Preparation method for preset gold-tin soldering ring of cover plate for ultrafast laser processing

Abstract

The invention provides a method for preparing a preset gold-tin soldering ring of a cover plate by using ultrafast laser processing, which comprises the following steps: S1, constructing a surface microstructure of the cover plate by using the ultrafast laser: selecting the cover plate with a specified size, processing by using the ultrafast laser, constructing the microstructure on the surface of the cover plate under nitrogen / argon mixed protective gas, and forming the microstructure on the surface of the cover plate by using the ultrafast laser; then, surface metallization treatment is completed through a multi-stage plating process, and a composite plating layer is formed; and S2, vacuum hot-pressing pre-melting of a welding ring: stamping a gold-tin alloy to form a sheet-shaped welding ring, and then pre-melting the gold-tin welding ring on the cover plate with the microstructure obtained in the step S1 by adopting a vacuum hot-pressing backflow process, so as to form the cover plate with the preset gold-tin welding ring. According to the invention, the assembly process of the cover plate and the gold-tin soldering ring can be simplified, and solder overflow and cover climbing during packaging of the cover plate and the substrate are inhibited.

Description

Technical Field

[0001] The present invention belongs to the technical field of precision manufacturing and electronic packaging, and particularly relates to a preparation method of a cover plate pre-placed gold-tin solder ring by ultrafast laser processing. Background Art

[0002] In the packaging of high-power devices and optoelectronic devices, gold-tin solder rings are often used for the packaging between the cover plate and the substrate (device). Traditional pre-placed cover plates often use screen printing, electroplating solder layers, multiple chip placements, and four-corner spot welding methods to pre-place gold-tin solder on the gold-plated cover plate. In addition to introducing flux organic matter that affects the airtightness of the packaging, multiple chip placements or spot welding positioning also bring problems such as inaccurate assembly positioning, welding misalignment, solder overflow, and solder climbing on the cover plate. Additionally, if fixtures are used to improve the assembly alignment accuracy, it will increase the process complexity and weaken the mass production capacity.

[0003] In order to simplify the packaging process, a cover plate with a pre-placed gold-tin solder ring has been developed, which can be directly assembled and packaged with the substrate, eliminating the step of mounting the gold-tin solder ring. Patent Document 1 (CN115302130B) records the precise assembly and pre-placement process of the cover plate and the solder ring through a grooved positioning mold, but it still cannot solve the problem of solder overflow and climbing on the gold-plated layer of the cover plate during the welding process between the pre-placed cover plate and the substrate. Patent Document 2 (CN118714785A) records a gold-tin solder cover plate with a locally non-gold-plated layer in the middle of one side, which hinders the spreading and overflow of solder along the gold-plated layer during the welding of the cover plate and the substrate by setting a "gold-free ring area" in the center of the cover plate. However, the "gold-free ring area" of this technology needs to be processed additionally. For example, after the entire area of the cover plate is gold-plated, additional laser processing or CNC processing technology is required to precisely remove gold locally in a ring shape on the cover plate. This not only wastes the precious metal coating but also reduces the strength of the pre-placed cover plate.

[0004] Therefore, there is an urgent need to design a preparation method that can precisely pre-place the gold-tin solder ring, control solder overflow during the welding process between the pre-placed cover plate and the substrate, and enhance the strength of the pre-placed cover plate. Summary of the Invention

[0005] In response to the above problems, the inventors have repeatedly and carefully studied and found that: an integrated preparation method of a cover plate pre-placed gold-tin solder ring with high precision, low thermal damage, high strength, and excellent welding performance, which solves the problems of low alignment accuracy, welding misalignment, solder overflow, and solder climbing during the preparation by traditional processes, and increases the strength of the pre-placed cover plate, thereby meeting the requirements of high service reliability and high-precision packaging.

