Copper powder-liquid gallium composite low-temperature metal solder and preparation method and welding application thereof

Abstract

The invention relates to the technical field of electronic packaging, in particular to a copper powder-liquid gallium composite low-temperature metal solder and a preparation method and welding application thereof.The copper powder-liquid gallium composite low-temperature metal solder is composed of, by weight, 60-90% of liquid gallium, 10-40% of copper powder, an accelerant and a process additive, and the accelerant and the process additive are both optional components. When welding interconnection needs to be carried out, welding flux is applied to the back face of a metalized substrate or a chip, the chip is mounted, heat preservation is carried out for 10-240 min under the inert or reducing atmosphere at the temperature of 30-260 DEG C and the pressure of 1-5 MPa, and a Cu-Ga intermetallic compound is generated on an interface / bulk phase in situ, and a compact welding seam is formed. Through verification, the solder can generate a stable intermetallic compound and a communication metal network in situ in the assembly process so as to improve the shear strength and the long-term reliability, and meanwhile, the material cost, the manufacturing process cost, the manufacturability and the consistency are considered.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electronic packaging, in particular to a copper powder-liquid gallium composite low-temperature metal solder and a preparation method and welding application thereof. BACKGROUND

[0002] Power semiconductor devices (such as Si, SiC, GaN) work under high junction temperature, high heat flux density and repeated thermal / power cycle conditions, which puts forward comprehensive requirements of low assembly temperature, high thermal conductivity, high strength and high reliability for the performance of the chip-substrate welding layer (die-attach). The ideal welding layer should be able to realize metallurgical connection under the condition of lower temperature and smaller pressure, and at the same time maintain low porosity and stable interface structure in long-term service thermal cycle and high temperature aging.

[0003] Existing chip interconnect materials mainly include Sn-based solders, AuSn eutectic solders, silver sintered materials, and transient liquid phase (TLP) interconnect systems using metals such as Sn / In / Ga. Each approach involves significant trade-offs in terms of process window, cost, reliability, and substrate compatibility. Sn-based solders (such as SAC-based solders) have advantages such as low cost and high maturity, low melting point and reflow temperature, and good assemblability. However, under high junction temperatures or extensive thermal cycling, the solder joint is prone to creep and fatigue cracking, the interfacial Cu–Sn / Ni–Sn intermetallic compound (IMC) continues to grow, and failure problems such as Kirkendall voids and electromigration (EM) become prominent, making it difficult to meet the application requirements of higher temperatures and longer lifespans. AuSn eutectic solder (typical composition 80Au / 20Sn, melting point approximately 280℃): Offers good thermal conductivity and reliability, with stable interfacial reactions; however, the high price of gold leads to high material and surface metallization costs. Additionally, the high process temperature may introduce thermal stress and warpage risks, and the Au-Sn brittle phase ratio and residual stress require precise control. Silver sintered materials: Sintered layers achieve high thermal conductivity and high temperature resistance reliability, but often require high sintering temperatures (typically ≥200℃) and / or high pressures (several MPa to tens of MPa). They are highly sensitive to equipment and substrate flatness and surface cleanliness; porosity control, batch consistency, and silver migration risks also require close attention, resulting in higher material and process costs. TLP bonding (using Sn / In, etc., as transient liquid phases): Achieves a balance between low-melting-point assembly and high-temperature service by melting low-melting-point metals at lower temperatures and reacting with Cu / Ni, etc., to generate high-melting-point IMC. However, Sn-based TLPs are prone to problems such as excessively thick IMC brittle layers, voids, and stress caused by volume changes; In-based TLPs have lower melting points and better wettability, but In's low strength and creep may limit high-temperature and long-life applications. Liquid gallium and its alloys: Gallium is liquid near room temperature, exhibiting good wettability and thermal conductivity, and has been widely explored for use in interconnects or interface materials. However, gallium surfaces easily form stable oxide films, inhibiting wetting and diffusion reactions; liquid metals suffer from "pumping / leakage" and insufficient geometric fixation; there is a risk of penetration embrittlement to metals such as aluminum; and the lack of low-residue, reproducible interface activation and fixation mechanisms leads to uncertainties in long-term stability, manufacturability, and reliability.

