A lead-free solder powder for reflow soldering and a method for producing the same

Abstract

This invention belongs to the field of electronic packaging and soldering materials technology, and relates to a lead-free solder powder for reflow soldering and its preparation method. This lead-free solder powder is based on a Sn-Ag-Cu alloy system, and achieves melt purification, β-Sn grain refinement, and interface IMC stabilization by adding trace amounts of Ce and Ni elements. Its composition, by mass percentage, is: Ag 2.9%–3.1%, Cu 0.45%–0.55%, Ce 0.01%–0.10%, Ni 0.01%–0.05%, with the balance being Sn and impurities. During preparation, a Ce-Ni master alloy is used to introduce microalloying elements, followed by vacuum melting, inert gas atomization, and surface treatment to obtain spherical solder powder. The resulting solder powder has low oxygen content, good wettability, slow interface IMC growth after reflow, high solder joint strength, and excellent thermal cycling reliability, making it suitable for high-reliability electronic packaging applications.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electronic packaging and soldering materials, and relates to a lead-free solder powder for reflow soldering and a preparation method thereof. BACKGROUND

[0002] With the continuous evolution of electronic packaging technology towards high density, miniaturization and lead-free, the reflow soldering process as the core link in the surface mount technology (SMT) puts unprecedentedly stringent requirements on the comprehensive performance of solder materials. Since the implementation of the RoHS directive, lead-free solder has gradually replaced the traditional tin-lead alloy to become mainstream. Among them, the Sn-Ag-Cu (SAC) series alloy, especially SAC305 (Sn-3.0Ag-0.5Cu), is widely used in consumer electronics, communication equipment and automotive electronics due to its relatively excellent mechanical properties, weldability and controllable cost. The alloy forms dispersed Ag3Sn and Cu6Sn5 intermetallic compounds (IMC) strengthening phases in the tin matrix through silver and copper, which to some extent makes up for the problem of decreased wettability and increased melting point caused by lead-free, and exhibits good process adaptability under initial reflow conditions.

[0003] However, with the increasingly complex service environment of electronic products, especially the widespread application of multiple reflow soldering processes (such as double-sided assembly, repair) and high temperature and humidity accelerated aging conditions, the traditional SAC series solder has exposed deep-seated internal defects in long-term reliability. At the interface between the solder and the copper pad, the Cu6Sn5 type IMC layer continues to grow during thermal cycling or high-temperature aging, not only the thickness increases significantly, but also the grain tends to coarsen, forming a continuous and brittle interface layer. Due to the significant difference in thermal expansion coefficient between the layer and the matrix solder and copper foil, the layer is extremely prone to induce micro-cracks under thermal stress, becoming the main origin of mechanical fatigue and even fracture failure of the solder joint. Further, since the melting point of lead-free solder is generally higher than that of tin-lead alloy (SAC305 liquidus about 217℃), the high-temperature reflow process intensifies the interface reaction kinetics, making the IMC overgrowth problem particularly prominent under multiple thermal shocks.

[0004] At the same time, high-temperature processes also pose a severe challenge to the wettability behavior of the solder. Compared with lead-containing solder, the spreading rate and final wetting angle of SAC alloy on copper substrate are both poor, which is not only due to its higher surface tension, but also closely related to the easy oxidation of the molten solder surface. During reflow, molten tin is prone to react with trace oxygen in the environment to form SnO xThe oxide film hinders effective metallurgical bonding of the solder with the pads, causes insufficient wetting, and further causes soldering defects such as a false solder, bridging, porosity, and the like, which seriously affect the electrical connection reliability and appearance consistency of the solder joint. Although the prior art often alleviates the problem by optimizing the activity of the flux or introducing an inert atmosphere, such external means are difficult to fundamentally improve the oxidation resistance of the solder body and the controllability of the interface reaction.

