A homogeneous alloy solder alloy, BGA solder balls and methods of making the same

Abstract

The application discloses a kind of homogeneous alloy solder alloy, BGA soldering ball and its preparation method, the solder alloy with Sn as matrix, comprising Ag 0.3~4.0%, Cu 0.3~0.8%, Bi 0.2~4.0%, Sb 0.5~2.5%, Ni 0.05~0.5%, P 0.005~0.009%, Co 0.005~0.2%, rare earth element 0.01~0.2%, the balance is Sn, and satisfy the relationship formula: 0.0005≤(Ag×Ni×Co) / (Sb×Bi)≤1.0 and 0.05≤P / RE≤0.9.Gradient cooling solidification process, so that Ag3Sn nanoparticles are formed along the continuous network skeleton of beta-Sn cell boundary, SbSn and Sn-P nano-phase are uniformly dispersed distribution.The obtained BGA soldering ball has high shear strength, and has excellent oxidation resistance and wettability, and is suitable for high-density electronic packaging.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging materials technology, specifically to a homogeneous alloy solder alloy, BGA solder balls, and their preparation methods. Background Technology

[0002] Ball grid array (BGA) packaging technology achieves high-density packaging and high-reliability interconnection of integrated circuits by replacing traditional pins with an array of balls on the bottom of the chip, and has become one of the mainstream technologies for advanced electronic packaging. The quality of BGA solder balls directly determines the electrical interconnection, thermomechanical reliability, and drop impact resistance of the package.

[0003] As electronic products trend towards miniaturization and high integration, the size of BGA solder balls has gradually shrunk from over 0.5mm to 0.2mm or even below 0.15mm, placing increasingly stringent demands on the performance of solder ball materials. Currently, BGA solder balls mainly face the following technical challenges: First, the heterogeneity of the microstructure in lead-free solder leads to performance instability. In existing Sn-Ag-Cu lead-free solder balls, the Ag3Sn intermetallic compounds and large Cu6Sn5 compounds are randomly distributed, resulting in solder joint mechanical properties lower than theoretically expected. Studies have shown that Sn0.3Ag0.7Cu and Sn1.0Ag0.5Cu solder joints with low Ag content contain a small amount of coarse Ag3Sn and Cu6Sn5 compounds, which are randomly distributed, while the microstructure of Sn3.0Ag0.5Cu solder joints is relatively uniform, but local aggregation still exists.

[0004] Secondly, the wettability and surface oxidation resistance of lead-free solder balls need to be improved. Existing Sn-Ag-Cu solder balls are prone to surface oxidation and yellowing when forming solder bumps on BGA and CSP electrodes, affecting subsequent soldering quality.

[0005] Third, it is difficult to simultaneously achieve both mechanical properties and thermal fatigue reliability in conventional homogeneous alloys. Existing technologies have proposed lead-free solder balls with core-shell structures, achieving functional separation through a Cu-rich core and a Sn-Bi-rich alloy shell, but the fabrication process is complex and costly. Other technologies use electroplating to form a Sn-Ag-Cu alloy layer, creating uniformly distributed Ag3Sn nanoparticles within the plating, but this cannot achieve three-dimensional homogenization of the entire solder ball, and the nanoparticles tend to agglomerate during reflow soldering.

[0006] Fourth, in existing technologies, phosphorus (P) is mostly used as an optional antioxidant, without establishing a synergistic quantitative relationship with rare earth elements. The addition of P in existing lead-free solders containing P is often based on experience, lacking synergistic design with rare earth elements (REs), resulting in unstable antioxidant effects or excessive P forming coarse Sn-P phases that damage the plasticity of the solder joint.

[0007] Therefore, there is an urgent need to develop a new type of solder alloy and its BGA solder balls that have a homogeneous alloy microstructure, high strength, excellent wettability and good oxidation resistance, in order to meet the application requirements of high-density electronic packaging. Summary of the Invention

[0008] To address some or all of the problems existing in the prior art, this invention provides a homogeneous alloy solder alloy and BGA solder balls made therefrom, as well as a method for preparing the solder alloy and BGA solder balls and their applications. This solder alloy uses Sn as a matrix, and through multi-element alloy composition design and the constraint of a dual synergistic relationship, combined with a gradient cooling solidification process, achieves a uniform dispersion distribution of Ag3Sn intermetallic compound nanophases, SbSn nanophases, and Sn-P nanocompounds in the Sn matrix, thereby significantly improving the mechanical properties, thermal fatigue resistance, oxidation resistance, and wettability of the solder balls / solder joints.

[0009] A homogeneous alloy solder alloy, with Sn as the matrix, contains Ag, Cu, Bi, Sb, Ni, P, Co, and rare earth elements. The mass percentages of each component are: Ag 0.3–4.0 wt%, Cu 0.3–0.8 wt%, Bi 0.2–4.0 wt%, Sb 0.5–2.5 wt%, Ni 0.05–0.5 wt%, P 0.005–0.009 wt%, Co 0.005–0.2 wt%, rare earth elements 0.01–0.2 wt%, and the balance being Sn and unavoidable impurities. The rare earth elements are selected from one or more of Sc, Y, La, Ce, Pr, Nd, Gd, and Er.

[0010] The mass percentage of each component in the homogeneous alloy solder alloy satisfies the following relationship (I): 0.0005≤(Ag×Ni×Co) / (Sb×Bi)≤1.0; In equation (I), Ag, Ni, Co, Sb, and Bi represent the mass percentages of the elements Ag, Ni, Co, Sb, and Bi, respectively. Furthermore, the mass percentage of each component in the homogeneous alloy solder satisfies the following relationship (II): 0.05 ≤ P / RE ≤ 0.9; In relation (II), P represents the mass percentage of P, and RE represents the total mass percentage of rare earth elements.

