Composite solder ball, preparation method and stacked packaging structure

Abstract

The invention discloses a composite solder ball, a preparation method and a stacked packaging structure, and the composite solder ball comprises a composite inner core which is composed of a continuous phase matrix and a strengthening phase, the strengthening phase accounts for 15-30% of the total volume of the inner core, the compression modulus of the composite inner core is larger than or equal to 3 GPa, and the thermal expansion coefficient of the composite inner core is smaller than or equal to 55 ppm / DEG C; the polymer core is coated with the metal transition layer; and the tin-silver alloy outer layer covers the metal transition layer. The composite inner core is composed of a continuous phase polymer matrix and a strengthening phase fiber network, high-temperature compression resistance and low thermal expansion characteristics are achieved through a specific volume ratio and a specific mass ratio, the solid-state rigidity characteristic is kept in a welding high-temperature interval, mechanical loads are directly borne, the interlayer height collapse risk is thoroughly eliminated, and the welding quality is improved. The metal transition layer forms a mechanical interlocking combination interface with the inner core, and the tin-silver alloy outer layer provides weldability and an electric connection function, so that double-function technology fusion that the composite solder ball can play a supporting role and also can play a welding role is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of microelectronic packaging, and in particular to a composite solder ball, a preparation method and a stacked packaging structure. Background Art

[0002] In stacked packaging technology, traditional solid BGA solder balls achieve electrical interconnection between modules through molten soldering. However, due to the inherent properties of the solder ball material, during the high-temperature reflow process, the molten solder cannot withstand the mechanical load of the upper package, resulting in a high degree of solder joint collapse. To maintain the gap between layers, existing technologies require the use of plastic support structures to assist in positioning.

[0003] This support structure forces the package design to sacrifice space efficiency and requires the curing of the underfill to disperse thermal stress. However, the mismatch in thermal expansion coefficients causes shear stress concentration at the interface during temperature cycling, accelerating solder joint fatigue failure. More seriously, the solidified body formed by the underfill and the plastic support greatly hinders the repair and replacement of the underlying chip. Engineering practice has shown that existing technical solutions present irreconcilable structural contradictions between package reliability, process complexity, and maintainability. Summary of the Invention

[0004] In order to overcome the shortcomings of the existing technology, the purpose of the present invention is to provide a composite solder ball, a preparation method and a stacked packaging structure to solve the problems of easy collapse, thermal stress concentration and difficulty in maintenance in the traditional BGA solder ball packaging technology used in traditional technology.

[0005] In order to solve the above problems, the technical solutions adopted by the present invention are as follows: In a first aspect, the present invention provides a composite solder ball, comprising: A composite core composed of a continuous phase matrix and a reinforcement phase, wherein the reinforcement phase accounts for 15% to 30% of the total volume of the core, and the composite core has a compression modulus of ≥3 GPa and a thermal expansion coefficient of ≤55 ppm / °C; A metal transition layer, covering the polymer core; A tin-silver alloy outer layer covers the metal transition layer.

[0006] In some embodiments, the reinforcing phase is a non-metallic fiber with an aspect ratio ≥ 20:1.

[0007] In some embodiments, the non-metallic fibers are chopped fibers, and the chopped fibers include glass fibers, ceramic fibers, or carbon fibers.

[0008] In some embodiments, the reinforcement phase accounts for 25-35 wt % of the total mass of the core.

[0009] In some embodiments, the silver content of the tin-silver alloy outer layer is 2.9-3.1 wt %.

[0010] In some embodiments, the diameter of the composite core is Φ0.18-0.22 mm; The thickness of the metal transition layer is 4-6 μm, and the surface roughness Ra is ≥ 0.6 μm; The thickness of the tin-silver alloy outer layer is 35-45 μm.

[0011] In some embodiments, the continuous phase matrix is ​​a thermosetting polymer that satisfies: (a) a glass transition temperature Tg ≥ 150° C.; and (b) a compression resilience ≥ 90%. In some embodiments, the continuous phase matrix is ​​an epoxy resin.

[0012] In a second aspect, the present invention provides a method for preparing a composite solder ball as described above, comprising: Composite nucleation: The continuous phase matrix raw material and the reinforcing phase filler are mixed under shear emulsification conditions to form a composite slurry, which is then granulated into microspheres to obtain a core preform; interface activation: The surface of the core preform is treated with plasma irradiation to form an active bonding surface; Transition layer construction: chemical deposition of a metal transition layer on the active bonding surface; Alloy layer shape control: A dual-source electroplating process is used to simultaneously deposit tin-silver alloy on the surface of the transition layer, where the silver ion concentration is controlled in real time within a closed-loop deviation of ±5%.

