Highly reliable electronic package solder balls and methods for making the same

Abstract

This invention provides a high-reliability electronic packaging solder ball and its preparation method. The solder ball has a core-shell structure, comprising: a core, which is composed of a copper alloy containing at least one of chromium and zirconium as precipitation strengthening elements, and at least one of silicon, niobium, and rare earth elements as trace stabilizing elements, with a total content of trace stabilizing elements of 0.01%–0.15 wt%; nanoscale precipitates are distributed in the copper alloy matrix, and the trace stabilizing elements are segregated at the interface between the nanoscale precipitates and the matrix; an interface control layer, formed on the outer surface of the core, used to suppress the diffusion and migration of copper atoms from the core to the outer solder layer; and a cladding layer, which covers the outer surface of the interface control layer, and is composed of a tin-based solder alloy with a melting point lower than that of the copper alloy. This invention overcomes the limitations of existing core-shell structure solder balls in terms of reliability and manufacturing consistency, meeting the growing demand for high-reliability solder joint interconnection in advanced electronic packaging.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging materials technology, specifically to a high-reliability electronic packaging solder ball and its preparation method. Background Technology

[0002] As electronic products continue to evolve towards miniaturization, high integration, and high power density, the reliability of solder joint interconnects in electronic packaging faces increasingly severe challenges. In advanced packaging forms such as ball grid array (BGA) and chip-scale packaging, the solder ball plays a crucial role—it is responsible for both power conduction and heat dissipation, and it also acts like a small pillar to support the chip on the substrate. However, with increasingly narrower package pitches and smaller solder joints, the stress distributed across each solder joint is becoming more concentrated, placing high demands on the strength and structural stability of the solder ball material.

[0003] Current practices involve enhancing performance through precipitation strengthening within the copper alloy core. Simply put, this involves growing tiny reinforcing particles within the alloy to increase hardness. The problem is that researchers often only focus on the initial effect of these particles – the finer and denser the better – neglecting how they change under repeated heating. In reality, the interface between these reinforcing particles and the surrounding matrix degrades with temperature changes, gradually loosening from a tightly bonded state. Once this transformation occurs, the particles easily grow larger and coarser, diminishing the strengthening effect. Simply making the particles small initially does not guarantee their long-term stability. Furthermore, excess alloying elements often remain in the copper alloy matrix. These elements act like nutrients, continuously feeding the reinforcing particles during heating, accelerating their growth and further weakening the material's properties.

[0004] In summary, existing technologies for core-shell solder balls have significant shortcomings, both in the formulation design of the core alloy and the long-term stability control of reinforcing particles. Therefore, there is an urgent need for a systematic technical solution that coordinates alloy composition design, microstructure control, interface structure design, and fabrication processes to overcome the limitations of existing core-shell solder balls in terms of reliability and manufacturing consistency, and to meet the growing demand for highly reliable solder joint interconnects in advanced electronic packaging. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide a highly reliable electronic packaging solder ball and its preparation method, so as to solve the technical problems faced by the core-shell structure electronic packaging solder ball in the prior art, such as insufficient thermal stability of core mechanical properties, imperfect control of copper solder interface reaction and lack of adaptability of preparation process.

[0006] To address the above problems, a first aspect of the present invention provides a high-reliability electronic packaging solder ball, wherein the solder ball has a core-shell structure and comprises: The core is composed of a copper alloy containing at least one of chromium and zirconium as precipitation strengthening elements, and at least one of silicon, niobium, and rare earth elements as trace stabilizing elements, the total content of which is 0.01%-0.15 wt%. The copper alloy matrix contains nanoscale precipitates, and the trace stabilizing elements are segregated at the interface between the nanoscale precipitates and the matrix. An interface control layer is formed on the outer surface of the core to suppress the diffusion and migration of copper atoms in the core to the outer solder layer. A cladding layer is applied to the outer surface of the interface control layer, and the cladding layer is composed of a tin-based solder alloy with a melting point lower than that of the copper alloy.

[0007] A second aspect of the present invention provides a method for preparing the above-mentioned high-reliability electronic packaging solder balls, comprising the following steps: S1. Alloy smelting: Copper alloy raw materials containing at least one precipitation strengthening element from chromium and zirconium and at least one trace stabilizing element from silicon, niobium and rare earth elements are smelted in a protective atmosphere with an oxygen content of ≤5 ppm to obtain copper alloy melt, and the melt is purified to reduce the oxide inclusion content. S2. Uniform Droplet Spraying into Balls: The copper alloy melt is passed through a high-temperature resistant ceramic nozzle with a wetting angle greater than 90° to form a continuous jet. Controlled-frequency vibration is applied to the jet to break it into uniform droplets. During flight, the droplets spherize due to surface tension and move at a 10... 3 -10 5 Rapid solidification at a cooling rate of K / s yields spherical copper alloy core microspheres, in which the alloying elements are in a supersaturated solid solution state. S3. Aging strengthening treatment: The core microspheres are subjected to aging treatment to precipitate the precipitation strengthening elements in the supersaturated solid solution state to form a nanoscale precipitate phase, and pre-stabilization aging is performed to evolve the precipitate phase to a thermodynamically relatively stable state. S4. Preparation of interface control layer: The surface of the aged core microspheres is cleaned and activated, and an interface control layer for inhibiting the outward diffusion of copper atoms is formed on its surface; S5. Solder coating: A tin-based solder alloy layer is coated on the outer surface of the interface control layer to form a core-shell structure solder ball.

[0008] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention achieves a harmonious balance between high strength and high thermal stability in a copper alloy core by introducing at least one of chromium and zirconium as precipitation strengthening elements and combining it with an alloy composition system designed with at least one of silicon, niobium, and rare earth elements as trace stabilizing elements. During aging, the precipitation strengthening elements precipitate from the supersaturated solid solution to form nanoscale precipitates. These high-density, dispersed precipitates constitute an effective network hindering dislocation movement in the matrix, fundamentally improving the yield strength and creep resistance of the copper alloy core. This invention creatively introduces trace stabilizing elements and concentrates them at the interface between the nanoscale precipitate and the copper matrix, thereby actively controlling the thermal stability of the precipitate at the atomic level. After the trace stabilizing elements are enriched at the interface, they effectively reduce the migration activity of the interface through elastic and chemical interactions between solute atoms and mismatched dislocations at the interface. This atomic-scale interface pinning mechanism ensures that the precipitate maintains basic stability in size and distribution even under the high-temperature exposure of reflow soldering and subsequent thermal cycling, thus guaranteeing the persistence of the core strengthening effect throughout the entire service life of the solder ball.

[0009] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0010] Figure 1 This is a cross-sectional schematic diagram of a high-reliability electronic packaging solder ball according to an embodiment of this application.

[0011] Figure 2 This is a flowchart of a method for preparing a high-reliability electronic packaging solder ball in one embodiment of this application. Detailed Implementation

[0012] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0013] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0014] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0015] The applicant discovered: Currently, lead-free tin-based solder alloys are widely used in the electronic packaging field for solder joint interconnection. However, solder balls composed of a single tin-based solder alloy have inherent shortcomings in terms of mechanical properties. Tin-based solders have relatively low elastic modulus and yield strength. Under alternating loads such as thermal cycling and mechanical vibration, creep deformation and fatigue crack initiation easily occur inside the solder joint, leading to gradual degradation and eventual failure. When chip power density increases, causing the service temperature of the solder joint to rise, the creep rate of tin-based solder accelerates further, making the reliability degradation problem more prominent. In addition, after pure tin-based solder joints undergo multiple reflow soldering thermal cycles, the microstructure inside the solder becomes significantly coarsened. Among them, the coarsening of the lamellar eutectic structure continuously reduces the mechanical load-bearing capacity of the solder matrix, making it difficult to meet the service requirements of long-life, high-reliability electronic products.

