A high-conductivity copper alloy nuclear welding ball, its preparation method and application

Abstract

This invention provides an ultra-high conductivity copper alloy core solder ball, its preparation method, and its application. The core comprises a core and a solder shell covering the core. The core is a copper alloy formed with copper as the matrix and at least one of silver and tin as strengthening elements. The total amount of the strengthening element added is configured to be below 70% of the room temperature equilibrium solid solubility limit of that element in copper, so that all the strengthening elements exist in the copper face-centered cubic lattice in the form of substitutional solid solution atoms without precipitation. The matrix maintains a single-phase solid solution structure without precipitated second phases. The solder shell is a solder alloy with a melting point lower than that of the core alloy. This invention selects a solid solution strengthening technique, which can achieve superior strength and creep resistance compared to pure copper while maintaining extremely high conductivity.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging materials technology, specifically to an ultra-high conductivity copper alloy core bonding ball, its preparation method, and its application. Background Technology

[0002] With the continuous evolution of electronic information technology, chip operating frequencies have been constantly increasing, and signal transmission rates have jumped from hundreds of megabits per second in the early days to tens or even hundreds of gigabits per second today. At such high transmission rates, package interconnect solder joints not only bear the simple functions of electrical conduction and mechanical fixation, but have also become indispensable active components in the signal transmission link. The resistance and inductance of the solder joint itself, as well as the electromagnetic coupling between it and adjacent solder joints, have a significant impact on signal integrity and transmission quality. Any impedance discontinuity or insufficient conductivity at any solder joint will lead to signal reflection, transmission delay, and power loss, ultimately manifesting as a deterioration of the signal eye diagram and an increase in the bit error rate. Therefore, high-frequency, high-speed package interconnects place unprecedentedly stringent requirements on the conductivity of solder ball materials.

[0003] In the core-shell structure solder ball technology route, the choice of core material directly determines the overall performance of the solder joint. While the widely used pure copper core currently possesses good electrical and thermal conductivity, its conductivity still falls short of application requirements. Especially under high-frequency operating conditions, the skin effect forces current to concentrate in a very thin layer on the conductor's surface. At this point, the microstructure of the core material's surface layer—including grain structure, compositional uniformity, and defect density—has amplified its impact on the effective resistivity. Because pure copper cores lack alloying technology, their surface layer microstructure cannot optimize the conductivity of the skin layer. Simultaneously, pure copper has limited yield strength and creep resistance. In fine-pitch packaging, as solder joint sizes continue to shrink, the thermomechanical stress shared by each solder joint actually increases, making the mechanical support capacity of the pure copper core increasingly insufficient under thermal cycling conditions.

[0004] In summary, there is an urgent need for a systematic technical solution to fundamentally meet the comprehensive requirements of high-frequency and high-speed electronic packaging for ultra-high conductivity of solder joints, signal integrity, and long-term reliability. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide an ultra-high conductivity copper alloy core welding ball, its preparation method and application, so as to solve the technical problems faced in the field of core-shell structure welding balls in the prior art, such as the difficulty in synergistically optimizing conductivity and strength in high-frequency scenarios.

[0006] To address the aforementioned problems, a first aspect of the present invention provides a high-reliability electronic packaging solder ball, comprising a core and a solder shell covering the core; The core is a copper alloy formed with copper as the matrix and at least one of silver and tin as the reinforcing element. The total amount of the reinforcing element is configured to be less than 70% of the room temperature equilibrium solid solubility limit of the element in copper, so that the reinforcing element exists in the copper face-centered cubic lattice in the form of substitutional solid solution atoms without precipitation. The matrix maintains a single-phase solid solution structure without precipitated second phase. The solder shell is a solder alloy with a melting point lower than that of the core alloy.

[0007] In some embodiments, the copper alloy is a solid solution strengthened copper-silver binary alloy, wherein the silver content is 0.03-0.12 wt%, and the balance is copper and unavoidable impurities.

[0008] In some embodiments, the copper alloy is a solid solution strengthened copper-tin-phosphorus ternary alloy, wherein the tin content is 0.05-0.35 wt%, the phosphorus content is 0.01-0.04 wt%, and the balance is copper and unavoidable impurities.

[0009] In some embodiments, the core has a gradient microstructure with increasing grain size from the outside to the inside in a radial cross section. The core includes a core and a fine-grained region located on the surface layer. The average grain size of the fine-grained region does not exceed 5 μm. The average grain size of the core is more than 3 times that of the fine-grained region. The radial thickness of the fine-grained region is not less than 2 times the skin depth of the solder ball at the target operating frequency band.

[0010] In some embodiments, a diffusion barrier layer is provided between the core and the solder shell layer. The diffusion barrier layer is a nickel-phosphorus alloy thin film formed in situ on the surface of the core by electroless plating, with a phosphorus content of 6-9 wt% and a thickness of 0.5-2 μm.