[0006] The present invention adopts the following technical solutions:

[0007] A preparation method of a cover plate pre - deposited gold - tin solder ring for ultrafast laser processing, comprising the following steps:

[0008] S1. Constructing the surface microstructure of the cover plate by ultrafast laser:

[0009] Select a cover plate of a specified size, process it with an ultrafast laser, construct a microstructure on the surface of the cover plate under a mixed protective gas of nitrogen / argon, clean the processed cover plate using ultrasonic and plasma cleaning processes, and then complete the surface metallization treatment by a multi - stage plating process to form a composite coating;

[0010] S2. Vacuum hot - pressing and reflow pre - melting the solder ring:

[0011] Stamp the gold - tin alloy into a sheet - shaped solder ring so that the size of the solder ring matches the cover plate, and then use the vacuum hot - pressing and reflow process to pre - melt the gold - tin solder ring on the cover plate with the microstructure obtained in step S1, thereby forming a cover plate with a pre - deposited gold - tin solder ring.

[0012] Among them, the cover plate material described in step 1 is selected from one of molybdenum - copper alloy, tungsten - copper alloy, kovar alloy or ceramic; the ceramic is alumina ceramic.

[0013] In step 1, the ultrafast laser is a femtosecond or picosecond - level pulsed laser, and the laser wavelength requirement is selected according to different cover plate materials to be processed. When it is an alloy - type material, the 1064nm wavelength is selected, and when it is a ceramic - type material, the 515nm wavelength is selected; preferably, a laser beam with a pulse width < 10ps is used for spiral progressive scanning to form the surface microstructure;

[0014] The shape of the microstructure described in step 1 is one of "valley - bump" shape, "thimble" shape or "wave" shape, and the arrangement mode of the microstructure is one of array - type, horizontal or vertical arrangement type, circular arrangement type and random arrangement type. The depth - to - width ratio of the microstructure on the cover plate is less than 5:1.

[0015] Among them, the depth of the valley of the "valley - bump" - shaped microstructure is required to be 30 - 50μm, the distance between two bumps is 30 - 75μm, the width of the bump is 30 - 50μm, and the inclination angle of the bump wall is required to be > 85°.

[0016] In step 1, the cover plate with the formed microstructure is cleaned by ultrasonic and plasma compound cleaning. During ultrasonic cleaning, anhydrous ethanol or dilute hydrofluoric acid solution (concentration 0.1% - 2%) is used, and during plasma cleaning, argon and oxygen mixed in a ratio of 4:1 are used.

[0017] In Step 1, multi-stage plating of the cleaned cover plate is carried out by means of magnetron sputtering or electrochemical deposition; at this time, different coating preparation methods need to be selected according to the aspect ratio of the microstructure. When the aspect ratio < 5:1, magnetron sputtering plating or electrochemical deposition can be selected, and when 5:1 ≤ aspect ratio < 10:1, electrochemical deposition is adopted.

[0018] Among them, the material of the AuSn solder ring described in Step 2 is Au20Sn alloy foil.

[0019] In Step 2, the requirement for the difference between the inner and outer frames of the solder ring size is 1 - 2 mm, and the requirement for the solder ring thickness is 1.2 - 1.5 times the depth of the microstructure of the cover plate.

[0020] In Step 2, the pre-setting process adopts vacuum hot press reflow soldering, and the welding vacuum degree is not less than 10 -3 Pa, and two-stage heating rates are set. Pressure is applied at the peak reflow temperature zone, and the applied pressure is 0.3 - 0.5 MPa. In addition, the cooling rate requirement is the same as the heating rate.

[0021] The two-stage heating rates are as follows: the heating rate in the first stage is controlled at 30 °C / min until it reaches 150 °C and then holds for 1.5 min. The second stage heats up at a rate of 50 °C / min to 300 °C and starts to hold. The holding time is synchronized with and consistent with the pressure application. At this time, hot pressing is applied, the pressure is controlled at 0.3 - 0.5 MPa, and the time is controlled at 30 - 90 s.

[0022] Among them, the ratio of the "valley" depth of the microstructure of the cover plate to the solder ring thickness is controlled at 1:1.5 - 1.8.