[0016] To improve the mechanical properties of solder joints, the industry has explored the technical route of introducing a high-modulus metal core into the solder ball to form a core-shell structure. Copper, due to its excellent electrical and thermal conductivity and high mechanical strength, has become a preferred candidate for the core material. However, using pure copper as the core material faces a key metallurgical contradiction: during reflow soldering, the direct contact between the copper core and the outer tin-based solder will result in a violent metallurgical reaction, with a large number of copper atoms diffusing towards the solder side and forming a copper-tin intermetallic compound layer. This intermetallic compound layer is not only highly brittle, but also continues to thicken during repeated thermal cycles, accompanied by the formation of Kirkendal voids. The accumulation of these voids at the interface will severely weaken the bonding force between the copper core and the solder, ultimately becoming a preferred site for crack initiation and accelerating solder joint failure. Therefore, how to effectively control the interfacial reaction between the copper core and the solder and suppress the excessive diffusion and migration of copper atoms has become a key technical problem that needs to be solved in the design of core-shell structure solder balls.

[0017] Even though the diffusion problem of copper atoms is mitigated to some extent by the design of the interface barrier layer, the mechanical stability of the copper core itself remains a deep-seated factor restricting the long-term reliability of solder balls. After the high-temperature exposure of ordinary pure copper or conventional copper alloy cores during reflow soldering, their grain structure may soften due to recrystallization, leading to a decrease in the core's mechanical support for the solder joint. If precipitation strengthening mechanisms are introduced through alloying to improve the strength of the copper alloy core, the thermal stability of the precipitates during service becomes a crucial issue. Nanoscale precipitates in precipitation-strengthened copper alloys are thermodynamically metastable. During the high-temperature exposure of reflow soldering and subsequent thermal cycling, the precipitates exhibit a thermodynamic driving force for coarsening and growth. Once the precipitates coarsen significantly, their pinning effect on dislocation movement will be greatly weakened, the strength of the copper alloy core will degrade, and the mechanical performance advantages of core-shell structure solder balls will gradually be lost.

[0018] Current technologies for enhancing the core performance of copper alloys through precipitation strengthening lack effective means for actively controlling the thermal stability of the precipitates. Conventional precipitation strengthening treatments focus only on the initial size and density of the precipitates to achieve high age-hardness, without fully considering the evolution of the precipitates under subsequent thermal exposure conditions. The interfacial structure between the precipitates and the matrix, especially the transition from a coherent to a non-coherent interface, directly determines the ease with which the precipitates coarsen and migrate. Without effective control of this interfacial transition process, the initial small size of the precipitates alone is insufficient to ensure their stability during thermal cycling. Furthermore, the residual supersaturated solid solution elements in the copper alloy matrix provide solute replenishment for the continued growth of the precipitates during thermal exposure, which is also a significant factor accelerating precipitate coarsening.

[0019] On the other hand, the fabrication process of core-shell structured solder balls also faces multiple challenges. The fabrication of copper alloy core microspheres requires balancing sphericity, dimensional uniformity, and alloy composition uniformity. Traditional atomization powdering methods struggle to simultaneously meet the requirements of high sphericity and narrow particle size distribution in microspheres, and compositional segregation during rapid solidification can affect the subsequent aging strengthening effect. Furthermore, the aging heat treatment of large batches of microspheres suffers from contact sintering between microspheres and uneven heating. These process limitations make it difficult to achieve stable, large-scale production of core-shell structured solder balls with excellent overall performance.

[0020] In summary, existing technologies in the field of core-shell electronic packaging solder balls face several technical bottlenecks, including insufficient core alloy design and precipitate stability control, imperfect regulation of copper solder interface reactions, and inadequate adaptability of fabrication processes. Therefore, a systematic technical solution that integrates alloy composition design, microstructure control, interface structure design, and fabrication process methods is urgently needed to overcome the limitations of existing core-shell solder balls in terms of reliability and manufacturing consistency, and to meet the growing demand for highly reliable solder joint interconnects in advanced electronic packaging.

[0021] In view of this, refer to Figure 1 One embodiment of this application provides a high-reliability electronic packaging solder ball, the solder ball having a core-shell structure, comprising: Core 100 is made of copper alloy, which contains at least one of chromium and zirconium as precipitation strengthening elements, and at least one of silicon, niobium and rare earth elements as trace stabilizing elements. The total content of trace stabilizing elements is 0.01%-0.15 wt%. Nanoscale precipitates are distributed in the copper alloy matrix, and trace stabilizing elements are segregated at the interface between the nanoscale precipitates and the matrix. Interface control layer 200 is formed on the outer surface of core 100 and is used to suppress the diffusion and migration of copper atoms in core 100 to outer solder. The cladding layer 300 covers the outer surface of the interface control layer 200 and is composed of a tin-based solder alloy with a melting point lower than that of copper alloy.

[0022] It should be noted that in existing core-shell solder balls, the mechanical properties of the copper core 100 lack long-term stability under heat exposure conditions, and the interfacial metallurgical reaction between the copper core 100 and the solder is difficult to control effectively. Therefore, the solder ball in this embodiment adopts a core-shell structure, including three functional layers: a copper alloy core 100, an interface control layer 200, and a tin-based solder coating layer 300. The precipitation strengthening elements in the copper alloy core 100 provide a dispersed strengthening effect by forming nanoscale precipitates in the matrix, resulting in mechanical properties of the copper alloy core 100 higher than pure copper. Trace stabilizing elements segregate at the precipitate interface, pinning the precipitate interface through solute-interface interaction and inhibiting the coarsening and migration of the precipitates under heat exposure conditions. This ensures the long-term thermal stability of the strengthening effect of the core 100, adapting to potential repeated heating scenarios. The interface control layer 200 acts as a physical barrier to copper atom diffusion, effectively suppressing excessive metallurgical reactions between the copper and solder, and preventing abnormal thickening of intermetallic compounds and the formation of Kirkendal voids. The synergistic combination of the three functional layers enables the solder ball to simultaneously possess high mechanical reliability, good electrothermal transfer performance, and excellent interfacial bonding stability.

[0023] As one implementation method, how to optimize the content and ratio of the precipitation strengthening elements chromium and zirconium to ensure that the nucleation density and size distribution of the two precipitates match each other during the aging process, jointly constructing an optimal dispersion-impeding network for dislocation movement. Based on this problem, this embodiment further specifies that the mass percentage of chromium in the copper alloy is 0.3%-1.2%, the mass percentage of zirconium is 0.03%-0.25%, and the balance is copper, trace stabilizing elements, and unavoidable impurities; wherein the mass ratio of chromium content to zirconium content is (3-15):1. The technical effect of this mass ratio setting is that chromium mainly desorbs in the copper matrix as a body-centered cubic chromium precipitate, while zirconium mainly desorbs as a copper-zirconium compound precipitate. The nucleation driving force and growth kinetics of the two precipitates are different. By controlling the mass ratio of chromium to zirconium, the two types of precipitates can be simultaneously precipitated with similar nucleation densities and size distributions during the aging process, forming a dispersion-impeding network in the matrix where the two precipitates intertwine. Compared with single precipitate systems, this composite dispersion strengthening mechanism can more effectively hinder dislocation movement on different slip systems, thereby achieving higher overall strengthening efficiency.

[0024] As one implementation method, when the trace stabilizing element contains rare earth elements, how can we prevent the formation of rare earth-zirconium compounds between the rare earth elements and zirconium, thus avoiding the depletion of the effective solid-solution zirconium content in the matrix that participates in precipitation strengthening? To solve this problem, in this embodiment, when the trace stabilizing element contains rare earth elements, the rare earth elements are selected from lanthanum and / or cerium, and the mass ratio of rare earth elements to zirconium is ≤1:2, so as to avoid the formation of rare earth-zirconium compounds between the rare earth elements and zirconium, which would excessively deplete the effective solid-solution zirconium content in the matrix that participates in precipitation strengthening.