[0004] From a substrate compatibility perspective, power device back electrodes and substrates often employ multilayer metallization structures such as Cu / Ni / Ag, Cu / Ni / Au, and Ti / Ni / Ag. Diffusion barrier layers such as TiW and TiN are required to suppress poor interdiffusion and embrittlement. Interconnect materials should exhibit good wetting / reaction and controllable interface structure within these metallization systems. From a process perspective, mass-production die-attach processes favor flux-free or low-residue processes (such as inert / micro-reduction or formic acid atmospheres) to avoid porosity, ion contamination, and long-cycle failures caused by volatile residues. Simultaneously, to reduce thermal shock to the chip and substrate, assembly temperature and pressure need to be controlled within the most moderate possible window. From a reliability perspective, the solder layer must withstand thermal cycling, power cycling, and high-temperature aging within a wide range of -40 to 150°C and beyond. Low solder porosity, controllable thickness and composition of the interface IMC layer, stable shear / tensile / fatigue performance, and good electrical and thermal conductivity retention are required. In summary, existing technologies generally face the following contradictions: it is difficult to simultaneously meet the requirements of low-temperature / low-pressure assembly and high-temperature / long-life service; high material / process costs (such as AuSn, some silver sintering), or insufficient reliability under high temperature and cycling conditions (such as some Sn / In systems), or bottlenecks in fixed and long-term stability in liquid metal routes (such as overflow and oxide film problems of pure gallium or gallium-based alloys). Summary of the Invention

[0005] The purpose of this invention is to provide a copper powder-liquid gallium composite low-temperature metal solder, its preparation method and welding application, which realizes the metal metallurgical interconnection of power chip-substrate under mild conditions such as 30-260℃ and 1-5MPa, and at the same time obtains a weld structure with high shear strength and low porosity.

[0006] To achieve the above objectives, the present invention provides a copper powder-liquid gallium composite low-temperature metal solder, which is composed of liquid gallium, copper powder, accelerator and process aids, wherein the weight percentage of liquid gallium is 60-90 wt%, the weight percentage of copper powder is 10-40 wt%, and the accelerator and process aids are optional components.

[0007] The promoter is selected from one or more of Sn, In, Ag, and Au metals, and is configured as a 10–200 nm thin layer coated on the surface of the copper powder and / or a solute dissolved in the liquid gallium phase.

[0008] The process aid is one or more of a volatile solvent, thixotropic agent, or dispersant, and is selected in a weight percentage of 0–5 wt%.

[0009] The copper powder-liquid gallium composite low-temperature metal solder is a silvery-white viscous paste that can generate Cu–Ga intermetallic compounds in situ between copper and gallium and form a continuous metal network under heating and pressure-assisted conditions.

[0010] Furthermore, the present invention also proposes a method for preparing copper powder-liquid gallium composite low-temperature metal solder, which includes the following steps:

[0011] Step 1: Activate liquid gallium by removing the film under an inert or reducing atmosphere;

[0012] Step 2: Dry, degas, and surface-activate the copper powder to form a Sn / In / Ag thin layer;

[0013] Step 3: Mix liquid gallium with copper powder and optional process additives at 2000 rpm for 5 min in an inert atmosphere or vacuum environment.

[0014] Step 4: Degas under reduced pressure and seal in nitrogen-filled packaging, then refrigerate at low temperature.

[0015] Optionally, during the execution of step 1, mechanical stirring and the introduction of 1–5 vol% hydrochloric acid vapor are used for membrane removal.

[0016] Optionally, the drying process in step 2 is specifically to dry for 10-20 minutes at 100–140℃ and ≤100Pa.

[0017] Optionally, the stirring speed in step 3 is 1000-2000 rpm, and the time is 5-20 min.

[0018] Furthermore, this invention also proposes a welding application of copper powder-liquid gallium composite low-temperature metal solder. Using the copper powder-liquid gallium composite low-temperature metal solder, the copper powder-liquid gallium composite low-temperature metal solder is applied to the back of a metallized substrate or chip, the chip is mounted, and the temperature is maintained at 30–260℃, 1–5MPa, inert or reducing atmosphere for 10–240 min, so that Cu–Ga intermetallic compounds are generated in situ at the interface / bulk phase and a dense weld is formed.

