A method for preparing indium-based soft soldering sheet by vacuum hot pressing forming

Abstract

The application discloses a method for preparing an indium-based soft soldering sheet by vacuum hot pressing, belongs to the technical field of thermal interface materials for chip packaging and IGBT power module welding material preparation, and is characterized in that pure indium powder or indium-tin mixed alloy powder is uniformly laid in a graphite mold, the graphite mold is vibrated or knocked, and the pure indium powder or indium-tin mixed alloy powder is laid flat and densely; the graphite mold is fixed in a vacuum hot pressing machine, a pressure of 10 MPa is applied to the vacuum hot pressing machine, vacuum is drawn to 1*10 ‑3 Pa, a temperature rising rate is controlled to be less than 5 DEG C / min, when the temperature rises to the melting point of indium or the melting temperature of the indium-tin mixed alloy, a pressure maintaining state is kept for 0.5-1 hours, then the mold is cooled to room temperature along with the furnace, the mold is taken out, and a pure indium soldering sheet or an indium-tin-based multi-element alloy soldering sheet is obtained. The method has simple process and low energy consumption, and the performance of the prepared soft thin soldering sheet is obviously improved.

Description

Technical Field

[0001] This invention belongs to the technical field of thermal interface materials for chip packaging and methods for preparing welding materials for IGBT power modules, specifically relating to a method for preparing indium-based soft solder sheets by vacuum hot pressing. Background Technology

[0002] With the rapid development of artificial intelligence (AI), high-performance computing (HPC), and intelligent electric vehicle industries, the market size of IGBT and global server chip packaging technologies will grow rapidly. Market demand for IGBT and server chip packaging technologies has driven the development of large-size, high-performance chips. However, both high-performance server chips and high-power IGBT chips face challenges related to high power consumption and large-size chip warpage. High power consumption and chip warpage affect heat dissipation, leading to a decline in the electrical performance of integrated circuit chips. Developing suitable thermal interface materials for chip packaging and thermally conductive indium-based bonding materials for IGBT power modules can solve the heat dissipation and warpage problems of large-size chips, improving chip performance and service reliability.

[0003] Pure indium foil and indium tin-based solder sheets can be used as thermal interface materials for chip packaging and as soldering materials for IGBT power modules, respectively. Compared to conventional tin-based solder alloys, pure indium foil is one-quarter as hard as pure lead, resulting in both pure indium and indium tin-based solder sheets being relatively soft. Furthermore, to meet different soldering temperatures, indium tin-based solder sheets require microalloying, adding high-melting-point metals such as Cu, Sb, and Ag (see US Patent US20220362890A1, SNIN SOLDER ALLOYS). Considering that thermal resistance is related to the thickness of the solder sheet, reducing the thickness lowers the thermal resistance, leading to a demand for thinner solder sheets with a thickness of 50-200 micrometers. However, producing these solder sheets presents challenges due to difficulties in rolling equipment processing, resulting in low processing accuracy and low yield. Major domestic manufacturers primarily employ the traditional method of mixing and melting metals with different melting points, casting them into ingots, and then rolling them in multiple passes to prepare alloy weld sheets (see Chinese Patent CN103639620A, which discloses a method for preparing Sn-Bi brittle alloy preformed weld sheets). However, this method for preparing indium-based soft alloy weld sheets suffers from problems such as weld sheet sticking to the rollers and poor surface quality, especially for thin sheets smaller than 200 micrometers, which are difficult to process. Some domestic research institutions use multiple rolling processes of alloy sheets with different melting points to prepare weld sheets as composite materials (see Chinese Patent CN112440029A), but this method suffers from drawbacks such as high porosity after welding. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for preparing indium-based soft alloy solder sheets by vacuum hot pressing, thereby improving the difficulties in processing and forming 50-100 μm indium-based soft alloy solder sheets and enhancing the welding performance of indium sheets for chip packaging and welding materials for IGBT power modules.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A method for preparing indium-based flexible solder sheets by vacuum hot pressing involves uniformly spreading pure indium powder or indium-tin alloy powder in a graphite mold, vibrating or tapping the graphite mold to make the pure indium powder or indium-tin alloy powder spread evenly and densely; fixing the graphite mold in a vacuum hot press, applying a pressure of 10 MPa to the vacuum hot press, and evacuating to a vacuum level of 1×10⁻⁶. -3 Pa, the heating rate is controlled within 5℃ / min; when the temperature rises to the melting point of indium or the melting temperature of indium-tin mixed alloy, maintain the pressure and hold for 0.5~1 hour, then the mold is cooled to room temperature with the furnace, the mold is removed, and pure indium solder sheet or indium-tin based multi-element alloy solder sheet is obtained.