[0025] Specifically, rare earth elements have a strong chemical affinity for zirconium, a transition element. If the amount of rare earth elements added is too high relative to zirconium, they will preferentially form thermodynamically stable rare earth-zirconium compounds in the matrix. These compounds not only do not participate in dispersion strengthening themselves, but also consume the effective solid-solution zirconium content in the matrix originally used to form nanoscale copper-zirconium compound precipitates, leading to a decrease in the final precipitation strengthening effect. In this embodiment, by limiting the mass ratio of rare earth to zirconium to no more than 1:2, the amount of rare earth elements added is ensured to be at a reasonable level, which can play a stabilizing role in segregation pinning at the precipitate interface without compromising the integrity of the precipitation strengthening system due to excessive reaction with zirconium. Lanthanum and cerium, as light rare earth elements, have a moderate degree of mismatch between their atomic radii and the copper matrix, which is conducive to achieving effective solute segregation at mismatched dislocations at the precipitate interface.

[0026] In one implementation method, the solid solution residues of chromium and zirconium in the copper alloy matrix do not exceed 15% of their respective total additions. The remaining chromium and zirconium are dissolved into the matrix as nanoscale precipitates, allowing the copper alloy matrix to maintain the intrinsic conductive channel characteristics of a high-purity copper matrix. It should be noted that alloying element atoms dissolved in the copper matrix act as electron scattering centers, disrupting the lattice periodicity of copper and significantly reducing its conductivity. Controlling the solid solution residues at a low level means that most alloying elements have been successfully converted into precipitates, restoring the electron scattering environment in the matrix to a state close to pure copper. This design ensures that the nanoscale precipitates have a sufficient volume fraction to provide effective dispersion strengthening while maintaining high electrical and thermal conductivity in the copper alloy matrix, meeting the requirements of electronic packaging solder balls for the electrothermal transfer function of the core 100 material. More complete precipitation means less electron scattering by the matrix and better conductivity; simultaneously, a higher volume fraction of precipitates provides stronger resistance to dislocations and better mechanical properties.

[0027] As one implementation method, how to maintain the long-term stability of the size and distribution of the nanoscale precipitate under thermal exposure conditions and avoid the degradation of the strengthening effect due to precipitate coarsening is addressed. Therefore, in this embodiment, the nanoscale precipitate is in a semi-coherent state after pre-stabilization treatment, and its interface with the copper matrix is ​​composed of a partially mismatched dislocation network; the average size of the nanoscale precipitate is 5-15 nanometers, and its bulk density is not less than 10. 20The trace stabilizing elements are enriched at the mismatched dislocations at the semi-coherent interface, and pin the interface through solute-dislocation interaction, inhibiting the further transformation of the precipitated phase toward the incoherent state and the coarsening migration.

[0028] It should be noted that, in the evolution sequence of precipitates, although the fully coherent precipitates have the smallest size and the highest strengthening efficiency, their low interfacial energy makes them thermodynamically unstable. Under thermal exposure, they tend to grow rapidly and lose their coherence. The fully incoherent precipitates, on the other hand, have interfacial migration unconstrained by the crystal lattice and exhibit the fastest coarsening rate.

[0029] Therefore, in this embodiment, the precipitated phase is controlled in a semi-coherent state between the two. At this point, a partial mismatched dislocation network has formed at the interface, exhibiting a certain degree of thermodynamic stability, while still maintaining partial lattice matching with the matrix. The segregation of trace stabilizing element atoms in the core region of the mismatched dislocation firmly pins the mismatched dislocations through the interaction of elastic strain fields and chemical bonding effects, making it difficult for them to climb or slide to accommodate further growth of the precipitated phase, thereby effectively freezing the precipitated phase in an optimized semi-coherent state. Furthermore, the average size and bulk density parameters of the precipitated phase ensure the effective frequency and intensity of hindering dislocation movement, enabling the copper alloy core 100 to maintain a stable high strength level throughout its service life.

[0030] To intuitively characterize the interfacial structure of the aforementioned nanoscale precipitates and the segregation behavior of trace stabilizing elements, the inventors employed various advanced microscopic analysis methods to systematically observe and analyze the copper alloy core sample of Example 1 (Cu-0.8Cr-0.1Zr-0.05Si). Specific parameters for Example 1 are detailed below, and the specific results are as follows: (i) High-resolution transmission electron microscopy characterization of the interface structure between the precipitated phase and the matrix High-resolution transmission electron microscopy (HRTEM, accelerating voltage 200 kV) was used to observe copper alloy core thin film samples after pre-stabilization and aging treatment along the zone axis of the copper matrix. The HRTEM images clearly showed a large number of nanoscale precipitates, approximately 8-12 nm in size, uniformly dispersed within the copper matrix. The precipitates exhibited significant contrast differences in the images, forming a clear interface outline with the matrix region. Analysis of high-resolution lattice images of the interface region between a single precipitate and the copper matrix revealed the following characteristics: the lattice fringes within the precipitate and the lattice fringes of the copper matrix were not completely continuously aligned at the interface, but rather exhibited a periodic mismatch arrangement. Specifically, a lattice mismatch site appeared at the interface every approximately 8-12 copper matrix interplanar spacings, where the lattice fringes showed significant bending and interruption, forming typical mismatch dislocation core features. This periodically arranged mismatched dislocation forms a partially mismatched dislocation network, the existence of which directly proves that the precipitated phase and the copper matrix are in a semi-coherent interface state—neither completely coherent (all lattice points perfectly match) nor completely incoherent (there is no lattice correspondence at the interface).

[0031] Further comparative observations were conducted on the precipitate interface before and after pre-stabilization aging. In samples treated only with conventional aging (without pre-stabilization), the precipitate size was smaller (approximately 4-6 nm), and its interface with the matrix showed a continuous transition of lattice fringes in HRTEM images, with no obvious mismatched dislocations observed, indicating that the precipitate was in a completely coherent state at this time. In contrast, in samples treated with pre-stabilization aging, the precipitate size increased to the range of 8-12 nm, and the aforementioned periodic mismatched dislocation network formed at the interface. However, the lattice fringes maintained good continuous alignment in the non-mismatched regions, indicating that most areas of the interface still maintained a lattice-matched relationship with the matrix, achieving an interface structure that combines partial lattice matching and periodic mismatched dislocations.

[0032] Furthermore, the spatial distribution of the precipitated phases was statistically analyzed using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). In the HAADF-STEM images, due to the atomic number difference between the chromium and copper-zirconium compound precipitates and the copper matrix, the precipitates appear as diffusely distributed particles with different contrast to the matrix. Statistical analysis showed that the average equivalent diameter of the precipitates was 9.2 nm, with a standard deviation of 2.8 nm, and the particle size distribution approximated a log-normal distribution. Within a statistical region of 200 nm × 200 nm, the number of surface projections of the precipitates exceeded 180, corresponding to a stereochemically calculated volume number density of approximately 2.5 × 10². 0 Units / m³, meeting a minimum requirement of 10² 0The requirement is [number] precipitates / m³. The two precipitates (chromium precipitate and copper zirconium compound precipitate) are interwoven in the matrix, and no obvious precipitate aggregation or depletion regions were observed, indicating that the precipitation and prestabilization processes of the two precipitates are uniform and coordinated.

[0033] (II) Atomic probe chromatography and STEM-EDS characterization of trace element interfacial segregation To quantitatively characterize the distribution of trace stabilizing element silicon at the precipitate / matrix interface, three-dimensional atomic-scale compositional analysis of the copper alloy core sample from Example 1 was performed using atomic probe chromatography (APT). The APT sample was prepared as a needle tip using a dual-beam focused ion beam (FIB) system and field evaporation was performed at 50 K in laser pulse-assisted mode. The reconstructed three-dimensional atomic map results showed that multiple nanoscale precipitate regions could be clearly identified within an analysis volume of approximately 60 nm × 60 nm × 120 nm. The chromium precipitate was characterized as a rich cluster dominated by chromium atoms, while the copper-zirconium compound precipitate was characterized as a cluster region enriched with both copper and zirconium atoms.