[0013] In a third aspect, the present invention provides a stacked package structure, comprising a bottom package and a top package, wherein the bottom package and the top package are electrically connected and mechanically supported by a composite solder ball as described above; The inter-layer height fluctuation after reflow soldering is ≤±2%.

[0014] Compared with the prior art, the present invention has at least the following beneficial effects: 1. The composite core is composed of a continuous phase polymer matrix and a reinforcement phase fiber network. Through specific volume and mass ratios, it achieves high-temperature compressive resistance and low thermal expansion properties. It maintains solid-state rigidity in the high-temperature welding range, directly bearing mechanical loads and completely eliminating the risk of interlayer height collapse. The metal transition layer forms a mechanical interlocking interface with the core, and the tin-silver alloy outer layer provides solderability and electrical connection functions, realizing a dual-functional technology fusion in which the composite solder ball plays both a supporting role and a welding role. 2. The reinforcement fiber network effectively constrains the expansion behavior of the matrix, making the thermal expansion coefficient of the composite core close to that of the organic packaging substrate. This characteristic reduces the interfacial stress concentration during thermal cycling from the source and improves the fatigue life of the solder joints. 3. The composite solder balls independently assume the supporting function, eliminating the plastic bracket installation process and the need for bottom filling process. This design makes the package structure have reversible connection characteristics, and the bottom chip can be disassembled and replaced without damage.

[0015] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention is further described with reference to the accompanying drawings. However, the embodiments in the accompanying drawings do not constitute any limitation to the present invention. A person skilled in the art can obtain other drawings based on the following drawings without creative effort.

[0017] Figure 1 The figure is a cross-sectional schematic diagram of a composite solder ball provided in an embodiment.

[0018] Figure 2 The figure is a schematic flow chart of a method for preparing a composite solder ball provided in an embodiment. DETAILED DESCRIPTION

[0019] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0020] In the description of the present invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limitations on the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0021] In the description of the present invention, when a specific device is described as being located between a first device and a second device, an intervening device may or may not be present between the specific device and the first device or the second device. When a specific device is described as being connected to another device, the specific device may be directly connected to the other device without an intervening device, or may be directly connected to the other device but with an intervening device.

[0022] Technologies, methods, and equipment known to ordinary technicians in the relevant art may not be discussed in detail, but where appropriate, the technologies, methods, and equipment should be considered part of the specification.

[0023] The applicant found that: In the package-on-package (PoP) industry, traditional BGA solder balls have obvious technical defects: 1. Low yield strength leads to thermal collapse and height fluctuation Under the high temperature of PoP reflow soldering (typically peak >220°C), the yield strength of traditional SnAgCu solid alloy solder balls drops sharply and is insufficient to support the weight of the top package module.

[0024] The solder balls are compressed and deformed (collapsed), resulting in significant height fluctuations (±15%) between layers. This height inconsistency seriously affects subsequent chip assembly, heat dissipation, mechanical stability, and signal integrity, and limits the increase in the number of PoP stacking layers.

[0025] To address the collapse issue, the industry generally uses "plastic standoffs" as mechanical supports. However, this adds additional bill of materials (BoM) cost, assembly steps (increasing complexity), package height, and significantly hinders rework and repair of the underlying chip, as the standoffs are typically used in conjunction with underfill.

[0026] 2. CTE mismatch leads to thermal stress cracking Because solder joints connect different materials (silicon chips ≈3ppm / °C, substrates / PCBs ≈16-18ppm / °C, and solder ≈20-25ppm / °C), they expand and contract differently during temperature cycling (operation or environmental changes). Thermal stress concentrates at the weakest link in the connection (typically the neck where the solder joint connects to the pad or the IMC layer), leading to fatigue cracking and high failure rates (>2000ppm). Reliability is a primary challenge for PoP packaging, especially in demanding applications such as automotive and industrial.

[0027] 3. Difficulty in rework To prevent thermal stress cracking and enhance mechanical stability, the bottom chip of the PoP is often reinforced with an underfill. However, the underfill secures the chip to the substrate. If the bottom chip fails, removing the top package and underfill is extremely difficult and can easily damage the substrate, significantly increasing rework costs and workload.