[0019] This invention provides a copper powder-liquid gallium composite low-temperature metal solder, composed of liquid gallium, copper powder, accelerator, and process aids. The liquid gallium comprises 60–90 wt% by weight, and the copper powder comprises 10–40 wt% by weight. The accelerator and process aids are optional components. When welding interconnects are required, the solder is applied to a metallized substrate or the back of a chip, the chip is mounted, and the mixture is held at 30–260°C, 1–5 MPa, and an inert or reducing atmosphere for 10–240 minutes. This allows for the in-situ formation of Cu–Ga intermetallic compounds at the interface / bulk phase, resulting in a dense weld. Verification has shown that the solder of this invention can generate stable intermetallic compounds and interconnected metal networks in-situ during assembly, improving shear strength and long-term reliability, while also considering material and process costs, manufacturability, and consistency. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the composition and morphology of a copper powder-liquid gallium composite low-temperature metal solder according to the present invention.

[0022] Figure 2 This is a schematic diagram of the process flow for preparing a copper powder-liquid gallium composite low-temperature metal solder according to the present invention.

[0023] Figure 3 This is a schematic diagram of the welding layer interconnection application process of copper powder-liquid gallium composite low-temperature metal solder in a specific embodiment of the present invention.

[0024] Figure 4 This is a SEM image of a specific embodiment 1 of the present invention.

[0025] Figure 5 This is an EDS diagram of a specific embodiment 1 of the present invention.

[0026] Figure 6 This is a statistical diagram illustrating the change in shear strength as a function of the particle size and proportion of copper powder in the solder, according to a specific embodiment of the present invention. Detailed Implementation

[0027] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0028] Please see Figure 1 The present invention provides a copper powder-liquid gallium composite low-temperature metal solder, which is composed of liquid gallium, copper powder, accelerator and process aid, wherein the weight percentage of liquid gallium is 60-90 wt%, the weight percentage of copper powder is 10-40 wt%, and the accelerator and process aid are optional components.

[0029] The promoter is selected from one or more of Sn, In, Ag, and Au metals, and is configured as a 10–200 nm thin layer coated on the surface of the copper powder and / or a solute dissolved in the liquid gallium phase.

[0030] The process aid is one or more of a volatile solvent / thixotropic agent / dispersant, selected at a weight percentage of 0–5 wt%. The process aid is used to adjust viscosity (25°C, 10s). -1 (20–200 Pa·s).

[0031] The copper powder-liquid gallium composite low-temperature metal solder is a silvery-white viscous paste. Under heating conditions, Cu–Ga intermetallic compounds can be generated in situ between copper and gallium to form a continuous metal network.

[0032] In specific embodiments, the copper powder particle size can be divided into three categories: 10–20 μm, 20–30 μm, and 30–40 μm, among which copper powder with a particle size range of 10–20 μm exhibits the best binding performance. In further embodiments, the copper powder can also be submicron-sized (e.g., 100 nm to 1 μm) or nano-sized copper powder with even smaller particle sizes to expand the application range.

[0033] Furthermore, the present invention also proposes a method for preparing copper powder-liquid gallium composite low-temperature metal solder, which includes the following steps:

[0034] Step 1: Activate liquid gallium by removing the film under an inert or reducing atmosphere;

[0035] Step 2: Dry, degas, and surface-activate the copper powder to form a Sn / In / Ag thin layer;

[0036] Step 3: Mix liquid gallium with copper powder and optional process additives at 2000 rpm for 5 min in an inert atmosphere or vacuum environment.

[0037] Step 4: Degas under reduced pressure and seal in nitrogen-filled packaging, then refrigerate at low temperature.

[0038] During the execution of step 1, mechanical stirring and the introduction of 1–5 vol% hydrochloric acid vapor are used for membrane removal.

[0039] The drying process in step 2 is specifically to dry for 10-20 minutes at 100–140℃ and ≤100Pa.

[0040] The stirring speed in step 3 is 1000-2000 rpm, and the time is 5-20 min.