[0005] The above problem lies in that the conventional SAC alloy system lacks a mechanism for actively regulating the interface reaction kinetics and the purity of the melt in the composition design. On one hand, the intrinsic growth characteristics of Cu6Sn5 IMC determine that it is difficult to self-limit under the action of heat; on the other hand, the presence of residual oxygen and impurity elements in the melt not only deteriorates the wettability, but also promotes the precipitation of coarse IMC as a heterogeneous nucleation core. Therefore, it is difficult to take into account the initial soldering quality and long-term thermal mechanical reliability of the solder joint by simply relying on process parameter adjustment or flux improvement. Under this background, how to simultaneously realize the refinement and stabilization of the interface IMC, the improvement of the oxidation resistance of the melt, and the improvement of the wettability by precisely adjusting the alloy composition without significantly increasing the melting point or damaging the processability has become a key bottleneck restricting the development of high-reliability lead-free solder. SUMMARY

[0006] To achieve the above-mentioned purposes, the present application provides a lead-free solder powder for reflow soldering and a preparation method thereof. The lead-free solder powder is based on a Sn-Ag-Cu ternary alloy system, and a trace amount of rare earth element cerium (Ce) and transition metal nickel (Ni) are introduced for composite micro-alloying to construct a new lead-free solder system with interface IMC stabilization, high purity of the melt, and excellent wettability, thereby simultaneously solving the key technical problems such as excessive growth of the interface intermetallic compound (IMC), solder joint embrittlement, and insufficient wettability of the conventional SAC solder under the conditions of multiple reflow and high-temperature aging without significantly increasing the liquidus temperature.

[0007] The chemical composition of the lead-free solder powder according to the present application is as follows in terms of mass percentage: Ag 2.9% to 3.1%, Cu 0.45% to 0.55%, Ce 0.01% to 0.10%, Ni 0.01% to 0.05%, and the balance of Sn and unavoidable impurity elements, wherein the total amount of the impurity elements is less than 0.05%.

[0008] Preferably, the composition of the solder powder is Ag 3.0%, Cu 0.5%, Ce 0.05%, Ni 0.03%, and the balance of Sn.

[0009] The Ce element is introduced in the form of an element or an intermediate alloy, which reacts with dissolved oxygen in the melt during smelting to generate high-melting-point and high-stability CeO2 or Ce2O3 oxide particles, the density of the oxide is greater than that of the molten tin matrix, and the oxide can be effectively settled at the bottom of the melt, thereby reducing the content of free oxygen in the melt and improving the purity of the melt.

[0010] The Ni element is completely solid-solved in the tin matrix during alloy smelting, and diffuses to the interface region between the solder and the copper pad during subsequent reflow soldering, partially replaces Cu atoms in Cu6Sn5 IMC, and forms (Cu, Ni)6Sn5 type composite intermetallic compound. The composite IMC has smaller lattice distortion energy and higher interface energy barrier, and its growth kinetics is inhibited. Under the reflow condition of 260 DEG C x 1 h, the thickness of the interface IMC layer is controlled to be 2.0-3.5 microns; after high-temperature aging at 150 DEG C x 500 h, the thickness of the IMC layer increases to 4.0-6.0 microns, and the grain size is maintained at 0.8-1.5 microns, without continuous coarsening or crack initiation. In contrast, the thickness of the IMC layer of the conventional SAC305 solder under the same conditions can reach more than 8.0 microns, the grain size is more than 3.0 microns, and obvious microcracks are accompanied.

[0011] Further, the synergistic effect of Ce and Ni is reflected in three aspects: first, after the Ce purifies the melt, the content of SnO x The formation of the film reduces the surface tension of the molten solder; second, the pure melt provides a more stable thermodynamic environment for the directional diffusion of Ni atoms to the interface, avoiding the interference of impurity elements on the diffusion path; third, the refined beta-Sn matrix and the uniformly dispersed (Cu, Ni)6Sn5 IMC together form a strong and tough microstructure, which improves the tensile strength and shear strength of the solder joint.

[0012] The application also provides a preparation method of the lead-free solder powder, comprising the following steps:

[0013] Step 1: raw material preparation. The high-purity metal raw materials are weighed according to the above-mentioned proportion, wherein the purity of Sn is greater than or equal to 99.99%, the purity of Ag is greater than or equal to 99.99%, the purity of Cu is greater than or equal to 99.95%, the purity of Ce is greater than or equal to 99.9%, and the purity of Ni is greater than or equal to 99.95%. All the raw materials are ultrasonically cleaned in acetone for 10 minutes before use to remove surface oil stains and oxide films, and then dried in a 60 DEG C vacuum drying box for 2 hours.

