A laser connection method for micro solder joints of functional chips

Abstract

The present invention relates to a laser connection method for micro-solder joints of a functional chip. The method comprises: embedding the functional chip in a fixture, placing the end of a lead to be welded at the center of a pad of the functional chip, applying solder paste to the surface of the pad with the lead, the solder paste completely covering the pad, and the solder paste having a thickness of 0.2-0.5 mm; performing laser soft soldering on the solder paste to form micro-solder joints connecting the solder paste, the pad, and the lead, and forming a uniform and continuous intermetallic compound at the interface between the pad and the solder paste. Laser welding parameters include: a laser power of 25W-40W and a welding time of 20ms-40ms. Using a semiconductor laser in combination with solder paste soft soldering can solve the problems of pad detachment and micro-solder joint failure caused by too little heat input, or micro-solder joint burnout caused by too much heat input. The method is suitable for ultra-thin pads, in particular, and achieves stable, high-quality, and efficient connection of the micro-solder joints of the functional chip. The tensile strength of the micro-solder joints is greater than 12.05MPa.

Description

Technical Field

[0001] The invention belongs to the field of laser welding, and in particular relates to a laser connection method for micro-welding points of a functional chip. Background Art

[0002] Functional chips are silicon wafers containing integrated circuits and play a vital role in microelectronics and industrial production. These chips require connections to components or leads at pads. Soldering irons or wave soldering are currently the most common methods used domestically and internationally. However, soldering irons generate a large amount of heat, making them unsuitable for soldering precision instruments. This results in low efficiency and poor soldering quality. While wave soldering is suitable for mass production of components and boasts high production efficiency, it is not suitable for use near the thermally sensitive elements of functional chips. Furthermore, this method is subject to numerous factors that affect soldering quality, making it difficult to achieve stable output and meeting the micro-solder point connection requirements of functional chips.

[0003] Lasers offer a high level of accuracy and control. Due to their high energy density and the fact that the laser beam can illuminate an area down to the micron level, they have significant advantages in welding tiny components or tiny solder joints. They are suitable for high-precision, high-quality welds such as medical devices. However, the following technical issues still exist when laser-connecting micro-solder joints on functional chips:

[0004] (1) The lasers commonly used in industry have strong penetrating power and concentrated energy. The pad is a thin layer of copper. When welding, especially ultra-thin pads, thermal sensitive components, and functional chip components with small gaps between adjacent micro pads, it is difficult to achieve fine internal grains and uniform structure due to the uneven heat distribution of the laser power light source and improper heat input control, which can easily cause pad detachment, micro solder joint failure or micro solder joint burnout. After the micro solder joint is formed, it is easy to cause the quality of the micro solder joint to deteriorate when heated again, making it difficult to achieve stable and high-quality connection of the micro solder joints of the functional chip.

[0005] (2) The surface roughness of the functional chip to be soldered is not properly controlled and the chip loading fixture has insufficient precision. If the lead is not in the center of the pad, the tensile strength of the micro solder joint will be reduced or the lead will be exposed outside the micro solder joint, affecting the conductivity and mechanical properties of the micro solder joint.

[0006] (3) The side length of the micro pad of the functional chip is small and the pad is thin. The solder paste can flow better than the liquid tin of the wire at high temperature, is evenly distributed at the solder joint, and has less pollution to the welding tools. At the same time, the heating temperature of the wire is generally above 450℃, and the temperature of the solder paste is only 180-300℃, so the heat input is smaller, which can avoid the burning of the ultra-thin pad under high heat input. However, the printing position, deposition thickness and coating uniformity of the solder paste are not good, and the laser power device and the solder paste are not properly matched. If too little solder paste is applied, the micro solder joints will be very small, the pads will not be completely covered, and the tensile strength will be affected. Too much solder paste will not only prolong the required welding process and easily cause the pads to fall off, but also the micro solder joints formed will be too large, which is not conducive to molding. Summary of the Invention

[0007] The present invention aims to solve at least one of the above technical problems to a certain extent. The present invention provides a laser connection method for micro solder joints of functional chips, which is suitable for ultra-thin pads in particular, to achieve stable and high-quality efficient connection of micro solder joints of functional chips.

