Pulse current auxiliary copper soldering paste rapid bonding method

Abstract

The invention discloses a pulse current assisted rapid bonding method for copper soldering paste. Comprising the following steps: coating a to-be-bonded surface of a lower substrate with micro-nano mixed copper soldering paste, and pre-drying to form a soldering paste layer; and placing the upper substrate above the soldering paste layer, in an atmospheric environment, applying an axial pressure of 0-5 MPa to the upper substrate or the lower substrate, synchronously applying a pulse current with a peak current of 0.5-1.8 kA and a duty ratio of 70%-90%, carrying out three pulse periods in total, and carrying out total duration of 1-10 s to complete bonding. According to the method disclosed by the invention, bonding with low time consumption, high bonding strength and low porosity can be realized in an atmospheric environment and under a low-pressure condition, and a high-reliability and low-cost solution is provided for power device packaging.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electronic packaging, in particular to a method for rapid bonding of pulse current assisted copper solder paste. BACKGROUND

[0002] The hot-press bonding process is a similar technology to the pulse current assisted copper solder paste low-pressure rapid bonding, and has been widely researched and applied in the field of advanced packaging. The process is to make the interface of the to-be-bonded material atomically diffuse to achieve connection under high temperature and high pressure environment. The principle is to use heating to make the material atoms obtain enough kinetic energy to break through the interface energy barrier, and at the same time, to apply pressure to promote the plastic deformation and close fit of the contact surface, so as to form a metallurgical bond at the interface. The main influencing parameters are temperature, pressure and time. The technology faces many technical bottlenecks. First, the material compatibility is poor. For organic substrates with poor temperature resistance or devices containing polymer packaging layers, high temperature will cause material aging or decomposition, and it is difficult to achieve high-quality bonding of copper and ceramic, semiconductor and other dissimilar materials, and the interface is prone to form pores or intermetallic compounds, affecting the reliability. Secondly, the process efficiency is low. Traditional hot-press bonding often requires several minutes to several hours of holding time, which cannot meet the demand of large-scale production, and the equipment needs to maintain a high-temperature and high-pressure environment, which consumes a lot of energy and has high investment cost. These limitations make it difficult for hot-press bonding to meet the needs of advanced electronic packaging, especially the manufacturing of high-density, thin and multi-functional devices, which seriously restricts the large-scale application of the technology. SUMMARY

[0003] In view of the shortcomings of the prior art, the present application provides a method for rapid bonding of pulse current assisted copper solder paste. The method of the present application combines segmented pulse current and micro-nano mixed copper solder paste to achieve rapid and high-reliability bonding, especially suitable for interconnection packaging of high-temperature and high-frequency power devices.

[0004] The technical solution adopted by the present application is as follows: The method of the present application comprises the following steps: applying micro-nano mixed copper solder paste on the to-be-bonded surface of the lower substrate, pre-drying to form a solder paste layer; placing the upper substrate above the solder paste layer, applying an axial pressure of 0-5 MPa in the atmospheric environment, synchronously applying a pulse current with a peak current of 0.5-1.8 kA, a pulse period of 0.2-3.33 s and a duty cycle of 70%-90%, for a total of three pulse periods and a total duration of 1-10 s, to complete the bonding.

[0005] Preferably, the components of the micro-nano mixed copper solder paste include micron-sized copper particles and nano-sized copper particles; the mass ratio of the micron-sized copper particles to the nano-sized copper particles is 60:40.

[0006] Preferably, the particle size of the micron-sized copper particles ranges from 1 to 5 microns; the particle size of the nano-sized copper particles ranges from 50 to 300 nanometers.

[0007] Preferably, the micro-nano hybrid brazing paste is coated by screen printing method, and the coating thickness is 50-120 μm.

[0008] Preferably, the pre-drying process is to keep at an ambient temperature of 60-150 ℃ for 1-20 min.

[0009] Preferably, the axial pressure is 0.5-4 MPa.

[0010] Preferably, the parameters of the pressure and pulse current are selected from at least one of the following combination schemes: a) the pressure is 0.5 MPa, the peak current of the pulse current is 1.2 kA, the duty cycle is 80%-90%, and the pulse current is applied for 5 s; b) the pressure is 1 MPa, the peak current of the pulse current is 0.8-1.5 kA, the duty cycle is 70%-90%, and the pulse current is applied for 3-7 s; c) the pressure is 2 MPa, the peak current of the pulse current is 1.2-1.8 kA, the duty cycle is 70%-90%, and the pulse current is applied for 3-5 s; d) the pressure is 3 MPa, the peak current of the pulse current is 0.8-1.5 kA, the duty cycle is 80%-90%, and the pulse current is applied for 5-10 s; e) the pressure is 4 MPa, the peak current of the pulse current is 1.2 kA, the duty cycle is 80%-90%, and the pulse current is applied for 7 s.