[0034] One-dimensional concentration profiles (proxigram analysis) were plotted at the interface between the chromium precipitate and the copper matrix. The results showed that in the copper matrix region far from the precipitate, the silicon concentration remained at approximately 0.02–0.03 at% (close to the average atomic percentage of silicon in the alloy). As the analysis position gradually moved closer to the precipitate / matrix interface from the matrix side, a significant peak enrichment of silicon concentration appeared at the interface, reaching a peak concentration of 0.8–1.2 at%, approximately 30–50 times the average concentration in the matrix. The full width at half maximum (FWHM) of this concentration peak was approximately 1.5–2.0 nm, comparable to the size of the mismatched dislocation core region observed by HRTEM. After crossing the interface into the interior of the chromium precipitate, the silicon concentration rapidly decreased to near zero. Similar proxigram analysis was performed on the interface of the copper-zirconium compound precipitate. Silicon also exhibited a significant concentration peak at the interface of this type of precipitate (peak concentration approximately 0.6–1.0 at%), but the peak width and height were slightly lower than at the chromium precipitate interface. The APT results clearly confirm that silicon, as a trace stabilizing element, mainly segregates in the interface region between the precipitate and the matrix, rather than being uniformly distributed in the matrix or dissolved in the interior of the precipitate.

[0035] As a supplement and cross-validation to the APT analysis, elemental surface mapping analysis was performed on the precipitate interface region using aberration-corrected scanning transmission electron microscopy combined with energy dispersive spectroscopy (Cs-corrected STEM-EDS). Guided by HAADF-STEM images, high spatial resolution EDS elemental mapping scans were performed on the region containing a single precipitate and its surrounding matrix (pixel dwell time 200 ms, multi-frame accumulation to improve signal-to-noise ratio). The Cr elemental mapping clearly showed the high enrichment of chromium within the precipitate; the Cu and Zr elemental mappings confirmed the locations of the matrix and the copper-zirconium compound precipitates, respectively. Although the Si elemental mapping showed a weak absolute signal intensity due to the extremely low silicon content, after multi-frame accumulation and background subtraction, an annular or shell-like enhancement pattern of the silicon signal was still observed at the interface region between the precipitate and the matrix—that is, the enhanced region of the silicon signal was not located at the center of the precipitate or inside the matrix, but rather concentrated at the edge contour of the precipitate. Line scan signals extracted from the interface regions of multiple precipitated phases were superimposed and statistically analyzed. The EDS signal intensity of silicon at the interface was about 3-5 times that of the matrix background signal, which is in order of magnitude consistent with the segregation factor given by APT analysis (the spatial resolution and detection sensitivity of EDS are lower than those of APT, so the lower absolute factor is normal).

[0036] The aforementioned HRTEM / HAADF-STEM microstructure characterization and APT / STEM-EDS elemental analysis results corroborate each other, fully demonstrating the following two key technical characteristics from an experimental perspective: First, after pre-stabilization and aging treatment, the nanoscale precipitated phase is indeed in a semi-coherent state, and its interface with the copper matrix consists of a periodically arranged network of partially mismatched dislocations; second, trace stabilizing elements (represented by silicon) do indeed segregate at the interface between the precipitated phase and the matrix, with their segregation locations coinciding with the core regions of mismatched dislocations, and their segregation concentration significantly higher than the average level of the matrix. These two experimental observations provide direct microstructure and chemical composition evidence for the technical mechanism of "solute-dislocation interaction pinning the interface to suppress precipitated phase coarsening" proposed in this invention.

[0037] As one implementation method, after pre-stabilization treatment, the residual solid solution content of chromium in the copper alloy matrix does not exceed 0.05 wt%, and the residual solid solution content of zirconium does not exceed 0.01 wt%, so that the matrix lacks a source of solute supply for the existing precipitates to continue to grow during reflow soldering heat exposure.

[0038] It should be noted that the inventors discovered that even if the precipitate interface has been pinned and stabilized, if a high concentration of solid solution alloying elements remains in the matrix, these solute atoms may still diffuse and accumulate towards the existing precipitate during heat exposure, providing material for its continued growth. Therefore, in this embodiment, the residual solid solution content in the matrix is ​​further reduced to an extremely low level, because the coarsening and growth of the precipitate requires solute atoms in the matrix to continuously migrate and replenish the precipitate through diffusion. Once the solid solution alloying elements in the matrix have been sufficiently desoluble into the precipitate, there is a lack of solute source in the matrix required for the continued growth of the existing precipitate. Even under the high-temperature exposure conditions of reflow soldering, the solute concentration gradient around the precipitate is extremely small, and the diffusion driving force is insufficient to promote the effective growth of the precipitate. This solute starvation mechanism, together with the interface pinning mechanism in the above embodiment, provides dual protection, jointly suppressing the coarsening of the precipitate from both the driving force and kinetic levels. The former cuts off the material basis of coarsening by eliminating solute supply, while the latter prevents coarsening through the structural pathway of interface pinning. The synergy of the two enables the nanoscale precipitated phase to have excellent dimensional stability under thermal exposure conditions.

[0039] In one implementation, the interface control layer 200 is a nickel layer or a nickel-phosphorus alloy layer with a thickness of 1-5 micrometers. After reflow soldering, the interface between the interface control layer 200 and the coating layer 300 forms a (Cu,Ni)6Sn5 type intermetallic compound layer with a continuous, flat and dense morphology.

[0040] In detail, nickel and nickel-phosphorus alloys are effective diffusion-blocking materials. The diffusion coefficient of nickel atoms in the copper lattice is much lower than that of copper atoms in tin. Therefore, the nickel layer can significantly slow down the rate at which copper atoms migrate through the interface control layer 200 to the solder side. During reflow soldering, the (Cu,Ni)6Sn5 type intermetallic compound formed by the reaction of nickel and tin-based solder has higher thermodynamic stability and a lower interface growth rate compared to pure Cu6Sn5. Its continuous, flat, and dense morphology means that there are no stress concentration sites and micropores at the interface caused by irregular growth of intermetallic compounds. The presence of phosphorus in the nickel-phosphorus alloy layer further improves the amorphization degree of the nickel layer, eliminates fast diffusion short-circuit channels such as grain boundaries, and enhances the blocking effect on copper atom diffusion. The thickness of the interface control layer 200 is set in the range of 1-5 micrometers to balance the effectiveness of the blocking effect and avoid the accumulation of interface stress caused by excessive nickel layer thickness. By selecting appropriate material and thickness of the interface control layer 200, it can effectively suppress copper atom diffusion while forming an interface microstructure with good mechanical integrity after reflow soldering.

[0041] In one embodiment, the solder alloy of the cladding layer 300 is selected from Sn-Ag-Cu alloy, Sn-Ag alloy or Sn-Cu alloy, and the thickness of the cladding layer 300 is 10-100 micrometers.

[0042] Among them, Sn-Ag-Cu alloys possess the best comprehensive mechanical properties and creep resistance due to their inclusion of silver and copper eutectic components; Sn-Ag alloys exhibit good wettability and high shear strength; Sn-Cu alloys have lower cost and good metallurgical compatibility with copper substrates. The thickness of the cladding layer 300, ranging from 10 to 100 micrometers, requires balancing several considerations: Too thin a thickness will result in insufficient solder to form complete solder joint wetting and filling during reflow soldering, affecting soldering reliability; too thick a thickness will weaken the volume proportion of the copper alloy core 100 in the overall solder ball, reducing the core 100's contribution to the mechanical properties of the solder joint. Within this thickness range, the cladding layer 300 provides sufficient solder to meet the reflow soldering interconnection needs while maintaining the dominant position of the copper alloy core 100 in the solder ball, allowing the mechanical reinforcement effect of the core-shell structure to be fully utilized.