[0041] The specific process flow diagram is as follows: Figure 2 As shown.

[0042] Furthermore, this invention also proposes a welding application of copper powder-liquid gallium composite low-temperature metal solder. Using the copper powder-liquid gallium composite low-temperature metal solder, the copper powder-liquid gallium composite low-temperature metal solder is applied to the back of a metallized substrate or chip, the chip is mounted, and the temperature is maintained at 30–260℃, 1–5MPa, inert or reducing atmosphere for 10–240 min, so that Cu–Ga intermetallic compounds are generated in situ at the interface / bulk phase and a dense weld is formed.

[0043] Please see Figure 3 The metallization system of the chip and the substrate is selected from Cu / Ni / Au, Ti / Ni / Ag, Ni–P, TiW, TiN or a multilayer combination thereof.

[0044] The coating method is screen printing, stencil printing or dispensing, the wet film thickness is 120-220μm and the final weld thickness is 60-200μm.

[0045] Through welding applications, the present invention can generate a power semiconductor welding assembly, including a chip with a metallized back electrode and a metallized substrate, wherein a weld obtained by the method in the application is provided therebetween, the weld containing a metal network in which a continuous phase of Cu–Ga intermetallic compound is connected to copper particles.

[0046] Furthermore, the present invention provides several embodiments for further illustration:

[0047] Example 1 (No accelerator / no organic additives)

[0048] Mixture ratio: Ga 85wt%, Cu 15wt%; Cu particle size: 30-40μm.

[0049] Preparation: Mix at 2000 rpm for 5 min in a vacuum.

[0050] Interconnection: Coated with a 100-120μm wet film; SiC chip (Ti / Cu), substrate DBC (Cu); 180℃, 5MPa, 10 min.

[0051] Results: Room temperature shear strength ≈ 30 MPa; Electrical conductivity ≈ 2 × 10⁻⁶ MPa 6S / m; porosity ≤10%; cross-section shows continuous Cu–Ga phase and metal network.

[0052] Example 2 (Powder Surface Coating)

[0053] Based on Example A, Cu powder was electroplated with ~50–100 nm Ag.

[0054] Conditions: 140℃, 1MPa, 5 min;

[0055] Results: Initial shear ≥40 MPa; sintering time shortened, interface spreading improved.

[0056] Example 3 (Gallium Alloy)

[0057] Ga – 1wt%In; the rest is the same as A;

[0058] Conditions: 140°C, 1MPa, 5 min.

[0059] Results: Shear strength ≥ 40 MPa; sintering initiation temperature decreased.

[0060] Example 4 (Copper powder with smaller particle size)

[0061] Cu particle size: submicron (e.g., 100nm to 1μm) or nanoscale copper powder

[0062] Conditions: 100℃, 1MPa, 5 min

[0063] Results: Higher shear strength, faster sintering speed, and higher electrical conductivity.

[0064] Example 5 (Copper powder in different shapes)

[0065] Based on Example A, Cu powder exhibits different shapes (flakes, threads, etc.).

[0066] Conditions: 140℃, 1MPa, 5 min;

[0067] Results: Initial shear ≥ 40 MPa; longer sintering time.

[0068] like Figure 4 and Figure 5 The images shown are SEM and EDS images of Example 1, in which the thickness of the Cu–Ga intermetallic compound layer at the interface is 60-200 μm and the overall porosity is ≤10%, containing four phases: Cu, CuGa2, Cu9Ga4, and Ga rich phase (more phases will exist if more metal additives are added).

[0069] Figure 6This is a statistical graph showing the change in shear strength of the component as a function of the particle size and proportion of copper powder in the solder.

[0070] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0071] 1. In the field of power device interconnection, silver sintering and copper sintering technologies are mainly used, and there is no clear technology for interconnection using gallium metal.

[0072] 2. Existing research on copper-gallium interconnects mainly focuses on the interconnection of copper with pure liquid gallium or gallium-based alloys, and there is no research on copper-gallium hybrid solders.

[0073] 3. Existing research on copper-gallium interconnects has difficulty in solving the overflow problem of liquid gallium metal. This study effectively solved this problem by adjusting the mixing ratio of copper powder and liquid gallium.