[0011] Preferably, the bonding method is used to prepare any one of the following devices: a) bonding a chip to a DBC board; b) bonding a DBC board to a heat sink.

[0012] The present application has the following advantages: 1. The method of the present application can realize low consumption time, high bonding strength and low porosity under atmospheric environment (without protective gas atmosphere) and low pressure conditions, and provides a high-reliability and low-cost solution for power device packaging.

[0013] 2. The nearly pure copper interconnection head (copper interconnection layer) formed by the method of the present application has a very high melting point (1083 ℃) and excellent electrical conductivity and thermal conductivity, and can withstand high-temperature working environment and effectively transmit large current; at the same time, the pulse current rapid bonding process creates a dense, firm and high-quality interface metallurgical bond, ensuring the long-term reliability and stability of the joint under high temperature, thermal cycling and high frequency working conditions. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 A schematic diagram of a pulsed current bonding device; Figure 2 A flow chart of a pulsed current bonding process; Figure 3 A schematic diagram of sample shearing; Figure 4 A SEM scanning image after shearing in Example 8 of the present application; Figure 5 A schematic diagram of a pulsed current in Example 8 of the present application. DETAILED DESCRIPTION

[0015] The present application will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0016] In order to more clearly illustrate the present application, the present method will be described in detail below in conjunction with specific embodiments, and it should be particularly pointed out that the protection scope of the present application is not limited to the content described in the embodiments.

[0017] The principle of the present application is specifically as follows: the present application utilizes the "low-temperature activation" of nano copper powder and the "skeleton filling" of micron copper powder in a micro-nano hybrid copper solder paste, and then utilizes the "rapid and precise temperature control" and "field-induced activation" effects of a segmented pulsed current, to realize the instantaneous sintering densification of the solder paste and the rapid diffusion of interface atoms under the combined action of pressure and Joule heat, so that a high-strength metallurgical bonding joint is formed in a very short time.

[0018] The segmented pulsed current described in the present application refers to applying different strength electric pulses according to different time stages for the entire bonding process. Specifically, the segmented pulsed current has three cycles, i.e. it is divided into three segments, and the duty cycles of each pulse cycle are the same. The first segment removes the organic carrier in the solder paste and preliminarily activates the surface of the particles; the second segment generates a large amount of Joule heat, so that the nano particles are preferentially melted and the rapid diffusion and sintering between the micron particles are induced, and at the same time the electric field can reduce the atomic diffusion energy barrier and strongly promote the interface metallurgical bonding; the third segment controls the stable formation of the joint organization (the connection between the surfaces to be bonded). The mode of precise control in stages realizes a high-speed and low-heat-input bonding process.

[0019] The present application provides a pulsed current assisted rapid bonding method. The schematic diagram of the method of the present application is as shown in Figure 1As shown in the figure: 1, marked with "+", is the positive electrode, which is an alloy with good conductivity and high temperature resistance, used to introduce pulse current; 2, represents a pressure applying component, the arrow indicates that it can apply pressure downward, usually connected to an axial pressure control system, to apply mechanical pressure to the sample below; 3, marked with "-", is the negative electrode, which forms a loop with the positive electrode to enable pulse current to flow through the middle processing area; 4, the base platform of the device, which serves to support and fix other components; 5, represents the sample to be processed, which is a multilayer structure composed of a chip and a solder paste layer, which undergoes physical or chemical changes under the combined action of pulse current and pressure to achieve the purpose of soldering and bonding processing.

[0020] The method comprises the following steps: applying a micro-nano mixed copper solder paste on the bonding surface of the lower substrate, pre-drying to form a solder paste layer; placing the upper substrate above the solder paste layer, applying a constant axial pressure of 0-5 MPa to the upper substrate or the lower substrate in an atmospheric environment, synchronously applying a pulse current with a peak current of 0.5-1.8 kA, a pulse period of 0.2-3.33 s, and a duty cycle of 70%-90%, for a total of three pulse periods, with a total duration of 1-10 s, to complete the bonding.

[0021] The pulse period includes a conduction time and an intermittent time. When the pulse current is 0, it is in the intermittent phase, and when the pulse current is greater than 0, it is in the conduction phase.

[0022] The duty cycle refers to the proportion of the conduction time in one pulse period.

[0023] The axial pressure refers to the pressure perpendicular to the bonding interface.

[0024] Preferably, the components of the micro-nano mixed copper solder paste include micron-sized copper particles and nano-sized copper particles; the mass ratio of the micron-sized copper particles to the nano-sized copper particles is 60:40.

