Kovar cover plate cladded with gold-tin alloy and preparation method of Kovar cover plate

Abstract

The invention discloses a Kovar cover plate cladded with gold-tin alloy and a preparation method of the Kovar cover plate, and relates to the technical field of electronic packaging. The preparation method comprises the following steps: S1, carrying out surface metallization treatment on a kovar cover plate to obtain a metallized cover plate; s2, the gold-tin solder is formed through mechanical punching, and a gold-tin soldering sheet is prepared; s3, the gold-tin soldering sheet is placed on the metalized cover plate, and a cover plate with gold and tin placed is obtained; and S4, the cover plate with the placed gold and tin is placed under an infrared laser welding machine to be subjected to laser cladding, and the Kovar cover plate cladded with the gold and tin alloy is prepared. According to the Kovar cover plate cladded with the gold-tin alloy and the preparation method of the Kovar cover plate, the problem that the cladding time is long when a tunnel furnace is used for heating is solved, and the influence of long-time welding flux in a molten state on the accuracy of subsequent welding and positioning with a tube shell is avoided; and the problem of subsequent welding temperature rise or poor welding caused by the welding flux and too much gold-plated layer erosion is avoided.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electronic packaging, in particular to a cladding gold-tin alloy kovar cover plate and a preparation method thereof. BACKGROUND

[0002] Gold-tin alloy (Au80Sn20) has a melting point of 280℃. Due to its good airtightness, high welding strength, strong corrosion resistance, good thermal fatigue and creep resistance, and matching thermal expansion coefficient (CTE) with ceramics and kovar alloy, it becomes the 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, especially in scenes with strict requirements on airtightness, high temperature resistance and long-term reliability.

[0003] Kovar alloy (Kovar, Fe54Ni29Co17) as the core structural part of electronic packaging, its role is to form a thermal mechanical match with hard glass, ceramics (such as aluminum oxide, aluminum nitride) and semiconductor materials such as silicon and gallium arsenide through a low coefficient of thermal expansion (CTE = 5.5-6.5 ppm / ℃), effectively inhibiting interfacial stress in temperature cycling (-55℃~450℃), avoiding packaging cracking or delamination failure. At the same time, the kovar surface is treated by multi-layer plating (Ni / Au or Ni / Pd / Au), and forms a high-strength metallurgical bond with gold-tin eutectic solder, forming an airtight package (leakage rate <1x10 -8 atm·cc / s) and a high thermal conductivity path (thermal conductivity = 57 W / m·K), meeting the harsh requirements of packaging reliability in the fields of optoelectronics, high-power modules and aerospace. It is currently an irreplaceable key material in the field of electronic packaging.

[0004] The most commonly used scene at present is to place a gold tin solder frame on the shell, and then place the metalized Kovar cover plate on the soldering sheet. The shell is passed through the reflow furnace, the gold tin is melted, and then the shell and the cover plate are metallurgically combined. This process has strict tolerances for the placement position of the soldering sheet and the Kovar cover plate, and generally uses a precision manipulator for accurate placement. Currently, some solder production enterprises will pre-locate the gold tin solder on the Kovar cover plate, and the packaging enterprise only needs to place the pre-located solder on the Kovar cover plate and the shell once, and then weld. Greatly reduces the possibility of solder overflow or insufficient cover plate sealing due to positioning problems. At present, a large part of the pre-locating process uses cladding technology. The gold tin soldering sheet or solder frame formed by mechanical punching is positioned and placed on the Kovar cover plate through a jig, and the gold tin is rapidly melted and metallurgically combined with the metalized Kovar cover plate through an atmosphere furnace or pulse heating, so as to fix the soldering sheet and the Kovar cover plate. The defect of this method is that the reaction of gold tin solder with the gold plated layer on the Kovar cover plate is difficult to control. Because the solder must go through the process of heating-melting-cooling in the furnace, the reaction of the gold plated layer with the gold tin solder is very rapid, which easily leads to excessive dissolution of the gold plated layer into the gold tin solder, thereby increasing the melting point of the gold tin solder and causing difficulties in subsequent welding with the shell. At the same time, because the molten solder has good flowability and surface tension, the solder after cladding will present an uneven arch shape with high middle and low sides, which is easy to cause uneven flow of solder during welding with the shell, resulting in an overall tilt effect. Therefore, it is urgent to seek a method for quickly pre-locating gold tin solder on a metalized Kovar cover plate. SUMMARY