[0014] Step 2: Vacuum induction melting. Add Sn, Ag, and Cu to a graphite crucible in sequence and place it in a vacuum induction melting furnace. Evacuate to ≤5×10⁻⁶. -2 High-purity argon gas was introduced as a protective atmosphere to maintain the furnace pressure at 0.05 MPa. The temperature was increased to 300°C at a rate of 15°C / min and held for 10 minutes to allow Ag and Cu to completely dissolve in the molten Sn, forming a uniform Sn-Ag-Cu master alloy melt.

[0015] Step 3: Addition of microalloying elements. A Ce-Ni master alloy block (Ce:Ni mass ratio 5:3), preheated to 200℃, is added to the center region of the melt through the feeding port. The master alloy is prepared by melting Ce and Ni in a vacuum arc furnace, exhibiting uniform composition and no oxide inclusions. After addition, the melt temperature is raised to 320℃, and the melt is electromagnetically stirred at 300 rpm for 15 minutes to ensure that the Ce and Ni elements are fully dissolved and uniformly distributed.

[0016] Step 4: Slag Removal and Casting. After stopping stirring, let stand for 5 minutes to allow high-density oxide inclusions such as CeO2 to settle to the bottom of the crucible. Pour the clean upper layer of melt into a copper mold preheated to 200°C, and cool to room temperature to obtain a cylindrical ingot with a diameter of 50 mm and a height of 30 mm.

[0017] Step 5: Inert atmosphere atomization powder production. The ingot is crushed into fragments with a particle size ≤5mm and loaded into the melting chamber of a high-pressure inert gas atomization device. A vacuum of ≤1×10⁻⁶ is applied. -2 After passing through a pressure of 0.1 MPa, high-purity nitrogen is introduced to maintain the pressure at 0.1 MPa. Medium-frequency induction heating is used to reach 350°C to completely melt the fragments. Then, high-purity nitrogen at 4.0 MPa is injected through a nozzle to break the melt into micron-sized droplets. The droplets rapidly cool and solidify during their descent, collecting in a stainless steel powder container at the bottom.

[0018] Step 6: Sieving and Surface Treatment. The obtained solder powder is sieved through a standard sieve, and spherical particles with a particle size range of 20–38 μm are selected as the finished product. The solder powder is then placed in a sealed container, and a nitrogen mixture containing 0.5 vol% formic acid vapor is introduced. The mixture is treated at 60°C for 30 minutes to remove trace amounts of oxide film from the powder surface, improving its dispersibility and storage stability in solder paste. Immediately after treatment, the powder is encapsulated under nitrogen protection.

[0019] In a preferred embodiment of the present invention, the amount of Ce-Ni master alloy added is 0.06% to 0.15% based on the total mass of the final solder powder, to ensure that the concentrations of Ce and Ni in the melt fall within the target ranges of 0.01% to 0.10% and 0.01% to 0.05%, respectively. The melting point of the master alloy is 980°C, which is much higher than the solder melting temperature, thus preventing excessive volatilization of Ce at high temperatures. Actual measurements show that by using the master alloy addition method, the Ce element recovery rate can reach over 92%, and the Ni element recovery rate exceeds 98%.

[0020] In another preferred embodiment of the present invention, during the high-pressure inert gas atomization process, the nitrogen pressure is controlled at 3.5–4.5 MPa, the melt superheat is 30–50°C, and the nozzle orifice diameter is 0.8 mm. Under these process parameters, the resulting solder powder has a sphericity ≥0.92, an oxygen content ≤80 ppm, a loose packing density of 5.8–6.1 g / cm³, and a tap density of 6.3–6.6 g / cm³, meeting the performance requirements of Type 4 solder powder in the J-STD-006B standard.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] This invention constructs a synergistic strengthening mechanism integrating melt purification, grain refinement, and interface stabilization by precisely introducing Ce and Ni microalloying elements into a Sn-Ag-Cu matrix. This mechanism addresses the core issues faced by lead-free solders in high-reliability applications, such as uncontrolled interfacial IMC growth, insufficient wettability, and degradation of thermomechanical properties. The prepared solder powder possesses comprehensive advantages, including uniform composition, high sphericity, low oxygen content, excellent wettability, and strong long-term thermal stability, making it suitable for advanced electronic manufacturing fields with stringent requirements for solder joint reliability, such as high-density packaging, automotive electronics, and 5G communication modules. Detailed Implementation