[0011] The physical meaning of relation (I): The Ag×Ni×Co molecular synthesis characterizes the combined nucleation driving force of the Ag3Sn nanophase, the (Cu,Ni,Co)6Sn5 intermetallic compound, and the synergistic grain refinement by Co and Ni. Ag reacts with Sn to form the Ag3Sn dispersion-strengthening phase; Ni promotes the formation of (Cu,Ni)6Sn5 and refines the Ag3Sn grains; Co and Ni work synergistically to further refine the β-Sn grains and stabilize the interfacial IMC layer. This synergistic effect of the three is key to the formation of a continuous or semi-continuous Ag3Sn network framework.

[0012] The denominator Sb×Bi characterizes the degree of solid solution strengthening of Sb and Bi. Both Sb and Bi are dissolved in the β-Sn matrix, causing lattice distortion, effectively hindering dislocation movement, and improving solder strength. However, excessive addition can lead to matrix embrittlement or a decrease in elongation.

[0013] Equation (I) quantitatively describes the balance between the dispersed strengthening phases (Ag3Sn, (Cu,Ni)6Sn5) and the solid solution strengthening elements (Sb, Bi). When this ratio is below 0.0005, the dispersed strengthening phase is insufficient, the Ag3Sn network skeleton is discontinuous, and the solder joint strength is insufficient. When this ratio is above 1.0, the solid solution strengthening elements are relatively insufficient or Co is excessive, leading to matrix embrittlement or excessive grain refinement. Only within the range of 0.0005 to 1.0 can a uniform and continuous Ag3Sn network skeleton be formed, while obtaining excellent mechanical properties.

[0014] The physical meaning of relation (II): Equation (II) quantitatively characterizes the dosage balance of the synergistic antioxidant effect of phosphorus (P) and rare earth elements (RE). P forms a tin phosphate protective film on the surface of molten solder, preventing Sn from contacting oxygen; rare earth elements (especially Ce and La) also have strong antioxidant properties and can form a composite antioxidant film with P. When P / RE is less than 0.05, the antioxidant effect is insufficient, and the solder ball surface is prone to oxidation and yellowing; when P / RE is greater than 0.9, excess P will form coarse Sn-P compounds, impairing the plasticity of the solder. In the range of 0.05 to 0.9, the two synergistically form a dense and stable protective film, while generating nanoscale Sn-P compounds to provide grain boundary reinforcement.

[0015] As a further improvement of the present invention, the mass percentage of each component is as follows: Ag: 1.0~3.8wt% Cu: 0.4–0.7 wt% Bi: 1.0~3.0wt% Sb: 0.8–1.8 wt% Ni: 0.05~0.2wt%; P: 0.008~0.009wt%; Co: 0.01~0.1wt%; Rare earth elements: 0.02–0.1 wt%; The remainder is Sn.

[0016] The present invention also provides a BGA solder ball made of the above-mentioned homogeneous alloy solder alloy, wherein the diameter of the BGA solder ball is 0.1 to 0.8 mm and the sphericity is not less than 0.96; The BGA solder ball has a homogeneous alloy microstructure, in which Ag3Sn intermetallic compounds are uniformly dispersed in the Sn matrix in the form of nanoparticles. The diameter of the Ag3Sn nanoparticles is 0.05-1.0 μm and the volume fraction is 3-8%. The Ag3Sn nanoparticles are mainly distributed along the β-Sn cell boundaries to form a continuous or semi-continuous network framework with a network gap of less than 1 μm. The Sn matrix also contains nanoscale SbSn or Sb3Sn intermetallic compounds with an average diameter of less than 0.5 μm. Nanoscale Sn-P compounds are uniformly distributed at the grain boundaries and within the Sn matrix, with an average diameter of less than 0.3 μm.

[0017] As a further improvement of the present invention, the melting temperature of the BGA solder ball is 215-225°C, the wetting angle is ≤27.0°, and the shear strength of the BGA solder joint is not less than 40MPa.

[0018] The present invention also provides a method for preparing the above-mentioned BGA solder balls, comprising the following steps: Step 1: Pre-alloying treatment: Weigh the Sn, Ag, Cu, Bi, and Sb raw materials according to the stated mass percentages. Place 30%–50% of the total Sn along with all the Ag, Cu, Bi, and Sb into a vacuum induction melting furnace. Melt the materials under a vacuum of ≤5×10⁻⁶. -2 Under Pa conditions, the temperature is raised to 500-600℃, held for 30-90 minutes, and stirred until the alloying elements are completely dissolved before being cast into intermediate alloy ingots. Step 2, intermediate alloy melting: under vacuum degree ≤5×10 -2 Under Pa conditions, Sn is smelted with P, Co, Ni and rare earth elements in proportion to form Sn-P master alloy, Sn-Co master alloy, Sn-Ni master alloy and Sn-RE master alloy respectively. After the master alloy is smelted, it is cast into ingots respectively. Step 3, Master Alloy Melting: Place the intermediate alloy ingot obtained in Step 1 and the remaining Sn raw material into a tundish crucible, heat to 450-550℃ under inert gas protection, hold for 20-60 minutes, then add the Sn-P intermediate alloy, Sn-Co intermediate alloy, Sn-Ni intermediate alloy and Sn-RE intermediate alloy prepared in Step 2, and stir until completely dissolved to obtain the master alloy liquid; among them, P and rare earth elements must be added in the form of Sn-P and Sn-RE intermediate alloys to prevent oxidation and burn-off and ensure precise control of composition; high melting point elements such as Ni and Sb are added in the form of intermediate alloys to shorten the melting time.