[0005] To solve the problems of excessive reaction of solder with plated layer (gold plated layer and nickel plated layer) and inconsistent flatness of molten solder due to high temperature (300-330℃) cladding, the present application provides a Kovar cover plate cladded with gold tin alloy and a preparation method thereof.

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

[0007] In a first aspect, a preparation method of a Kovar cover plate cladded with gold tin alloy is provided, comprising the following steps:

[0008] S1. Surface metallization treatment is performed on the Kovar cover plate to prepare a plated layer on the Kovar cover plate, obtaining a metalized cover plate;

[0009] S2. Gold tin solder is formed by mechanical punching to obtain a gold tin soldering sheet;

[0010] S3. The gold tin soldering sheet is placed on the metalized cover plate with a tolerance offset of 20μm or less, obtaining a cover plate with gold tin placed;

[0011] S4. The cover plate with gold tin placed is placed under the infrared laser welding machine for laser cladding to obtain the kovar cover plate with gold tin alloy cladding.

[0012] Preferably, in step S2, the outer frame size of the gold tin soldering sheet is adapted to the metalized cover plate to be soldered, and the inner frame size of the gold tin soldering sheet is adapted to the tube shell to be soldered.

[0013] Preferably, in step S3, the gold tin solder is accurately placed on the metalized cover plate by manual placement or mechanical hand placement.

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

[0015] The lower the pulse width of the laser, the higher the energy of a single point, the lower the heat effect, the more concentrated the energy, and the better the effect. That is, in terms of effect, femtosecond laser > picosecond laser > nanosecond laser.

[0016] Further, in step S1, the plating layer is one of a Ni / Au plating layer and a Ni / Pd / Au plating layer.

[0017] Preferably, in step S4, the cover plate with gold tin placed is placed under the focusing distance of the infrared laser welding machine for laser cladding.

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

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

[0020] The plated nickel layer is arranged between the kovar cover plate and the gold plating layer.

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

[0022] Further, in step S4, the working mode of the infrared laser welding machine is pulse mode or continuous mode.

[0023] Preferably, the working mode of the infrared laser welding machine is continuous mode.

[0024] Further, in step S4, the setting parameters of the infrared laser welding machine are as follows: the coverage area for welding is set to be greater than 20%-25% of the area of the metalized cover plate.

[0025] Preferably, the setting parameters of the infrared laser welding machine further include: the setting pattern is a rectangle.

[0026] Further, in step S4, the setting parameters of the infrared laser welding machine are as follows: the welding mode is set as filling welding, the welding running pattern is a parallel line, and the interval is set as 0.02mm-0.05mm.

[0027] Further, in step S4, the setting parameters of the infrared laser welding machine are as follows: the laser average power is 50-60W, the setting laser power is 40%-50% of the laser average power, the pulse frequency is 300kHZ-350kHZ, the running speed is 2000mm / s-2500mm / s, and the pulse width is 5ns-8ns.

[0028] Further, in step S4, laser cladding is performed while using nitrogen to perform purging.

[0029] In the laser cladding process, nitrogen (high-purity nitrogen) is used for purging, which can greatly reduce the oxidation of the solder surface, and the purging of nitrogen can accelerate the solidification of the solder and prevent excessive reaction of the solder with the gold plating layer.

[0030] In a second aspect, a cladding gold-tin alloy Kovar cover plate is provided, which is prepared by the preparation method of the cladding gold-tin alloy Kovar cover plate according to the first aspect.

