solder @Sn-Bi core-shell composite solder obtained in Step 3 is removed by vacuum filtration, washed with deionized water, and dried to obtain high-reliability core-shell composite solder powder. The barrel plating process parameters are as follows: The plating is carried out in a barrel plating equipment at room temperature; the plating time is 600 ~ 1800 s; and the current density is 1 ~ 4 A / dm³. 2 The process employs unidirectional square wave pulsed DC electroplating with a duty cycle of 10% to 50% and a frequency of 100 to 500 Hz. The anode is a platinum sheet with a purity greater than 99%, the cathode is Sn-based brazing filler metal, and the barrel plating speed is 0.5 to 10 rpm.

2. The preparation method according to claim 1, characterized in that, The Sn-Bi alloy plating solution also includes polyethylene glycol and Bi. 3+ The molar ratio of polyethylene glycol is 0.01 ~ 30 :

1.

3. The preparation method according to claim 1, characterized in that, In step 1, replacing the solder with Sn-based solder of 100 μm ~ 1000 μm can produce high-reliability standard-diameter BGA composite solder balls.