[0074] 4. The copper-gallium metal solder composition designed in this study greatly shortens the sintering time, from several hours or tens of hours for the original interconnection of copper with pure liquid gallium or gallium-based alloys to several minutes.

[0075] The specific data results are as follows:

[0076] Low-temperature / low-pressure sintering: a continuous metal network is formed at 30–260℃ and ≤5MPa;

[0077] High performance: Excellent room temperature shear strength (up to ~70MPa), electrical conductivity ≥2.0×10⁻⁶ 6 S / m;

[0078] Reliability: High strength retention after 1000 thermal cycles at -40–150℃, porosity ≤10%;

[0079] Process compatibility: Suitable for both screen printing and dispensing, and compatible with metallization of Cu / Ni / Au, Ti / Ni / Ag, etc.

[0080] Green and environmentally friendly: lead-free, low halogen; organic carrier is optional or can be omitted.

[0081] The above description discloses only one or more preferred embodiments of the present invention, and should not be construed as limiting the scope of the present invention. Those skilled in the art will understand that all or part of the processes of the above embodiments can be implemented, and equivalent changes made in accordance with the claims of the present invention are still within the scope of the invention.

Claims

1. A copper powder-liquid gallium composite low-temperature metal solder, characterized in that, It is composed of liquid gallium, copper powder, accelerator and process aids, wherein the weight percentage of liquid gallium is 60-90 wt%, the weight percentage of copper powder is 10-40 wt%, and the accelerator and process aids are optional components. The promoter is selected from one or more of Sn, In, Ag, and Au metals, and is configured as a 10–200 nm thin layer coated on the surface of the copper powder and / or a solute dissolved in the liquid gallium phase.

2. The copper powder-liquid gallium composite low-temperature metal solder as described in claim 1, characterized in that, The process aid is one or more of a volatile solvent / thixotropic agent / dispersant, and the weight percentage selected is 0–5 wt%.

3. The copper powder-liquid gallium composite low-temperature metal solder as described in claim 2, characterized in that, The copper powder-liquid gallium composite low-temperature metal solder is a silvery-white viscous paste. Under heating and pressure-assisted conditions, Cu–Ga intermetallic compounds can be generated in situ between copper and gallium to form a continuous metal network.

4. A method for preparing a copper powder-liquid gallium composite low-temperature metal solder, used to prepare the copper powder-liquid gallium composite low-temperature metal solder as described in any one of claims 1 to 3, characterized in that, Includes the following steps: Step 1: Activate liquid gallium by removing the film under an inert or reducing atmosphere; Step 2: Surface activation and drying / degassing of copper powder can be performed to form a Sn / In / Ag thin layer; Step 3: Mix liquid gallium with copper powder and optional process additives at 2000 rpm for 5 min in an inert atmosphere or vacuum environment. Step 4: Degas under reduced pressure and seal in nitrogen-filled packaging, then refrigerate at low temperature.

5. The method for preparing copper powder-liquid gallium composite low-temperature metal solder as described in claim 4, characterized in that, During the execution of step 1, mechanical stirring and the introduction of 1–5 vol% hydrochloric acid vapor are used for membrane removal.

6. The method for preparing copper powder-liquid gallium composite low-temperature metal solder as described in claim 4, characterized in that, The drying process in step 2 is specifically to dry for 10-20 minutes at 100–140℃ and ≤100Pa.

7. The method for preparing copper powder-liquid gallium composite low-temperature metal solder as described in claim 4, characterized in that, The stirring speed in step 3 is 1000-2000 rpm, and the time is 5-20 min.

8. A welding application of a copper powder-liquid gallium composite low-temperature metal solder, using the copper powder-liquid gallium composite low-temperature metal solder as described in any one of claims 1 to 3, characterized in that, The copper powder-liquid gallium composite low-temperature metal solder is applied to the metallized substrate or the back of the chip, the chip is mounted, and the mixture is kept at 30–260°C, 1–5MPa, inert or reducing atmosphere for 10–240 min to generate Cu–Ga intermetallic compounds in situ at the interface / bulk phase and form a dense weld.

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