[0007] Furthermore, the pure indium powder is a spherical powder with a purity of 99.999% and a diameter of 25-50 μm; the pure indium solder sheet prepared has a purity of 99.999%, a thickness of 50 μm ± 5 μm, a spreading area fraction of not less than 98.76%, and a post-soldering void ratio of not more than 0.68%.

[0008] Further, the indium-tin mixed alloy powder is a mixture of Sn48In52 eutectic alloy powder, Sn42Bi58 eutectic alloy powder, SnSb5 master alloy powder, SnCu2 master alloy powder, and SnNi0.5 master alloy powder, with a particle size of 25-50 μm; the prepared indium-tin based multi-element alloy solder sheet has the composition of SnIn14Bi5Sb1.5Cu0.07Ni0.05 or SnIn17Bi5Sb1.5Cu0.07Ni. 0.05, wherein the SnIn14Bi5Sb1.5Cu0.07Ni0.05 alloy weld sheet has a melting point of 199.2℃, a post-weld spreading area fraction of not less than 94.39%, and a post-weld void ratio of not more than 0.77%; the SnIn17Bi5Sb1.5Cu0.07Ni0.05 alloy weld sheet has a melting point of 201.5℃, a post-weld spreading area fraction of not less than 88.4%, and a post-weld void ratio of not more than 0.88%; the thickness of the alloy weld sheet is 100 μm ± 5 μm.

[0009] Furthermore, the graphite mold is a high-purity graphite layered mold, including a base with a concave cavity, a mold top cover, and a middle layer gasket frame with adjustable thickness disposed between the top cover and the base; the upper opening of the concave cavity of the base has a flange, the middle layer gasket frame is a frame that can be fitted over the flange, and the top cover has a pressure head that can pass through the middle layer gasket frame and be inserted into the concave cavity, the pressure head being tightly fitted to the four walls of the concave cavity; the surface roughness of the graphite mold is Ra 0.1μm or less.

[0010] Furthermore, the inner cavity of the graphite mold has a length of 20 cm and a width of 15 cm, with a dimensional accuracy within 0.005 mm.

[0011] The present invention has the following significant advantages:

[0012] (1) The present invention adopts vacuum hot pressing process, which simplifies the process, shortens the preparation cycle of welding sheets, reduces the loss in the rolling process, and improves the metal utilization rate compared with the traditional rolling forming process.

[0013] (2) The present invention adopts a vacuum hot pressing process, which effectively avoids the introduction of oxidation and impurities. The prepared solder sheet has a high surface smoothness, no wrinkles, low void rate after welding, and excellent spreading performance.

[0014] (3) The thickness and precision of the indium-based soft alloy sheet prepared by the present invention are flexible and controllable, and the minimum thickness and tolerance can reach 50 μm ± 5 μm. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the structure of the graphite mold used in the method of the present invention;

[0016] Figure 2 The melting characteristic curves are for Examples 1, 2, and 3;

[0017] Figure 3 Images showing the spread of the alloy weld sheet prepared in Example 1 after welding;

[0018] Figure 4 Images showing the spread of the alloy weld sheet prepared in Example 2 after welding;

[0019] Figure 5 Images showing the spread of the alloy weld sheet prepared in Example 3 after welding;

[0020] Figure 6 Image showing the spread of the alloy weld sheet prepared in Comparative Example 1 after welding;

[0021] Figure 7 Image showing the post-weld spread of the alloy weld sheet prepared in Comparative Example 2;

[0022] Figure 8 Image showing the post-weld spread of the alloy weld sheet prepared in Comparative Example 3;

[0023] Figure 9 The following is a statistical chart showing the fractional area of ​​the welded sheets after welding for Examples 1, 2, 3, Comparative Examples 1, 2, and 3.