[0011] In some embodiments, the solder shell is selected from Sn-Ag-Cu, Sn-Bi, or Sn-Sb solder alloys, with a thickness of 5-20 μm; the core particle size is 0.05-0.76 mm, sphericity ≥93%, and particle size distribution Cpk ≥1.33.

[0012] A second aspect of the present invention provides a method for preparing solder balls as described above, comprising: S1. Based on the room temperature equilibrium solid solubility data of the selected strengthening element in copper, the amount of the strengthening element added is set to less than 70% of the solid solubility limit; the prepared copper alloy raw material is heated to above the liquidus line under an inert atmosphere with a vacuum degree of not less than 10-2 Pa or an oxygen content of not more than 5 ppm for melting, and the superheat of the melt is controlled within the range of 50-150℃, so that the strengthening element is completely dissolved in the copper liquid to form a single-phase melt with uniform composition; S2. The melt is passed through a high-temperature resistant ceramic or sapphire nozzle with an inner diameter of 50-300 μm to form a continuous laminar jet. A piezoelectric ceramic actuator applies periodic vibrations of frequency f to the jet. The frequency f is determined according to the optimal perturbation condition of Rayleigh instability, f = v / (4.508 × d), based on the jet diameter d and the jet velocity v. The droplets are cooled by flight in an inert cooling atmosphere to obtain a spherical core. S3. After forming, the core does not undergo any aging heat treatment steps above 200°C and proceeds to the subsequent coating process; S4. A low-melting-point solder alloy layer is coated on the outer surface of the core to obtain a core-shell structure solder ball.

[0013] In some embodiments, in step S2, the inert cooling atmosphere is helium, and the droplet's flight cooling distance in the helium atmosphere is 0.3-1.5m.

[0014] In some embodiments, step S3 further includes a homogenization annealing step before entering the coating process: determining the annealing temperature Td based on the solid solubility-temperature phase diagram data of the selected alloy system, wherein Td is the solid solubility temperature ±20°C corresponding to the actual amount of strengthening element added, so that the annealing process is always in the single-phase solid solution region. The core matrix is ​​held at the annealing temperature Td for 1-10 minutes and then cooled to room temperature at a rate of not less than 50°C / min. After annealing, the core matrix is ​​observed by transmission electron microscopy to still maintain a single-phase solid solution structure without the precipitation of a second phase.

[0015] A third aspect of the present invention provides an application of the solder balls as described above, wherein the solder balls are applied in a high-density ball grid array, the array including signal solder ball positions carrying high-speed differential signals and adjacent ground solder ball positions; the signal solder ball positions are configured with solder balls having the single-phase solid solution copper alloy core, and the ground solder ball positions are configured with solder balls having the copper-nickel alloy core, wherein the nickel content is 5-30 wt%.

[0016] Compared with the prior art, the beneficial effects of the present invention include at least the following: This invention selects the solid solution strengthening technical route. After silver or tin atoms enter the normal lattice sites of the copper face-centered cubic lattice by substitution, although it causes a slight change in the lattice parameters, the matrix still maintains a continuous single-phase solid solution structure, and there is no heterogeneous interface between the second phase and the matrix. The scattering of charge carriers by solid solution atoms mainly comes from the weak perturbation of the electron wave function by the local elastic strain field caused by them at the lattice sites. This scattering cross section is much smaller than the scattering at the second phase interface. Therefore, solid solution strengthened copper alloys can obtain strength and creep resistance superior to pure copper while maintaining an extremely high conductivity level.

[0017] The present invention will be further described in detail below with reference to specific embodiments. Detailed Implementation

[0018] The applicant discovered: In high-frequency and high-speed signal transmission applications, especially in the packaging and interconnection of RF front-end modules, high-speed serializer / deserializer interface chips, and high-performance processor chips in fifth-generation and sixth-generation mobile communication systems, the requirements for the conductivity of solder ball materials are becoming increasingly stringent.

[0019] Currently, the pure copper core widely used in the industry has significant performance deficiencies. Therefore, some researchers have adopted a technical solution using precipitation-strengthened copper alloys as the core material. Significant strength enhancements are achieved by introducing nanoscale chromium and zirconium precipitates into a copper matrix, and thermal stability is improved by adding trace stabilizing elements such as silicon, niobium, and rare earth elements to pin the precipitate interfaces. However, from the perspective of high-frequency, high-speed signal transmission, precipitation-strengthened copper alloy cores present a fundamental technical contradiction. While nanoscale precipitates impart high strength to the material, the numerous heterogeneous interfaces between the precipitates and the copper matrix themselves act as scattering sources for charge carriers. Free electrons, during transmission, are scattered at each precipitate / matrix interface due to abrupt changes in lattice structure and electron potential energy, leading to a shortened mean free path and increased resistivity. Higher volume fractions and smaller sizes of the precipitates mean a higher interface density per unit volume, resulting in more severe damage to conductivity. This means that precipitation-strengthened copper alloys inevitably sacrifice some conductivity while achieving high strength. For high-frequency, high-speed packaging applications where signal transmission quality is paramount, this loss of conductivity is unacceptable.

