Method for preparing Sn58Bi / Cu welding joint with assistance of electric pulse

Abstract

The invention relates to the technical field of Sn58Bi welding, and particularly discloses a method for preparing a Sn58Bi / Cu welding joint under the assistance of electric pulses, which comprises the following steps of: heating the Sn58Bi / Cu joint to be welded to 160-180 DEG C within 40-60 seconds, and preserving heat in the temperature environment; welding operation is conducted in the heat preservation period, and the welding operation comprises the steps that pressure and electric pulses are applied to the Sn58Bi / Cu connector to be welded; and after the welding operation is finished, air cooling is performed. According to the method, the mode of applying pressure and electric pulse to the Sn58Bi / Cu joint to be welded is adopted, so that the welding quality is improved, and interface holes are reduced; by controlling the time and the current density of the electric pulse treatment, the microstructure of the joint is improved, and the shear strength is improved.

Description

Technical Field

[0001] The invention relates to the technical field of Sn58Bi welding, and in particular to a method for preparing a Sn58Bi / Cu welding joint with the assistance of electric pulses. Background Art

[0002] With the acceleration of the update and iteration of consumer electronic products, the advanced technology that has been born has ensured that electronic devices can obtain more in a smaller volume and thus obtain higher computing power by shrinking the transistor architecture. However, the degree of shrinkage of the transistor architecture is about to face the limitations of physical size and process tolerance. Therefore, 3D packaging technology is designed to solve this problem in IC packaging. 3D packaging technology mainly builds complex chip systems by assembling multiple chips or heterogeneous integration in a single packaging process. In this process, the electrical connection between the stacked chips is mainly achieved through wire bonding. The three-dimensional stacking will increase the wire bonding length and reduce the overall reliability. A new process (silicon through via) was proposed to solve this problem. A metal layer deposited on silicon is directly connected to the SOC (system-level) chip through the redistribution (RDL) technology. However, in order to ensure the overall reliability of the package after multi-layer connection in this technology, a series of high-reliability solder alloys with different melting points are required. For example, the melting point of the connection metal of the first layer of the package should be the highest, and the melting point of the subsequent layers will decrease accordingly, so as to ensure the stability of the front solder joints when the subsequent layers are packaged.

[0003] Among traditional solder alloys, commercial lead-free solder alloys with melting points above 150°C have been developed in large quantities, such as the Sn-Ag-Cu series and the Sn multi-element alloy series. However, there are relatively few alloy solders with melting points below 150°C, mainly the Sn58Bi series and the Sn-In series. The In element is less commercially considered due to its high price. Therefore, the Sn58Bi alloy has become a representative of low melting point solder alloys and has received a lot of attention.

[0004] The microstructure of Sn58Bi alloy is similar to the eutectic alloy of Sn-Pb. As a solder alloy, its main characteristics are good wettability, high conductivity, high strength, low price, and the interface between it and the metal copper is thinner than the Sn-Ag-Cu series due to the segregation barrier effect of Bi. Therefore, its solder joint strength is higher. However, the large amount of Bi phase in Sn58Bi reduces the plasticity and service reliability of the solder joint. First, during the service process of the solder joint, as the Sn in the solder layer continuously reacts with the Cu substrate to generate Cu 6 Sn 5 At the interface IMC, the unreacted Bi phase is concentrated at the interface, which hinders the formation of IMC. At the same time, the hard and brittle Bi-rich phase also reduces the toughness of the solder joint.

[0005] Research has found that electric pulses can have a nurturing effect on molten metal, thereby improving its post-solidification structure. Therefore, electric pulse-assisted welding can achieve the effect of improving the melt state of the weld interface during the welding process, and the small amount of Joule heat generated by the electric pulse can assist the melting welding process. However, the existing electric pulse-assisted welding technology focuses on the heat treatment of the welded joint after welding with large pulse current, which is not suitable for solder alloys that have no solid phase change, low melting point, and do not require heat treatment.

[0006] Therefore, it is urgent to provide a method for preparing Sn58Bi / Cu welding joints with the assistance of electric pulses to solve the problems existing in the above-mentioned prior art. Summary of the invention

[0007] The purpose of the present invention is to provide a method for preparing Sn58Bi / Cu welding joints with the assistance of electric pulses, so as to solve the problems existing in the above-mentioned prior art.