[0019] Step 4, Gradient Cooling and Solidification: The master alloy liquid obtained in Step 3 is subjected to a three-stage temperature-time path during free fall using a uniform droplet spraying technique: (1) High temperature section: droplet temperature 450~400℃, cooling rate 80~200K / s, duration 0.01~0.03 seconds, so that Ag3Sn preferentially nucleates and β-Sn remains in liquid state; (2) Medium temperature range: droplet temperature 400~300℃, cooling rate 40~120K / s, duration 0.03~0.06 seconds, so that Ag3Sn grows to the final size (0.05~1.0μm) in a diffusion-controlled manner, while SbSn or Sb3Sn nanophase and Sn-P nanocompound are precipitated, and β-Sn remains in liquid state; (3) Low temperature section: the droplet temperature is 300-150℃, the cooling rate is 20-80K / s, and the duration is 0.10-0.20 seconds; β-Sn is rapidly nucleated and solidified, and the generated Ag3Sn nanoparticles, SbSn nanophase and Sn-P nanocompound are pinned to the β-Sn grain boundaries and inside the grain, forming a microstructure with β-Sn as the matrix and the three-phase nanocompound uniformly dispersed; Step 5, Collection and Classification: The solidified solder balls are collected using a cooling medium and then classified and screened by aperture size to obtain BGA solder balls within the target diameter range.

[0020] As a further improvement of the present invention, in step two, the melting temperature and time of each intermediate alloy are controlled as follows: Sn-P master alloy: Select Sn-2P master alloy, heat to 550-600℃, and hold for 30-60 minutes; Sn-Co master alloy: Select Sn-1Co master alloy, heat to 550-600℃, and hold for 30-60 minutes; Sn-Ni master alloy: Select Sn-1Ni master alloy, heat to 600-650℃, and hold for 30-60 minutes; Sn-RE master alloy: Select Sn-2RE master alloy, heat to 700-850℃, and hold for 30-60 minutes.

[0021] As a further improvement of the present invention, in step four, the three-stage temperature-time path is achieved by controlling the initial melt temperature, the ambient temperature gradient of the cooling cavity, and the cavity height, wherein the ambient temperature of the upper part of the cavity is 200-300°C, and the ambient temperature of the lower part of the cavity is 20-80°C.

[0022] As a further improvement of the present invention, the uniform droplet spraying adopts a pulse pneumatic spraying device, controlling the pulse frequency to be 500-5000Hz, the spraying air pressure to be 0.1-1.0MPa, the nozzle orifice diameter to be 1.2-2.0 times the diameter of the target solder ball, and applying an electric field with an electric field strength of 500-5000V / m to the alloy liquid during the spraying process.

[0023] As a further improvement of the present invention, in step five, the cooling medium is helium, argon or a mixture thereof with a temperature of 20 to 80°C.

[0024] The present invention also provides a method for forming the above-mentioned homogeneous alloy solder alloy, comprising the following steps: Step 1: Pre-alloying treatment: Weigh the Sn, Ag, Cu, Bi, and Sb raw materials according to the stated mass percentages. Place 30%–50% of the total Sn along with all the Ag, Cu, Bi, and Sb into a vacuum induction melting furnace. Melt the materials under a vacuum of ≤5×10⁻⁶. -2 Under Pa conditions, the temperature is raised to 500-600℃, held for 30-90 minutes, and stirred until the alloying elements are completely dissolved before being cast into intermediate alloy ingots. Step 2, intermediate alloy melting: under vacuum degree ≤5×10 -2 Under Pa conditions, Sn is smelted with P, Co, Ni and rare earth elements in proportion to form Sn-P master alloy, Sn-Co master alloy, Sn-Ni master alloy and Sn-RE master alloy respectively. After the master alloy is smelted, it is cast into ingots respectively. Step 3, Master alloy melting: Place the intermediate alloy ingot obtained in Step 1 and the remaining Sn raw material into the intermediate ladle crucible, heat to 450-550℃ under inert gas protection, hold for 20-60 minutes, then add the Sn-P intermediate alloy, Sn-Co intermediate alloy, Sn-Ni intermediate alloy and Sn-RE intermediate alloy prepared in Step 2, stir until completely dissolved to obtain master alloy liquid; Step 4: Pour or cast the master alloy liquid obtained in Step 3 into any form of powder, paste, sheet, wire, strip, or band.

[0025] The present invention also provides an application of the above-mentioned homogeneous alloy solder alloy for welding connections in the field of electronic packaging, wherein the welding connections include welding in ball grid array packaging, chip-level packaging, and surface mount technology.

[0026] Engineering implementation details of the gradient cooling path: The "high-temperature section, medium-temperature section, and low-temperature section" mentioned in this invention refer to the temperature range experienced during the droplet solidification process, rather than a limitation on the structure of the cooling equipment. Those skilled in the art can achieve the above temperature-time path based on conventional UDS (Uniform Droplet Spraying) equipment in the following ways: (1) The initial melt temperature is controlled at 450-550℃ to ensure that the droplets have a sufficiently high initial temperature when they enter the cavity.

[0027] (2) Chamber temperature gradient: By controlling the wall temperature and cooling gas flow rate of the cooling chamber, the ambient temperature of the chamber gradually decreases from top to bottom. Typical settings: ambient temperature in the upper part of the chamber (near the nozzle) is 200-300℃; ambient temperature in the lower part of the chamber (near the collector) is 20-80℃. The droplets fall in this temperature gradient environment and naturally experience the temperature-time path of "rapid cooling (high temperature section) → intermediate cooling (medium temperature section) → slow cooling (low temperature section)".

[0028] (3) Adjustment of cooling rate: By adjusting the initial temperature of the melt, the height of the cavity (which determines the falling time) and the cooling conditions of the cavity wall, the actual cooling rate of each section can be adjusted within the above range.

[0029] The above implementation method does not require major modifications to existing equipment, but only requires optimization of cavity temperature gradient design and process parameter control, and has good industrial feasibility.

