Transient liquid phase connection method based on composite laminated structure

Abstract

The invention relates to a transient liquid phase connection method based on a composite laminated structure, belongs to the technical field of power semiconductor packaging interconnection, and mainly solves the problems of poor joint reliability and poor high-temperature service performance caused by insufficient solder wettability and narrow process window in the traditional transient liquid phase connection technology. According to the method, a symmetric composite laminated structure of a Cu substrate / Sn-Cu soldering paste / Cu-Ni alloy sheet / Sn-Cu soldering paste / Cu substrate is constructed, optimized printing process parameters are combined, and the good wetting characteristic of the Sn-Cu soldering paste and the microalloying effect of the Ni element in the Cu-Ni alloy sheet are utilized in the welding process, so that the interface bonding quality is remarkably improved, and the formation of (Cu, Cu)-Cu-Ni alloy sheets at low temperature is promoted; and a high-melting-point and compact all-intermetallic compound joint mainly comprising Ni) Sn. The obtained joint is low in porosity and high in heat conductivity and has excellent mechanical and thermal reliability at high temperature, the process is compatible with an existing production line, and a high-feasibility and low-cost packaging solution is provided for a high-power semiconductor device.

Description

Technical Field

[0001] This invention relates to a transient liquid phase bonding method based on a composite stacked structure, belonging to the field of welding technology for power semiconductor package interconnects. Background Technology

[0002] With the rapid development of third-generation semiconductor technologies, represented by silicon carbide and gallium nitride, power semiconductor devices are evolving towards higher power density, higher thermal conductivity, and higher reliability, which places higher demands on electronic packaging interconnect technologies. Transient liquid phase bonding technology, due to its unique advantage of achieving "low-temperature connection and high-temperature operation," has shown significant potential in the field of power electronic packaging.

[0003] However, traditional transient liquid phase bonding technology based on pure tin faces significant challenges in practical applications. At typical bonding temperatures, pure tin solder exhibits relatively poor wettability, requires high substrate flatness and surface roughness, and incomplete reactions can easily lead to residual low-melting-point phases in the joint, affecting its high-temperature performance. Therefore, the industry urgently needs a new bonding method that can improve solder wettability, reduce bonding temperature, and is easy to industrialize. Summary of the Invention

[0004] The purpose of this invention is to overcome the problems of insufficient solder wettability, high welding temperature, and the need to improve joint quality in existing transient liquid phase bonding technologies, and to provide a transient liquid phase bonding method based on a composite layered structure. This method aims to improve the interface bonding quality and effectively enhance the density and reliability of the joint by using Sn-Cu solder paste instead of traditional pure tin solder and combining optimized printing process parameters.

[0005] To achieve the above objectives, the present invention adopts the following technical solution.

[0006] The core of this invention lies in the design of a symmetrical composite layered structure: Cu substrate / Sn-Cu solder paste / Cu-Ni alloy sheet / Sn-Cu solder paste / Cu substrate. By introducing Sn-Cu solder paste (where the mass fraction of Cu does not exceed 1%) to replace traditional pure tin solder, and combining it with a Cu-Ni alloy sheet (where the mass fraction of Ni does not exceed 10%) as a key functional layer, the superior wetting properties of Sn-Cu solder paste are utilized to promote the spread of the liquid phase on the substrate surface. Simultaneously, by precisely controlling the printing process parameters, the uniformity and consistency of solder paste deposition are ensured, thereby achieving a denser interfacial bond at a lower temperature.

[0007] The method includes the following steps in sequence.

[0008] A first Cu substrate and a second Cu substrate are provided as components to be joined. A first layer of Sn-Cu solder paste (thickness from 1 micrometer to 250 micrometers) is formed on the upper surface of the first Cu substrate using a printing process. A Cu-Ni alloy sheet (thickness from 5 micrometers to 500 micrometers) is precisely placed on the upper surface of the first layer of Sn-Cu solder paste. A second layer of Sn-Cu solder paste (thickness from 1 micrometer to 250 micrometers) is formed on the upper surface of the Cu-Ni alloy sheet using a printing process. The second Cu substrate is then placed on top of the second layer of Sn-Cu solder paste to construct a symmetrical composite stacked structure. The composite structure is then subjected to transient liquid phase bonding treatment.