[0008] To achieve the above object, the present invention provides a method for preparing Sn58Bi / Cu welding joints by electric pulse assistance, comprising the following steps:

[0009] The Sn58Bi / Cu joint to be welded is heated to 160°C to 180°C within 40s to 60s and kept warm at this temperature;

[0010] Performing a welding operation during the heat preservation period, wherein the welding operation includes applying pressure and electric pulses to the Sn58Bi / Cu joint to be welded;

[0011] After the welding operation is completed, air cooling is carried out.

[0012] Preferably, the pressure applied to the Sn58Bi / Cu joint to be welded during the welding operation is 0.1 MPa to 0.5 MPa.

[0013] Preferably, the pressure applied to the Sn58Bi / Cu joint to be welded during the welding operation is 0.22 MPa.

[0014] Preferably, the voltage of the electric pulse applied to the Sn58Bi / Cu joint to be welded during the welding operation is 0 to 100V and the frequency is 0 to 40Hz.

[0015] Preferably, the electric pulse current density applied to the Sn58Bi / Cu joint to be welded during the welding operation is 66.6A / cm 2 ~133.3A / cm 2 .

[0016] Preferably, the time for performing electric pulse treatment on the Sn58Bi / Cu joint to be welded during the welding operation is 40s to 80s.

[0017] Preferably, the heating and welding operations of the Sn58Bi / Cu joint to be welded are both performed in an auxiliary reflow soldering device.

[0018] Preferably, the auxiliary reflow soldering device comprises:

[0019] A transmission belt, used for conveying the Sn58Bi / Cu joint to be welded, the transmission belt is provided with a groove for placing the Sn58Bi / Cu joint to be welded, and the transmission belt is connected to the positive electrode of a pulse power supply; hot air outlets are provided on both sides of the transmission belt, and the hot air outlets are used to heat the Sn58Bi / Cu joint to be welded;

[0020] An electric pressing head is arranged just above the transmission belt, the electric pressing head is connected to the negative electrode of the pulse power supply, and the electric pressing head is used to apply pressure and electric pulses to the Sn58Bi / Cu joint to be welded;

[0021] A controller, the electric pressure head is electrically connected to the controller, and the controller is used to control the opening and closing of the electric pressure head.

[0022] Preferably, a thermocouple is provided inside the transmission belt, and the thermocouple is used to detect the temperature of the hot air blown out of the hot air outlet, and the thermocouple is electrically connected to the controller.

[0023] Preferably, the insulation time is 100s to 200s.

[0024] Compared with the prior art, the present invention has the following advantages and technical effects:

[0025] The method for preparing Sn58Bi / Cu welding joints with the assistance of electric pulses provided by the present invention improves the welding quality and reduces interface holes by applying pressure and electric pulses to the Sn58Bi / Cu joints to be welded; and improves the microstructure of the joint and enhances the shear strength by controlling the time and current density of the electric pulse treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0027] Figure 1 It is a structural schematic diagram of the auxiliary reflow soldering device of the present invention;

[0028] Figure 2The electron microscope images of the cross-section microstructure of the joints of the comparative examples and embodiments of the present invention;

[0029] Figure 3 The shear strength of the comparative example and the embodiment after 180°C reflow soldering of the present invention;

[0030] Figure 4 The shear strength of the comparative example and the embodiment after 160°C reflow soldering of the present invention;

[0031] In the figure: 1. Negative pole of pulse power supply; 2. Positive pole of pulse power supply; 3. Sn58Bi / Cu joint to be welded; 4. Electric pressing head; 5. Transmission belt; 6. Hot air outlet; 7. Controller. DETAILED DESCRIPTION

[0032] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. The present invention will be described in detail below with reference to the drawings and in combination with the embodiments.

[0033] The present invention provides a method for preparing a Sn58Bi / Cu welding joint with the aid of electric pulse, comprising the following steps:

[0034] The Sn58Bi / Cu joint 3 to be welded is heated to 160°C to 180°C within 40s to 60s. This process is a preheating process, which can activate the flux and make the joint temperature evenly distributed. Then, the joint is kept warm under this temperature environment.

[0035] During the holding period, a welding operation is performed, wherein the welding operation includes applying pressure and electric pulses to the Sn58Bi / Cu joint 3 to be welded;

[0036] After the welding operation is completed, air cooling is carried out.

[0037] According to a further optimized solution, the pressure applied to the Sn58Bi / Cu joint 3 to be welded during the welding operation is 0.1 MPa to 0.5 MPa.