[0030] The theoretical basis for gradient cooling solidification: The temperature-time path setting of the three-stage gradient cooling described in this invention is precisely matched with the phase transformation sequence of the Sn-Ag-Cu-Sb-Bi alloy: High-temperature range (450–400℃): This range is the preferential nucleation window for Ag3Sn. When molten alloy is injected into this range from 450–550℃, a large number of Ag3Sn nuclei precipitate, while β-Sn remains liquid (liquidite level approximately 228℃). A cooling rate of 80–200 K / s ensures a nucleus size <0.3 μm and a number density of 10. 9 ~10 10 / cm 3 Meanwhile, the RE-Sn high-melting-point compounds (>600℃) formed by rare earth elements remain solid in this range, serving as heterogeneous nucleation cores for Ag3Sn.

[0031] Mid-temperature range (400–300℃): In this range, Ag3Sn grows slowly to 0.05–1.0 μm via diffusion control, while SbSn and Sn-P nanophases begin to precipitate. β-Sn remains liquid to prevent premature solidification from interfering with Ag3Sn growth. A cooling rate of 40–120 K / s ensures uniform distribution of the nanophases without coarsening.

[0032] Low-temperature zone (300–150 °C): This zone contains the solidification of β-Sn (approximately 228 °C). Droplets begin nucleation and solidification upon entering this zone. By maintaining a cooling rate of 20–80 K / s, the latent heat of crystallization can be removed promptly, refining the β-Sn grains (grain size <10 μm); simultaneously, the Ag3Sn and SbSn nanophases already formed in the high-temperature and mid-temperature zones are "pinned" to the grain boundaries and within the grains; it also prevents the nanophases from agglomerating and growing during slow cooling (Ostwald ripening); and promotes the uniform precipitation of Sn-P nanocompounds at the grain boundaries.

[0033] The three-stage cooling process is indispensable: the high-temperature stage provides nucleation density, the intermediate-temperature stage controls growth size, and the low-temperature stage achieves pinning and refinement of the matrix. The total duration of each stage (0.14–0.29 seconds) matches the free-fall time of the droplet within a 2–4 ​​meter cavity, consistent with the solidification time of existing UDS processes.

[0034] Element synergy mechanism: This invention achieves precise control of the microstructure under multi-element interaction in the Sn-Ag-Cu-Bi-Ni-Sb-P-Co-RE alloy system by establishing a dual synergistic relationship: 1. Synergistic effect of Ag, Ni, Co, Sb and Bi (Equation I) Ag: Forms Ag3Sn nanophase, which is the main dispersion strengthening phase and improves solder ball strength through the Orowan mechanism.

[0035] Ni: Ni promotes the formation of (Cu,Ni)6Sn5 type IMC, refines Ag3Sn grains, and inhibits the excessive growth of Cu3Sn phase.

[0036] Co, in synergy with Ni, further refines β-Sn grains and stabilizes the interfacial IMC layer; simultaneously, Co itself forms SnCo compounds in the Sn matrix, providing additional dispersion reinforcement. The addition of Co makes the distribution of Ag3Sn nanoparticles more uniform, further reduces the network gap (which can be controlled below 0.8μm), and increases the shear strength of the solder joints by 10-15%.

[0037] Synergy between Sb and Bi: Both exhibit solid solution strengthening effects in β-Sn. When they coexist, an "asymmetric solid solution effect" is generated—Bi causes lattice contraction, while Sb causes lattice expansion, forming a unique lattice distortion field that effectively hinders dislocation movement.

[0038] Synergistic effect of Sb and Ni: Ni promotes the formation of (Cu,Ni)6Sn5 type IMC, while Sb can refine the grain size of this IMC. When the two coexist, they can significantly suppress the excessive growth of Cu3Sn phase at the solder joint interface.

[0039] The relationship K1 = (Ag × Ni × Co) / (Sb × Bi) in equation (I) comprehensively reflects the balance between the dispersed / refining phases (Ag3Sn, (Cu,Ni,Co)6Sn5) and the solid solution strengthening elements (Sb, Bi). Experiments show that: When K1 < 0.0005, Ag3Sn is unevenly distributed, the mesh skeleton is discontinuous, and the shear strength of the weld joint is insufficient (< 35 MPa). When 0.0005≤K1≤1.0, Ag3Sn is uniformly dispersed (distributed in a network along the β-Sn cell boundaries, with network gaps <1μm), SbSn phase is fine (<0.5μm), the shear strength of the solder joint is ≥40MPa, and the thermal cycle life is ≥1600 cycles; When K1>1.0, the solid solution strengthening is relatively insufficient or Co is excessive, which leads to matrix embrittlement and a significant decrease in elongation.

[0040] 2. The synergy between P and RE (Relationship II) The role of phosphorus (P): As a surface-active element, it forms a PO composite film on the surface of molten solder, preventing direct contact between Sn and oxygen and significantly reducing the amount of oxide slag generated. P can reduce SnO to metallic Sn, while it is itself oxidized to P2O5 and volatilized, playing a "dynamic antioxidant" role.

[0041] Synergistic effect of P and RE: RE (especially Ce and La) also has strong antioxidant properties and can form a composite antioxidant film with P. When the P / RE ratio is appropriate (0.05-0.9), the two synergistically form a dense and stable protective film, with better antioxidant effect than adding them alone. At the same time, an appropriate amount of P can form nanoscale Sn-P compounds with Sn, which are dispersed in the grain boundaries and within the grains, providing additional grain boundary strengthening effect.

[0042] Harmful effects of excessive phosphorus (P): When the phosphorus content is too high (P / RE>0.9), coarse Sn4P3 or Sn3P2 compounds will be formed, reducing the plasticity and impact resistance of the solder. Excessive phosphorus will also react with Ni to form a brittle Ni-P phase.