[0043] Reference Figure 2 Another embodiment of this application provides a method for preparing electronic packaging solder balls as described above, comprising the following steps: S1. Alloy Melting: Copper alloy raw materials containing at least one precipitation strengthening element from chromium and zirconium, and at least one trace stabilizing element selected from silicon, niobium, and rare earth elements, are melted under a protective atmosphere with an oxygen content ≤ 5 ppm to obtain a copper alloy melt. The melt is then purified to reduce the oxide inclusion content. The strict control of the low-oxygen atmosphere is because chromium and zirconium are both strong oxidizing elements, which readily react with oxygen at the melting temperature to form oxide inclusions. These inclusions not only consume the effective alloying element content but also become heterogeneous nucleation cores during subsequent balling, leading to uneven solidification structure and reducing the strength and fatigue life of the core as mechanical defects. The purification treatment further removes residual oxide particles from the melt to ensure its purity.

[0044] S2. Uniform Droplet Spraying into Balls: The copper alloy molten material is passed through a high-temperature resistant ceramic nozzle with a wetting angle greater than 90° to form a continuous jet. Controlled-frequency vibrations are applied to the jet to break it into uniform droplets. During flight, the droplets spherize due to surface tension and move at a speed of 10... 3 -10 5Rapid solidification at a cooling rate of K / s yields spherical copper alloy core microspheres with alloying elements in a supersaturated solid solution state. A wetting angle greater than 90° between the ceramic nozzle and the copper alloy melt ensures the melt passes through the nozzle in a non-wetting state, preventing melt adhesion to the nozzle inner wall and thus avoiding blockage and jet instability. Controlled frequency vibration causes the jet to break into uniformly sized droplets at a defined wavelength under Rayleigh instability, resulting in a microsphere population with an extremely narrow particle size distribution. The rapid solidification at the high cooling rate prevents alloying elements from undergoing long-range diffusion segregation, freezing them in a supersaturated solid solution state. This provides an ideal compositional homogeneity prerequisite for the uniform and high-density nucleation of precipitates during subsequent aging treatment.

[0045] S3. Aging Strengthening Treatment: The core microspheres undergo aging treatment to induce precipitation strengthening elements in the supersaturated solid solution state to precipitate and form nanoscale precipitates. Pre-stabilization aging is then performed to evolve the precipitates to a thermodynamically relatively stable state. The aging treatment causes chromium and zirconium to desolvate from the supersaturated matrix, uniformly nucleate within the matrix, and grow to nanoscale sizes, forming a dispersed network that hinders dislocation movement. The significance of the pre-stabilization aging step lies in orderly advancing the precipitates from the initially fully coherent metastable state to a semi-coherent stable state. Combined with the segregation and enrichment of trace stabilizing elements at the interface, this locks in the size and interface structure of the precipitates, enabling them to withstand disturbances from subsequent reflow soldering and thermal exposure during service.

[0046] S4. Preparation of the Interface Control Layer: The surface of the aged core microspheres is cleaned and activated to form an interface control layer on their surface to inhibit the outward diffusion of copper atoms. The surface cleaning and activation step removes oxide films and contaminants that may form on the surface of the microspheres during the aging process, ensuring the bonding force between the interface control layer and the copper alloy core. The formation of the interface control layer provides an outer surface with controllable chemical properties and reactivity for subsequent solder coating, making the interface metallurgical behavior of the entire core-shell structure under design control.

[0047] S5. Solder Coating: A tin-based solder alloy layer is coated onto the outer surface of the interface control layer to form a core-shell structured solder ball. The solder coating layer endows the solder ball with the ability to melt and wet at reflow soldering temperatures and form a metallurgical bond with the solder pad, completing the final preparation of the core-shell structured solder ball.

[0048] The above five steps, from alloy composition control, microsphere morphology and microstructure control, precipitate stability control, interface diffusion control to solder function assignment, form a complete preparation technology chain, establish a complete preparation process flow with each step connected and matched, and realize the batch stable preparation of core-shell structured solder balls with the above-mentioned microstructure and interface structure characteristics.

[0049] In one implementation method, the aging treatment in step S3 is carried out in a fluidized bed mode, that is, the core microspheres are heated and kept warm in a suspended fluidized state in an upward inert airflow to avoid contact sintering between microspheres and to ensure the uniformity of heat treatment temperature of batch microspheres; the cooling stage is switched to cold inert airflow to achieve uniform cooling.

[0050] The technical principle of fluidized bed technology applied to microsphere heat treatment lies in the following: when the velocity of the rising airflow reaches the critical fluidization velocity, the microsphere particles are lifted by the airflow and detached from the stacked contact state, with each particle in an independent suspended motion state within the airflow. This state fundamentally eliminates the opportunity for solid-state contact between microspheres, preventing the necking and sintering phenomenon between microspheres caused by surface diffusion at aging temperatures. Simultaneously, the microspheres suspended in the airflow have a large heat exchange area and a very high convective heat transfer coefficient with the heat carrier airflow, enabling each microsphere to reach the set aging temperature uniformly and rapidly, with minimal temperature differences between batches of microspheres. The high uniformity of the aging temperature directly ensures the consistency of the size, density, and distribution of the precipitated phase in the core microspheres of the batch, which is crucial for the batch-to-batch consistency of the final solder ball product performance. The cooling stage switches to a cold, inert airflow, again utilizing the efficient and uniform heat exchange characteristics of the fluidized bed state to achieve rapid and uniform cooling, precisely terminating the growth process of the precipitated phase and preventing excessive growth and coarsening of the precipitated phase in some microspheres due to uneven cooling.

[0051] It should be noted that the gas flow rate setting is a key parameter for the stable operation of the fluidized bed. For copper alloy microspheres with a particle size of 80-300 μm and a density of approximately 8.9 g / cm³, the critical fluidization velocity of argon gas at an aging temperature of 400-500°C is approximately 0.02-0.15 m / s (increasing with increasing particle size). In actual operation, the gas velocity is set to 1.5-3.0 times the critical fluidization velocity to ensure that all microspheres are fully suspended and maintain a stable bubbling fluidization state, while avoiding excessively high gas velocities that could cause microspheres to be blown out of the bed. For uniformly jetted microspheres with a narrow particle size distribution (coefficient of variation ≤3%), the above-mentioned gas velocity range is sufficient to ensure that all microspheres simultaneously achieve a good fluidization state. If the particle size distribution of individual batches is slightly wider, the particles are first classified by vibrating screen before aging treatment. The microspheres are divided into 2-3 narrow distribution ranges according to particle size and processed in batches. The gas velocity is set independently for each batch to eliminate the problem of fluidization deviation or entrainment caused by the difference in critical fluidization velocity of microspheres of different particle sizes.

[0052] Controlling the purity of the inert atmosphere is crucial for protecting the microsphere surface from oxidation. Chromium and zirconium are both strong oxidizing elements; even trace amounts of oxygen at the aging temperature can cause the formation of chromium oxide or zirconium oxide films on the microsphere surface. Therefore, in addition to the purity requirements of the carrier gas itself, the following measures are taken: the fluidized bed reactor is purged with carrier gas for at least 30 minutes before heating, reducing the residual air oxygen content at the outlet to below 1 ppm; all reactor connection flanges are sealed with metal gaskets to prevent air infiltration; an online oxygen analyzer is installed at the gas outlet to monitor the oxygen content in the exhaust gas in real time, triggering an alarm and automatically increasing the carrier gas flow rate for dilution once the detected value exceeds 2 ppm.