[0009] The mass fraction of Ni in the Cu-Ni alloy sheet does not exceed 10%, and its thickness is controlled between 5 micrometers and 500 micrometers; the mass fraction of Cu in the Sn-Cu solder paste does not exceed 1%, and the thickness of the first layer of Sn-Cu solder paste and the second layer of Sn-Cu solder paste are both between 1 micrometer and 250 micrometers.

[0010] The Sn-Cu solder paste is printed using stencil printing technology under the following optimized parameters: ambient temperature 20-26℃, humidity 40%–60% RH, squeegee speed 20-30 mm / s, squeegee pressure 0.35-0.45 kg / cm², squeegee angle 45°-55°, stencil-substrate distance 0-0.1 mm, and demolding speed 0.1-1 mm / s. This parameter combination ensures sufficient filling of the solder paste within the stencil openings while avoiding stencil deformation due to excessive pressure. The Sn-Cu solder paste used has a viscosity of 160-220 Pa·s at 25℃, and the powder particle size is either No. 3 powder (25-45 microns) or No. 4 powder (20-38 microns). The area ratio of the printing stencil openings is greater than 0.66, and the aspect ratio is greater than 1.5, which facilitates good demolding and shaping of the solder paste.

[0011] The present invention has the following advantages.

[0012] (1) By implementing Sn-Cu solder paste layers with optimized printing process parameters, high-precision thickness control within the range of 1–250 micrometers can be achieved. Compared to pure tin solder, Sn-Cu solder paste has superior wettability, and its liquid phase can spread more fully on the surface of Cu-Ni alloy sheets and Cu substrates. Precise coordination of squeegee angle, pressure, and speed ensures uniform deposition of the solder paste, while controlled demolding speed effectively prevents sharpening or defects in the printed pattern. These factors work together to reduce the stringent requirements for the flatness and surface roughness of Cu-Ni alloy sheets, widen the process window, and significantly reduce joint porosity. The Cu element in the solder paste promotes the growth of (Cu,Ni)6Sn5.

[0013] (2) The excellent wettability and fluidity of Sn-Cu solder paste, combined with optimized printing parameters, result in a denser solder joint. This dense microstructure contributes to higher thermal conductivity, improving the thermal conductivity and long-term reliability of the joint.

[0014] (3) The Sn-Cu solder paste and its optimized printing process used in this invention are highly compatible with existing mature industrial solder paste printing equipment. The entire method is simple and does not require major modifications to existing production lines or the introduction of expensive equipment, thus showing good prospects for industrial application. The Sn-Cu solder paste itself has a low cost, making this solution have a significant cost advantage in large-scale production. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the composite stacked structure before transient liquid phase connection in an embodiment of the present invention.

[0016] Figure 2 This is a cross-sectional scanning electron microscope image (low magnification) of the connector obtained by transient liquid phase connection in an embodiment of the present invention.

[0017] Figure 3 for Figure 2 Scanning electron microscope image (high magnification) of the microstructure of the joint shown in the rectangular region.

[0018] Figure 4 The graph shows the change in thermal conductivity of the welded joint obtained in the embodiment of the present invention in the temperature range of 25°C to 250°C. Detailed Implementation

[0019] The present invention will be further described below with reference to embodiments and accompanying drawings. Unless otherwise specified, the methods described are conventional methods, and the raw materials are all available from publicly available commercial sources.

[0020] The materials selected in this embodiment are as follows: Cu-Ni alloy sheet is Cu-5 wt%Ni white copper foil with a thickness of 100 micrometers; Sn-Cu solder paste is Sn-0.7 wt%Cu alloy; the substrate is copper with a purity of 99.99 wt% and a size of 10*10*1.5 mm.

[0021] The experimental steps are as follows: First, a first and a second Cu substrate are provided; a first layer of Sn-Cu solder paste is formed on the surface of the first Cu substrate using a printing process. The printing process parameters are set as follows: ambient temperature 23℃, humidity 50% RH, squeegee speed 25 mm / s, squeegee pressure 0.4 kg / cm², squeegee angle 50°, distance between stencil and substrate 0.05 mm, and demolding speed 0.5 mm / s; the viscosity of the Sn-Cu solder paste used is approximately 190 Pa·s (25℃), the powder is No. 4 powder (20-38 micrometers), and the printing thickness is 30 micrometers. Then, a Cu-5 wt% Ni alloy sheet with a thickness of 100 micrometers is precisely placed on the surface of the solder paste; next, a second layer of Sn-Cu solder paste is printed on the surface of the alloy sheet, with the same process parameters as the first layer and a thickness of 30 micrometers; finally, the second Cu substrate is covered, thus forming a composite stacked structure, such as... Figure 1 As shown. After fixing the structure with a fixture, it was placed in a vacuum reflow oven and held at 250°C for 10 minutes for transient liquid phase bonding treatment.