[0023] This invention provides a lead-free solder powder for reflow soldering and its preparation method. The core of this method lies in introducing trace amounts of the rare earth element cerium and the transition metal nickel into a Sn-Ag-Cu ternary alloy system to construct a novel lead-free solder system possessing intermetallic compound stabilization, high melt purity, and excellent wetting properties. This technical solution effectively suppresses excessive thickening and coarsening of the intermetallic compound (IMC) at the solder joint interface during repeated reflow soldering and high-temperature service without significantly increasing the liquidus temperature. Simultaneously, it significantly improves the wettability and oxidation resistance of the solder, thereby comprehensively enhancing the initial soldering quality and long-term thermomechanical reliability of the solder joint. The technical solution of this invention will be described in detail below with reference to specific embodiments and comparative examples to ensure that those skilled in the art can fully understand and implement this invention.

[0024] All examples are based on the Sn-Ag-Cu ternary alloy system, with trace amounts of Ce and Ni added (in the following mass percentage ranges: Ag 2.9%–3.1%, Cu 0.45%–0.55%, Ce 0.01%–0.10%, Ni 0.01%–0.05%, with the balance being Sn). Microalloying elements were introduced using a Ce-Ni master alloy (mass ratio 5:3). The alloys were prepared through vacuum melting, inert gas atomization (nitrogen pressure 3.5–4.5 MPa, melt superheat 30–50 °C, nozzle orifice diameter 0.8 mm), and surface treatment (nitrogen treatment at 60 °C containing 0.5 vol% formic acid vapor for 30 minutes). Specific parameters and properties are as follows:

[0025]

[0026] Comparative Example 1 is a traditional Sn-Ag-Cu lead-free solder (free of Ce and Ni), while Comparative Examples 2 and 3 are solders with Ce or Ni exceeding the patent's limits. The preparation process is the same as the examples except for the composition. The specific parameters and performance are as follows:

[0027]

[0028] Performance comparison table:

[0029]

[0030] The overall advantages of the embodiments within the scope of this invention:

[0031] All embodiments outperform the conventional SAC305 (Comparative Example 1):

[0032] The oxygen content is lower (50-75 ppm vs 100 ppm), and the melt purification effect is significant, which is attributed to the reaction of Ce with dissolved oxygen in the melt to generate high-melting-point oxides and then precipitate.

[0033] The sphericity is higher (0.93~0.95 vs 0.88), which meets the requirements of high-precision electronic packaging for solder powder morphology, thanks to the synergistic regulation of inert gas atomization process and Ce-Ni master alloy;

[0034] It has better wettability (surface tension 478~485mN / m vs 520mN / m, wetting angle 28~32° vs 38°), reducing defects such as cold solder joints and bridging, and reducing melt oxidation and optimizing surface condition due to Ce;

[0035] The interface IMC is more stable (2.2–3.0 μm after reflow vs 4.0 μm, 4.5–5.5 μm after aging vs 8.2 μm), and Ni forms (Cu,Ni)6Sn5 composite IMC to inhibit growth;

[0036] It has stronger mechanical properties and thermal stability (tensile strength 42-46 MPa vs 38 MPa, thermal cycling strength retention rate 85-90% vs 68%), and the refined β-Sn grains and composite IMC together improve the fatigue resistance of the solder joint.

[0037] Negative effects of excessive Ce / Ni:

[0038] Although Comparative Example 2 (Ce exceeding 0.10%) and Comparative Example 3 (Ni exceeding 0.05%) outperformed traditional SAC305 in some indicators (such as the wettability of Comparative Example 2), they had key drawbacks:

[0039] The sphericity decreased (0.89-0.90 vs. 0.93-0.95 in the example), and the excess Ce / Ni was prone to forming heterogeneous inclusions during atomization, which destroyed the sphericity of the particles;

[0040] The IMC has poor long-term stability (7.0-7.5 μm after aging vs. 4.5-5.5 μm in the example), and the excessive Ce / Ni disrupts the interfacial reaction equilibrium, accelerating the coarsening of the IMC.

[0041] Insufficient mechanical properties and thermal stability (tensile strength 40-41 MPa vs 42-46 MPa in example, retention rate 72-75% vs 85-90%), excessive elements introduce brittle phases, reducing the weld joint's resistance to thermal cycling.