[0008] The technical solution adopted by the present invention to solve its technical problem is:

[0009] A laser connection method for micro solder joints of a functional chip, the method comprising:

[0010] Install the functional chip in the fixture, and place the end of the lead to be welded at the center of the pad of the functional chip;

[0011] Apply solder paste to the surface of the pad with the lead, and completely cover the pad with solder paste. The thickness of the solder paste should be 0.2-0.5mm. Controlling the thickness of the solder paste can avoid too little solder paste, which will cause the micro solder joints to be too small and fail to completely cover the pad, thus affecting the tensile strength; and avoid too much solder paste, which will cause the welding process to be prolonged, easily cause the pad to fall off, and form micro solder joints that are too large and not conducive to forming.

[0012] Laser soldering is performed on the solder paste. The laser heats and melts the solder paste to form micro solder joints connecting the solder paste to the pad and lead. The side length of the micro solder joint is ≤1.6mm. The lead is located inside the micro solder joint. A uniform and continuous intermetallic compound is formed at the interface between the pad and the solder paste. The laser welding parameters include: laser power of 25W-40W, welding time of 20ms-40ms.

[0013] Controlling laser welding parameters can be combined with solder paste soft soldering, which does not require any auxiliary tools compared to soldering with a soldering iron or wave soldering. It can also make the intermetallic compound between the solder joint and the copper foil of the pad continuous and more consistent in thickness, and the formed eutectic structure evenly distributed, ensuring the bonding force between the solder joint and the pad, thereby ensuring the conductivity and mechanical properties of the solder joint. At the same time, it can avoid the solder paste from forming a reliable solder joint due to the small heat input when the laser power is lower than 25W or the welding time is lower than 20ms; and avoid the solder joint from burning due to excessive heat input when the laser power is greater than 40W or the welding time is greater than 40ms.

[0014] Furthermore, before the functional chip is mounted on the fixture, the surface of the pad to be soldered is cleaned and polished to a surface roughness of R a 0.1μm≤R a ≤0.3μm. A certain surface roughness can enable the solder paste to better bond with the copper pad during laser heating, thereby increasing the mechanical properties of the solder joint. At the same time, it can avoid the surface roughness exceeding 0.3μm, which causes the pad to be too thin and reduces the bonding strength of the solder joint.

[0015] Furthermore, acetone is used for cleaning to remove surface oil stains, and sandpaper is used to remove the oxide layer to further improve the bonding strength of the solder joints.

[0016] Furthermore, the fixture is manufactured using 3D printing technology. The fixture includes a groove and several lead holes located above the groove side. The groove is clearance-matched with the edge of the functional chip. The assembly size tolerance of the functional chip, that is, the fixture accuracy, is ≤0.1mm. It is used to form a high-precision fixture through 3D printing and improve laser welding accuracy.

[0017] Furthermore, the ratio of the distance D1 between the end of the lead hole and the center of the pad to the side length D2 of the functional chip is 10-15, and the head end of the lead can pass through the lead hole and be placed above the pad, and the tail end of the lead is flush with the end of the lead hole, which is used to further control the accuracy of the position of the end of the lead to be welded in the center of the pad of the functional chip, avoiding the lead end being too shorter than the center position of the pad, resulting in a decrease in the tensile performance of the solder joint, and avoiding the lead end being too longer than the center position of the pad, resulting in the lead being exposed outside the solder joint and affecting the electrical performance of the solder joint.

[0018] Furthermore, the solder paste is applied using a screen printing method, which includes:

[0019] Cover the functional chip with the silk screen. The mesh of the silk screen should correspond to the pads with leads. The mesh size should be consistent with the pad size, so that the solder paste can accurately and completely cover the pads.

[0020] Dip the scraper into the solder paste, control the scraper angle at 60°-80°, and print the solder paste from the mesh to the pad surface in the same direction. Remove the screen after printing. By controlling the scraper angle and direction, you can control the printing position, deposition thickness and coating uniformity of the solder paste. This can prevent the scraper angle from being too low, causing the solder paste to flow to both sides under pressure and cause waste, and avoid the scraper angle from being too large, which is not conducive to printing the solder paste.