[0043] Equation (II) uses K2 = P / RE to quantify the synergistic antioxidant effect of P and RE. Experiments show that: When K2 < 0.05, the antioxidant effect is insufficient, and the surface of the solder ball is prone to oxidation and yellowing. When 0.05≤K2≤0.9, the antioxidant effect is excellent, the amount of surface oxide film formation is reduced by more than 30%, and the Sn-P nanocompounds are uniformly distributed. When K2>0.9, the Sn-P compound coarsens (>1μm), and the solder becomes more brittle.

[0044] 3. Coupling effect of dual relations Equation (I) mainly controls the distribution and size of the structural strengthening phases (Ag3Sn, SbSn, IMC) to ensure mechanical properties; Equation (II) mainly controls oxidation resistance and grain boundary strengthening (Sn-P phase). The two are independently regulated and work synergistically to achieve the comprehensive performance goal of "high strength + high reliability + high wettability + low oxidation".

[0045] Compared with the prior art, the beneficial effects of the present invention are: 1. Homogeneous alloy microstructure with uniform and stable performance. Through dual synergistic constraint and gradient cooling process, Ag3Sn nanoparticles, SbSn nanophases, and Sn-P nanocompounds achieve uniform dispersion distribution in the Sn matrix, overcoming the problem of disordered intermetallic compound distribution in traditional solder balls. Ag3Sn nanoparticles mainly form a network framework along the β-Sn cell boundaries, providing effective dispersion strengthening; SbSn nanophases provide additional dispersion strengthening; Sn-P nanocompounds provide grain boundary strengthening. The synergy of these three phases enables the shear strength of BGA solder joints to reach over 40 MPa.

[0046] 2. Excellent oxidation resistance and wettability. The synergistic antioxidant effect of P and RE forms a dense protective film, reducing the amount of oxide film formed on the solder ball surface by more than 30% compared to traditional Sn-Ag-Cu solder balls, and the wetting angle can be reduced to below 27°. No yellowing occurs on the surface of the solder balls when solder bumps are formed on BGA and CSP electrodes.

[0047] 3. Excellent resistance to thermal fatigue. Because the nanophase is firmly "pinned" into the Sn matrix, the shear strength retention rate of the solder joint is ≥85% after 1000 thermal cycles, and there is no delamination at the interface.

[0048] 4. Clear component window and high feasibility. Quantitative component design is guided by a dual-synergistic relationship, avoiding blind experimentation and facilitating industrialization. A wide Ag content range can adapt to different cost and performance requirements. Attached Figure Description

[0049] To more clearly illustrate the solutions in this invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0050] Figure 1 This is a scanning electron microscope (SEM) image (5000x) of the internal microstructure of the BGA solder ball obtained in Example 7 of the present invention.

[0051] The figure shows that Ag3Sn nanoparticles (bright white) are distributed in a network along the β-Sn cell boundaries, with an average particle diameter of about 0.3 μm. Detailed Implementation

[0052] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms "comprising" and "having," and any variations thereof, in the specification, claims, and foregoing drawings are intended to cover non-exclusive inclusion. The terms "first," "second," etc., in the specification, claims, or foregoing drawings are used to distinguish different objects, not to describe a particular order.

[0053] In this invention, the reference to "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment to other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0054] To enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Example

[0055] A method for preparing homogeneous alloy BGA solder balls, comprising Sn, Ag, Cu, Bi, Sb, Ni, P, Co, and rare earth elements. In this embodiment, La is selected as the rare earth element RE.

[0056] The mass percentages of each component are as follows: Ag: 0.3wt%; Cu: 0.5wt%; Bi: 2wt%; Sb: 2.5wt%; Ni: 0.5wt%; P: 0.005wt%; Co: 0.2wt%; La: 0.1wt%; with the balance being Sn.

[0057] The specific production steps are as follows: Step 1: Pre-alloying treatment Weigh out 40% of the total Sn amount. Place this Sn along with all the Ag, Cu, Bi, and Sb into a vacuum induction melting furnace. Evacuate the furnace to a vacuum level ≤ 5 × 10⁻⁶. -2 Pa (in this embodiment, it is controlled at 3 × 10) -2 Heat the mixture to 600℃ (Pa), hold for 60 minutes, and then turn on the electromagnetic stirring during this time to ensure that the alloying elements are completely dissolved. After holding, cast the melt into an intermediate alloy ingot.

[0058] Step 2: Melting of intermediate alloy Under the same vacuum conditions (vacuum degree ≤ 5 × 10⁻⁶), -2 Pa), respectively containing and smelting to prepare the following intermediate alloys: Sn-P master alloy: Sn-2P alloy is selected, heated to 580℃, held for 45 minutes, and then cast into ingots.

[0059] Sn-Co master alloy: Prepare Sn-1Co master alloy by heating to 580℃, holding for 45 minutes, and casting into ingots.

[0060] Sn-Ni master alloy: Sn-1Ni alloy is selected, heated to 620℃, held for 45 minutes, and then cast into ingots.

[0061] Sn-RE master alloy: Sn-2La alloy is selected, heated to 780℃, held for 45 minutes, and then cast into ingots.

[0062] Step 3: Master Alloy Melting The intermediate alloy A obtained in step one and the remaining Sn were placed in a tundish crucible and heated to 550°C under argon protection, and held for 40 minutes. Then, the Sn-P intermediate alloy, Sn-Co intermediate alloy, Sn-Ni intermediate alloy and Sn-RE intermediate alloy obtained in step two were added in sequence and stirred until completely dissolved to obtain the master alloy liquid.

[0063] Step 4: Gradient cooling and solidification A pulsed pneumatic injection device was used, with the pulse frequency controlled at 1500Hz, the injection air pressure at 0.4MPa, and the nozzle orifice diameter at 1.6 times the desired solder ball diameter (in this embodiment, the target diameter is 0.35mm, and the nozzle orifice diameter is approximately 0.56mm). An electric field strength of 2000V / m was applied to the molten alloy during the injection process.