[0042] This invention solves the problems of excessive IMC growth, poor wettability, and insufficient thermal stability of traditional lead-free solders by integrating Ce melt purification, Ni interface stabilization, and β-Sn grain refinement, without changing the basic melting point of Sn-Ag-Cu alloy (which is close to SAC305). It is suitable for fields with stringent requirements for solder joint reliability, such as high-density packaging, automotive electronics, and 5G communication modules.

[0043] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A lead-free solder powder for reflow soldering, characterized in that, The solder powder is based on the Sn-Ag-Cu ternary alloy system and contains trace amounts of rare earth element Ce and transition metal Ni. Its chemical composition by mass percentage is as follows: Ag 3.0%, Cu 0.5%, Ce 0.05%, Ni 0.03%, with the balance being Sn. In the solder powder, the Ce element reacts with dissolved oxygen in the melt during the smelting process to generate high-melting-point oxide particles, which then settle to the bottom of the melt, thereby reducing the oxygen content of the melt. During the reflow soldering process, Ni diffuses to the interface between the solder and the copper pad, partially replacing Cu atoms in Cu6Sn5IMC to form (Cu,Ni)6Sn5 type intermetallic compound. The solder powder is obtained by atomization with inert gas, with a particle size of 20-38 μm, an oxygen content of ≤80 ppm, and a sphericity of ≥0.92; The Ce element is introduced in the form of a Ce-Ni master alloy, wherein the mass ratio of Ce to Ni in the Ce-Ni master alloy is 5:3 and the melting point is 980℃. The amount of Ce-Ni master alloy added is 0.08% based on the total mass of the solder powder.

2. The lead-free solder powder according to claim 1, characterized in that, The β-Sn primary phase grain size of the solder powder is 5–15 μm.

3. The lead-free solder powder according to claim 1, characterized in that, Under reflow conditions of 260℃×1h, the thickness of the IMC layer formed at the interface between the solder and the copper pad is 2.0~3.5μm; After high-temperature aging at 150℃ for 500 hours, the thickness of the IMC layer is 4.0–6.0 μm, and the IMC grain size is 0.8–1.5 μm.

4. The lead-free solder powder according to claim 1, characterized in that, The solder powder has a surface tension of 480 mN / m at 250°C, a spreading area of ​​85–90 mm², and a wetting angle of 28°–32°.

5. A method for preparing lead-free solder powder as described in any one of claims 1 to 4, characterized in that, Includes the following steps: Step S1: Weigh Sn, Ag, Cu and Ce-Ni master alloy according to the ratio. The raw materials are ultrasonically cleaned with acetone for 10 minutes and vacuum dried at 60°C for 2 hours. Step S2: Add Sn, Ag, and Cu to a graphite crucible and heat under a vacuum ≤ 5 × 10⁻⁶. -2 High-purity argon gas was introduced to 0.05 MPa under Pa, and the temperature was raised to 300℃ and held for 10 minutes to form a uniform Sn-Ag-Cu master alloy melt. Step S3: Add the Ce-Ni master alloy preheated to 200℃ into the melt, raise the temperature to 320℃, and stir electromagnetically at 300 rpm for 15 minutes; Step S4: After standing for 5 minutes, pour the mixture into a 200°C copper mold to obtain an ingot; Step S5: After crushing the ingot, atomize it into powder under an inert atmosphere. The atomizing nitrogen pressure is 3.5-4.5 MPa, the melt superheat is 30-50℃, and the nozzle orifice diameter is 0.8 mm. Step S6: Sieve to obtain 20-38μm solder powder, and treat it with nitrogen gas containing 0.5 vol% formic acid vapor at 60°C for 30 minutes, followed by nitrogen protection encapsulation.

6. The preparation method according to claim 5, characterized in that, In step S3, the Ce-Ni master alloy is obtained by melting pure Ce and pure Ni in a vacuum electric arc furnace and then crushing and sieving it into particles of 0.4 to 0.6 mm.

7. The preparation method according to claim 5, characterized in that, In step S5, the atomization process is performed under a vacuum ≤ 1 × 10⁻⁶. -2 High-purity nitrogen gas was introduced at 0.1 MPa, and the melt was heated to 350°C and then atomized by nitrogen injection at 4.0 MPa.

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