[0021] Furthermore, the solder paste is a tin-based solder paste, which makes it easy to control the laser irradiation time and output power.

[0022] Furthermore, the solder paste is preferably Sn 63 Pb 37 The solder paste is a medium-temperature tin-lead solder paste with moderate viscosity, good wetting properties, fluidity and welding properties, and a melting point of 183°C. It can avoid the difficulty in controlling the solder paste deposition shape due to too low viscosity, and avoid the mesh wall hanging, missed printing, collapsed edges or bridging due to too high viscosity, resulting in incomplete coverage of the pad.

[0023] Furthermore, a semiconductor laser power device is used for laser welding, and the power range is 0-200W. Compared with other laser power devices, it has the advantage of uniform spot energy distribution, which can further improve the stability and quality of the micro-solder points of the functional chip. The spot diameter at the focal position is 0.5-1mm, preferably 0.8mm. The focal position is at the center of the pad, which can concentrate the laser energy, focus accurately, avoid micro-solder point burning, and further improve the welding tensile strength.

[0024] Furthermore, the functional chip is a thermosensitive element, and soft soldering can be applied near the thermosensitive element. After the micro solder joints are formed, they are no longer heated, which can avoid reheating and causing the solder joint quality to deteriorate.

[0025] Furthermore, there are multiple pads and leads; the side length of the pad is 1 to 1.6 mm, and the preferred pad width*length dimension is 1*1.6 mm; the gap between adjacent pads is 0.1-0.5 mm, and the preferred spacing from the end of the lead hole to the center of the pad is 10-10.8 mm. The welding track moves along the multiple pads of the functional chip to form suitable micro-solder joints, further improving the electrical performance and tensile strength.

[0026] Furthermore, the thickness of the pad is ≤0.5 mm, and the above method is used to connect the micro solder joints at the thin pad, which can avoid excessive heat input causing the pad to burn.

[0027] Furthermore, the micro solder joints have uniform structure, are consistent in size with the pads, and have a tensile strength greater than 12.05 MPa, which can significantly improve the tensile strength compared to existing soldering iron or wave soldering.

[0028] A functional chip is manufactured by the laser connection method of functional chip micro-solder points described in any one of the above.

[0029] Furthermore, the functional chip includes a double-sided printed circuit board, and the double-sided micro solder joints are connected by repeating the steps of the laser connection method, and the repeated operation stabilizes the model.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) Using appropriate laser power and welding time to control heat input, a uniform and continuous intermetallic compound can be formed at the interface between the pad and the solder paste. Compared with soldering iron or wave soldering, no auxiliary tools are required, and the solder paste has less pollution to the welding tools. It solves the problems of pad detachment, micro solder joint failure caused by too little heat input, or micro solder joint burnout caused by too much heat input. In combination with semiconductor lasers, the uniformity of heat distribution of the light source is further improved. It is suitable for ultra-thin pads, thermal sensitive elements, and functional chip components with small gaps between adjacent micro pads, which is conducive to improving work efficiency and finished product quality and reducing costs.

[0032] (2) A high-precision fixture is used to fit the functional chip, so that the end of the lead to be welded can be accurately placed in the center of the pad of the functional chip, so that the lead is located in an appropriate position inside the micro-solder point, solving the problem that the lead is not in the center of the pad, resulting in a decrease in the tensile performance of the micro-solder point or the lead is exposed outside the micro-solder point. Combined with the appropriate roughness of the surface to be welded, the conductivity and mechanical properties of the micro-solder point are further improved.

[0033] (3) The screen printing method is used to control the solder paste to completely cover the pad and the appropriate solder paste thickness. Laser soft soldering is performed on top of the solder paste to form micro solder joints with solder paste and pad, lead connection, and side length ≤1.6mm. This can avoid the burning of ultra-thin pads under high heat input, and solve the problem of improper cooperation between laser power device and solder paste leading to decreased tensile strength, low welding efficiency and unfavorable forming. The tensile strength of micro solder joints is greater than 12.05MPa, which can significantly improve the tensile strength compared with existing soldering iron or wave soldering.