[0025] Preferably, the particle size range of the micron-sized copper particles is 1-5 μm; the particle size range of the nano-sized copper particles is 50-300 nm.

[0026] Preferably, the micro-nano mixed copper solder paste is coated using a screen printing method, and the coating thickness is 50-120 μm.

[0027] Preferably, the pre-drying process is as follows: maintaining at an ambient temperature of 60-150℃ for 1-20 min.

[0028] Preferably, the axial pressure is 0.5-4 MPa.

[0029] Preferably, the parameters of the pressure and the pulse current are selected from at least one of the following combination schemes: a) The pressure is 0.5 MPa, the peak current of the pulse current is 1.2 kA, the duty cycle is 80%~90%, and the pulse current application time is 5 s; b) The pressure is 1 MPa, the peak current of the pulse current is 0.8~1.5 kA, the duty cycle is 70%~90%, and the pulse current application time is 3~7 s; c) The pressure is 2MPa, the peak current of the pulse current is 1.2~1.8kA, the duty cycle is 70%~90%, and the pulse current application time is 3~5s; d) The pressure is 3MPa, the peak current of the pulse current is 0.8~1.5kA, the duty cycle is 80%~90%, and the pulse current application time is 5~10s; e) The pressure is 4 MPa, the peak current of the pulse current is 1.2 kA, the duty cycle is 80%~90%, and the pulse current application time is 7 s.

[0030] Furthermore, such as Figure 2 As shown, the bonding method of the present invention can be used to fabricate any of the following devices: a) Attach the chip to the DBC board; b) Attach the DBC board to the heatsink.

[0031] Furthermore, the bonding structure of the device obtained by the present invention has a shear strength greater than 60 MPa and a porosity of less than 2%.

[0032] Specifically, during the bonding process, both the positive and negative electrodes of the pulse power module are in contact with the lower substrate.

[0033] Specific embodiments of the present invention are as follows: Example 1 like Figure 2 As shown, this embodiment uses the following steps to bond the copper simulation chip to the copper substrate: Step 1, Solder paste coating: The bonding surface of the substrate is sanded with 5000# sandpaper to make the surface roughness Ra≤1μm. Micro-nano hybrid copper solder paste is then coated on the bonding surface of the substrate; the coating thickness is 80μm.

[0034] In this embodiment, the center diameter of the micron-sized copper particles is 2 μm; the particle size distribution range of the nano-sized copper particles is 50~100 nm. The solvent for the micro-nano hybrid copper solder paste is a composite solvent of ethylene glycol and terpineol.

[0035] Step 2, Pre-drying: Pre-dry the micro-nano hybrid copper solder paste at 100°C for 10 min to remove solvents (ethylene glycol and terpineol) and reduce porosity. Step 3, pulse current assisted bonding: the upper substrate was placed above the solder paste layer, and a constant axial pressure of 0 MPa was applied to the upper substrate or the lower substrate in an atmospheric environment, and a pulse current with a peak current of 0.5 kA, a pulse period of 0.33 s, a duty cycle of 90%, and a total duration of 1 s was applied synchronously to complete the bonding.

[0036] Next, the remaining examples and comparative examples were all completed by using the same materials and processes as Example 1, and the parameters in Tables 1 and 2 were controlled to complete the bonding between the copper simulation chip and the copper substrate. In Comparative Example 7, the micron copper particles and the solvent in Example 1 were used to prepare the micron copper solder paste. In Comparative Example 8, the nanometer copper particles and the solvent in Example 1 were used to prepare the nanometer copper solder paste.

[0037] In each example and comparative example, the pulse period can be calculated according to the total time and the duty cycle. The pulse current schematic used in Example 8 is shown in Figure 5 .

[0038] Table 1 Table 2 After the above examples and comparative examples were completed, the bonding structures obtained in each example and comparative example were respectively subjected to shear strength testing and porosity testing.

[0039] The process of shear strength testing is shown in Figure 3 , and specifically, the sample to be tested (i.e., the obtained bonding structure) was fixed in the fixed clamp of the shear force tester, and a push head was used to apply a pushing force to the upper substrate of the bonding structure, and the pushing force (i.e., the shear force) applied when the connecting layer was broken was recorded synchronously. The shear strength value of the connecting layer was calculated by dividing the measured shear force by the connecting area.

[0040] The process of porosity testing is as follows: first, the sample to be tested (i.e., the obtained bonding structure) was embedded with epoxy resin and cured at room temperature for 13 hours. After curing, a metallographic cutting machine was used to cut the sample along a direction perpendicular to the surface of the sample to expose the interconnection interface. Then, the cutting section was subjected to rough grinding, fine grinding and polishing in sequence to obtain a smooth and clean connecting layer section. The prepared section was imaged using a scanning electron microscope (SEM) to obtain a high-resolution micrograph. Finally, the SEM image was processed by MATLAB software. In the generated gray-scale image, the pores in the connecting layer region appear dark. By calculating the area proportion of these dark pore regions in the entire target connecting layer image region, the porosity of the connecting layer can be obtained.

