A method of controlling micro linear weld spot size

Abstract

The present application relates to the technical field of material preparation and connection, and discloses a method for controlling the size of a micro linear welding spot, which comprises the following steps: obtaining a plurality of copper bars with a specific structure, and the end surface of the copper bar is square; pretreating the copper bars to obtain welding copper bars, treating the surface of the welding copper bars to obtain treated copper bars; fixing two treated copper bars according to a preset spatial position, and placing the two treated copper bars in a drying dish to wait for solidification; filling the treated copper bars with solder paste after the treated copper bars are solidified; welding the treated copper bars to obtain butt welding bars; fixing the butt welding bars on a PCB to perform thinning treatment, and obtaining the butt welding bars after the thinning treatment; and performing polishing treatment on the butt welding bars close to the required size to control the size of the micro linear welding spot. The present application guarantees that the surface of the welding spot is fully polished by using a specially-made welding carrier and a polishing platform, and the linear welding spot size is complete and the image is clear.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of material preparation and connection, and particularly relates to a method capable of controlling the size of a micro linear solder joint. BACKGROUND

[0002] Solder joints are an indispensable part of microelectronic interconnection, playing a role in mechanical connection, electrical connection and signal transmission. With the development of the electronic packaging industry, the microelectronic packaging space is reduced, the number of components is increased, the solder joint size is reduced, and the chip heat generation is intensified. In the remelting process, intermetallic compounds are generated between the solder and the metallization layer of the solder pad. During the service process of the components, the solder joint has to withstand a higher current density. Under the drive of thermodynamic and kinetic factors, the IMCs will continuously grow or dissolve, and the morphology, size, crystal orientation and thickness will change, which may cause the solder joint to fail, thereby affecting the service life and reliability of the components.

[0003] It has been shown that when the current density flowing through the solder joint exceeds the threshold value (104 A / cm2) of electromigration, the collision between the flowing electrons and the metal atoms causes the directional movement of the atoms due to the exchange of momentum, and the metal atoms diffuse from the cathode to the anode, resulting in the generation of voids at the cathode and the initiation of micro-cracks to cause the cracking failure of the solder joint. The Sn-based lead-free solder joint prepared by remelting usually presents a single crystal or twin crystal structure. Since the beta-Sn has a body-centered tetragonal structure (a=b=0.5832, c=0.3182, c / a=0.546), the diffusion of copper atoms in the Sn grains with different crystal orientations is obviously anisotropic. For example, at 25℃, the diffusion rate of Cu along the c-axis of the beta-Sn lattice is 2*10-6 cm2 / s, which is 500 times of the diffusion rate of Cu along the a-axis and b-axis. This orientation diffusion behavior will seriously affect the electromigration behavior of the solder joint. The single crystal solder joint of the Sn-based solder with the c-axis parallel to the current direction is prone to premature failure, and the growth rate of the interface IMC is about 10 times of that of the single crystal solder joint or the twin crystal solder joint with the c-axis perpendicular to the current direction. At present, it is a thermodynamic problem to deeply understand and predict the growth mode of the Sn dendrite. After the interconnection is completed, each solder joint has a unique crystal orientation, so some solder joints will inevitably fail prematurely due to the unfavorable orientation of the beta-Sn grains during the use of the electronic product, thereby reducing the service life of the electronic product. It can be seen that the grain orientation of the solder joint will seriously affect the service reliability thereof, and therefore, the research on the evolution behavior of the interface IMC during the remelting state electromigration process of the interface IMC of the solder joint will greatly improve the understanding level of the interface reaction behavior of the solder joint. In the actual application of the electronic product, the solder joint is seriously affected by the thermal stress due to insufficient heat dissipation or uneven temperature, which affects the micro-morphology of the solder joint, and in severe cases, may cause the melting of the solder joint. Therefore, it is necessary to find a suitable means to fully dissipate heat during the electromigration process of the solder joint and reduce the influence of the thermal stress on the electromigration behavior of the solder joint. SUMMARY

[0004] To solve the above technical problems, the present application provides a method for controlling the size of a micro linear solder joint, which solves the heat dissipation problem during the power-on process by a specific welding form and reducing the size of the solder joint after welding, and controls the size of the solder joint to be at least 100 microns by a specially designed welding carrier and polishing platform, so that the surface of the solder joint can be fully polished, and the linear solder joint meets the requirements of complete size and clear image during scanning electron microscope and electron backscatter diffraction tests.