[0034] In summary, the present invention is easy to control, has high yield and efficiency, is suitable for ultra-thin pads in particular, can ensure the bonding force between micro-solder points and pads, ensure conductivity and mechanical properties, and can achieve stable and high-quality connection of micro-solder points of functional chips. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which:

[0036] Figure 1 is a flow chart of the method of the present invention;

[0037] Figure 2 It is a structural diagram of the clamp of the present invention;

[0038] Figure 3 Schematic diagram of the functional chip of the present invention mounted on a fixture;

[0039] Figure 4 It is a schematic diagram of the functional chip and screen printing of the present invention.

[0040] Markings in the figure: fixture 1, groove 101, lead hole 102, silk screen 3, mesh 301, scraper 4, solder paste 5, functional chip 6, pad 601, lead 7.

[0041] Figure 5 This is a morphology diagram of the laser micro-welding point in Example 1 of the present invention.

[0042] Figure 5 (a) shows the morphology of the bonding position between the micro solder joint and the pad; Figure 5 (b) shows the internal morphology of the micro solder joint. DETAILED DESCRIPTION

[0043] The following describes embodiments of the present invention in detail, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and are not to be construed as limiting the present invention.

[0044] In the description of the present invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "axial," "radial," "vertical," "horizontal," "inner," "outer," and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended only to facilitate the description of the present invention and simplify the description. They do not indicate or imply that the devices or elements referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limiting the present invention. In the description of the present invention, "multiple" and "several" mean two or more, unless otherwise expressly and specifically defined.

[0045] Example 1

[0046] like Figure 1-4 As shown, a preferred embodiment of the laser connection method of a functional chip micro solder joint of the present invention includes the following steps:

[0047] S1: Take the functional chip 6 to be soldered and measure it. Set the size of the groove 101 of the fixture 1 according to the measured size of the functional chip 6. The width and length of the four pads 601 are all measured to be 1*1.6mm. The gap between adjacent pads 601 is 0.5mm. The thickness of the pad 601 is 0.5mm. The distance from the end of the lead hole 102 to the center of the pad 601 is set to 10mm according to the size and position of each pad 601. Prepare the fixture 1 including multiple grooves 101 and multiple lead holes 102 located above and on the sides of the grooves 101 using a 3D printing device. The accuracy of the fixture 1 is ≤0.1mm.

[0048] S2: The surface of the functional chip 6 to be soldered is first cleaned with acetone to remove the oil on the surface, and then polished with sandpaper to remove the oxide layer and make the roughness of the surface of the pad 601 to be soldered R a Maintained at 0.1μm≤R a ≤0.3μm;

[0049] S3: Installing multiple functional chips 6 in the grooves 101 of the fixture 1 respectively;

[0050] S4: Pass the lead wire 7 of the sensor to be soldered through the lead wire hole 102 of the fixture 1 and place it above the soldering pad 601. The tail end of the lead wire 7 is flush with the end of the lead wire hole 102, and the lead wire 7 is placed at the center of the soldering pad 601 of the functional chip 6.

[0051] S5: Cover the screen 3 on the functional chip 6. The mesh 301 of the screen 3 corresponds to the pad 601 where the lead 7 is placed. The size of the mesh 301 is consistent with the size of the pad 601. Use a scraper 4 to dip Sn 63 Pb 37 The solder paste 5 of the material is printed from the mesh 301 to the surface of the pad 601 by controlling the angle of the scraper 4 and making the scraper 4 print the solder paste 5 in the same direction. After printing, the screen 3 is removed.