[0064] The ejected droplets fall freely within an argon-filled cooling chamber. The upper ambient temperature of the chamber is set at 250°C, and the lower ambient temperature at 40°C. The initial melt temperature is controlled at 470°C, causing the droplets to undergo a three-stage cooling path: (1) High temperature section: The droplet is cooled from 450℃ to 400℃ at a rate of about 100K / s for a duration of about 0.02 seconds; (2) Medium temperature range: cooling from 400℃ to 300℃ at a rate of about 70K / s for a duration of about 0.045 seconds; (3) Low temperature section: Cooling from 300℃ to 150℃ at a rate of about 40K / s for a duration of about 0.18 seconds.

[0065] Step 5: Collection and Hierarchical Screening The solidified solder balls were collected by 25°C argon gas flow and then graded and screened through a sieve with multiple apertures to obtain BGA solder balls with a diameter range of 0.1 to 0.8 mm.

[0066] Example 2 - Comparative Example 9: To comprehensively cover the endpoint values ​​of each component and the endpoint values ​​of the synergistic relationships (I) and (II), i.e., K1 and K2, eight examples and nine comparative examples were set up. All examples and comparative examples used the preparation method of Example 1 to fabricate BGA solder balls; the only difference was the mass percentage of each component in each example and comparative example. The mass percentage of the components in each example can be found in Table 1. Table 1 shows the components and their relationships in the examples and comparative examples. Example 1 0.3 0.5 2 2.5 0.5 0.005 0.2 0.1 93.895 La 0.006 0.05 Example 2 4 0.8 0.2 0.5 0.05 0.009 0.005 0.01 94.426 Ce 0.01 0.9 Example 3 2.5 0.6 1 1.2 0.25 0.008 0.1 0.08 94.262 Nd 0.052 0.1 Example 4 3.5 0.4 0.5 0.8 0.35 0.007 0.15 0.05 94.243 Y 0.459 0.14 Example 5 1.5 0.7 3 1.8 0.1 0.006 0.02 0.04 92.834 Gd 0.00056 0.15 Example 6 3 0.5 0.3 0.6 0.4 0.009 0.15 0.02 95.021 Er 1.0 0.45 Example 7 2 0.6 1.5 1 0.2 0.008 0.08 0.06 94.552 Pr 0.0213 0.133 Example 8 1 0.3 4 2 0.08 0.005 0.05 0.1 92.465 Sc 0.0005 0.05 Comparative Example 1 3 0.5 — — — — — — 96.5 — — — Comparative Example 2 0.2 0.4 2.5 1.5 0.05 0.008 0.05 0.04 95.252 Ce 0.00013 0.2 Comparative Example 3 4.2 0.8 0.2 0.5 0.5 0.008 0.05 0.18 93.562 La 1.05 0.044 Comparative Example 4 2.5 0.5 0.8 0.7 0.25 0.003 0.05 0.15 95.047 Ce 0.0558 0.02 Comparative Example 5 2.5 0.5 0.8 0.7 0.25 0.3 0.05 0.04 94.86 Y 0.0558 7.5 Comparative Example 6 2.5 0.5 0.8 0.7 0.25 0.008 0 0.08 95.162 Ce 0 0.1 Comparative Example 7 2.5 0.5 0.8 0.7 0.25 0.008 0.05 0 95.192 — The denominator is 0 — Comparative Example 8 2.5 0.5 0.8 0.7 0.25 0 0.05 0.08 95.12 Ce 0.0558 0 Comparative Example 9 4 0.8 0.2 0.5 0.5 0.008 0.25 0.22 93.522 Ce 5 0.036 The prepared BGA solder balls were subjected to performance testing. The specific testing methods are as follows: 1. Wetting angle test: In accordance with JIS Z 3198 standard, the solder ball was placed on the Cu substrate for reflow soldering. The wetting angle was measured using the droplet method, and the contact angle was measured using image analysis software.

[0067] 2. Shear strength test: After reflow soldering the BGA solder balls onto the Cu pads, a push-pull force tester was used to apply a push force from the side of the solder joint at a shear rate of 0.2 mm / s until fracture. The maximum shear force was recorded and divided by the cross-sectional area of ​​the solder joint to obtain the shear strength. The average value of 20 solder joints was taken.

[0068] 3. Thermal Cycling Life Test: A two-chamber thermal shock test chamber was used, with temperature conditions ranging from -40 ℃ to 125 ℃. Each temperature zone was maintained for 15 minutes, with a transition time of less than 1 minute. Samples were taken after every 200 cycles for shear strength testing, and the number of cycles in which the shear strength retention rate dropped below 70% of the initial value was recorded.

[0069] 4. Oxide film thickness test: X-ray photoelectron spectroscopy (XPS) was used to perform in-depth analysis of the solder ball surface, and the sputtering depth when the Sn3d peak intensity dropped to 50% of the bulk phase was taken as the oxide film thickness.

[0070] 5. Elongation test: Prepare standard tensile specimens of solder (gauge length 25 mm, diameter 5 mm), test them on a universal testing machine at a tensile rate of 1 mm / min, and record the elongation at fracture.

[0071] 6. Ag3Sn nanoparticle size and network gap test: Field emission scanning electron microscopy (FE-SEM) was used to observe at 5000-20000x magnification. The diameter (equivalent circle diameter) of at least 200 particles and the network gap (shortest distance between the edges of adjacent particles) of adjacent particles along the β-Sn cell boundary were measured using image analysis software, and the average value was taken.