[0005] To achieve the above object, the present application provides a method for controlling the size of a micro linear solder joint, comprising the following steps:

[0006] Preparation of a copper rod to be welded;

[0007] Surface treatment of the copper rod to be welded to obtain a treated copper rod;

[0008] According to the preset spatial position, two treated copper rods are fixed and placed in a dry dish to wait for solidification, the treated copper rods are filled with solder paste, and the treated copper rods are welded to obtain butt welding rods.

[0009] The butt welding rods are fixed on the PCB board for thinning treatment to obtain the butt welding rods after thinning treatment, and the butt welding rods close to the required size are subjected to polishing treatment to control the size of the micro linear welding points.

[0010] Optionally, the preparation of the copper rod to be welded comprises: obtaining a copper rod with a target structure, and performing pre-welding treatment on the copper rod with the aid of a cold insert to obtain the copper rod to be welded.

[0011] Optionally, the copper rod with a target structure is a copper rod with a 90° angle.

[0012] Optionally, the pre-welding treatment on the copper rod with the aid of a cold insert to obtain the copper rod to be welded comprises: using a cold insert resin solution to make a cold insert, treating the cold insert, fixing the copper rod with double-sided tape and hot melt adhesive, polishing the welding surface of the copper rod, and obtaining the copper rod to be welded.

[0013] Optionally, the method for treating the surface of the copper rod to be welded comprises: removing organic matter and oxides attached to the surface of the copper rod to be welded.

[0014] Optionally, the removal of organic matter and oxides attached to the surface of the copper rod to be welded uses acetone solution and hydrochloric acid alcohol solution.

[0015] Optionally, the method for fixing two treated copper rods according to the preset spatial position comprises: placing two treated copper rods on the PCB board, welding planes perpendicular to the PCB board, two welding planes aligned and placed in parallel, the distance between the two welding planes kept at a preset distance, and using high-temperature glue to fix the two treated copper rods.

[0016] Optionally, the solder paste uses Sn-3.0Ag-0.5Cu.

[0017] Optionally, the method for fixing the butt welding rods on the PCB board for thinning treatment to obtain the butt welding rods after thinning treatment comprises: locally polishing the butt welding rods, fixing the butt welding rods on the side of the PCB board with epoxy resin, using a trapezoidal polishing tool combined with sandpaper to thin the surface of the butt welding rods, and obtaining the butt welding rods after thinning treatment.

[0018] Optionally, the method for polishing the butt welding rod close to the required size comprises: using coarse polishing liquid and fine polishing liquid to polish the butt welding rod close to the required size, wherein the coarse polishing liquid is 0.3-0.5 mu m alpha-alumina suspension liquid, and the fine polishing liquid is 0.02-0.05 mu m silicon dioxide suspension liquid.

[0019] The present application discloses a method for controlling the size of a micro linear welding spot, which can make the butt welding rod maintain a certain rigidity while reducing the size, prevent damage in the polishing process and damage of the copper rod by the wire clamp during power-on in the subsequent process by using customized welding copper rods and polishing tools; the welding spot observation surface has large areas at both ends, the observation surface of the welding spot reduces the influence caused by uneven force during polishing and polishing, ensures the polishing quality, and meets the requirements of complete size and clear image of the welding spot surface during scanning electron microscope and electron backscatter diffraction testing. The method has wide application value. BRIEF DESCRIPTION OF DRAWINGS