[0038] The solution is further optimized, and the pressure applied to the Sn58Bi / Cu joint 3 to be welded during the welding operation is 0.22 MPa.

[0039] According to a further optimized solution, the voltage of the electric pulse applied to the Sn58Bi / Cu joint 3 to be welded during the welding operation is 0 to 100 V and the frequency is 0 to 40 Hz.

[0040] Further optimization scheme, the electric pulse current density applied to the Sn58Bi / Cu joint 3 to be welded during the welding operation is 66.6A / cm 2 ~133.3A / cm 2 .

[0041] According to a further optimization scheme, the time for performing electric pulse treatment on the Sn58Bi / Cu joint 3 to be welded during the welding operation is 40s to 80s.

[0042] When the electric pulse starts, pressure starts to be applied simultaneously, and when the electric pulse stops, pressure and heating stop simultaneously.

[0043] The scheme is further optimized, and the heating and welding operations of the Sn58Bi / Cu joint 3 to be welded are both performed in an auxiliary reflow welding device.

[0044] Further optimization scheme, such as Figure 1 As shown, the auxiliary reflow soldering device includes:

[0045] The transmission belt 5 is used to transport the Sn58Bi / Cu joint 3 to be welded. The transmission belt 5 is provided with a groove for placing the Sn58Bi / Cu joint 3 to be welded, and the transmission belt 5 is connected to the positive electrode 2 of the pulse power supply; hot air outlets 6 are provided on both sides of the transmission belt 5, and the hot air outlets 6 are used to heat the Sn58Bi / Cu joint 3 to be welded;

[0046] The electric pressing head 4 is arranged just above the transmission belt 5, and the electric pressing head 4 is connected to the negative electrode 1 of the pulse power supply, and the electric pressing head 4 is used to apply pressure and electric pulses to the Sn58Bi / Cu joint 3 to be welded;

[0047] The controller 7 is electrically connected to the electric pressure head 4 , and the controller 7 is used to control the opening and closing of the electric pressure head 4 .

[0048] As a further optimization scheme, a thermocouple is provided inside the transmission belt 5 , and the thermocouple is used to detect the temperature of the hot air blown out of the hot air outlet 6 , and the thermocouple is electrically connected to the controller 7 .

[0049] The solution was further optimized and the insulation time was 100s to 200s.

[0050] The method for preparing Sn58Bi / Cu solder joints with electric pulse assistance provided by the present invention is carried out using an auxiliary reflow soldering device, and the specific process is as follows:

[0051] First, the auxiliary reflow soldering device is started, and the Sn58Bi / Cu joints 3 to be soldered are placed in the special grooves on the transmission belt 5 of the auxiliary reflow soldering device; when each Sn58Bi / Cu joint 3 to be soldered is transmitted to the bottom of the electric pressure head 4, the hot air outlets 6 on both sides of the transmission belt 5 blow out hot air, and the hot air temperature is raised to the set welding temperature within 60 seconds; after the thermocouple built into the transmission belt 5 detects that the hot air temperature reaches the set welding temperature for 100 seconds, the electric pressure head 4 is pressed down, and an electric pulse is applied to the Sn58Bi / Cu joint 3 to be soldered, and the electric pressure head 4 also applies pressure to the Sn58Bi / Cu joint 3 to be soldered. After 60 seconds of electric pulse and pressure assisted soldering, the soldering is completed, and the transmission belt 5 transmits the soldered Sn58Bi / Cu joint out of the equipment for air cooling.

[0052] The present invention processes the Sn58Bi / Cu joint 3 to be welded by electric pulse pressure-assisted welding. Under the action of an external pulse electric field, the alloy melt in the reflow welding state is induced by the external electric field to produce more "atomic clusters". This "atomic cluster" acts as a nucleation embryo in the subsequent alloy solidification process, changing the original alloy solidification microstructure. Moreover, since the metallurgical reaction between Sn and Cu begins to occur at the moment the two come into contact, when the pulse electric field is applied in the later stage of reflow welding, the intermetallic compound Cu already exists at the interface of the Sn58Bi / Cu joint. 6 Sn 5 The thin IMC feature of the Sn58Bi / Cu joint interface is retained. At the same time, the pressure applied to the Sn58Bi / Cu joint not only ensures the connectivity of the circuit, but also improves the quality of welding and reduces interface holes.