[0031] The present application has the following advantages:

[0032] The gold-tin solder is placed on the cover plate and laser cladding is performed, the high energy applied by the laser on the surface of the solder is used to quickly metallurgically combine the gold-tin solder with the gold plating layer on the Kovar cover plate, and the surface is quickly solidified, thereby cladding the gold-tin solder on the Kovar cover plate, which has the following advantages: 1. The problem of long heating and cladding time in an atmosphere furnace is avoided, which causes the solder to react violently with the gold plating layer, excessive internal IMC (IMC: intermetallic compound, mainly (NiAu) Sn compound) is caused, the solder has high brittleness, and the growth of IMC in the solder also increases the melting point of the solder. 2. The solder will spontaneously flow into an arch shape with high middle and low sides due to the influence of solder flowability and surface tension in the molten state for a long time, which affects the positioning accuracy of subsequent welding to the shell. 3. The gold-tin solder deviates from the eutectic point composition due to excessive reaction of the solder with the gold plating layer, which causes the subsequent welding temperature to rise or the welding to be poor. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed in the embodiment description. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort.

[0034] Figure 1 Microstructure diagram of the cladding gold-tin alloy kovar cover plate prepared by the preparation method of the embodiment 1 of the present application;

[0035] Figure 2 Solder morphology diagram prepared by the preparation method of the embodiment 1 of the present application;

[0036] Figure 3 Microstructure diagram of the cladding gold-tin alloy kovar cover plate prepared by the preparation method of the comparative example 1 of the present application;

[0037] Figure 4 Solder morphology diagram prepared by the preparation method of the comparative example 1 of the present application. DETAILED DESCRIPTION

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

[0039] It should be understood that the terms “include” and “contain” as used in the specification and the appended claims indicate the presence of the described features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or collections thereof.

[0040] It should also be understood that the terms used in the present application specification are only for the purpose of describing specific embodiments and are not intended to limit the present application. As used in the present application 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 be further understood that the term “and / or” as used in the present application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations, and includes these combinations.

[0042] In order to more fully understand the technical content of the present application, the technical solutions of the present application will be further introduced and explained in combination with specific embodiments.

[0043] Example 1

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

[0045] S1. Surface metallization treatment is performed on a Kovar cover plate (size: 10 mm*10 mm*0.25 mm) to prepare a plating layer thereon, the plating layer being a Ni / Au plating layer, to obtain a metallized cover plate; wherein the surface metallization treatment is performed by chemical plating to prepare a plating Ni layer and by electroplating to prepare a plating Au layer, the plating Ni layer (plating nickel layer) in the Ni / Au plating layer having a thickness of 3 μm, and the plating Au layer (plating gold layer) in the Ni / Au plating layer having a thickness of 0.3 μm; the plating nickel layer is arranged between the Kovar cover plate and the plating gold layer;

[0046] S2. Gold-tin solder (made of the following components by weight percentage: Au 80%, Sn 20%) is mechanically punched to form a gold-tin solder piece (outer frame size: 10 mm*10 mm*0.05 mm, inner frame size: 8 mm*8 mm*0.05 mm), the outer frame size of the gold-tin solder piece being adapted to the metallized cover plate to be soldered, and the inner frame size of the gold-tin solder piece being adapted to the tube shell to be soldered;

[0047] S3. The gold-tin solder piece is placed on the metallized cover plate by a mechanical hand placement method (using a precision clamp), with a tolerance of 20 μm or less, to obtain a cover plate with the gold-tin solder piece placed thereon;

[0048] S4. The cover plate with the gold-tin solder piece placed thereon is placed under the focal distance of an infrared picosecond laser welding machine to perform laser cladding, and high-purity nitrogen gas (purity: 99.999%) is used as a protective gas to perform purging at a flow rate of 20 L / min, to cover the soldering area to obtain a Kovar cover plate with gold-tin alloy cladding.