[0020] The accompanying drawings, which form a part of the present application, are intended to provide further understanding of the present application and are incorporated herein for explanation by way of the non-limiting examples. In the drawings:

[0021] Figure 1 A schematic view of a structure for polishing and polishing the end surface of the copper rod of the embodiment of the present application adhered to a specially designed cold insert;

[0022] Figure 2 A schematic view of the embodiment of the present application for filling solder paste in the gap between the two copper rods to be welded fixed on the PCB;

[0023] Figure 3 A schematic view of the embodiment of the present application for fixing the butt welding rod of the customized PCB;

[0024] Figure 4 A schematic view of the trapezoidal polishing platform of the embodiment of the present application;

[0025] Figure 5 A schematic view of the linear welding spot after the final polishing and polishing of the embodiment of the present application;

[0026] Figure 6 A flowchart of the method for controlling the size of a micro linear welding spot of the embodiment of the present application;

[0027] Wherein 1 is a cold insert, 2 is a copper rod, 3 is a PCB, and 4 is solder paste. DETAILED DESCRIPTION

[0028] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0029] It should be noted that the steps shown in the flowchart of the drawings can be executed in a computer system such as a set of computer executable instructions, and although the logical order is shown in the flowchart, in some cases, the steps shown or described herein can be executed in a different order.

[0030] As shown in the embodiment, a method capable of controlling the size of a micro linear welding spot is provided, comprising the following steps: Figure 6

[0031] A copper rod 2 to be welded is prepared;

[0032] The surface of the copper rod 2 to be welded is treated to obtain a treated copper rod 2;

[0033] According to a predetermined spatial position, two treated copper rods 2 are fixed and placed in a drying dish to wait for solidification. Sn3.0Ag0.5Cu solder paste 4 is filled into the gap of the treated copper rods 2 after solidification, and the treated copper rods 2 are welded to obtain a butt joint welding rod.

[0034] The butt joint welding rod is fixed on a PCB 3 for thinning treatment to obtain a thinned butt joint welding rod. The butt joint welding rod close to the required size is polished to control the size of the micro linear welding spot.

[0035] Specifically, a copper rod 2 with a specific structure is obtained by wire cutting, and the end face of the copper rod 2 is square. A cold insert 1 with a size consistent with the copper rod 2 is made to assist the butt joint copper rod 2 in pre-welding pretreatment to obtain a copper rod 2 to be welded. The copper rod 2 to be welded needs to use organic solution and decomposition solution to remove organic matter and oxides attached to the welding surface. The organic solvent and the decomposition solution are acetone solution and hydrochloric acid alcohol solution, respectively. According to a predetermined spatial position, two copper rods 2 to be welded are fixed using high-temperature glue and placed in a drying dish to wait for solidification to ensure the size of the welding seam width. After solidification, slightly excessive Sn3.0Ag0.5Cu solder paste 4 is filled into the gap of the copper rod 2, and welding operation is performed to obtain a one-dimensional linear butt joint welding rod. The treated butt joint welding rod is fixed to the side of a specially made PCB 3 using a customized epoxy resin curing agent, and one end face is aligned with the surface of the PCB. The gap between the tail end of the butt joint welding rod and the PCB 3 is also filled with epoxy resin. A customized trapezoidal polishing tool is used to polish the surface of the copper rod 2 on the side of the PCB 3 for thinning. The size of the copper rod 2 will not be reduced as a whole, and a certain rigidity is maintained. When the required size is approached, fine sandpaper is used for polishing. The side of the butt joint welding rod is polished to ensure the final size of the welding spot and the smoothness of the observation surface.

[0036] ​In this embodiment, the Cu / Sn3.0Ag0.5Cu / Cu butt joint with a cross-sectional size of 100 μm x 100 μm and a weld width of 100 μm is taken as an example to illustrate the manufacturing process and the specially customized copper bar 2 and polishing tool.