[0052] S6: Adjust the parameters of the laser brazing equipment: laser power, welding time, focus position, spot diameter of 0.8 mm, and focus position at the center of the pad 601;

[0053] S7: Laser soldering is performed on the solder paste 5. The solder paste 5 is melted by laser heating and then welded to form a micro solder joint connecting the solder paste 5 with the pad 601 and the lead 7. The micro solder joint has uniform structure and the same size as the pad 601. The lead 7 is located inside the micro solder joint. A uniform and continuous intermetallic compound is formed at the joint surface between the pad 601 and the solder paste 5. The functional chip 6 is removed from the fixture 1 to obtain the finished product. The maximum tensile force and tensile strength that the finished micro solder joint can withstand are measured.

[0054] Comparative Example 1: A functional chip 6 with micro soldering points soldered by a commercially available electric soldering iron.

[0055] The difference between Example 2-3 and Comparative Example 2-4 and Example 1 is that the laser welding parameters are different.

[0056] Example 4:

[0057] A laser connection method for micro solder joints of a functional chip is provided, which differs from Example 1 in that the distance between the end of the lead hole 102 and the center of the solder pad 601 is 10.5 mm.

[0058] The difference between Examples 5-6 and Comparative Example 5 and Example 4 lies in the different laser welding parameters.

[0059] The laser parameters of the embodiment and the comparative example were compared, and the maximum tensile force and tensile strength of the micro solder joints were measured according to the Japanese microelectronic component industry standard JIS Z3198. The results are shown in Table 1 below:

[0060] Table 1 Comparison results of the embodiment and the comparative example

[0061]

[0062] By the attached Figure 5 As can be seen from Table 1, due to the high energy density of the laser and the area that the laser beam can irradiate at the micron level, soft soldering near the thermosensitive element does not require any auxiliary tools compared to soldering with a soldering iron or wave soldering, and the solder paste 5 causes less pollution to the soldering tools. The grains inside the micro-solder joints are small and the structure is uniform. A thin, uniform and continuous intermetallic compound is formed at the interface between the pad 601 and the solder paste 5, indicating reliable welding quality.

[0063] A high-precision fixture 1 is used to fit and load the functional chip 6, so that the end of the lead 7 to be welded can be accurately placed at the center of the pad 601 of the functional chip 6, so that the lead 7 is located at an appropriate position inside the micro-weld point, thereby solving the problem that the lead 7 is not at the center of the pad 601, resulting in a decrease in the tensile performance of the micro-weld point or the lead 7 is exposed outside the micro-weld point. Combined with the appropriate roughness of the surface to be welded, the conductivity and mechanical properties of the micro-weld point are further improved. Combined with the semiconductor laser, the uniformity of the heat distribution of the light source is further improved, which can enhance the comprehensive performance of the weld point.

[0064] From the comparison results of comparative example 1 and embodiments 1-6, it can be seen that the tensile strength of the micro solder joints of the present invention is greater than 12.05 MPa, which can significantly improve the tensile strength compared with the existing soldering iron brazing; the screen 3 printing method is used to control the solder paste 5 to completely cover the pad 601 and the appropriate thickness of the solder paste 5, the thickness of the solder paste 5 is 0.2-0.5 mm, which can avoid too little solder paste 5 causing the micro solder joints to be too small and unable to completely cover the pad 601, thereby affecting the tensile strength; from the comparison results of comparative example 2 and embodiments 1-3, it can be seen that too much solder paste 5 can be avoided, which can avoid the required welding time process being prolonged, easily causing the pad 601 to fall off, and the formed micro solder joints being too large and not conducive to molding.

[0065] From the comparison results of Comparative Example 3 with Examples 1-3, and the comparison results of Comparative Example 5 with Examples 4-6, it can be seen that the present invention adopts appropriate laser power and welding time to control heat input, controls the laser power to 25W-40W, and the welding time to 20ms-40ms, which can solve the problem of micro-weld point burning caused by too long welding time and too high welding power input.

[0066] From the comparison results of Comparative Example 4 and Examples 1-3, it can be seen that the present invention adopts appropriate laser power and welding time to control heat input, controls the laser power to 25W-40W, and the welding time to 20ms-40ms, which can solve the problem that when the laser power is lower than 25W or the welding time is lower than 20ms, the heat input is small, resulting in the solder paste 5 failing to form a reliable solder joint.