[0020] Furthermore, from the perspective of the physical mechanism of high-frequency signal transmission, existing technologies suffer from a generally overlooked problem: the impact of the microstructure of the solder ball core surface layer on signal loss under the skin effect. When the signal frequency enters the gigahertz band, the skin depth shrinks to the micrometer or even submicrometer level, and the high-frequency current flows almost entirely within the extremely thin outermost layer of material in the core. If the grain structure of this surface layer is coarse, and solid solution elements undergo severe segregation at a few grain boundaries, localized high resistivity regions will form within the skin layer, increasing transmission loss. However, existing copper alloy core solder ball technologies have not specifically designed a gradient for the microstructure of the core surface layer, nor have they proposed optimization strategies for the matching relationship between skin depth and surface layer microstructure characteristics.

[0021] Furthermore, in high-density ball grid array (BGA) packages, as solder ball spacing shrinks to hundreds of micrometers or even lower, electromagnetic crosstalk between adjacent high-speed signal solder joints becomes another critical factor affecting signal integrity. Existing core-shell solder ball solutions only focus on optimizing the performance of individual solder joints, lacking consideration from the perspective of array-level system design on how to utilize differentiated configurations of solder ball core materials to suppress electromagnetic coupling between channels. This means that even if the transmission performance of a single solder joint is optimized to an ideal level, the signal integrity of the entire array may still be limited by adjacent-channel crosstalk.

[0022] In summary, existing technologies in the field of core-shell structure solder balls face several limitations, including the difficulty in synergistically optimizing conductivity and strength in high-frequency scenarios, the lack of specific design for the skin effect in the core surface layer structure, and the absence of array-level crosstalk suppression methods. There is an urgent need for a systematic technical solution that coordinates the selection of strengthening mechanisms for the core alloy, gradient design of the microstructure, precise control of interface diffusion, and differentiated array-level configurations to fundamentally meet the comprehensive requirements of high-frequency, high-speed electronic packaging for ultra-high conductivity, signal integrity, and long-term reliability of solder joints.

[0023] In view of this, this application provides an ultra-high conductivity copper alloy core solder ball with a core-shell structure, including a spherical core and a solder shell layer covering the core; The core is made of a copper alloy, which is formed with copper as the matrix and at least one of silver and tin as the reinforcing element. The total amount of the reinforcing element is configured to be less than 70% of the room temperature equilibrium solid solubility limit of the element in copper, so that the reinforcing element exists in the copper face-centered cubic lattice in the form of substitutional solid solution atoms without precipitation. The matrix maintains a single-phase solid solution structure without precipitated second phase. The local elastic strain field caused by the solid solution atoms in the copper lattice generates solid solution drag resistance to dislocation slip, thereby improving the yield strength and creep resistance of the matrix. The spherical core is prepared by uniform droplet spraying, and the core does not undergo any aging heat treatment steps above 200°C after preparation, so as to avoid the solid solution elements from desolvating from the supersaturated state due to thermal activation diffusion and thus destroying the single-phase solid solution structure. The solder shell is a solder alloy with a melting point lower than that of the core alloy, so that the shell melts during reflow soldering to achieve wetting interconnection while the core remains solid.

[0024] It should be noted that, in order to achieve near-pure copper-like ultra-high conductivity and superior mechanical strength in the solder ball core, and to ensure that these two properties remain stable throughout the solder ball's fabrication and service thermal history, this embodiment employs a technical approach encompassing three interrelated levels: First, at the material level, this embodiment selects a solid solution-strengthened copper alloy as the core material. The copper alloy contains at least one strengthening element selected from silver and tin, and its total addition is controlled to be below 70% of the room temperature equilibrium solid solubility limit of copper. The principle behind this design is that when the content of the solid solution element is lower than the equilibrium solid solubility of copper within its service temperature range, the alloy system does not have supersaturation to drive precipitation reactions. Therefore, the strengthening element will exist stably in the copper face-centered cubic lattice in the form of substitutional solid solution atoms, and the matrix will maintain a single-phase solid solution structure without precipitated second phases. At this time, in this single-phase core, the substitutional solid solution atoms induce a local elastic strain field in the copper lattice. This strain field interacts with the elastic stress field of moving dislocations, generating solid solution drag resistance. This requires higher applied stress for dislocations to overcome the resistance and continue to slide, thereby improving the alloy's yield strength and creep resistance. At the same time, since there is no heterogeneous interface between the precipitated second phase and the matrix, free electrons will not encounter interface scattering when transporting within the matrix, and the electron mean free path is basically undisturbed. The conductivity of the alloy remains close to that of pure copper.