[0053] In one implementation method, step S4 involves a two-step surface cleaning and activation process: first, the aged microspheres are immersed in a 5%-10% (v / v) dilute hydrochloric acid solution for 30-60 seconds to remove the chromium oxide and zirconium oxide films formed on the surface of the microspheres during the aging process; then, they are rapidly transferred to a 1%-3% (v / v) hydrochloric acid-palladium chloride activation solution (palladium chloride concentration 0.1-0.5 g / L) for 20-40 seconds to allow the microspheres to adsorb catalytically active palladium nuclei, providing uniformly distributed nucleation sites for subsequent electroless nickel plating. After activation, the microspheres are thoroughly rinsed with deionized water and rapidly transferred to the plating process under nitrogen protection, with an intermediate exposure time not exceeding 5 minutes to prevent the copper alloy surface from re-oxidizing in the air and reducing the adhesion of the plating layer.

[0054] Preferably, the interface control layer is formed using a chemical nickel-phosphorus alloy plating method. The basic formulation of the chemical plating solution is as follows: 20-30 g / L nickel sulfate as the nickel source, 15-25 g / L sodium hypophosphite as the reducing agent and phosphorus source, 15-20 ml / L lactic acid and 10-15 g / L sodium citrate as complexing agents to control the concentration of free nickel ions and the deposition rate, and 8-12 g / L sodium acetate as a buffer. The pH value of the plating solution is controlled within the range of 4.6-5.2 (adjusted with ammonia or dilute sulfuric acid), the plating temperature is 80-90°C, and the plating time is adjusted according to the target thickness, typically ranging from 10-40 minutes, corresponding to a deposition rate of approximately 8-15 μm / h. Under these process conditions, the phosphorus content in the nickel-phosphorus alloy coating is 8-12 wt%, belonging to a high-phosphorus amorphous nickel-phosphorus alloy. Its amorphous structure eliminates fast diffusion short-circuit channels such as grain boundaries, and its diffusion blocking effect on copper atoms is superior to that of crystalline pure nickel coatings.

[0055] In one implementation method, in step S5, the solder coating employs an electrochemical deposition method to deposit a tin-based solder alloy layer by layer on the outer surface of the interface control layer. Taking the Sn-3.0Ag-0.5Cu alloy coating layer as an example, a stepwise electrodeposition-diffusion alloying process is adopted: firstly, a tin layer, a silver layer, and a copper layer are sequentially electrodeposited on the surface of the core microspheres of the pre-plated nickel-phosphorus interface control layer; then, low-temperature diffusion annealing is used to allow the three metal layers to diffuse into each other, forming a uniform Sn-Ag-Cu alloy. The electrodeposition is carried out in a drum electroplating apparatus, which utilizes the rotation of a porous drum to continuously tumble the microspheres in the plating solution, ensuring uniform contact between the surface of each microsphere and the current field.

[0056] The plating solution composition for tin electrodeposition is: 30-50 g / L stannous methanesulfonate (as Sn²⁺). + Methylsulfonic acid (60-100 ml / L) is used as a conductive salt and complexing agent, and an appropriate amount of brightener is added. The plating bath temperature is 20-30°C, the cathode current density is 0.5-2.0 A / dm², and the plating time is adjusted according to the total thickness of the target solder layer. The silver and copper layers are deposited from potassium silver cyanide plating bath and acidic copper sulfate plating bath, respectively, and their thicknesses are determined according to the composition ratio of Sn-3.0Ag-0.5Cu. After the deposition of each layer, the microspheres are diffused annealed at 150-180°C in a nitrogen atmosphere for 2-6 hours to allow tin, silver, and copper atoms to diffuse into each other and form a uniform Sn-Ag-Cu alloy coating layer. The annealing temperature is lower than the melting point of tin (232°C), and alloying is completed in the solid state, thereby maintaining the morphological integrity of the coating layer.

[0057] The final quality of the coating layer is inspected in the following ways: SEM cross-sectional observation is used to confirm that the coating layer thickness is within the target range (10-100 micrometers) and is uniform; EDS line scanning or area scanning is used to confirm that Sn, Ag, and Cu elements are uniformly distributed in the coating layer and that the composition ratio meets the design requirements; and differential scanning calorimetry (DSC) is used to determine the melting temperature of the coating layer to verify whether the alloying is complete.

[0058] The beneficial effects of the above embodiments are as follows: 1. By incorporating reinforcing elements such as chromium and zirconium into the copper alloy core, along with small amounts of silicon, niobium, or rare earth elements as stabilizers, the core material possesses both high strength and excellent heat resistance. During heat treatment, the reinforcing elements precipitate from the alloy, forming numerous extremely fine reinforcing particles. These particles are densely dispersed throughout the matrix, acting like a net to firmly prevent microscopic deformation within the material, fundamentally improving the core's strength and creep resistance. More importantly, because many alloying elements have transformed from being "dissolved in copper" into independent particles, the copper matrix itself returns to a near-pure copper state, with minimal impact on electrical and thermal conductivity. In other words, the solder ball core is both robust and unobstructed, achieving both mechanical support and electrothermal transfer.

[0059] 2. More importantly, this invention creatively utilizes trace amounts of stabilizing elements to provide an additional layer of reinforcement to these reinforcing particles. These stabilizing elements actively aggregate at the interface between the reinforcing particles and the copper matrix, locking the interface at the atomic level. Ordinary reinforcing particles grow larger and coarser during repeated heating, gradually diminishing their reinforcing effect. The stabilizing elements in this invention act like a ring of nails on the interface, significantly inhibiting particle growth. As a result, even after the solder balls experience the high temperatures of reflow soldering and repeated temperature changes during long-term use, these reinforcing particles remain small and uniform, and the core strength does not significantly decrease throughout the entire service life. This approach of proactively imparting heat resistance stability to the material at the microstructural level breaks free from the limitations of traditional methods that only pursue initial performance while ignoring subsequent structural changes during use.

[0060] 3. This invention also incorporates an interface control layer between the copper alloy core and the outer tin-based solder, further enhancing the long-term reliability of the solder ball from the perspective of interface reaction control. This barrier effectively prevents the rapid diffusion of copper atoms to the solder side, avoiding excessive reaction between copper and tin. During reflow soldering, the intermetallic compound layer formed between the interface control layer and the solder is smooth, dense, and firmly bonded, unlike irregularly grown compounds that can create stress concentrations and microcracks at the interface. Simultaneously, this barrier effectively suppresses the formation of Kirkendal voids, eliminating the risk of sudden brittle fracture of the solder joint due to void accumulation at the interface. The interface control layer, core reinforcement design, and solder coating layer work together to comprehensively optimize the mechanical, electrical, and thermal properties of the entire core-shell solder ball.

[0061] 4. Regarding the preparation method, this invention employs a uniform droplet jetting spheroidization technique to prepare copper alloy core microspheres, replacing the traditional atomization method. This technique precisely controls the vibration frequency to break the metal jet into droplets of uniform size, ensuring high sphericity and uniform size distribution of the microspheres from the source. The rapid solidification process freezes the alloying elements in a supersaturated state, laying a good microstructure foundation for the uniform precipitation of strengthening particles during subsequent heat treatment. The heat treatment stage uses a fluidized bed approach, allowing the microspheres to be suspended in an inert gas flow and uniformly heated. This avoids the adhesion and sintering between microspheres and ensures consistent heat treatment effects for each microsphere, solving the long-standing problems of uneven heating and adhesion in traditional methods. The pre-stabilization aging step allows the strengthening particles to evolve to a relatively stable state, while simultaneously allowing excess solid solution elements in the matrix to fully precipitate, locking in the strengthening state of the core at both the microstructure and composition levels, enabling it to withstand thermal shock without significant degradation during subsequent reflow soldering and service. The entire process, from melting, pelletizing, heat treatment to surface treatment, forms a complete technology chain. Each link is closely connected, ensuring the consistency and repeatability of the final product performance.