[0022] The treated joint was characterized, such as Figure 2 and Figure 3 As shown, the solder layer has been completely transformed into intermetallic compounds, mainly forming two phases: (Cu,Ni)6Sn5 and (Cu,Ni)3Sn. The microstructure is dense, with a porosity of less than 5%. The thermal conductivity of this joint at 25℃, 50℃, 75℃, 100℃, 150℃, 200℃, and 250℃ are 144.730, 157.591, 170.681, 183.233, 192.000, 193.537, and 187.862 W / (m·K), respectively. Figure 4 As shown in the figure, it maintains excellent thermal conductivity even at temperatures exceeding the melting point of Sn-0.7 Cu; its electrical conductivity is 29.391 MS·m⁻¹, and its room temperature shear strength reaches 46.82 MPa.

[0023] The above embodiments demonstrate that the present invention, using Sn-Cu solder paste combined with optimized printing process parameters, can form high-performance all-metal compound joints at relatively low temperatures, with process controllability and joint reliability superior to the traditional pure tin system.

Claims

1. A transient liquid phase bonding method based on a composite laminated structure, characterized in that, Includes the following steps: A first Cu substrate and a second Cu substrate are provided; a first layer of Sn-Cu solder paste is formed on the upper surface of the first Cu substrate by a printing process; a Cu-Ni alloy sheet is placed on the upper surface of the first layer of Sn-Cu solder paste; a second layer of Sn-Cu solder paste is formed on the upper surface of the Cu-Ni alloy sheet by a printing process; a second Cu substrate is placed on the upper surface of the second layer of Sn-Cu solder paste, thereby forming a composite structure in which the first Cu substrate, the first layer of Sn-Cu solder paste, the Cu-Ni alloy sheet, the second layer of Sn-Cu solder paste and the second Cu substrate are stacked in sequence; the composite structure is subjected to transient liquid phase bonding treatment, so that the Sn-Cu solder paste undergoes a metallurgical reaction with the Cu-Ni alloy sheet and the Cu substrate through liquid phase diffusion, and is completely transformed into an intermetallic compound bonding joint mainly composed of (Cu,Ni)6Sn5 and (Cu,Ni)3Sn.

2. The method according to claim 1, characterized in that: The mass fraction of Ni in the Cu-Ni alloy sheet does not exceed 10%, and its thickness ranges from 5 micrometers to 500 micrometers; the mass fraction of Cu in the Sn-Cu solder paste does not exceed 1%, and the thickness of the first layer of Sn-Cu solder paste and the second layer of Sn-Cu solder paste are both from 1 micrometer to 250 micrometers.

3. The method according to claim 1, characterized in that, The printing process is carried out under the following conditions: ambient temperature 20-26℃, humidity 40%–60% RH, squeegee speed 20-30 mm / s, squeegee pressure 0.35-0.45 kg / cm², squeegee angle 45°-55°, distance between stencil and substrate 0-0.1 mm, and demolding speed 0.1-1 mm / s; the Sn-Cu solder paste used has a viscosity of 160-220 Pa·s at 25℃, and the powder particle size is No. 3 powder (25-45 micrometers) or No. 4 powder (20-38 micrometers); the area ratio of the printing stencil opening is greater than 0.66, and the aspect ratio is greater than 1.

5.

4. The method according to claim 1, characterized in that: Compared to the transient liquid phase bonding method using pure tin, the Sn-Cu solder paste used in this invention has better wettability, requires a lower soldering temperature, reduces the requirements for the flatness and surface roughness of Cu-Ni alloy sheets, and can promote the growth of (Cu,Ni)6Sn5; the resulting welded joint has a porosity of less than 5% and a thermal conductivity of more than 140 W / (m·K).

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