[0025] Then, at the process level, it is stipulated that the core will not undergo any aging heat treatment steps above 200°C after preparation. The principle behind this restriction is that even if the alloy composition is within the equilibrium solid solubility limit, the actual concentration of solid solution elements may exceed the equilibrium value at that temperature in local areas and become supersaturated due to the extremely high cooling rate during rapid solidification. If high-temperature aging treatment is applied to the core in subsequent processes, thermally activated diffusion will enable these supersaturated solid solution atoms to acquire sufficient migration ability to locally aggregate and nucleate, thereby destroying the single-phase solid solution structure. Therefore, limiting the subsequent heat treatment temperature to below 200°C helps maintain the diffusion coefficient of solid solution atoms at an extremely low level, kinetically preventing any possible precipitation process.

[0026] Finally, at the structural level, the solder balls employ a core-shell structure. The melting point of the outer solder shell is lower than that of the core alloy, allowing the shell solder to melt first during reflow soldering and wet the pads to form a metallurgical interconnect, while the core remains solid, forming a rigid support within the solder joint. This functional partitioning design allows the core's ultra-high conductivity and mechanical strength to be fully utilized within the solder joint.

[0027] Optionally, as a possible implementation, the copper alloy is a solid solution strengthened copper-silver binary alloy, wherein the silver content is 0.03-0.12 wt%, and the balance is copper and unavoidable impurities; wherein, the upper limit of the silver content is set to 0.12 wt%, that is, controlled below 70% of the solid solubility limit, so that the silver atoms are in a thermodynamically stable solid solution state throughout the entire thermal process from room temperature to the peak temperature of reflow soldering, and do not have the thermodynamic driving force for precipitation.

[0028] It should be noted that both silver and copper have face-centered cubic crystal structures. When silver atoms substitute for electrons in normal lattice sites of copper, the resulting lattice distortion is limited due to the small difference in their lattice constants. From the perspective of electron scattering theory, the scattering cross-section of substitutional solid solution atoms on conduction electrons mainly depends on the difference in valence electron numbers and atomic sizes between solute atoms and matrix atoms. Silver and copper both belong to Group 1B elements, have the same number of valence electrons, and a difference in atomic radius of approximately 12.6%, which is considered a mild lattice perturbation. Therefore, the additional scattering cross-section of silver atoms on free electrons in the copper matrix is ​​much smaller than the scattering caused by interstitial solid solution elements or second-phase interfaces, allowing the alloy to maintain extremely high conductivity. The lower limit for silver content is set at 0.03 wt%, a concentration sufficient to produce a measurable solid solution strengthening effect in the copper lattice, resulting in an alloy yield strength higher than that of pure copper under the same conditions.

[0029] Optionally, in applications requiring higher solid solution strengthening efficiency than the copper-silver system, as a possible implementation, the copper alloy is a solid solution strengthened copper-tin-phosphorus ternary alloy, wherein the tin content is 0.05-0.35 wt%, i.e., below 70% of the solid solubility limit, ensuring that tin remains in a stable solid solution state within the copper matrix. Tin atoms have a slightly larger atomic radius than silver atoms, thus inducing a stronger elastic strain field in the copper lattice, resulting in a higher solid solution strengthening efficiency per unit concentration than silver. The phosphorus content is 0.01-0.04 wt%, with the balance being copper and unavoidable impurities. Phosphorus plays a dual role in this alloy system: during the smelting stage, phosphorus acts as a deoxidizer, reacting with dissolved oxygen in the molten copper to generate phosphorus oxides that float to the surface and are discharged from the melt, reducing the oxygen content and oxide inclusion density in the matrix, and decreasing carrier scattering and electron mean free path shortening effects caused by oxide particles; after the deoxidation reaction, a small amount of phosphorus remaining in the matrix exists in solid solution form, providing additional solid solution strengthening. The upper limit of phosphorus content is set at 0.04 wt%, which is less than 80% of the solid solubility limit of phosphorus in copper, which is about 0.05 wt%. This ensures that the portion of phosphorus remaining in the matrix after fulfilling its deoxidation function in the smelting stage does not exceed the solid solubility limit, thus avoiding the formation of brittle Cu3P precipitates.

[0030] In addition, under high-frequency signal transmission conditions, besides the alloy type design, the spatial design of the core microstructure is also very important, because the spatial design of the core microstructure directly affects the effective resistivity within the skin layer, thereby reducing the insertion loss of high-frequency signal transmission.

