A cladding kovar cover plate of gold-tin alloy and a method for manufacturing the same

Abstract

This invention discloses a Kovar cover plate clad with a gold-tin alloy and its preparation method, relating to the field of electronic packaging technology. The preparation method includes the following steps: S1. Metallizing the surface of a Kovar cover plate to obtain a metallized cover plate; S2. Mechanically stamping gold-tin solder to obtain gold-tin solder sheets; S3. Placing the gold-tin solder sheets on the metallized cover plate to obtain a cover plate with gold-tin inclusions; S4. Placing the cover plate with gold-tin inclusions under an infrared laser welding machine for laser cladding to obtain a Kovar cover plate clad with a gold-tin alloy. This invention provides a Kovar cover plate clad with a gold-tin alloy and its preparation method, avoiding the problem of long cladding time in tunnel furnaces, avoiding the impact of prolonged solder molten state on the accuracy of subsequent welding and positioning with the casing, and avoiding the problem of increased welding temperature or poor welding due to excessive gold plating caused by solder erosion.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging technology, and in particular to a Kovar cover plate clad with a gold-tin alloy and its preparation method. Background Technology

[0002] Gold-tin alloy (Au80Sn20) has a melting point of 280℃. Due to its advantages such as good hermeticity, high weld strength, strong corrosion resistance, and excellent resistance to thermal fatigue and creep, coupled with a coefficient of thermal expansion (CTE) that matches ceramics and Kovar alloys, it has become a core welding material in the field of high-reliability electronic packaging. It is widely used in optoelectronic devices, MEMS sensors, aerospace electronics, power semiconductors, and other fields, and is particularly irreplaceable in scenarios with stringent requirements for hermeticity, high-temperature resistance, and long-term reliability.

[0003] Kovar alloy (Fe54Ni29Co17) serves as a core structural component in electronic packaging. Its low coefficient of thermal expansion (CTE = 5.5-6.5 ppm / ℃) allows for thermomechanical matching with hard glass, ceramics (such as alumina and aluminum nitride), and semiconductor materials (such as silicon and gallium arsenide). This effectively suppresses interfacial stress during temperature cycling (-55℃ to 450℃), preventing package cracking or delamination failure. Simultaneously, the Kovar surface undergoes multi-layer plating (Ni / Au or Ni / Pd / Au) treatment, forming a high-strength metallurgical bond with gold-tin eutectic solder, resulting in a hermetic package (leakage rate < 1x10⁻¹⁰). -8 With its high thermal conductivity (atm·cc / s) and high thermal conductivity path (thermal conductivity = 57 W / m·K), it meets the stringent reliability requirements of optoelectronics, high-power modules, and aerospace fields. It is currently an irreplaceable key material in the field of electronic packaging.

[0004] The most common scenario currently involves placing a gold-tin solder frame on the casing, then placing a metallized Kovar cover plate on the solder pad. The casing then passes through a reflow oven, where the gold-tin is melted and metallurgically bonded to the casing and cover plate. This process requires precise placement of the solder pad and Kovar cover plate due to stringent tolerances. Currently, some solder manufacturers pre-place the gold-tin solder on the Kovar cover plate, allowing packaging companies to simply align the pre-soldered Kovar cover plate with the casing before soldering. This significantly reduces solder overflow or insufficient cover plate sealing that can occur due to positioning issues. A large portion of the current pre-placement process uses a cladding process, where mechanically punched gold-tin solder pads or frames are positioned on the Kovar cover plate using a fixture. The gold-tin is then rapidly melted and metallurgically bonded to the metallized Kovar cover plate using an atmosphere furnace or pulse heating, thus fixing the solder pad to the Kovar cover plate. The drawback of this method lies in the difficulty of controlling the reaction between the gold-tin solder and the gold plating on the Kovar cover plate. Because the solder must undergo a heating-melting-cooling process in the furnace, the reaction between the gold plating and the gold-tin solder is very rapid. This can easily lead to excessive dissolution of the gold plating into the gold-tin solder, causing the melting point of the gold-tin solder to rise and making subsequent welding to the casing difficult. Furthermore, due to the good flowability and surface tension of the molten solder, the cladding solder will exhibit an uneven arch shape, higher in the middle and lower at both ends. This can easily cause uneven solder flow and overall tilting during welding to the casing. Therefore, there is an urgent need to find a method for quickly pre-positioning the gold-tin solder onto the metallized Kovar cover plate. Summary of the Invention

[0005] To address the problems of excessive reaction between solder and plating layers (gold and nickel plating) and inconsistent flatness of the molten solder caused by high-temperature (300-330℃) cladding, this invention provides a Kovar cover plate clad with a gold-tin alloy and its preparation method.