[0024] Figure 10 The following is a statistical chart of the post-weld void rate for Examples 1, 2, 3, Comparative Examples 1, 2, and 3. Detailed Implementation

[0025] The present invention will be further described in detail below through embodiments, but the scope of protection of the present invention is not limited to the contents described in the embodiments.

[0026] The method for preparing pure indium soft solder sheets by vacuum hot pressing according to the present invention uses pure indium spherical powder with a purity of 99.999% and a diameter of 25-50 μm, and an oxygen content of less than 100 ppm to ensure good flowability and formability of the alloy. A graphite mold is used to hold the pure indium powder, which melts within the mold to form a metal sheet. The graphite mold can be any type of graphite mold capable of forming metal sheets; in this embodiment, a layered mold made of high-purity graphite is used, such as... Figure 1 As shown, the system includes a box-shaped base 1 with a concave cavity 1a, a mold top cover 3, and an adjustable-thickness intermediate layer gasket frame 2 positioned between the top cover and the base. The upper opening of the concave cavity of the base has a flange 1b. The intermediate layer gasket 2 is a frame that can be fitted over the flange 1b. The top cover 3 has a pressure head 3a that can pass through the intermediate layer gasket frame and be inserted into the concave cavity 1a. The pressure head fits tightly against the four walls of the concave cavity. A set of intermediate layer gasket frames 2 with different thicknesses can be prepared. By changing the intermediate layer gasket frames of different thicknesses, the depth of the pressure head pressing into the concave cavity can be adjusted, thereby achieving thickness adjustment of the soft metal sheet pressed within the base. The surface roughness of the graphite mold can be polished to below Ra 0.1μm.

[0027] In this embodiment, the inner cavity of the mold has a length of 20 cm and a width of 15 cm, with a dimensional accuracy within 0.005 mm.

[0028] Pure indium powder is evenly spread in a mold. The mold is then vibrated or gently tapped to ensure even powder distribution. The mold is then fixed in a vacuum hot press and evacuated to a vacuum level of 1×10⁻⁶. -3 A high vacuum is maintained at 10 MPa to remove interstitial gases from the indium powder, preventing porosity during heating, reducing alloy oxidation, and ensuring sufficient powder contact under pressure. The vacuum hot press furnace is heated while applying 10 MPa of pressure, with the heating rate controlled below 5°C / min. When the temperature reaches the melting point of the indium powder, it is held for 0.5 hours to allow for sufficient diffusion and increase density. The furnace is then kept under pressure and cooled to room temperature. The mold is opened, and a thin, soft indium sheet is removed to reduce internal stress and prevent cracking.

[0029] The method for preparing indium-based soft alloy solder sheets by vacuum hot pressing according to the present invention uses Sn48In52 eutectic alloy powder, Sn42Bi58 eutectic alloy powder, SnSb5 master alloy powder, SnCu2 master alloy powder, and SnNi0.5 master alloy powder with a pure diameter of 25-50μm and an oxygen content of less than 100ppm. These are mixed according to the multi-element alloy composition SnIn14Bi5Sb1.5Cu0.07Ni0.05 or SnIn17Bi5Sb1.5Cu0.07Ni0.05 to obtain SnIn14Bi5Sb1.5Cu0.07Ni0.05 alloy powder or SnIn17Bi5Sb1.5Cu0.07Ni0.05 alloy powder. Similarly, [the method is described in the original text]. Figure 1 The layered mold shown is used to prepare indium-based soft alloy solder sheets. The mixed alloy powder is evenly spread in the mold, and the powder distribution is further improved by vibration or light tapping of the mold. The mold is then fixed in a vacuum hot press, and a vacuum of 1×10⁻⁶ is applied. -3 The vacuum hot press furnace is heated while applying a pressure of 10 MPa, with the heating rate controlled within 5℃ / min. When the temperature reaches 250℃, it is held at that temperature for 1 hour, maintaining the pressurized state. The furnace is then cooled to room temperature. The mold is opened, and the thin indium tin-based soft alloy sheet is removed.