[0031] Therefore, in some possible embodiments, the core has a gradient microstructure with increasing grain size from the outside to the inside in the radial cross section, including a fine-grained region located in the surface layer, the average grain size of the fine-grained region not exceeding 5 μm, and the average grain size of its core being more than 3 times that of the fine-grained region.

[0032] Specifically, during the uniform droplet ejection process, a highly thermally conductive inert cooling atmosphere with a thermal conductivity of not less than 0.15 W / (m·K) is used, resulting in a significantly higher heat dissipation rate on the outer surface of the droplet compared to the core. The outer surface reaches nucleation supercooling first and begins to solidify. Due to the rapid heat dissipation rate and high solidification front advancement speed, the grains are frozen by the subsequent solidification interface before they have sufficient time to grow, forming a fine-grained region with an average grain size of no more than 5 μm. In contrast, the core, due to its lower cooling rate, has more time for grain growth, ultimately resulting in larger grain sizes. This establishes a gradient microstructure with increasing grain size from the outside to the inside in the radial direction of the core.

[0033] Furthermore, the radial thickness of the fine-grained region is not less than twice the skin depth of the solder ball at the target operating frequency band, ensuring that the skin layer where the high-frequency current is concentrated falls completely into the fine-grained region.

[0034] In some possible implementations, to establish a diffusion barrier between the solid solution-strengthened copper alloy core and the solder shell, thereby suppressing the excessive growth of intermetallic compounds caused by the copper-tin interface reaction and preventing the diffusion migration of dissolved silver or tin atoms in the core towards the solder layer to maintain the solid solution strengthening effect and conductivity uniformity in the near-interface region of the core, this embodiment provides a diffusion barrier layer between the core and the solder shell. The diffusion barrier layer is a nickel-phosphorus alloy thin film formed in situ on the core surface through electroless plating, with a phosphorus content of 6-9 wt% and a thickness of 0.5-2 μm. The selection of the phosphorus content range is based on the structural phase transformation law of nickel-phosphorus alloys: when the phosphorus content is below 6 wt%, the alloy is polycrystalline, and the penetrating grain boundaries between grains constitute short-circuit channels for rapid atomic diffusion. The grain boundary diffusion coefficient is usually several orders of magnitude higher than the bulk diffusion coefficient, significantly reducing the actual blocking efficiency of the barrier layer; when the phosphorus content is between 6-9 wt%, the diffusion barrier layer is more efficient. Within the wt% range, the alloy transforms into an amorphous structure, the atomic arrangement loses long-range order, and there are no defect structures such as grain boundaries. Atomic diffusion across the barrier layer can only occur through the bulk diffusion mechanism, and the effective diffusion coefficient is greatly reduced. When the phosphorus content exceeds 9 wt%, the internal stress of the film increases significantly with the increase of phosphorus content, which leads to a decrease in the bonding force between the film and the copper alloy core and the risk of cracking and peeling, thus losing the barrier function.

[0035] In addition, the thin film exhibits amorphous scattering characteristics when detected by X-ray scattering, that is, it only shows broad diffuse scattering peaks without sharp crystalline scattering peaks.

[0036] Optionally, the solder shell is selected from Sn-Ag-Cu, Sn-Bi, or Sn-Sb solder alloys, with a thickness of 5-20 μm; the core particle size is 0.05-0.76 mm, sphericity ≥93%, and particle size distribution Cpk ≥1.33.

[0037] Sn-Ag-Cu solders, due to the presence of silver and copper eutectic components, possess excellent comprehensive mechanical properties and creep resistance, making them suitable for conventional reflow soldering temperatures. Sn-Bi solders have lower melting points, making them suitable for low-temperature soldering of heat-sensitive devices. Sn-Sb solders exhibit high creep resistance and high-temperature stability, making them suitable for high junction temperature service conditions.