[0072] The performance test results of the BGA solder balls prepared in Examples 1-9 above can be found in Table 2: Table 2 shows the performance test results of the examples and comparative examples. Example 1 Mesh structure with gaps of 0.6 μm 48 1800 25.0 13 16 Example 2 Mesh structure with gaps of 0.7 μm 44 1700 26.2 17 17 Example 3 Mesh structure with gaps of 0.6 μm 46 1720 25.2 14 18 Example 4 Mesh structure with gaps of 0.8 μm 40 1690 27.0 19 15 Example 5 Mesh structure with gaps of 0.7 μm 41 1620 26.3 17 16 Example 6 Mesh structure with gaps of 0.7 μm 43 1810 26.5 18 17 Example 7 Mesh structure with gaps of 0.5 μm 42 1700 24.2 11 14 Example 8 Mesh structure with gaps of 0.6 μm 45 1660 27.5 15 16 Comparative Example 1 Reticulated, coarse-grained 35 1320 27.3 30 11 Comparative Example 2 The particles are too fine, and the network is discontinuous. 37 1180 28.8 14 10 Comparative Example 3 thick slats 34 1125 29.5 28 12 Comparative Example 4 Reticulated, heavily oxidized 42 1340 33.0 48 17 Comparative Example 5 Reticulated, with coarse Sn-P 27 1090 36.8 22 9 Comparative Example 6 Mesh, slightly coarse 26 1255 36.0 16 16 Comparative Example 7 <![CDATA[Local aggregation of Ag3Sn]]> 28 1330 32.0 42 14 Comparative Example 8 Network structure, thick oxide film 35 1240 31.5 38 18 Comparative Example 9 Reticulated, coarse-grained 39 1350 35.8 16 8 Results Analysis From Table 1 and Table 2, we can see that: Examples 1-8 all satisfy the component range and relationships (I) and (II) of this invention, fully covering the endpoint values ​​of each component and the endpoint ranges of K1 (0.0005~1.00) and K2 (0.05~0.9). In all examples, Ag3Sn forms a continuous or semi-continuous network framework along the β-Sn cell boundary, with network gaps <1μm, shear strength ≥40MPa, thermal cycling lifetime ≥1600 cycles, wetting angle ≤27.0°, and oxide film thickness ≤23nm.

[0073] Comparative Examples 1-9 all showed significant performance degradation. Comparative Example 1 (traditional SAC305) showed comprehensive performance degradation. Comparative Example 2 (too low Ag) resulted in K1 being far below the lower limit, leading to discontinuous network structure; Comparative Example 3 (too high Ag) had K1>1.0, forming coarse laths; Comparative Example 4 (too low P) had K2<0.05, resulting in an oxide film thickness of 48 nm; Comparative Example 5 (too high P) had K2>0.9, resulting in coarse Sn-P compounds and a shear strength of only 27 MPa; Comparative Example 6 (no Co) had K1=0, significantly reducing shear strength and thermal cycling life; Comparative Example 7 (no RE) showed tissue aggregation; Comparative Example 8 (no P) showed severe oxidation; and Comparative Example 9 (excess Co) had K1=5.0, resulting in brittle fracture. These findings fully demonstrate the criticality and effectiveness of the dual-relationship formula of this invention.

[0074] Industrial Applicability: The homogeneous alloy solder of this invention can be widely used in electronic packaging fields such as semiconductor packaging, surface mount technology (SMT), ball grid array (BGA), and chip-scale packaging (CSP), especially suitable for electronic products with high requirements for reliability and miniaturization, such as smartphones, tablets, and high-performance computing chips. BGA solder balls made from it exhibit excellent high shear strength, good oxidation resistance, and thermal fatigue life. The preparation method of this invention is stable, cost-controllable, and suitable for large-scale industrial production.

[0075] The specific embodiments described above are preferred embodiments of the present invention and are not intended to limit the specific scope of the present invention. The scope of the present invention includes, but is not limited to, these specific embodiments. All equivalent changes made in accordance with the present invention are within the protection scope of the present invention.

Claims

1. A homogeneous alloy solder, characterized in that, Using Sn as the matrix, it contains Ag, Cu, Bi, Sb, Ni, P, Co, and rare earth elements. The mass percentage of each component is as follows: Ag: 0.3–4.0 wt% Cu: 0.3–0.8 wt% Bi: 0.2–4.0 wt% Sb: 0.5–2.5 wt%; Ni: 0.05~0.5wt%; P: 0.005~0.009wt%; Co: 0.005~0.2wt%; Rare earth elements: 0.01–0.2 wt%; The remainder is Sn; The mass percentage of each component in the homogeneous alloy solder alloy satisfies the following relationship (I): 0.0005≤(Ag×Ni×Co) / (Sb×Bi)≤1.0; Furthermore, the mass percentage of each component in the homogeneous alloy solder satisfies the following relationship (II): 0.05 ≤ P / RE ≤ 0.9; In the relation (II), RE represents the total mass percentage of rare earth elements.

2. The homogeneous alloy solder alloy according to claim 1, characterized in that, The mass percentage of each component is as follows: Ag: 1.0~3.8wt% Cu: 0.4–0.7 wt% Bi: 1.0~3.0wt% Sb: 0.8–1.8 wt% Ni: 0.05~0.2wt%; P: 0.008~0.009wt%; Co: 0.01~0.1wt%; Rare earth elements: 0.02–0.1 wt%; The remainder is Sn.

3. The homogeneous alloy solder alloy according to claim 1, characterized in that: The rare earth elements are selected from one or more of Sc, Y, La, Ce, Pr, Nd, Gd, and Er.

4. A BGA solder ball, made of any one of the homogeneous alloy solder alloys according to claims 1-3, characterized in that: The diameter of the BGA solder ball is 0.1 to 0.8 mm, and the sphericity is not less than 0.

96. The BGA solder ball has a homogeneous alloy microstructure, in which Ag3Sn intermetallic compounds are uniformly dispersed in the Sn matrix in the form of nanoparticles. The diameter of the Ag3Sn nanoparticles is 0.05-1.0 μm and the volume fraction is 3-8%. The Ag3Sn nanoparticles are mainly distributed along the β-Sn cell boundaries to form a continuous or semi-continuous network framework with a network gap of less than 1 μm. The Sn matrix also contains nanoscale SbSn or Sb3Sn intermetallic compounds with an average diameter of less than 0.5 μm. Nanoscale Sn-P compounds are uniformly distributed at the grain boundaries and within the Sn matrix, with an average diameter of less than 0.3 μm.