[0030] The technical effects of this invention were analyzed by testing the melting characteristics, post-weld voids, and spreading performance of the soft solder sheets obtained in the following examples and comparative examples. The testing equipment was a STA449F5 calorimeter, and the test sample was 40 mg of solder sheet scrap. The temperature was raised to 250°C under Ar atmosphere, and then cooled to room temperature under Ar atmosphere, with both heating and cooling rates at 10 K / min. The solder sheets were welded using a formic acid vacuum reflow soldering machine (HX-HPK). The post-weld void rate was tested using a dage XD7500X-RAY. The spreading performance of the solder sheets was quantified using the spreading area fraction, and the spreading area fraction was statistically analyzed using ImageJ software. Example 1

[0031] Spherical indium powder with a purity of 99.999% and a diameter of 25~30μm was evenly spread in a graphite layered mold. The mold was then fixed in a vacuum hot press and evacuated to a vacuum level of 1×10⁻⁶. -3 The vacuum hot press furnace was heated to 156℃, the melting point of pure indium metal, at a pressure of 10 MPa. The heating rate was controlled at 5℃ / min, and the temperature was held for 0.5 hours. Then, the pressure was maintained while cooling to room temperature. The mold was removed and opened to obtain an indium-based soft alloy thin sheet with a thickness of 50 μm and a tolerance of 5 μm. The melting characteristics, post-soldering void ratio, and spreading performance of the sheet were tested. The melting characteristic curve of the sheet is shown in [Figure showing...]. Figure 2The spread area fraction was 98.76%, and the post-weld void rate was 0.68%, as shown in Tables 1 and 2. Figure 3 The image shows the post-weld spread of the alloy weld sheet prepared in this embodiment. It can be seen that the surface of the weld sheet is smooth and wrinkle-free, with low post-weld void ratio and good spreadability. Example 2

[0032] Spherical alloy powder with a diameter of 40~50μm, consisting of SnIn14Bi5Sb1.5Cu0.07Ni0.05, was evenly spread in a graphite layered mold. The mold was then fixed in a vacuum hot press and evacuated to a vacuum level of 1×10⁻⁶. -3 The vacuum hot press furnace was heated to 250℃, the pressure was set to 10 MPa, the heating rate was controlled at 5℃ / min, and the temperature was held for 1 hour. Then, the pressure was maintained and the furnace was cooled to room temperature. The mold was removed and opened to obtain a SnIn14Bi5Sb1.5Cu0.07Ni0.05 soft alloy thin sheet with a thickness of 100 μm and a tolerance of 5 μm. The peak temperature of the sheet was 199.2℃. The melting characteristics, post-weld void ratio, and spreading performance of the sheet were tested. The melting characteristic curve of the sheet is shown in [Figure showing...]. Figure 2 The post-weld spread area fraction was 94.39%, and the post-weld void rate was 0.77%, as shown in Tables 1 and 2. Figure 4 The image shows the post-weld spread of the alloy weld sheet prepared in this embodiment. It can be seen that the surface of the weld sheet is smooth and wrinkle-free, with low post-weld void ratio and good spreadability. Example 3

[0033] Spherical alloy powder with a diameter of 40~50μm, consisting of SnIn17Bi5Sb1.5Cu0.07Ni0.05, was evenly spread in a graphite layered mold. The mold was then fixed in a vacuum hot press and evacuated to a vacuum level of 1×10⁻⁶. -3 The vacuum hot press furnace was heated to 250℃, the pressure was set to 10 MPa, the heating rate was controlled at 5℃ / min, and the temperature was held for 1 hour. Then, the pressure was maintained and the furnace was cooled to room temperature. The mold was removed and opened to obtain a SnIn17Bi5Sb1.5Cu0.07Ni0.05 soft alloy thin sheet with a thickness of 100 μm and a tolerance of 5 μm. The peak temperature of the sheet was 201.5℃. The melting characteristics, post-weld void ratio, and spreading performance of the sheet were tested. The melting characteristic curve of the sheet is shown in [Figure showing...]. Figure 2 The post-weld spread area fraction was 88.4%, and the post-weld void rate was 0.88%, as shown in Tables 1 and 2. Figure 5 The image shows the post-weld spread of the alloy weld sheet prepared in this embodiment. It can be seen that the surface of the weld sheet is smooth and wrinkle-free, with low post-weld void ratio and good spreadability. Comparative Example 1