[0038] This application provides a method for preparing solder balls as described above, including: S1. Based on the room temperature equilibrium solid solubility data of the selected strengthening element in copper, the addition amount of the strengthening element is set to be below 70% of the solid solubility limit, thus establishing the thermodynamic basis of the single-phase solid solution from the source; the prepared copper alloy raw material is then subjected to a vacuum of not less than 10... -2 Melting is carried out under an inert atmosphere with an oxygen content not exceeding 5 ppm or above the liquidus line to prevent alloying elements from reacting with oxygen at high temperatures to form oxide inclusions. The superheat of the melt is controlled within the range of 50-150°C. If the superheat is too low, undissolved alloying element clusters may remain in the melt, resulting in local compositional inhomogeneity in the core after spraying. If the superheat is too high, the chemical erosion of the nozzle material by the melt will intensify, shorten the nozzle life, and may introduce nozzle compositional contamination. The strengthening elements are completely dissolved in the copper liquid to form a homogeneous single-phase melt. S2. The melt is passed through a high-temperature resistant ceramic or sapphire nozzle with an inner diameter of 50-300 μm to form a continuous laminar jet. The nozzle material has a temperature resistance 200°C higher than the alloy liquidus temperature to ensure that the melt does not solidify and blockage within the nozzle channel. A piezoelectric ceramic actuator applies periodic vibrations of frequency f to the jet. The frequency f is determined according to the optimal perturbation condition of Rayleigh instability, f = v / (4.508 × d), based on the jet diameter d and jet velocity v, where v is the jet velocity, d is the jet diameter, and 4.508 is the ratio of wavelength to diameter corresponding to the fastest growing wavenumber of axisymmetric perturbation in a cylindrical jet. This frequency causes the jet to break at a specific location within each perturbation cycle, producing uniformly sized droplets with extremely small volume variation coefficients. The droplets are cooled in an inert atmosphere with a thermal conductivity of not less than 0.15 W / (m·K) at a speed of 10... 3 -10 5 Rapid solidification occurs at a cooling rate of K / s, which prevents the solid solution elements from undergoing long-range diffusion segregation and precipitation during solidification. The solid solution elements are frozen in the copper lattice and remain in a single-phase supersaturated solid solution state. The droplets are cooled by flight in an inert cooling atmosphere to obtain a spherical core. S3. After forming, the core does not undergo any aging heat treatment steps above 200°C and proceeds to the subsequent coating process; S4. A low-melting-point solder alloy layer is coated on the outer surface of the core to obtain a core-shell structure solder ball.

[0039] Preferably, in order to achieve the formation of a radial gradient microstructure in the core in the process, in step S2, the inert cooling atmosphere is helium, which has a thermal conductivity about 6 times that of nitrogen, and can provide efficient convective heat dissipation in the droplet flight path; the flight cooling distance of the droplet in the helium atmosphere is 0.3-1.5m. The logic for setting this distance is to ensure that when the droplet reaches the collector, the outer surface has been completely solidified while the core is still in a semi-solidified state or has just been solidified, thereby establishing a solidification gradient in the radial direction with an outer layer cooling rate 3-10 times that of the core, so that a fine-grained region is formed in the outer layer.

[0040] Preferably, in step S3, before entering the coating process, a homogenization annealing step is also included: determining the annealing temperature Td based on the solid solubility-temperature phase diagram data of the selected alloy system, wherein Td is the solid solubility temperature ±20℃ corresponding to the actual amount of strengthening element added; The core matrix is ​​held at the annealing temperature Td for 1-10 minutes and then cooled to room temperature at a rate of not less than 50°C / min. After annealing, the core matrix is ​​observed by transmission electron microscopy to still maintain a single-phase solid solution structure without the precipitation of a second phase.

[0041] It should be noted that, in order to eliminate the micro-segregation of solid solution elements caused by rapid solidification of the core surface layer without damaging the single-phase solid solution structure and without causing grain coarsening, this embodiment uses low-temperature homogenization annealing to eliminate this segregation; by controlling the annealing temperature, the annealing process is always kept within the single-phase solid solution region, and there is no thermodynamic driving force for precipitation nucleation; by controlling the annealing time, sufficient diffusion kinetic energy is provided for the solid solution atoms segregated between dendrites to migrate from the high concentration region to the low concentration region, thereby achieving composition homogenization.

[0042] This application provides an application of the solder balls as described above. The solder balls are applied in a high-density ball grid array. The array includes signal solder ball positions carrying high-speed differential signals and adjacent ground solder ball positions. The signal solder ball positions are configured with solder balls using the single-phase solid solution copper alloy core to obtain the lowest transmission resistance. The ground solder ball positions are configured with solder balls using a copper-nickel alloy core with a nickel content of 5-30 wt%.

[0043] The solid solution of nickel in copper alters the electronic structure of the alloy, introducing a magnetic scattering mechanism. This causes the copper-nickel alloy to exhibit a higher surface resistance and a relative permeability greater than 1 than pure copper in the GHz band. Consequently, the grounding solder ball dampens and absorbs the electromagnetic coupling field that crosses the space between it and the adjacent signal solder ball at high frequencies.

[0044] The solder shell of the signal solder ball contains nanoscale ferrite particles. These ferrite particles are selected from NiZn ferrite or MnZn ferrite, with a particle size of 10-100 nm and a mass percentage of 0.1-1.0 wt%. The ferrite material is selected based on the following: NiZn ferrite has a significant imaginary permeability in the GHz band, and its domain walls undergo irreversible displacement in an alternating magnetic field, generating hysteresis loss and converting the high-frequency electromagnetic coupling energy passing through the solder layer into heat dissipation. The upper limit of 100 nm for the particle size ensures that the particle size is smaller than the typical interlamellar spacing of the tin-based eutectic structure in the solder alloy, thus avoiding adverse effects of the particles on the wetting flowability of the solder and the solder joint formation quality.