[0067] In summary, the present invention is suitable for functional chip 6 components, especially ultra-thin pads 601, thermal sensitive elements, and small gaps between adjacent micro pads 601. It is beneficial to improve work efficiency and finished product quality, reduce costs, and is easy to control, with high yield and efficiency. It can ensure the bonding force between micro solder joints and pads 601, ensure conductivity and mechanical properties, and achieve stable and high-quality connection of the micro solder joints of the functional chip 6.

[0068] The series of detailed descriptions listed above are only specific descriptions of feasible embodiments of the present invention. They are not intended to limit the scope of protection of the present invention. Any equivalent embodiments or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A laser connection method for micro solder joints of functional chips, characterized in that: The methods include: The surface to be soldered of the pad (601) is cleaned and polished to a surface roughness R a 0.1μm≤R a ≤0.3μm, the functional chip (6) is mounted in the fixture (1), the end of the lead wire (7) to be welded is placed at the center of the pad (601) of the functional chip (6), and the thickness of the pad (601) is ≤0.5mm; Applying solder paste (5) on the surface of the pad (601) on which the lead (7) is placed, so that the solder paste (5) completely covers the pad (601), and the thickness of the solder paste (5) is 0.2-0.5 mm; Laser soldering is performed on the solder paste (5) to form micro solder joints connecting the solder paste (5) with the pad (601) and the lead (7). The side length of the micro solder joint is ≤1.6 mm. A uniform and continuous intermetallic compound is formed at the interface between the pad (601) and the solder paste (5). The laser welding parameters include: laser power of 25W-40W, welding time of 20ms-40ms, and focus position at the center of the pad (601).

2. The laser connection method for functional chip micro solder joints according to claim 1, characterized in that: The fixture (1) is manufactured using 3D printing technology. The fixture (1) includes a groove (101) and a plurality of lead holes (102) located above the side of the groove (101). The groove (101) is clearance-matched with the edge of the functional chip (6). The accuracy of the fixture (1) is ≤0.1 mm. The ratio of the distance D1 from the end of the lead hole (102) to the center of the pad (601) to the side length D2 of the functional chip (6) is 10-15. The head end of the lead (7) can pass through the lead hole (102) and be placed above the pad (601), and the tail end of the lead (7) is flush with the end of the lead hole (102).

3. The laser connection method for micro solder joints of a functional chip according to claim 1, characterized in that: The solder paste (5) is applied using a screen (3) printing method, the screen (3) printing method including: The silk screen (3) is covered on the functional chip (6), the mesh (301) of the silk screen (3) corresponds to the pad (601) on which the lead (7) is placed, and the size of the mesh (301) is consistent with the size of the pad (601); Use a scraper (4) to dip the solder paste (5), control the angle of the scraper (4) to 60°-80°, and use the scraper (4) to print the solder paste (5) from the mesh (301) to the surface of the pad (601) in the same direction. After printing, remove the screen (3).

4. The laser connection method for functional chip micro solder joints according to claim 1, characterized in that: The solder paste (5) is a tin-based solder paste.

5. The laser connection method for functional chip micro solder joints according to claim 1, characterized in that: A semiconductor laser power device is used for laser welding, and the spot diameter at the focal position is 0.5-1mm.

6. The laser connection method for functional chip micro solder joints according to claim 1, characterized in that: The functional chip (6) is a heat-sensitive element, and the micro-soldering point is no longer heated after being formed.

7. The laser connection method for functional chip micro solder joints according to claim 1, characterized in that: There are a plurality of solder pads (601) and leads (7), the side length of the solder pads (601) is 1-1.6 mm, and the gap between adjacent solder pads (601) is 0.1-0.5 mm.

8. The laser connection method for functional chip micro solder joints according to claim 1, characterized in that: The micro solder joints have uniform structure, are consistent in size with the solder pads (601), and have a tensile strength greater than 12.05 MPa.

9. A functional chip, characterized in that: The functional chip (6) is manufactured by the laser connection method of the functional chip micro-solder joints according to any one of claims 1 to 8.

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