[0034] Pure indium metal with a purity of 99.999% was added to a vacuum furnace and heated to 250 °C. After holding at that temperature for 1 hour, it was cast into a mold to form an alloy ingot. The alloy ingot was then rolled into an indium sheet with a thickness of 50±30 μm. The solder sheet was tested for post-soldering void ratio and spreading performance. The spreading area fraction was 58.65%, and the post-soldering void ratio was 2.79% (see Tables 1 and 2). Figure 6 The image shows the post-weld spread of the alloy weld sheet prepared in this comparative example. It can be seen that the surface of the weld sheet is uneven and the post-weld void rate is high. Comparative Example 2

[0035] The SnIn14Bi5Sb1.5Cu0.07Ni0.05 alloy was heated to 400 ℃ in a solder melting furnace and held at that temperature for 2 hours before being cast into a mold to form an alloy ingot. The alloy ingot was then rolled into SnIn14Bi5Sb1.5Cu0.07Ni0.05 solder sheets with a thickness of 100±30 μm. The solder sheets were tested for post-soldering void ratio and spreading performance. The spreading area fraction was 64.59%, and the post-soldering void ratio was 1.48% (see Tables 1 and 2). Figure 7 The image shows the post-weld spread of the alloy weld sheet prepared in this comparative example. It can be seen that the surface of the weld sheet is uneven and the post-weld void rate is high. Comparative Example 3

[0036] The SnIn17Bi5Sb1.5Cu0.07Ni0.05 alloy was heated to 400 ℃ in a solder melting furnace and held at that temperature for 2 hours before being cast into a mold to form an alloy ingot. The alloy ingot was then rolled into SnIn17Bi5Sb1.5Cu0.07Ni0.05 solder sheets with a thickness of 100±30 μm. The solder sheets were tested for post-soldering void ratio and spreading performance. The spreading area fraction was 77.51%, and the post-soldering void ratio was 0.88% (see Tables 1 and 2). Figure 8 The image shows the post-weld spread of the alloy weld sheet prepared in this comparative example. It can be seen that the surface of the weld sheet is uneven and the post-weld void rate is high.

[0037] The melting characteristics test of the solder sheet in the above embodiment was performed using a STA449F5 weight loss calorimeter. The test sample was a 40 mg solder sheet scrap. The temperature was raised to 250°C under Ar atmosphere and then cooled to room temperature under Ar atmosphere. The heating and cooling rates were both 10 K / min.

[0038] The above embodiments and comparative examples tested the post-weld void rate and spreading performance of the solder sheets. The solder sheets were welded using formic acid vacuum reflow soldering HX-HPK. The post-weld void rate was tested using dage XD7500 X-RAY. The spreading performance of the solder sheets was quantified using spreading area fraction, and the spreading area fraction was statistically analyzed using Image J software.

[0039] Table 1. Statistical results of the spread area fraction of the welded sheet after welding

[0040] Table 2 Statistical results of void rate after welding of welded sheets

[0041] Figure 9 The statistical results of the weld sheet spreading area fraction after welding for Examples 1, 2, 3, Comparative Examples 1, 2, and 3 are shown. Figure 10 The statistics of post-weld void rates for Examples 1, 2, 3, Comparative Examples 1, 2, and 3 are shown.

[0042] As can be seen from the above embodiments and comparative examples, the method of the present invention effectively solves the shortcomings of traditional rolling forming processes, and effectively solves the problems of soft metals easily sticking to the rolls and wrinkling and surface patterns caused during the rolling process. Compared with the traditional melting and rolling process, the solder sheet prepared by the present invention has a high surface smoothness, no wrinkles, and less surface oxidation. The post-weld void ratio of the prepared indium-based soft alloy thin solder sheet can be reduced by about 50%, the solder sheet spreading area can be increased by about 1.5 times, the processing temperature is as low as 250°C, and the solder sheet performance is significantly improved, which can meet the requirements of welding high-performance server chips and IGBT high-power chips.