[0053] Comparative Example 1:

[0054] Preferably, Sn58Bi solder sheet, size 10*10*0.1mm, Cu substrate, size 10*10*1mm. Pre-mounted into a "sandwich" structure. Directly reflow at a peak reflow temperature of 180°C for 160s without starting the electric press head 4. Figure 2 a is the electron microscope image of the cross-section microstructure after welding. Figure 3 is the shear strength of the joint after reflow soldering.

[0055] Embodiment 1:

[0056] Preferred Sn58Bi solder sheet, size 10*10*0.1mm, Cu substrate, size 10*10*1mm. Pre-mounted into a "sandwich" structure. Reflow at a peak reflow temperature of 180°C, electric pulse treatment for 60s, and apply a pressure of 0.22MPa. Processing parameters: 5Hz20V, current density is about: 66.6A / cm 2 . Figure 2b is the electron microscope image of the cross-section microstructure after welding. Figure 3 is the shear strength of the joint after reflow soldering.

[0057] Embodiment 2:

[0058] Preferred Sn58Bi solder sheet, size 10*10*0.1mm, Cu substrate, size 10*10*1mm. Pre-mounted into a "sandwich" structure. Reflow at a peak reflow temperature of 180°C, electric pulse treatment for 60s, and apply a pressure of 0.22MPa. Processing parameters: 5Hz40V, current density is about: 133.3A / cm 2 . Figure 2 c is the electron microscope image of the cross-section microstructure after welding. Figure 3 is the shear strength of the joint after reflow soldering.

[0059] Comparative Example 2:

[0060] Preferably, Sn58Bi solder sheet, size 10*10*0.1mm, Cu substrate, size 10*10*1mm. Pre-mounted into a "sandwich" structure. Directly reflow at a peak reflow temperature of 160°C for 160s without starting the electric press head 4. Figure 2 d is the electron microscope image of the cross-section microstructure after welding. Figure 4 is the shear strength of the joint after reflow soldering.

[0061] Embodiment 3:

[0062] Preferred Sn58Bi solder sheet, size 10*10*0.1mm, Cu substrate, size 10*10*1mm. Pre-mounted into a "sandwich" structure. Reflow at a peak reflow temperature of 160°C, electric pulse treatment for 60s, and apply a pressure of 0.22MPa. Processing parameters: 5Hz20V, current density is about: 66.6A / cm 2 . Figure 2 e is the electron microscope image of the cross-section microstructure after welding. Figure 4 is the shear strength of the joint after reflow soldering.

[0063] Embodiment 4:

[0064] Preferred Sn58Bi solder sheet, size 10*10*0.1mm, Cu substrate, size 10*10*1mm. Pre-mounted into a "sandwich" structure. Reflow at a peak reflow temperature of 160°C, electric pulse treatment for 60s, and apply a pressure of 0.22MPa. Processing parameters: 10Hz20V, current density is about: 66.6A / cm 2 . Figure 2 f is the electron microscope image of the cross-section microstructure after welding. Figure 4 is the shear strength of the joint after reflow soldering.

[0065] Shear strength calculation formula:

[0066]

[0067] Where, τ is the shear stress (strength), unit MPa, F s is the maximum shear force, in N, and A is the shear area, in cm 2 . The shear strength of each joint is calculated according to the above formula.

[0068] in accordance with Figure 2 The cross-sectional microstructure of the joint in the figure shows that when the electric pulse pressure assisted reflow is performed at 180°C, when no electric pulse is applied, the organization presents an irregular eutectic structure (gray is Sn phase, white is Bi phase, black on the upper and lower sides is Cu, and gray between Cu and solder layer is Cu 6 Sn 5 IMC). When a 5Hz20V pulse current was applied, dendritic Sn appeared in the solder layer without obvious Bi-rich phase. The enlarged schematic diagram of the Sn dendrite showed that small pieces of white Bi phase were precipitated, which was the reason why its shear strength (59.5MPa) was improved relative to the untreated joint strength. Subsequently, when the electric pulse voltage was increased to 40V, due to the increase in current density and the increase in Joule heating effect, the interfacial metallurgical reaction intensified the Sn consumption and increased the large pieces of Bi-rich phase at the interface. This also led to a decrease in the joint shear strength (54.8MPa) compared with Example 1, but due to the presence of Sn dendrites containing precipitated Bi, the joint strength was slightly higher than that of Comparative Example 1 (53.4MPa).