[0045] The beneficial effects of the above embodiments are as follows: This invention fundamentally differs from existing precipitation-strengthened copper alloy bonding ball technology in its choice of strengthening mechanism, opting instead for solid solution strengthening, a route with intrinsic advantages in maintaining conductivity. In precipitation-strengthened systems, while nanoscale second-phase particles effectively hinder dislocation movement and significantly enhance strength, they simultaneously introduce numerous heterogeneous interfaces into the copper matrix. These interfaces' abrupt changes in lattice structure and electronic potential energy constitute unavoidable scattering sources during free electron transport. Each precipitate / matrix interface shortens the mean free path of charge carriers and increases resistivity. The solid solution strengthening mechanism employed in this invention fundamentally avoids this contradiction. When silver or tin atoms substitute into the normal lattice sites of the copper face-centered cubic lattice, although causing slight changes in lattice parameters, the matrix maintains a continuous single-phase solid solution structure, without any heterogeneous interfaces between the second phase and the matrix. The scattering of charge carriers by solid solution atoms mainly originates from the weak perturbation of the electron wavefunction by the local elastic strain field induced at the lattice sites. This scattering cross-section is much smaller than that of the second-phase interface scattering. Therefore, solid solution strengthened copper alloys can achieve superior strength and creep resistance compared to pure copper while maintaining extremely high conductivity. This ability to simultaneously improve strength and maintain conductivity within a single-phase structure is physically impossible to achieve with precipitation-strengthened systems.

[0046] This invention further ensures the thermodynamic stability of the single-phase solid solution microstructure throughout the entire preparation and service thermal history of the solder ball by strictly controlling the addition of strengthening elements to below 70% of the room-temperature equilibrium solid solubility limit in copper. When the content of solid-solid elements in the alloy is lower than its equilibrium solid solubility at a certain temperature, the alloy system at that temperature lacks the thermodynamic driving force for precipitation phase nucleation, i.e., the supersaturation is zero or even negative, and precipitation reaction cannot occur spontaneously thermodynamically. The 70% design margin provides sufficient safety margin for factors such as temperature fluctuations and local compositional variations, ensuring that even under brief thermal exposure to the peak temperature of reflow soldering, the solid-solid elements remain stably residing in the copper lattice without precipitating to form a second phase. In contrast, precipitation-strengthened alloys require the content of alloying elements to be set far above the solid solubility limit to obtain sufficient supersaturation to drive precipitation, and the two approaches form a fundamental opposition in alloy design philosophy. Combined with the process limitation of not undergoing aging treatment above 200°C, this invention constructs a dual guarantee of single-phase microstructure from both thermodynamic and kinetic perspectives.

[0047] This invention introduces a gradient microstructure design concept for the core surface layer, specifically for high-frequency signal transmission scenarios. During uniform droplet ejection, a high thermal conductivity inert cooling atmosphere is selected to ensure a higher solidification rate on the outer surface of the droplet compared to the core, naturally establishing a gradient microstructure with increasing grain size from the outside inwards along the radial direction of the core. This gradient design ensures that the skin layer, where high-frequency currents are concentrated, falls precisely within the fine-grained region of the outermost core. The fine-grained microstructure exhibits a high but uniform grain boundary density, and the segregation of solid solution atoms at each grain boundary is diluted due to the increased total grain boundary area, avoiding the problem of severe segregation at a few grain boundaries and the formation of localized high resistivity bands, as seen in coarse-grained microstructures. Combined with low-temperature homogenization annealing, this further eliminates solidification microsegregation, achieving a highly uniform composition and resistivity distribution within the skin layer, thereby reducing insertion loss in high-frequency signal transmission. This design method, which couples solidification process parameters with the physical requirements of signal transmission, has never been proposed in existing solder ball technology.

[0048] This invention also proposes a differentiated core material configuration strategy for high-density array packaging. A highly conductive single-phase solid solution copper alloy core is used at the high-speed differential signal solder ball positions to achieve the lowest signal transmission resistance. A copper-nickel alloy core is used at the adjacent ground solder ball positions. Utilizing the magnetic scattering mechanism introduced by the solid solution of nickel in copper, the ground solder ball exhibits high surface impedance and a permeability greater than 1 at high frequencies. This damping and absorption effect on the electromagnetic coupling field crossing the space between the signal ball and the ground ball reduces near-end and far-end crosstalk. This system design approach, which matches the optimal core material to different functional solder ball positions within the same array, elevates the optimization level of the solder ball from single-point performance to array-level signal integrity management, providing a new technical dimension for high-density, high-speed packaging.