[0062] The present invention will be further described in detail below with reference to specific embodiments: To comprehensively verify the technical effects of this invention, a total of 9 sets of embodiments and 6 sets of comparative examples were designed. The 9 sets of embodiments cover the technical solution space defined by the claims from different perspectives, including different alloy composition ratios, different types of stabilizing elements, and different content levels. The 6 sets of comparative examples each focus on a single variable comparison of the core technical features of this invention: Comparative Example 1 removed trace stabilizing elements; Comparative Example 2 omitted pre-stabilization aging treatment; Comparative Example 3 used a pure copper core; Comparative Example 4 eliminated the interface control layer; Comparative Example 5 exceeded the specified range for rare earth element addition; and Comparative Example 6 used traditional atomization spheroidization and fixed-bed heat treatment processes. Through this systematic comparative design, the independent contribution and synergistic effect of each technical feature of this invention to the final performance can be clearly presented.

[0063] Test conditions: Hardness testing was performed using the Vickers hardness test (load 100g); reflow soldering conditions were a peak temperature of 260°C and a holding time of 60s, for a total of 3 reflow soldering cycles; thermal cycling reliability testing conditions were -40°C to 125°C, with a holding time of 15min each time, for a total of 1000 cycles; shear strength testing was performed using the solder ball pushing method; precipitate size and distribution were observed using transmission electron microscopy (TEM); conductivity was tested using the eddy current method; the thickness of the intermetallic compound (IMC) layer and Kirkendal voids were observed using scanning electron microscopy (SEM) cross-section.

[0064] Table 1 lists the copper alloy core composition designs for each embodiment and comparative example, with the remainder being copper and unavoidable impurities.

[0065]

[0066] Table 2 summarizes the key preparation process parameters for each example and comparative example. All examples were melted under argon protection with an oxygen content ≤ 5 ppm.

[0067]

[0068] Table 3 lists the microstructure characteristics and mechanical properties of the copper alloy core for each group of solder balls. The hardness retention rate is calculated as (hardness after 3 reflows / initial hardness) × 100%.

[0069]

[0070] Table 4 lists the solder joint shear strength, thermal cycling reliability data, interface microstructure characteristics, and manufacturing consistency indicators for each group of solder balls. Thermal cycling conditions: -40°C to 125°C, 1000 cycles.

[0071]

[0072] Results analysis: (1) The key role of trace stabilizing elements (Example 1 vs Comparative Example 1) The alloy compositions of Example 1 and Comparative Example 1 differed only in the presence or absence of trace stabilizing elements (0.05% Si); all other conditions were identical. After three reflow soldering cycles, Example 1 maintained a hardness retention of 95.5%, with the precipitate size increasing only slightly from 8.2 nm to 9.5 nm. In contrast, the hardness retention of Comparative Example 1 plummeted to 81.1%, with the precipitate size coarsening from 8.0 nm to 18.5 nm, an increase of over 130%. Regarding solder joint reliability, Example 1 maintained a shear strength retention of 87.2% after 1000 thermal cycles, while Comparative Example 1 maintained only 69.1%. This comparison clearly demonstrates that the segregation and pinning effect of trace stabilizing elements at the precipitate interface is crucial for maintaining the thermal stability of the precipitate and the long-term mechanical properties of the core.

[0073] (2) The necessity of pre-stabilization aging treatment (Example 1 vs Comparative Example 2) Comparative Example 2, while containing the same stabilizing elements as Example 1, omitted the pre-stabilization aging step. Its initial hardness (182 HV) was even slightly higher than Example 1 (178 HV), because the unstabilized precipitates were finer (4.5 nm vs 8.2 nm), in a fully coherent peak-aged state. However, after three reflow soldering cycles, the hardness of Comparative Example 2 dropped sharply to 138 HV (retention rate only 75.8%), and the precipitate size increased dramatically to 22.0 nm. Simultaneously, the residual solid solutions of Cr and Zr in the matrix were as high as 0.12% and 0.028%, respectively, far exceeding the 0.038% and 0.008% of Example 1, providing sufficient solute supply for precipitate coarsening. This demonstrates that stabilizing elements alone, without pre-stabilization treatment, cannot achieve long-term stability of the precipitates—interfacial pinning and solute starvation mechanisms must function simultaneously.

[0074] (3) The fundamental improvement of core mechanical properties by precipitation enhancement (Example 1 vs Comparative Example 3) Comparative Example 3 used a pure copper core, with an initial hardness of only 65 HV and a yield strength of 135 MPa, far lower than the 178 HV and 435 MPa of Example 1. More importantly, the initial shear strength of the pure copper core solder ball was only 28.5 MPa, which further decreased to 16.2 MPa after 1000 thermal cycles (retention rate 56.8%). In addition, due to the lack of effective protection from the interface control layer, a violent metallurgical reaction occurred between the pure copper and the solder, resulting in an IMC layer thickness of up to 4.5 μm and the appearance of obvious Kirkendal voids. The precipitation-strengthened copper alloy core showed a qualitative leap in mechanical properties compared to the pure copper core, while maintaining a high conductivity of 82% IACS.

[0075] (4) The interface control layer ensures the long-term reliability of the solder joints (Example 1 vs Comparative Example 4) Comparative Example 4, without the Ni-P interface control layer, exhibited identical mechanical properties to Example 1 (same core formulation and process), but its solder joint reliability was drastically different. Without the interface control layer, the IMC layer thickness after reflow soldering increased from 1.8 μm in Example 1 to 5.2 μm, with numerous Kirkendal voids appearing. Although the initial shear strength was only slightly lower than Example 1 (44.0 vs 48.5 MPa), after 1000 thermal cycles, the shear strength of Comparative Example 4 rapidly decreased to 25.8 MPa (58.6% retention), while Example 1 maintained 42.3 MPa (87.2% retention). This result demonstrates that the interface control layer is indispensable for suppressing copper atom diffusion, controlling interfacial reactions, and ensuring the long-term mechanical integrity of the solder joint.

[0076] (5) Reasonable control of rare earth element addition (Example 4 vs Comparative Example 5) Example 4 added 0.04% Ce (Ce / Zr = 0.4:1, meeting the requirement of ≤1:2), and Comparative Example 5 added 0.15% Ce (Ce / Zr = 1.5:1, exceeding the specified range). In Comparative Example 5, the excess rare earth element formed a rare earth-zirconium compound with zirconium, consuming the effective solid-solution zirconium involved in precipitation strengthening. This resulted in an initial hardness of only 155 HV (176 HV in Example 4), a yield strength of only 375 MPa (430 MPa in Example 4), and a decrease in conductivity to 78% IACS. This verifies the necessity of limiting the mass ratio of rare earth to zirconium to ≤1:2 in this invention—exceeding this range will impair the precipitation strengthening effect.

[0077] (6) The process advantages of uniform droplet jetting into spheres and fluidized bed heat treatment (Example 1 vs Comparative Example 6) Comparative Example 6 uses traditional gas atomization to form microspheres and fixed-bed heat treatment instead of the uniform droplet spraying and fluidized bed scheme of this invention. Regarding manufacturing consistency, the sphericity of the microspheres in Comparative Example 6 is only 0.90~0.96 (Example 1 ≥ 0.98), and the particle size variation coefficient is as high as 8.5% (Example 1 only 2.1%). In terms of performance, due to uneven cooling rates during atomization leading to component segregation and uneven fixed-bed heat treatment temperature, the hardness retention rate of Comparative Example 6 is only 91.2% (vs 95.5%), and the coarsening of the precipitated phase is more pronounced (9.5→13.5nm vs 8.2→9.5nm). After 1000 cycles, the shear strength retention rate is only 81.1% (vs 87.2%). This indicates that precise control of the manufacturing process has a significant impact on the consistency and reliability of the final product performance.