[0041] Figure 4The SEM diagram of the cross section of the connecting layer obtained from Example 8 shows that the connecting layer cross section obtained by the method of the application has lower porosity.

[0042] The shear strength test results and porosity test results of each example are shown in Table 3.

[0043] Table 3 As can be seen from Table 3, the method of the application can realize low consumption time, high bonding strength and low porosity under atmospheric environment and low pressure conditions, and provides a high-reliability and low-cost solution for power device packaging. In addition, compared with other examples, Examples 2, 4-5 and 8-12 can achieve better results. The effect of Example 8 is the best.

[0044] In summary, the nano copper powder has extremely high surface activity, and its surface effect and melting point reduction effect make it melt and sinter at a lower temperature, playing the role of "binder" and "fast diffusion channel"; and the micron copper powder acts as the main filling skeleton to prevent excessive shrinkage of nano particles, and together provides a basis for forming a dense copper structure.

[0045] In addition, in the segmented pulse current, the first segment removes the organic carrier in the solder paste and preliminarily activates the surface of the particles; the second segment generates a large amount of Joule heat, causing the nano particles to melt preferentially and inducing rapid diffusion and sintering between micron particles, while the electric field may reduce the atomic diffusion energy barrier, strongly promoting the interface metallurgical bonding; the third segment controls the stable formation of the joint structure (the connecting part of the surfaces to be bonded). The mode of precise control in stages realizes a high-speed and low-heat-input bonding process.

[0046] It should be particularly pointed out that the above examples are only for the explanation and description of the technical solutions of the application and do not constitute a limitation on the protection scope. Those skilled in the art can modify or equivalently replace the technical solutions on the basis of the core principles of the application, as long as such changes do not deviate from the technical essence and protection scope defined in the claims, and should be included in the protection scope of the application.

Claims

1. A pulse current-assisted fast bonding method, characterized in that, Includes the following steps: Micro-nano hybrid copper solder paste is coated on the bonding surface of the lower substrate and pre-dried to form a solder paste layer. The upper substrate is placed above the solder paste layer, and under atmospheric conditions, an axial pressure of 0~5MPa is applied, and a pulse current with a peak current of 0.5~1.8kA and a duty cycle of 70%~90% is applied simultaneously for three pulse cycles, with a total duration of 1~10s, to complete the bonding.

2. The pulse current-assisted fast bonding method according to claim 1, characterized in that: The micro-nano hybrid copper solder paste comprises micron-sized copper particles and nano-sized copper particles; the mass ratio of micron-sized copper particles to nano-sized copper particles is 60:

40.

3. The pulse current-assisted fast bonding method according to claim 2, characterized in that: The micron-sized copper particles have a particle size range of 1~5μm; the nano-sized copper particles have a particle size range of 50~300nm.

4. The pulse current-assisted fast bonding method according to claim 2 or 3, characterized in that: The micro-nano hybrid copper solder paste was coated using a screen printing method, with a coating thickness of 50~120μm.

5. The pulse current-assisted fast bonding method according to claim 1, characterized in that: The pre-drying process is as follows: maintain an ambient temperature of 60~150℃ for 1~20 minutes.

6. The pulse current-assisted fast bonding method according to claim 1, characterized in that: The axial pressure is 0.5~4 MPa.

7. The pulse current-assisted fast bonding method according to claim 1, characterized in that: The parameters of the pressure and pulse current are selected from at least one of the following combinations: a) The pressure is 0.5 MPa, the peak current of the pulse current is 1.2 kA, the duty cycle is 80%~90%, and the pulse current application duration is 5 s; b) The pressure is 1 MPa, the peak current of the pulse current is 0.8~1.5 kA, the duty cycle is 70%~90%, and the pulse current application duration is 3~7 s; c) The pressure is 2 MPa, the peak current of the pulse current is 1.2~1.8 kA, the duty cycle is 70%~90%, and the pulse current application duration is 3~5 s; d) The pressure is 3 MPa, the peak current of the pulse current is 0.8~1.5 kA, the duty cycle is 80%~90%, and the pulse current application duration is 5~10 s; e) The pressure is 4 MPa, the peak current of the pulse current is 1.2 kA, the duty cycle is 80%~90%, and the pulse current application duration is 7 s.

8. The pulse current-assisted fast bonding method according to any one of claims 1 to 7, characterized in that: The bonding method is used to fabricate any of the following devices: a) Attach the chip to the DBC board; b) Attach the DBC board to the heatsink.

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