[0043] Unless otherwise stated, all percentages mentioned in this invention are mass percentages.

[0044] The vacuum hot press used in the method of this invention is existing technology equipment. The equipment used in the embodiment is a K-series model equipment manufactured by Zhengzhou Gongjiang Machinery Equipment Co., Ltd.

[0045] Compared to traditional single-metal smelting and rolling processes, the indium sheets and solder sheets obtained by this invention solve the problem of soft metals easily sticking to the rolls, avoid wrinkling and surface patterns caused by traditional rolling processes, and have less surface oxidation. The thickness ranges from 50 to 100 μm with a precision control of ±5 μm. The post-weld void ratio is reduced by nearly 50%, and the solder sheet spreading area is nearly 1.5 times larger. This effectively improves the difficulties in processing and forming 50-100 μm indium-based soft alloy solder sheets, and enhances the welding performance of indium sheets for chip packaging and IGBT power module welding materials. This invention features a simple process, low energy consumption, and significantly improved performance of the prepared soft thin solder sheets.

Claims

1. A method for preparing indium-based flexible solder sheets by vacuum hot pressing, characterized in that, Pure indium powder or indium-tin alloy powder is evenly spread in a graphite mold. The graphite mold is then vibrated or tapped to ensure the pure indium powder or indium-tin alloy powder is spread evenly and compacted. The graphite mold is then fixed in a vacuum hot press, and a pressure of 10 MPa is applied to the vacuum hot press, and a vacuum of 1×10⁻⁶ is drawn. -3 Pa, the heating rate is controlled within 5℃ / min; when the temperature rises to the melting point of indium or the melting temperature of indium-tin mixed alloy, maintain the pressure and hold for 0.5~1 hour, then the mold is cooled to room temperature with the furnace, the mold is removed, and pure indium solder sheet or indium-tin based multi-element alloy solder sheet is obtained. The pure indium powder is a spherical powder with a purity of 99.999% and a diameter of 25-50 μm; the prepared pure indium solder sheet has a purity of 99.999%, a thickness of 50 μm ± 5 μm, a spreading area fraction of not less than 98.76%, and a post-soldering void ratio of not more than 0.68%; The indium-tin mixed alloy powder is a mixture of Sn48In52 eutectic alloy powder, Sn42Bi58 eutectic alloy powder, SnSb5 master alloy powder, SnCu2 master alloy powder, and SnNi0.5 master alloy powder, with a particle size of 25-50 μm; the prepared indium-tin based multi-element alloy solder sheet has the composition of SnIn14Bi5Sb1.5Cu0.07Ni0.05 or SnIn17Bi5Sb1.5Cu0.07Ni0. 05, wherein the SnIn14Bi5Sb1.5Cu0.07Ni0.05 alloy weld sheet has a melting point of 199.2℃, a post-weld spreading area fraction of not less than 94.39%, and a post-weld void ratio of not more than 0.77%; the SnIn17Bi5Sb1.5Cu0.07Ni0.05 alloy weld sheet has a melting point of 201.5℃, a post-weld spreading area fraction of not less than 88.4%, and a post-weld void ratio of not more than 0.88%; the thickness of the alloy weld sheet is 100 μm ± 5 μm.

2. The method for preparing indium-based flexible solder sheets by vacuum hot pressing as described in claim 1, characterized in that, The graphite mold is a high-purity graphite layered mold, including a base (1) with a concave cavity (1a), a mold top cover (3), and a middle layer gasket frame (2) with adjustable thickness located between the top cover and the base; the upper opening of the concave cavity of the base has a flange (1b), the middle layer gasket frame (2) is a frame that can be fitted over the flange (1b), and the top cover (3) has a pressure head (3a) that can pass through the middle layer gasket frame and be inserted into the concave cavity (1a), the pressure head being tightly fitted to the four walls of the concave cavity; the surface roughness of the graphite mold is Ra 0.1μm or less.

3. The method for preparing indium-based flexible solder sheets by vacuum hot pressing as described in claim 2, characterized in that, The inner cavity of the graphite mold has a length of 20 cm and a width of 15 cm, with a dimensional accuracy within 0.005 mm.

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