[0037] As shown in FIG. 1, the linear solder joint butt copper bar 2 used in this example is a specially customized copper bar 2 with a 90° angle, the size of the end face is 600 μm x 600 μm, the length of both ends is 1 cm, and the linear cutting is used to manufacture the copper bar 2. Figure 1

[0038] The cuboid cold-embedded block 1 is manufactured in a 1 cm x 1 cm bottom surface mold using a cold-embedded resin solution. The cold-embedded resin has a fast hardening speed, a low heat release during the curing process, and a small thermal shrinkage. The complete flat surface is manufactured through a slicing process to facilitate the polishing process in the later stage. Then, the copper bar 2 can be fixed by using double-sided tape and hot melt adhesive, and the welding surface of the copper bar 2 is polished. In order to remove the organic contaminants and residual oxides on the surface of the copper bar 2 and ensure the welding quality, the polished copper bar 2 is placed in a prepared acetone solution and ultrasonically cleaned for 10 minutes to remove the organic substances on the surface. Then, the copper bar 2 is placed in a mixed solution of hydrochloric acid and alcohol and ultrasonically cleaned for 10 minutes. The mixed solution contains 10% hydrochloric acid by volume fraction, and the oxides on the surface of the copper bar 2 are washed away.

[0039] The polished copper bar 2 is placed on the PCB board 3, the welding plane is perpendicular to the PCB board 3, the size of the PCB board 3 is 20 mm x 30 mm x 1.5 mm, the two welding planes are aligned and placed in parallel, and the distance is kept at 100 μm. In order to ensure the position of the two welding rods during the welding process, the two copper bars 2 need to be fixed. In this embodiment, high-temperature adhesive is used for bonding and fixing, and the treated sample is placed in a drying dish for 12 h to completely dry the high-temperature adhesive.

[0040] In this embodiment, Sn3.0Ag0.5Cu solder paste 4 is used for welding the two copper bars 2. The solder paste 4 generally needs to be stored at low temperature, so before starting the welding in this embodiment, the solder paste 4 has been placed at room temperature for 4 hours to restore the viscosity of the solder paste 4, and the stirring is sufficient. As shown in FIG. 4, the solder paste 4 is fully filled into the gap of the copper bar 2. Figure 2

[0041] In this embodiment, a hot air repair workbench is used for reflow welding operation, the reflow temperature is 270°C, the reflow time is 30 s, and the air is naturally cooled. Finally, a one-dimensional linear butt welding rod of Sn3.0Ag0.5Cu solder is obtained.

[0042] ​​The size of the butt copper bar 2 obtained at this time is 600 μm x 600 μm, the surface has oxides and is attached with dry high-temperature glue, and therefore the side surfaces need to be polished flat before use. The copper bar 2 is attached to the cold insert 1 with double-sided adhesive, and the four side surfaces are polished flat and then the surface residual glue is removed with acetone solution.

[0043] In this example, because the final size of the solder joint is too small, polishing the copper bar 2 as a whole will cause the copper bar 2 to become poor in rigidity, and therefore local polishing is selected. As shown in FIG. 4, the butt copper bar 2 is fixed to the side surface of the concave PCB 3 with epoxy resin, and the upper surface is polished with a polishing tool as shown in FIG. 5, and at this step the PCB 3 is thinned together with the copper bar 2. The surface of the trapezoidal polishing platform is 1000# sandpaper, and when polishing is performed to near 100 μm, 1500#, 3000# and 7000# sandpaper are used for subsequent polishing. Figure 3 Figure 4 As shown in FIG. 6, the side surface is polished and thinned, the side surface is the observation surface of the solder joint, and 1000#, 1500#, 3000# and 7000# sandpaper are used in sequence to polish to near 100 μm, and then coarse polishing liquid and fine polishing liquid are used in sequence to polish in a polishing manner to obtain high-precision polishing. In this embodiment, the coarse polishing liquid is 0.3 μm to 0.5 μm α-alumina suspension liquid, and the fine polishing liquid is 0.02 μm to 0.05 μm silica suspension liquid. During the polishing process, the surface of the solder joint is observed with the polarizer of the orthogonal polarized light microscope to ensure that the observation surface of the solder joint is clean and smooth, and finally a one-dimensional linear butt solder joint with a size of 100 μm x 100 μm x 100 μm is obtained.