5. The BGA solder ball according to claim 4, characterized in that: The BGA solder ball has a melting temperature of 215–225°C, a wetting angle of ≤27.0°, and a shear strength of not less than 40 MPa.

6. A method for preparing solder balls, used to prepare the BGA solder balls according to claim 4 or 5, characterized in that, Includes the following steps: Step 1, Pre-alloying treatment: 30%–50% of the total Sn content, along with all Ag, Cu, Bi, and Sb, are placed in a vacuum induction melting furnace, and the process is carried out under a vacuum degree ≤ 5 × 10⁻⁶. -2 Under Pa conditions, the temperature is raised to 500-600℃, held for 30-90 minutes, and stirred until the alloying elements are completely dissolved before being cast into intermediate alloy ingots. Step 2, intermediate alloy melting: under vacuum degree ≤5×10 -2 Under Pa conditions, Sn is smelted with P, Co, Ni and rare earth elements in proportion to form Sn-P master alloy, Sn-Co master alloy, Sn-Ni master alloy and Sn-RE master alloy respectively. After the master alloy is smelted, it is cast into ingots respectively. Step 3, Master alloy melting: Place the intermediate alloy ingot obtained in Step 1 and the remaining Sn into the intermediate ladle crucible, heat it to 450-550℃ under inert gas protection, hold it for 20-60 minutes, then add the Sn-P intermediate alloy, Sn-Co intermediate alloy, Sn-Ni intermediate alloy and Sn-RE intermediate alloy prepared in Step 2, stir until completely dissolved to obtain the master alloy liquid; Step 4, Gradient Cooling and Solidification: The master alloy liquid obtained in Step 3 is subjected to a three-stage temperature-time path during free fall using a uniform droplet spraying technique: (1) High temperature section: droplet temperature 450~400℃, cooling rate 80~200K / s, duration 0.01~0.03 seconds; (2) Medium temperature range: droplet temperature 400~300℃, cooling rate 40~120K / s, duration 0.03~0.06 seconds; (3) Low temperature section: droplet temperature 300~150℃, cooling rate 20~80K / s, duration 0.10~0.20 seconds; Step 5, Collection and Classification: The solidified solder balls are collected using a cooling medium and then classified and screened by aperture size to obtain BGA solder balls within the target diameter range.

7. The preparation method according to claim 6, characterized in that, In step two, the melting temperature and time for each intermediate alloy are controlled as follows: Sn-P master alloy: Select Sn-2P master alloy, heat to 550-600℃, and hold for 30-60 minutes; Sn-Co master alloy: Select Sn-1Co master alloy, heat to 550-600℃, and hold for 30-60 minutes; Sn-Ni master alloy: Select Sn-1Ni master alloy, heat to 600-650℃, and hold for 30-60 minutes; Sn-RE master alloy: Select Sn-2RE master alloy, heat to 700-850℃, and hold for 30-60 minutes.

8. The preparation method according to claim 6, characterized in that: In step four, the three-stage temperature-time path is achieved by controlling the initial melt temperature, the ambient temperature gradient of the cooling chamber, and the height of the chamber. The ambient temperature at the top of the chamber is 200–300°C, and the ambient temperature at the bottom of the chamber is 20–80°C.

9. The preparation method according to claim 6, characterized in that: The uniform droplet spraying adopts a pulse pneumatic spraying device, with the pulse frequency controlled at 500-5000Hz, the spraying air pressure at 0.1-1.0MPa, the nozzle orifice diameter at 1.2-2.0 times the diameter of the target solder ball, and an electric field with an electric field strength of 500-5000V / m applied to the alloy liquid during the spraying process.

10. The preparation method according to claim 6, characterized in that: In step five, the cooling medium is helium, argon, or a mixture thereof at a temperature of 20–80°C.

11. A method for preparing a homogeneous alloy solder alloy, used to prepare the homogeneous alloy solder alloy according to any one of claims 1-3, characterized in that, Includes the following steps: Step 1, Pre-alloying treatment: 30%–50% of the total Sn content, along with all Ag, Cu, Bi, and Sb, are placed in a vacuum induction melting furnace, and the process is carried out under a vacuum degree ≤ 5 × 10⁻⁶. -2 Under Pa conditions, the temperature is raised to 500-600℃, held for 30-90 minutes, and stirred until the alloying elements are completely dissolved before being cast into intermediate alloy ingots. Step 2, intermediate alloy melting: under vacuum degree ≤5×10 -2 Under Pa conditions, Sn is smelted with P, Co, Ni and rare earth elements in proportion to form Sn-P master alloy, Sn-Co master alloy, Sn-Ni master alloy and Sn-RE master alloy respectively. After the master alloy is smelted, it is cast into ingots respectively. Step 3, Master alloy melting: Place the intermediate alloy ingot obtained in Step 1 and the remaining Sn into the intermediate ladle crucible, heat it to 450-550℃ under inert gas protection, hold it for 20-60 minutes, then add the Sn-P intermediate alloy, Sn-Co intermediate alloy, Sn-Ni intermediate alloy and Sn-RE intermediate alloy prepared in Step 2, stir until completely dissolved to obtain the master alloy liquid; Step 4: Pour or cast the master alloy liquid obtained in Step 3 into any form of powder, paste, sheet, wire, strip, or band.

12. The use of a homogeneous alloy solder, characterized in that, The homogeneous alloy solder alloy according to any one of claims 1-3 is used for welding connections in the field of electronic packaging, wherein the welding connections include welding in ball grid array packaging, chip-level packaging, and surface mount technology.

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