[0006] Specifically, the following technical solutions are included:

[0007] In a first aspect, a method for preparing a Kovar cover plate clad with a gold-tin alloy is provided, comprising the following steps:

[0008] S1. The Kovar cover plate is subjected to surface metallization treatment, and a coating is prepared on the Kovar cover plate to obtain a metallized cover plate;

[0009] S2. The gold-tin solder is mechanically stamped to form a gold-tin solder sheet;

[0010] S3. Place the gold-tin solder sheet on the metallized cover plate with an offset tolerance within 20μm to obtain the cover plate with the gold-tin solder placed on it;

[0011] S4. Place the cover plate with gold and tin in an infrared laser welding machine for laser cladding to obtain a Kovar cover plate clad with gold and tin alloy.

[0012] Preferably, in step S2, the outer frame size of the gold-tin solder sheet is adapted to the metallized cover plate to be welded, and the inner frame size of the gold-tin solder sheet is adapted to the tube shell to be welded.

[0013] Preferably, step S3 involves accurately placing the gold-tin solder onto the metallized cover plate by manual placement or robotic placement.

[0014] Preferably, in step S4, the infrared laser welding machine is selected from one of the following: infrared femtosecond laser welding machine, infrared picosecond laser welding machine, and infrared nanosecond laser welding machine.

[0015] Among these, the lower the pulse width of the laser, the higher the energy at a single point, resulting in lower thermal effects, more concentrated energy, and better performance. In terms of effectiveness, femtosecond lasers > picosecond lasers > nanosecond lasers.

[0016] Furthermore, in step S1, the coating is one of Ni / Au coating or Ni / Pd / Au coating.

[0017] Preferably, step S4 involves placing the cover plate with the gold and tin placed on it at the focal distance of the infrared laser welding machine for laser cladding.

[0018] Preferably, the surface metallization treatment is one or more of chemical plating and electroplating.

[0019] Furthermore, the coating is a Ni / Au coating, wherein the thickness of the Ni plating layer (nickel plating layer) in the Ni / Au coating is 1-8 μm, and the thickness of the Au plating layer (gold plating layer) in the Ni / Au coating is 0.1-3 μm.

[0020] The nickel plating layer is disposed between the Kovar cover plate and the gold plating layer.

[0021] Furthermore, in step S2, the gold-tin solder is made of the following components by weight percentage: Au 70%-80%, Sn 20%-30%.

[0022] Furthermore, in step S4, the infrared laser welding machine operates in either pulse mode or continuous mode.

[0023] Preferably, the infrared laser welding machine operates in continuous mode.

[0024] Furthermore, in step S4, the infrared laser welding machine is set with the following parameters: the welding coverage area is set to be 20%-25% larger than the area of ​​the metallized cover plate.

[0025] Preferably, the settings parameters for the infrared laser welding machine also include setting the graphic to a rectangle.

[0026] Furthermore, in step S4, the infrared laser welding machine is set with the following parameters: the welding mode is set to fill welding, the welding path is set to parallel lines, and the spacing is set to 0.02mm-0.05mm.

[0027] Furthermore, in step S4, the infrared laser welding machine is set with the following parameters: average laser power of 50-60W, laser power set to 40%-50% of average laser power, pulse frequency of 300kHz-350kHz, travel speed of 2000mm / s-2500mm / s, and pulse width of 5ns-8ns.

[0028] Furthermore, in step S4, nitrogen gas is used for purging while laser cladding is being performed.

[0029] In particular, the use of nitrogen (high-purity nitrogen) for purging during the laser cladding process can greatly reduce the oxidation of the solder surface. At the same time, the nitrogen purging accelerates the solidification of the solder and prevents the solder from reacting excessively with the plating layer.

[0030] In a second aspect, a Kovar cover plate clad with a gold-tin alloy is provided, which is prepared by the method for preparing a Kovar cover plate clad with a gold-tin alloy as described in the first aspect.

[0031] The beneficial effects of this invention are as follows:

[0032] Laser cladding is performed on a cover plate with gold and solder placed on it. High energy is applied instantaneously to the solder surface by a laser, rapidly bonding the gold and solder to the gold plating on the Kovar cover plate. Simultaneously, the surface solidifies quickly, cladding the gold and solder onto the Kovar cover plate. The advantages are: First, it avoids the problem of long cladding times in atmosphere furnaces, which can lead to violent reactions between the solder and the gold plating, resulting in excessive internal IMC (intermetallic compounds, mainly (NiAu)Sn compounds), causing significant brittleness in the solder. The growth of IMC in the solder also increases the melting point. Second, it avoids the solder spontaneously flowing into an arched shape (high in the middle, low at the sides) due to its fluidity and surface tension during prolonged molten state, which can affect the accuracy of subsequent welding to the casing. Third, it avoids the gold and solder shifting from their eutectic composition due to excessive gold plating erosion, which can lead to increased welding temperatures or poor welding. Attached Figure Description

[0033] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a microstructure diagram of the Kovar cap plate clad with gold-tin alloy prepared by the preparation method in Example 1 of the present invention.