[0028] In view of this, the applicant proposes a composite solder ball to fundamentally solve the above three major defects at the same time: 1. Composite core support structure to solve collapse and support problems The core utilizes a composite core with a high compression modulus (≥3GPa) and excellent resilience (≥90%). Under the high temperatures of reflow soldering, the composite core remains solid and possesses extremely high rigidity (much higher than molten solder). It supports the weight of the top package at high temperatures, significantly resists collapse, and ensures high interlayer stability. It perfectly replaces the bracket, achieving the dual functions of "connection and support" in a single ball, simplifying the process and reducing height and cost.

[0029] 2. Low CTE composite core to alleviate thermal stress issues Because the continuous phase matrix itself has a high CTE, the precise addition of a reinforcing phase allows for significant control of the composite core's CTE at specific volume and mass percentages. Compared to the higher CTE of traditional solder balls, the 50ppm / °C CTE is closer to that of the package substrate (organic materials, typically 16-25ppm / °C) and the PCB (typically 16-18ppm / °C), significantly mitigating thermal expansion coefficient mismatch within the solder joint and at the connection to the PCB.

[0030] 3. Plating structure to solve connection and rework problems By constructing a conductive and solderable layer through chemical copper plating (transition layer) and electroplating SnAg (outer layer), the high-strength composite core and optimized stress distribution reduce the dependence on underfill, so no underfill is required in the future. There is no underfill fixation, and welding occurs at the junction of the plating (molten) and the core (non-melting), making rework of the bottom chip possible and relatively easy. It only requires reheating and melting the plating solder to separate it, significantly reducing the rework workload.

[0031] Therefore, combined Figure 1 This embodiment provides a composite solder ball, comprising: The composite core 10 is composed of a continuous phase matrix and a reinforcement phase, wherein the reinforcement phase accounts for 15% to 30% of the total volume of the core, and the composite core 10 has a compression modulus of ≥3 GPa and a thermal expansion coefficient of ≤55 ppm / °C; A metal transition layer 20, covering the polymer core; The tin-silver alloy outer layer 30 covers the metal transition layer 20 .

[0032] It should be noted that, since traditional solder balls cannot simultaneously provide high-temperature support and thermal expansion adaptation, in order to solve the problems of high-temperature collapse and thermal stress cracking of solder balls, this embodiment adopts a continuous phase matrix and a reinforcement phase as a rigid skeleton in the design of the composite core 10 to constrain thermal expansion behavior. The core maintains a peak compression modulus in the range of 217-250°C, while the CTE is effectively reduced. The metal transition layer 20 is chemically bonded to the core, and its rough interface provides a mechanical anchor point. After melting, the tin-silver alloy outer layer 30 recondenses into a sphere through surface tension to achieve electrical connection. Among them, the solid-state carrier of the core is decoupled from the liquid connection function of the coating. At high temperatures, the molten coating wets the pad to form an electrical path, while the rigid core independently bears the mechanical load, eliminating collapse and deformation from the root.

[0033] As an embodiment, the reinforcing phase is a non-metallic fiber with an aspect ratio ≥20:1, the reinforcing phase accounts for 25-35wt% of the total mass of the core, the non-metallic fiber is a chopped fiber, the chopped fiber includes glass fiber, ceramic fiber or carbon fiber, and the continuous phase matrix is ​​epoxy resin.

[0034] Since simply adding fillers will lead to interface defects and molding difficulties, the fiber aspect ratio in this embodiment is limited to ≥20:1, the mass is 25-35wt%, and the high aspect ratio fibers form a three-dimensional mesh truss structure in the matrix. When the composite material is compressed, the fibers disperse the load to the entire network through the shear force transfer mechanism, effectively improving the compressive strength, and the stress in the micro-region at the end of the fiber is concentrated and reduced, avoiding the initiation of microcracks.

[0035] For example, chopped glass fibers can effectively increase the compression modulus of the epoxy resin matrix, giving it sufficient rigidity and strength to resist collapse at high temperatures. Additionally, chopped glass fibers have an extremely low CTE (approximately 5-6 ppm / °C), and dispersing them evenly in epoxy resin can significantly lower the expansion coefficient of the overall resin core, which is the core of solving thermal stress problems. Chopped fiber form is easier to achieve uniform distribution in a spherical micro-space than continuous fiber.

[0036] Because epoxy resin needs to be manufactured into high-precision microspheres, the addition of chopped glass fibers (micrometer-sized in length) helps maintain good fluidity during the epoxy resin melt ball-forming process, ensuring the roundness and precision of the microspheres. Long fibers or excessive filling can severely impair fluidity, resulting in unsatisfactory microsphere formation.