[0044] As shown in FIG. 6, the side surface is polished and thinned, the side surface is the observation surface of the solder joint, and 1000#, 1500#, 3000# and 7000# sandpaper are used in sequence to polish to near 100 μm, and then coarse polishing liquid and fine polishing liquid are used in sequence to polish in a polishing manner to obtain high-precision polishing. In this embodiment, the coarse polishing liquid is 0.3 μm to 0.5 μm α-alumina suspension liquid, and the fine polishing liquid is 0.02 μm to 0.05 μm silica suspension liquid. During the polishing process, the surface of the solder joint is observed with the polarizer of the orthogonal polarized light microscope to ensure that the observation surface of the solder joint is clean and smooth, and finally a one-dimensional linear butt solder joint with a size of 100 μm x 100 μm x 100 μm is obtained. Figure 5

[0045] The above is only a preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical scope disclosed by the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.​​

Claims

1. A method of being able to control the size of a micro linear weld, characterized in that, The method comprises the following steps: preparing a copper rod (2) to be welded; processing the surface of the copper rod (2) to be welded to obtain a treated copper rod (2); fixing two treated copper rods (2) at preset spatial positions and placing them in a drying dish to wait for solidification, filling the treated copper rods (2) with solder paste (4) after solidification, welding the treated copper rods (2), and obtaining a butt joint welding rod; fixing the butt joint welding rod on a PCB board (3) for thinning treatment to obtain a thinned butt joint welding rod, and performing polishing and grinding treatment on the butt joint welding rod close to the required size to control the size of the micro linear welding point; the preparation of the copper rod (2) to be welded comprises: obtaining a copper rod (2) with a target structure, and using a cold insert (1) to assist in pre-welding treatment of the copper rod (2) to obtain the copper rod (2) to be welded; the copper rod (2) with a target structure is a copper rod (2) with a 90° angle; the cold insert (1) is used to assist in pre-welding treatment of the copper rod (2) to obtain the copper rod (2) to be welded, which comprises: using a cold insert (1) made of a cold insert resin solution to process the cold insert (1), fixing the copper rod (2) with double-sided adhesive tape and hot melt adhesive, polishing the welding surface of the copper rod (2), and obtaining the copper rod (2) to be welded; the method for processing the surface of the copper rod (2) to be welded comprises: removing organic matter and oxides attached to the surface of the copper rod (2) to be welded; acetone solution and hydrochloric acid alcohol solution are used to remove organic matter and oxides attached to the surface of the copper rod (2) to be welded; the method for fixing two treated copper rods (2) at preset spatial positions comprises: placing two treated copper rods (2) on the PCB board (3) with the welding plane perpendicular to the PCB board (3), aligning and placing the two welding planes in parallel, keeping the distance between the two welding planes at a preset distance, and fixing the two treated copper rods (2) with high-temperature glue; the solder paste (4) is Sn-3.0Ag-0.5Cu; the method for fixing the butt joint welding rod on the PCB board (3) for thinning treatment to obtain a thinned butt joint welding rod comprises: locally polishing the butt joint welding rod, fixing the butt joint welding rod on the side of the PCB board (3) with epoxy resin, and using a trapezoidal polishing tool combined with sandpaper to perform thinning treatment on the surface of the butt joint welding rod to obtain a thinned butt joint welding rod.

2. The method of claim 1, wherein the method for polishing and grinding the butt joint welding rod close to the required size comprises: polishing and grinding the butt joint welding rod close to the required size with coarse polishing liquid and fine polishing liquid, wherein the coarse polishing liquid is 0.3-0.5 μm α-alumina suspension liquid, and the fine polishing liquid is 0.02-0.05 μm silica suspension liquid. ​

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