[0049] The working mode of the infrared picosecond laser welding machine is continuous mode (CW); the setting parameters of the infrared picosecond laser welding machine are as follows: the welding pattern is set to be rectangular, and the coverage area of the welding is set to be greater than 25% of the area of the metallized cover plate (to ensure complete coverage). The welding mode is set to be fill welding (Fill), the welding travel pattern is set to be parallel lines, the interval is set to be 0.02 mm, the laser power is set to be 40% (i.e. 24 W, which is 40% of the average power of the equipment, 60 W), the pulse frequency is set to be 300 kHZ, the travel speed is set to be 2000 mm / s, and the pulse width is set to be 5 ns.

[0050] The laser cladding process of the method for preparing a Kovar cover plate with gold-tin alloy cladding of Example 1 only takes about 0.5 seconds. The microstructure diagram of the Kovar cover plate with gold-tin alloy cladding prepared by the method of Example 1 is as shown in FIG. 1. 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 solder composition and melting point after cladding are detected, and the test results show that the Au content in the solder is significantly increased, and the Sn content is decreased (partially diffused into the matrix). The measured melting point is increased to 290-305°C.

[0060] The above is only a 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 various equivalent modifications or replacements within the technical range disclosed by the present application, and these modifications or replacements 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 making a cladded Kovar cover plate of a gold-tin alloy, characterized by, The method comprises the following steps: S1. Surface metallization treatment is performed on the Kovar cover plate to prepare a plated layer on the Kovar cover plate, thereby obtaining a metallized cover plate; S2. Gold-tin solder is formed by mechanical punching to obtain a gold-tin solder piece; S3. The gold-tin solder piece is placed on the metallized cover plate, with a tolerance of 20 μm or less, thereby obtaining a cover plate with gold-tin placed thereon; S4. The cover plate with gold-tin placed thereon is placed under an infrared laser welding machine to perform laser cladding, thereby obtaining a Kovar cover plate with gold-tin alloy cladded thereon.

2. The method of claim 1, wherein the overlaying of the Kovar cover plate with the gold-tin alloy is performed by, In step S1, the plated layer is one of a Ni / Au plated layer and a Ni / Pd / Au plated layer.

3. The method of claim 2, wherein the overlaying of the Kovar cover plate with the gold-tin alloy is performed by, In the Ni / Au plated layer, the thickness of the plated Ni layer is 1-8 μm, and the thickness of the plated Au layer is 0.1-3 μm.

4. The method of claim 1, wherein the overlaying of the Kovar cover plate with the gold-tin alloy is performed by, In step S2, the gold-tin solder is made of the following components by weight percentage: Au 70%-80% and Sn 20%-30%.

5. The method of claim 1 wherein the overlaying of the Kovar cover plate with the gold-tin alloy is performed by, In step S4, the working mode of the infrared laser welding machine is pulse mode or continuous mode.

6. The method of claim 1, wherein the overlaying of the Kovar cover plate with the gold-tin alloy is performed by, In step S4, the setting parameters of the infrared laser welding machine are as follows: the coverage area of welding is set to be greater than 20%-25% of the area of the metallized cover plate.

7. The method of claim 1, wherein the overlaying of the Kovar cover plate with the gold-tin alloy is performed by, In step S4, the setting parameters of the infrared laser welding machine are as follows: the welding mode is set to be filling welding, the welding travel pattern is parallel line, and the interval is set to be 0.02-0.05 mm.

8. The method of claim 1, wherein the overlaying of the Kovar cover plate with the gold-tin alloy is performed by, In step S4, the setting parameters of the infrared laser welding machine are as follows: the average laser power is 50-60 W, the laser power is set to be 40%-50% of the average laser power, the pulse frequency is 300-350 kHz, the travel speed is 2000-2500 mm / s, and the pulse width is 5-8 ns.

9. The method of claim 1 wherein the overlaying of the Kovar cover plate with the gold-tin alloy is accomplished by, In step S4, nitrogen is used for purging while laser cladding is performed.

10. A cladded Kovar cover plate of gold-tin alloy characterized in that, The method is prepared by the method for preparing the Kovar cover plate with gold-tin alloy cladded thereon according to any one of claims 1-9.

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