Claims

1. A high-conductivity copper alloy nuclear welding ball, characterized in that, Includes a core and a solder shell covering the core; The core is a copper alloy formed with copper as the matrix and at least one of silver and tin as the reinforcing element. The total amount of the reinforcing element is configured to be less than 70% of the room temperature equilibrium solid solubility limit of the element in copper, so that the reinforcing element exists in the copper face-centered cubic lattice in the form of substitutional solid solution atoms without precipitation. The matrix maintains a single-phase solid solution structure without precipitated second phase. The solder shell is a solder alloy with a melting point lower than that of the core alloy.

2. The solder ball as described in claim 1, characterized in that, The copper alloy is a solid solution strengthened copper-silver binary alloy, wherein the silver content is 0.03-0.12 wt%, and the balance is copper and unavoidable impurities.

3. The solder ball as described in claim 1, characterized in that, The copper alloy is a solid solution strengthened copper-tin-phosphorus ternary alloy, wherein the tin content is 0.05-0.35 wt%, the phosphorus content is 0.01-0.04 wt%, and the balance is copper and unavoidable impurities.

4. The solder ball as described in claim 1, characterized in that, The core has a gradient microstructure with increasing grain size from the outside to the inside in the radial cross section. The core includes a core and a fine grain region located on the surface layer. The average grain size of the fine grain region does not exceed 5 μm. The average grain size of the core is more than 3 times that of the fine grain region. The radial thickness of the fine grain region is not less than 2 times the skin depth of the solder ball at the target operating frequency band.

5. The solder ball according to any one of claims 1 to 4, characterized in that, A diffusion barrier layer is provided between the core and the solder shell layer. The diffusion barrier layer is a nickel-phosphorus alloy film formed in situ on the surface of the core by electroless plating, with a phosphorus content of 6-9 wt% and a thickness of 0.5-2 μm.

6. The solder ball as described in claim 5, characterized in that, The solder shell is selected from Sn-Ag-Cu, Sn-Bi, or Sn-Sb solder alloys, with a thickness of 5-20 μm; the core particle size is 0.05-0.76 mm, sphericity ≥93%, and particle size distribution Cpk ≥1.

33.

7. A method for preparing solder balls as described in any one of claims 1 to 6, characterized in that, include: S1. Based on the room temperature equilibrium solid solubility data of the selected strengthening element in copper, the amount of the strengthening element added is set to be below 70% of the solid solubility limit; the prepared copper alloy raw material is then subjected to a vacuum of not less than 10... -2 The copper is heated to above the liquidus line under an inert atmosphere with an oxygen content of no more than 5 ppm (Pa) for smelting. The superheat of the melt is controlled within the range of 50-150°C, so that the strengthening elements are completely dissolved in the copper melt to form a single-phase melt with uniform composition. S2. The melt is passed through a high-temperature resistant ceramic or sapphire nozzle with an inner diameter of 50-300 μm to form a continuous laminar jet. A piezoelectric ceramic actuator applies periodic vibrations of frequency f to the jet. The frequency f is determined according to the optimal perturbation condition of Rayleigh instability, f = v / (4.508 × d), based on the jet diameter d and the jet velocity v. The droplets are cooled by flight in an inert cooling atmosphere to obtain a spherical core. S3. After forming, the core does not undergo any aging heat treatment steps above 200°C and proceeds to the subsequent coating process; S4. A low-melting-point solder alloy layer is coated on the outer surface of the core to obtain a core-shell structure solder ball.

8. The method as described in claim 7, characterized in that, In step S2, the inert cooling atmosphere is helium, and the droplet's flight cooling distance in the helium atmosphere is 0.3-1.5m.

9. The method as described in claim 8, characterized in that, In step S3, before entering the coating process, a homogenization annealing step is also included: the annealing temperature Td is determined according to the solid solubility-temperature phase diagram data of the selected alloy system, wherein Td is the solid solubility temperature ±20°C corresponding to the actual amount of strengthening element added, so that the annealing process is always in the single-phase solid solution region. The core matrix is ​​held at the annealing temperature Td for 1-10 minutes and then cooled to room temperature at a rate of not less than 50°C / min. After annealing, the core matrix is ​​observed by transmission electron microscopy to still maintain a single-phase solid solution structure without the precipitation of a second phase.

10. An application of the solder ball as described in any one of claims 1 to 6, characterized in that, The solder balls are used in a high-density ball grid array, which includes signal solder ball positions carrying high-speed differential signals and adjacent ground solder ball positions. The signal solder ball positions are configured with solder balls using the single-phase solid solution copper alloy core, and the ground solder ball positions are configured with solder balls using a copper-nickel alloy core, with a nickel content of 5-30 wt%.