[0035] Figure 2 This is a diagram showing the morphology of the solder obtained by the preparation method in Example 1 of the present invention;

[0036] Figure 3 This is a microstructure diagram of the Kovar cover plate clad with gold-tin alloy prepared by the preparation method of Comparative Example 1 of the present invention.

[0037] Figure 4 This is a diagram showing the morphology of the solder prepared by the method of Comparative Example 1 of the present invention. Detailed Implementation

[0038] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] It should be understood that, when used in this specification and the appended claims, the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0040] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0041] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0042] To better understand the technical content of the present invention, the technical solution of the present invention will be further introduced and explained below with reference to specific embodiments.

[0043] Example 1

[0044] A method for preparing a Kovar cover plate clad with a gold-tin alloy includes the following steps:

[0045] S1. A Kovar cover plate (size: 10mm*10mm*0.25mm) is subjected to surface metallization treatment. A coating is prepared on the Kovar cover plate, wherein the coating is a Ni / Au coating, resulting in a metallized cover plate. The surface metallization treatment is performed by chemical plating to prepare the Ni layer and by electroplating to prepare the Au layer. The thickness of the Ni layer (nickel plating layer) in the Ni / Au coating is 3μm, and the thickness of the Au layer (gold plating layer) in the Ni / Au coating is 0.3μm. The nickel plating layer is disposed between the Kovar cover plate and the gold plating layer.

[0046] S2. The gold-tin solder (made from the following components by weight percentage: Au 80%, Sn 20%) is mechanically stamped to produce a gold-tin solder sheet (outer frame size 10mm*10mm*0.05mm, inner frame size 8mm*8mm*0.05mm). The outer frame size of the gold-tin solder sheet is adapted to the metallized cover plate to be welded, and the inner frame size of the gold-tin solder sheet is adapted to the tube shell to be welded.

[0047] S3. Using a robotic arm (precision fixture), place the gold-tin solder sheet onto the metallized cover plate with an offset tolerance within 20μm to obtain the cover plate with the gold-tin solder placed on it.

[0048] S4. Place the gold-tin cap plate at the focal distance of the infrared picosecond laser welding machine for laser cladding. At the same time, use high-purity nitrogen (purity of 99.999%) as a protective gas for purging at a flow rate of 20L / min to cover the welding area and obtain the Kovar cap plate with gold-tin alloy cladding.

[0049] The infrared picosecond laser welding machine operates in continuous wave (CW) mode. Its settings are as follows: the welding pattern is set to rectangle, and the welding coverage area is set to be greater than 25% of the metallized cover plate area (ensuring complete coverage). The welding mode is set to fill welding, the welding path is parallel lines with a spacing of 0.02mm, the laser power is 40% (i.e., 24W, which is 40% of the equipment's average power of 60W), the pulse frequency is 300kHz, the travel speed is 2000mm / s, and the pulse width is 5ns.

[0050] The laser cladding process for the Kovar cap plate clad with gold-tin alloy in Example 1 takes only about 0.5 seconds. The microstructure of the Kovar cap plate clad with gold-tin alloy obtained by the method in Example 1 is shown in the figure below. Figure 1As shown, the solder and the gold plating layer rapidly form a metallurgical bond. A thin and uniform (Ni,Au)SnIMC layer (approximately 1 μm thick) is formed at the interface.

[0051] The solder morphology obtained by the preparation method in Example 1 is shown in the figure below. Figure 2 As shown, the weld shape remained well after cladding, and the weld frame edges were clear. The surface flatness was high, with the maximum arch height difference ≤5μm. No obvious flow deformation occurred.

[0052] In addition, the composition and melting point of the solder after cladding were tested. The test results showed that the overall composition of the solder changed very little, and the measured melting point remained close to the eutectic point (280℃) of Au80Sn20. Subsequent welding compatibility was tested, and the results are as follows: After the Kovar cap plate with the gold-tin alloy cladding was aligned with the tube shell, conventional reflow soldering was performed. The molten solder flowed evenly, forming a high-quality sealing ring without tilting or overflow.