[0037] Micron-sized chopped glass fibers can effectively transfer loads. At the same time, their short nature makes it difficult for their ends to generate significant stress concentration points inside the material. They can better ensure the uniformity and long-term reliability of composite materials than long fibers, especially under cyclic stress.

[0038] Furthermore, a breakthrough in performance inflection point is achieved with a glass fiber content of 25-35wt% through the percolation threshold theory: when the mass content is <25wt%, the isolated distribution of fibers cannot form a continuous force transmission network, and the effect on improving the CTE and modulus of the matrix is ​​not significant enough, making it difficult to effectively reduce the CTE, and the improvement in anti-collapse strength at high temperatures may also be insufficient; when the mass content is >35wt%, the matrix cannot completely cover the fibers, resulting in interface defects, the viscosity of the resin mixture rises sharply, and the fluidity becomes very poor, which poses a huge challenge to the highly precise microsphere molding process (requiring high roundness and no defects). The glass fibers agglomerate and are unevenly distributed, affecting performance consistency and making it impossible to form a perfect sphere.

[0039] Therefore, 25-35wt% is the optimal balance between "effective CTE / modulus improvement," "processability (good flowability and spherical formability)," and "reliable mechanical properties of the final core." Below the lower limit (25%), the effect is insufficient, while above the upper limit (35%), it is difficult to mass-produce high-quality microspheres.

[0040] As an embodiment, the silver content of the tin-silver alloy outer layer 30 is 2.9-3.1 wt %.

[0041] It's important to note that pure tin (Sn) is relatively brittle. Adding an appropriate amount of silver (Ag) can significantly improve the ductility (toughness) and fatigue resistance of Sn-based solders. This is crucial for solder joints subjected to thermal cycling stresses, helping to absorb some of the stress and slow crack initiation and growth, thereby extending thermal cycling life.

[0042] Since the silver content directly affects the melting point, 2.9-3.1wt% Ag is a balanced ratio range that can achieve an almost eutectic or near-eutectic composition, keep the melting point constant (approximately 217-220°C), and achieve an optimal balance between the mechanical properties (strength, ductility) and fatigue resistance of the tin-silver alloy outer layer 30. Below 2.9% Ag, ductility and fatigue resistance begin to decline; above 3.1% Ag, the formed Ag3Sn particles are too large, which in turn reduces ductility and even slightly increases the melting point. By limiting the silver content, the following can be achieved: The coating is precisely melted and wetted and spread in the set temperature zone to ensure the stability of the reflow process; Avoid the increase of solid-liquid coexistence zone due to composition shift, prevent thermal tearing or void formation, that is, no risk of mushy zone; Precise composition control ensures that the surface tension of the molten alloy is controllable. Within the rigid support frame provided by the resin core, the molten SnAg layer can be precisely re-solidified into a perfect spherical shape under the action of surface tension, providing a guarantee for reflow self-shaping.

[0043] As an embodiment, the diameter of the composite core 10 is Φ0.18-0.22 mm, and the size fluctuation is controlled to ≤ 0.5 μm by microfluidic granulation technology to ensure high consistency between layers in the stacked package; The thickness of the metal transition layer 20 is 4-6 μm, and the surface roughness Ra is ≥ 0.6 μm. Plasma irradiation generates micron-scale pits on the epoxy surface, and the transition layer metal penetrates to form a mechanical locking structure; The thickness of the tin-silver alloy outer layer 30 is 35-45 μm. The current density gradient electroplating method is used to gradually change the SnAg grain size from columnar to equiaxed state, thereby eliminating the internal stress of the plating layer.

[0044] As an embodiment, the continuous phase matrix is ​​a thermosetting polymer that satisfies: (a) Glass transition temperature Tg ≥ 150°C, ensuring that the compressive elastic modulus retention rate is > 85% before the matrix enters the glass transition zone (molecular chain segments freeze) at the peak reflow temperature; (b) Compression resilience ≥ 90%. This limitation is due to the control of epoxy cross-linking density. After unloading, the entropy elasticity of the polymer chain drives the deformation recovery, completely eliminating the accumulation of plastic deformation. Second, combining Figure 2 This embodiment provides a method for preparing a composite solder ball as described above, comprising: Composite nucleation: The continuous phase matrix raw material and the reinforcing phase filler are mixed under shear emulsification conditions to form a composite slurry, which is then granulated into microspheres to obtain a core preform; interface activation: The surface of the core preform is treated with plasma irradiation to form an active bonding surface; Transition layer construction: chemically depositing a metal transition layer 20 on the active bonding surface; Alloy layer shape control: A dual-source electroplating process is used to simultaneously deposit tin-silver alloy on the surface of the transition layer, where the silver ion concentration is controlled in real time within a closed-loop deviation of ±5%.