[0078] (7) Applicability of different stabilizing elements (comparison of Examples 1 / 3 / 4 / 8) Examples 1 (Si), 3 (Nb), 4 (Ce), and 8 (La) employed different stabilizing elements, achieving hardness retention rates of 95.5%, 96.1%, 95.5%, and 95.4%, respectively. The coarsening of the precipitated phase and the retention rate of shear strength were similar. Example 3, containing Nb, performed slightly better, possibly due to the stronger chemical affinity of Nb atoms at the interface. Example 5 (Si+Nb), with its composite stabilizing elements, achieved the highest hardness retention rate (96.7%) and shear strength retention rate (89.0%), demonstrating the synergistic advantages of multi-element stabilization. Overall, the Si, Nb, and rare earth elements specified in this invention can effectively perform the function of pinning and stabilizing the precipitated phase interface.

[0079] (8) Validation of component range coverage (Examples 6 / 7) Examples 6 and 7 represent the lower and upper limits of the composition range defined in the claims, respectively. Although the initial hardness and strength of Example 6 (Cu-0.3Cr-0.05Zr-0.03Si) are lower than the intermediate formulation (148HV, 362MPa), the hardness retention rate is still 94.6%, the shear strength retention rate is 85.2%, and the conductivity is as high as 88% IACS, making it suitable for applications with higher conductivity requirements. Example 7 (Cu-1.2Cr-0.2Zr-0.1Si) achieved the highest initial hardness (192HV) and yield strength (472MPa), with a hardness retention rate of 95.3%, but the conductivity was relatively low (76% IACS). This indicates that within the composition range defined by this invention, strength and conductivity can be flexibly adjusted according to specific application requirements, while maintaining excellent thermal stability.

[0080] Comparing the above embodiments and comparative examples, it can be seen that the present invention, through the introduction of trace stabilizing elements in the alloy composition, pre-stabilization aging treatment, interface control layer design, and a preparation process combining uniform droplet spraying into spheres with fluidized bed heat treatment, effectively solves the technical bottlenecks of existing core-shell structure solder balls in terms of core mechanical properties, thermal stability, interface reaction control, and manufacturing consistency. All embodiments exhibit a hardness retention rate of over 94%, a shear strength retention rate of over 85% after 1000 thermal cycles, an IMC layer thickness controlled within 2 μm without Kirkendal voids, and microsphere sphericity ≥0.98 and particle size variation coefficient ≤2.4%, fully meeting the stringent requirements of advanced electronic packaging for highly reliable solder joint interconnection.

[0081] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A high-reliability electronic packaging solder ball, characterized in that, The solder ball has a core-shell structure, including: The core is composed of a copper alloy containing at least one of chromium and zirconium as precipitation strengthening elements, and at least one of silicon, niobium, and rare earth elements as trace stabilizing elements, the total content of which is 0.01%-0.15 wt%. The copper alloy matrix contains nanoscale precipitates, and the trace stabilizing elements are segregated at the interface between the nanoscale precipitates and the matrix. An interface control layer is formed on the outer surface of the core to suppress the diffusion and migration of copper atoms in the core to the outer solder layer. A cladding layer is applied to the outer surface of the interface control layer, and the cladding layer is composed of a tin-based solder alloy with a melting point lower than that of the copper alloy.

2. The electronic packaging solder ball as described in claim 1, characterized in that, The copper alloy contains 0.3%-1.2% chromium by mass and 0.03%-0.25% zirconium by mass, with the balance being copper, the trace stabilizing elements, and unavoidable impurities. The mass ratio of chromium to zirconium is (3-15):

1. This mass ratio is set so that the nucleation density and size distribution of the chromium precipitate and the copper zirconium compound precipitate match during the aging process, together forming a dispersed network that hinders dislocation movement.

3. The electronic packaging solder ball as described in claim 2, characterized in that, When the trace stabilizing element contains rare earth elements, the rare earth elements are selected from lanthanum and / or cerium, and the mass ratio of the rare earth elements to zirconium is ≤1:2, so as to avoid the formation of rare earth-zirconium compounds by the rare earth elements and zirconium, which would excessively consume the amount of solid-solution zirconium in the matrix that effectively participates in precipitation strengthening.

4. The electronic packaging solder ball as described in claim 2 or 3, characterized in that, In the copper alloy matrix, the solid solution residues of chromium and zirconium do not exceed 15% of their respective total additions, and the remaining chromium and zirconium are desoluble in the matrix in the form of nanoscale precipitates, so that the copper alloy matrix retains the intrinsic conductive channel characteristics of a high-purity copper matrix.

5. The electronic packaging solder ball as described in any one of claims 1 to 3, characterized in that, The nano-scale precipitated phase is in semi-coherent state after pre-stabilization treatment, and the interface between the nano-scale precipitated phase and the copper matrix is composed of partial misfit dislocation network; the average size of the nano-scale precipitated phase is 5-15 nanometers, and the bulk density is not less than 10 20 / m³; the trace stabilizing element is enriched at the misfit dislocation of the semi-coherent interface, pins the interface through solute-dislocation interaction, and inhibits further transformation of the precipitated phase to incoherent state and coarsening migration.

6. The electronic packaging solder ball as described in claim 5, characterized in that, After the pre-stabilization treatment, the residual solid solution content of chromium in the copper alloy matrix does not exceed 0.05 wt%, and the residual solid solution content of zirconium does not exceed 0.01 wt%, resulting in a lack of solute supply source in the matrix for the existing precipitates to continue to grow during reflow soldering heat exposure.

7. The electronic packaging solder ball as described in any one of claims 1 to 3, characterized in that, The interface control layer is a nickel layer or a nickel-phosphorus alloy layer with a thickness of 1-5 micrometers. After reflow soldering, the interface between the interface control layer and the coating layer forms a (Cu,Ni)6Sn5 type intermetallic compound layer with a continuous, flat and dense morphology.

8. The electronic packaging solder ball as described in claim 7, characterized in that, The solder alloy of the cladding layer is selected from Sn-Ag-Cu alloy, Sn-Ag alloy or Sn-Cu alloy, and the thickness of the cladding layer is 10-100 micrometers.

9. A method for preparing electronic packaging solder balls as described in any one of claims 1-8, characterized in that, Includes the following steps: S1. Alloy smelting: Copper alloy raw materials containing at least one precipitation strengthening element from chromium and zirconium and at least one trace stabilizing element from silicon, niobium and rare earth elements are smelted in a protective atmosphere with an oxygen content of ≤5 ppm to obtain copper alloy melt, and the melt is purified to reduce the oxide inclusion content. S2. Uniform Droplet Spraying into Balls: The copper alloy melt is passed through a high-temperature resistant ceramic nozzle with a wetting angle greater than 90° to form a continuous jet. Controlled-frequency vibration is applied to the jet to break it into uniform droplets. During flight, the droplets spherize due to surface tension and move at a 10... 3 -10 5 Rapid solidification at a cooling rate of K / s yields spherical copper alloy core microspheres, in which the alloying elements are in a supersaturated solid solution state. S3. Aging strengthening treatment: The core microspheres are subjected to aging treatment to precipitate the precipitation strengthening elements in the supersaturated solid solution state to form a nanoscale precipitate phase, and pre-stabilization aging is performed to evolve the precipitate phase to a thermodynamically relatively stable state. S4. Preparation of interface control layer: The surface of the aged core microspheres is cleaned and activated, and an interface control layer for inhibiting the outward diffusion of copper atoms is formed on its surface; S5. Solder coating: A tin-based solder alloy layer is coated on the outer surface of the interface control layer to form a core-shell structure solder ball.

10. The preparation method according to claim 9, characterized in that, The aging treatment described in step S3 is carried out in a fluidized bed mode, that is, the core microspheres are heated and kept warm in a suspended fluidized state in an upward inert airflow to avoid contact sintering between microspheres and to ensure the uniformity of heat treatment temperature of batch microspheres; the cooling stage is switched to cold inert airflow to achieve uniform cooling.

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