[0069] During 160℃ electric pulse assisted reflow soldering, when electric pulse assisted soldering was not applied, the microstructure of Comparative Example 2 was the same as that of Comparative Example 1, which was an obvious eutectic structure. The difference was that the structure was slightly coarsened due to the shorter cooling time than that of Comparative Example 1. When 5Hz20V pulse current was applied to assist soldering, a small amount of Sn dendrites containing precipitated Bi appeared in the structure, resulting in a slight increase in strength (52.8MPa) compared to Comparative Example 2 (52.2MPa). When the processing voltage remained unchanged and the frequency was increased to 10Hz, the number of Sn dendrites in the middle structure of the solder layer increased, and no blocky Bi-enriched phase was observed (the Bi phase was serpentine), resulting in an increase in strength (54.9MPa).

[0070] By controlling the time and current density of the electric pulse treatment, the improvement of the joint microstructure and the enhancement of the shear strength were achieved.

[0071] The above are only preferred specific implementations of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing Sn58Bi / Cu welding joints by electric pulse assistance, characterized in that: The following steps are involved: The Sn58Bi / Cu joint (3) to be welded is heated to 160°C to 180°C within 40s to 60s, and kept warm at this temperature; During the heat preservation period, a welding operation is performed, wherein the welding operation includes applying pressure and electric pulses to the Sn58Bi / Cu joint (3) to be welded; After the welding operation is completed, air cooling is carried out.

2. The method for preparing Sn58Bi / Cu welding joints with electric pulse assistance according to claim 1, characterized in that: The pressure applied to the Sn58Bi / Cu joint (3) to be welded during the welding operation is 0.1MPa to 0.5MPa.

3. The method for preparing Sn58Bi / Cu welding joints by electric pulse assistance according to claim 2, characterized in that: The pressure applied to the Sn58Bi / Cu joint (3) to be welded during the welding operation is 0.22 MPa.

4. The method for preparing Sn58Bi / Cu welding joints with electric pulse assistance according to claim 1, characterized in that: During the welding operation, the voltage of the electric pulse applied to the Sn58Bi / Cu joint (3) to be welded is 0 to 100V and the frequency is 0 to 40Hz.

5. The method for preparing Sn58Bi / Cu welding joints by electric pulse assistance according to claim 4, characterized in that: The electric pulse current density applied to the Sn58Bi / Cu joint (3) to be welded during the welding operation is 66.6A / cm 2 ~133.3A / cm 2 .

6. The method for preparing Sn58Bi / Cu welding joints by electric pulse assistance according to claim 4, characterized in that: During the welding operation, the time for performing electric pulse treatment on the Sn58Bi / Cu joint (3) to be welded is 40s to 80s.

7. The method for preparing Sn58Bi / Cu welding joints by electric pulse assistance according to claim 1, characterized in that: The heating and welding operations of the Sn58Bi / Cu joint (3) to be welded are both carried out in an auxiliary reflow welding device.

8. The method for preparing Sn58Bi / Cu welding joints by electric pulse assistance according to claim 7, characterized in that: The auxiliary reflow soldering device comprises: A transmission belt (5) is used to transport the Sn58Bi / Cu joint (3) to be welded, the transmission belt (5) is provided with a groove for placing the Sn58Bi / Cu joint (3) to be welded, and the transmission belt (5) is connected to the positive electrode (2) of the pulse power supply; hot air outlets (6) are provided on both sides of the transmission belt (5), and the hot air outlets (6) are used to heat the Sn58Bi / Cu joint (3) to be welded; An electric pressing head (4) is arranged directly above the transmission belt (5), the electric pressing head (4) is connected to the negative electrode (1) of the pulse power supply, and the electric pressing head (4) is used to apply pressure and electric pulses to the Sn58Bi / Cu joint (3) to be welded; A controller (7), the electric pressure head (4) is electrically connected to the controller (7), and the controller (7) is used to control the opening and closing of the electric pressure head (4).

9. The method for preparing Sn58Bi / Cu welding joints by electric pulse assistance according to claim 8, characterized in that: A thermocouple is arranged inside the transmission belt (5), and the thermocouple is used to detect the temperature of the hot air blown out of the hot air outlet (6). The thermocouple is electrically connected to the controller (7).

10. The method for preparing Sn58Bi / Cu welding joints by electric pulse assistance according to claim 1, characterized in that: The insulation time is 100s to 200s.

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