[0045] It should be noted that in the composite nucleation stage, the reinforcing phase filler is subjected to dynamic fluid stress dispersion in the shear emulsification field, and the high shear rate peels off the fiber agglomerates. Combined with the interfacial tension self-constraint effect of microfluidic granulation, 25-40 vol% of the short-cut fibers are evenly dispersed in the continuous phase matrix, achieving a rigid-thermal matching core with a compression modulus ≥3 GPa and a thermal expansion coefficient ≤55 ppm / °C of the composite core 10; in the interfacial activation stage, oxygen-containing plasma is used to bombard the surface of the preform, and highly active oxygen free radicals preferentially etch the CH bonds in the epoxy resin, generating micron-sized pits on the surface and simultaneously grafting carboxyl functional groups. This process constructs a mechanical anchoring morphology and chemical adsorption sites with Ra ≥0.6 μm. It lays a strong bonding foundation for the subsequent metal layer; the transition layer is constructed using an electroless chemical deposition process, which catalytically reduces metal ions on the activated surface and uses surface active sites to guide the directional deposition of metal atoms to achieve nano-scale coating integrity of the 4-6μm transition layer, avoiding core swelling caused by electrolyte penetration; the alloy layer shape control process innovation adopts a dual-source electroplating architecture, in which the tin source electrolyte and the silver source electrolyte are physically isolated by an ion exchange membrane but coupled in the current loop. The silver ion concentration sensor is used to provide real-time feedback to adjust the pulse electroplating duty cycle, suppressing fluctuations in the deposited silver content within the range of ±0.1wt%, and simultaneously achieving uniform growth of a coating with a thickness of 35-45μm and precise control of the silver content of 2.9-3.1wt%.

[0046] In a third aspect, this embodiment provides a stacked package structure, including a bottom package and a top package, wherein the bottom package and the top package are electrically connected and mechanically supported by a composite solder ball as described above; The inter-layer height fluctuation after reflow soldering is ≤±2%.

[0047] Among them, the top package is a BGA or CSP package. This embodiment uses composite solder balls to achieve connection and support at the same time. It is directly applied in the BGA / CSP field without using plastic support structure and bottom filling glue, and eliminating auxiliary structure.

[0048] Specifically, the composite solder balls form an array of support pillars within the PoP: during reflow, the outer SnAg layer melts and wets the solder pads, while the core remains solid, supporting the top package. Because the core's compressive modulus (3 GPa) exceeds the package's modulus (1.5 GPa), height fluctuations are solely due to the core's dimensional tolerance (±1.5 μm). Replacing traditional plastic supports with this structure reduces interlayer thermal resistance by 40°C. Furthermore, eliminating the underfill process reduces the rework temperature from 200°C to 183°C (the melting point of SnAg), enabling non-destructive chip removal.

[0049] For example: In this embodiment, a composite core 10 with a diameter of Φ0.20 mm is used, which is composed of bisphenol A epoxy resin and 30 wt% chopped glass fiber reinforcement. The chopped glass fiber accounts for 35% of the total volume of the core. The diameter of the chopped glass fiber is Φ0.5 μm, the aspect ratio is 50:1, and the compression resilience is greater than 95%. The film was activated by oxygen-nitrogen plasma (O2 / N2=3:1, 300W / 5min), followed by chemical deposition of a 5μm copper transition layer with a surface roughness of Ra=0.8μm to enhance bonding strength. An outer layer of SnAg3.0 alloy was then electroplated with a silver content of 3.0±0.1wt%, a thickness of 40μm, and a finished product diameter of Φ0.30mm. Finally, reflow shaping was performed at 230℃×30s to liquefy and resolidify the coating into a perfect sphere.

[0050] The composite solder balls prepared in the above embodiment have a collapse strength of 105 MPa at 250°C, while conventional BGA solder balls have a collapse strength of only 28 MPa at 250°C. This embodiment improves this by 275%, achieving a PoP layer height fluctuation of ≤±1.5%. Conventional BGA solder balls have a PoP layer height fluctuation of ≤15%, a fluctuation reduced tenfold by this embodiment. Furthermore, after accelerated thermal cycling at -40°C to 125°C, the solder joint life exceeded 5,000 cycles, while conventional BGA solder balls had a thermal cycle life of no more than 1,500 cycles. This represents a 233% improvement over conventional solutions, simultaneously achieving three disruptive effects: 1) The solder ball diameter is self-corrected to Φ0.300±0.001mm after reflow shaping (roundness error ≤0.3μm); 2) The core CTE is ≈50ppm / °C (close to 18ppm / °C of PCB), which reduces thermal stress by 70%; 3) The single-ball integrated connection and support functions replace the plastic bracket and underfill process, reducing the PoP package rework time by 80% and the stacking thickness by 40%.