[0053] Comparative Example 1

[0054] Comparative Example 1 provides a method for preparing a Kovar cover plate clad with a gold-tin alloy. Comparative Example 1 uses a conventional cladding process to pre-place gold-tin solder to prepare the Kovar cover plate, specifically including the following steps: pre-placing gold-tin solder using a conventional cladding process to obtain gold-tin solder sheets; placing the metallized cover plate obtained in step S1 of Example 1 into a dedicated graphite fixture; and placing the gold-tin solder sheets from Comparative Example 1 onto the metallized cover plate to obtain the assembly.

[0055] Place the components in an atmosphere furnace with nitrogen (99.99% purity) and set the following cladding parameters:

[0056] The temperature was increased to 310°C at a rate of 10°C / s (30°C above the eutectic point). The temperature was then held at 320°C for 60 seconds (until the solder was completely melted). The temperature was then cooled to room temperature (25°C) at a rate of 5°C / s to obtain the Kovar cover plate of the gold-tin alloy clad in Comparative Example 1.

[0057] The cladding process of the Kovar cap plate clad with gold-tin alloy in Comparative Example 1 lasted longer than 120 seconds. The microstructure of the Kovar cap plate clad with gold-tin alloy obtained by the method in Comparative Example 1 is shown in the figure below. Figure 3 As shown, the solder reacts violently with the gold plating. A thick and uneven (Ni,Au)SnIMC layer (4-10 μm thick) forms at the interface. The Au plating layer is almost completely dissolved and consumed. Coarse primary AuSn phases are clearly visible inside the solder.

[0058] The solder morphology diagram prepared by the method in Comparative Example 1 is shown below. Figure 4 As shown, due to the surface tension and fluidity of the solder in a prolonged molten state, it forms a distinct left-right arch shape, with a height difference of 15-20 μm between the center and the edge. The edges of the solder frame are blurred.

[0059] In addition, the composition and melting point of the solder after cladding were tested. The test results showed that the Au content in the solder increased significantly, while the Sn content decreased (partially diffused into the matrix). The measured melting point rose to 290-305℃.

[0060] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing a Kovar cover plate clad with a gold-tin alloy, characterized in that, Includes the following steps: S1. The Kovar cover plate is subjected to surface metallization treatment, and a coating is prepared on the Kovar cover plate to obtain a metallized cover plate; S2. The gold-tin solder is mechanically stamped to form a gold-tin solder sheet; S3. Place the gold-tin solder sheet on the metallized cover plate with an offset tolerance within 20μm to obtain the cover plate with the gold-tin solder placed on it; S4. Place the gold-tin cap plate under an infrared laser welding machine for laser cladding to obtain a Kovar cap plate clad with gold-tin alloy; the setting parameters of the infrared laser welding machine are as follows: average laser power is 50-60W, the laser power is set to 40%-50% of the average laser power, the pulse frequency is 300kHz-350kHz, the travel speed is 2000mm / s-2500mm / s, and the pulse width is 5ns-8ns; the infrared laser welding machine is selected from one of infrared femtosecond laser welding machines, infrared picosecond laser welding machines, and infrared nanosecond laser welding machines.

2. The method for preparing the Kovar cover plate clad with gold-tin alloy as described in claim 1, characterized in that, In step S1, the coating is one of Ni / Au coating or Ni / Pd / Au coating.

3. The method for preparing the Kovar cover plate clad with gold-tin alloy as described in claim 2, characterized in that, The coating is a Ni / Au coating, wherein the thickness of the Ni layer in the Ni / Au coating is 1-8 μm, and the thickness of the Au layer in the Ni / Au coating is 0.1-3 μm.

4. The method for preparing the Kovar cover plate clad with gold-tin alloy as described in claim 1, characterized in that, In step S2, the gold-tin solder is made of the following components by weight percentage: Au 70%-80%, Sn 20%-30%.

5. The method for preparing the Kovar cover plate clad with gold-tin alloy as described in claim 1, characterized in that, In step S4, the infrared laser welding machine operates in either pulse mode or continuous mode.

6. The method for preparing the Kovar cover plate clad with gold-tin alloy as described in claim 1, characterized in that, In step S4, the infrared laser welding machine is set with the following parameters: the welding coverage area is set to be 20%-25% larger than the area of ​​the metallized cover plate.

7. The method for preparing the Kovar cover plate clad with gold-tin alloy as described in claim 1, characterized in that, In step S4, the infrared laser welding machine is set with the following parameters: the welding mode is set to fill welding, the welding path is set to parallel lines, and the spacing is set to 0.02mm-0.05mm.

8. The method for preparing the Kovar cover plate clad with gold-tin alloy as described in claim 1, characterized in that, In step S4, nitrogen gas is used for purging while laser cladding is being performed.

9. A Kovar cover plate clad with a gold-tin alloy, characterized in that, The Kovar cover plate clad with gold-tin alloy as described in any one of claims 1-8 is prepared by the method for preparing the Kovar cover plate.

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