[0051] This embodiment systematically solves the technical contradictions of mechanical stability, thermal reliability and maintainability in stacked packaging through the synergistic solution of "composite core 10-plating" through structural innovation and micron-level process control.

[0052] In summary, compared with the prior art, the above embodiment has at least the following technical advantages: The composite core 10 is composed of a continuous polymer matrix and a reinforcement fiber network. Its specific volume and mass ratios achieve high-temperature compressive strength and low thermal expansion properties. It maintains solid-state rigidity in the high-temperature welding range, directly bearing mechanical loads and completely eliminating the risk of interlayer height collapse. The metal transition layer 20 forms a mechanically interlocking interface with the core, while the tin-silver alloy outer layer 30 provides solderability and electrical connection, achieving a dual-functionality fusion of the composite solder ball, which serves both as a support and a soldering function. The reinforcement fiber network effectively constrains the matrix expansion behavior, making the thermal expansion coefficient of the composite core 10 close to that of the organic packaging substrate. This characteristic reduces the interfacial stress concentration during thermal cycling from the source, thereby improving the fatigue life of the solder joints. The composite solder balls independently assume the supporting function, eliminating the plastic bracket installation process and the need for bottom filling process. This design makes the package structure have reversible connection characteristics, and the underlying chip can be disassembled and replaced without damage.

[0053] The above embodiments are only preferred embodiments of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and replacements made by technicians in this field on the basis of the present invention fall within the scope of protection required by the present invention.

Claims

1. A composite solder ball, characterized in that: include: A composite core composed of a continuous phase matrix and a reinforcement phase, wherein the reinforcement phase accounts for 15% to 30% of the total volume of the core, and the composite core has a compression modulus of ≥3 GPa and a thermal expansion coefficient of ≤55 ppm / °C; A metal transition layer, covering the polymer core; A tin-silver alloy outer layer covers the metal transition layer.

2. The composite solder ball according to claim 1, wherein: The reinforcing phase is a non-metallic fiber with an aspect ratio of ≥20:

1.

3. The composite solder ball according to claim 2, wherein: The non-metallic fibers are chopped fibers, and the chopped fibers include glass fibers, ceramic fibers or carbon fibers.

4. The composite solder ball according to claim 1, wherein: The reinforcement phase accounts for 25-35 wt% of the total mass of the core.

5. The composite solder ball according to any one of claims 1 to 4, characterized in that: The silver content of the tin-silver alloy outer layer is 2.9-3.1 wt %.

6. The composite solder ball according to claim 5, wherein: The diameter of the composite core is Φ0.18-0.22mm; The thickness of the metal transition layer is 4-6 μm, and the surface roughness Ra is ≥ 0.6 μm; The thickness of the tin-silver alloy outer layer is 35-45 μm.

7. The composite solder ball according to claim 6, wherein: The continuous phase matrix is ​​a thermosetting polymer that satisfies: (a) Glass transition temperature Tg ≥ 150°C; (b) Compression resilience ≥ 90%.

8. The composite solder ball according to claim 7, wherein: The continuous phase matrix is ​​epoxy resin.

9. A method for preparing a composite solder ball according to any one of claims 1 to 8, characterized in that: include: Composite nucleation: The continuous phase matrix raw material and the reinforcing phase filler are mixed under shear emulsification conditions to form a composite slurry, which is then granulated into microspheres to obtain a core preform; interface activation: The surface of the core preform is treated with plasma irradiation to form an active bonding surface; Transition layer construction: chemical deposition of a metal transition layer on the active bonding surface; alloy layer shape control: a dual-source electroplating process is used to simultaneously deposit a tin-silver alloy on the surface of the transition layer, where the silver ion concentration is controlled in real time within a closed-loop within a deviation of ±5%.

10. A stacked package structure comprising a bottom package and a top package, characterized in that: The bottom package and the top package are electrically connected and mechanically supported by a composite solder ball according to any one of claims 1 to 8; The inter-layer height fluctuation after reflow soldering is ≤±2%.

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