[0023] Advantages of the present invention

[0024] The preparation method of the present invention includes two main steps: laser structuring of the surface microstructure of the cover plate and reflow pre-melting of the AuSn solder ring. First, an array of microstructures is formed on the cover plate by an ultrafast laser, and the area of the microstructure region is the size of the AuSn solder ring; the cover plate with the microstructure is first nickel-plated and then gold-plated; the AuSn alloy is rolled and stamped into an AuSn solder ring; the AuSn solder ring and the cover plate are assembled relying on the inner circle of the microstructure; the assembled cover plate-solder ring is placed in a vacuum eutectic reflow furnace for reflow pre-melting, so as to pre-place the AuSn solder ring on the cover plate to form a pre-placed cover plate. Specifically, through the ultrafast laser processing technology, the present invention proposes a method of using femtosecond or picosecond-level laser processing to construct a "valley-protrusion" shaped microstructure on the surface of the cover plate, and then using thermal compression reflow soldering to pre-melt the AuSn solder ring on the cover plate, thereby preparing a cover plate with a pre-placed AuSn solder ring. The processing process of the ultrafast pulsed laser is approximately cold processing, so no thermal stress is generated while constructing the microstructure on the surface of the cover plate material, thus protecting the phase characteristics of the cover plate material from being affected by the processing. In addition, by constructing the microstructure on the surface of the cover plate, its surface area can be increased. When pre-melting and soldering with the solder subsequently, the wettability of the liquid solder to the microstructure region is increased, promoting the metallurgical bonding between the solder and the cover plate, thereby increasing the post-welding strength of the pre-placed cover plate.

[0025] Ultrasonic and plasma cleaning after laser processing can remove the nanoscale recast layer remaining on the surface of the cover plate after processing. Ultrasonic cleaning with dilute hydrofluoric acid combined with plasma cleaning of the alloy cover plate surface can remove the recast layer of laser processing and the "amorphous" (glass state) metal nanoparticles generated by the rapid cooling of laser processing; ethanol ultrasonic combined with plasma cleaning can remove the nanoscale ceramic particles remaining on the surface of the ceramic material after laser processing. The surface of the cover plate after such cleaning is more conducive to the subsequent deposition coating step and increases the bonding force between the coating and the cover plate.

[0026] Vacuum thermal compression reflow soldering is used to pre-place the solder on the surface of the cover plate. By controlling the heating rate in two stages, the gas and incompletely volatilized ethanol cleaning agent in the cover plate and the coating before welding can be removed during the heat preservation after the first-stage heating. Pressurization after the second-stage heating can enhance the wetting rate of the liquid solder metal to the surface of the cover plate and reduce the porosity after welding.

[0027] When the pre-placed cover plate is subjected to secondary reflow soldering with the substrate, due to the existence of the microstructure, it hinders the overflow and climbing of the liquid solder to area 2, ensuring the precision and airtightness of the package. Description of the Drawings

[0028] Figure 1 Schematic diagram of ultrafast laser processing of the cover plate (left) and schematic diagram of the surface microstructure of the cover plate (right)

[0029] Figure 2Top view of the structure of the surface processing area and the unprocessed area of the pre-set cover plate (upper), schematic diagram of the packaging structure of the cover plate and the substrate (lower)

[0030] Figure 3 Cross-sectional schematic diagram of the pre-set cover plate

[0031] Reference signs

[0032] 1 Microstructure area (or solder pre-placement area) on the cover plate after ultrafast laser processing, 2 Unprocessed area, 3 Cover plate, 4 Cover plate microstructure layer, 5 Pre-set solder ring layer, 6 Packaging substrate welded to the pre-set cover plate, 7 Cover plate substrate, 8 Functional layer, 9 Nickel plating layer, 10 Gold plating layer, 11 Pre-set AuSn solder ring Specific embodiments

[0033] The following specifically describes the embodiments of the present invention through the drawings and examples. However, the examples are only an example for illustration and do not constitute a limitation on the protection scope.

[0034] The preparation method of the pre-set AuSn solder ring on the cover plate by ultrafast laser processing of the present invention specifically includes the following steps:

[0035] 1) Constructing the surface microstructure of the cover plate by ultrafast laser

[0036] For the original cover plate (mainly composed of kovar / molybdenum copper / tungsten copper / aluminum oxide ceramic), select the cover plate after stamping, and select an ultrafast laser of femtosecond or picosecond level (preferably laser pulse width < 10 ps) for processing. The heat affected zone generated by traditional nanosecond-level laser processing will cause microcracks in the substrate (processing thermal stress up to 200 - 300 MPa), and at the same time, the surface is prone to residual contaminating slag (typical contaminating layer thickness > 5 μm). While lasers above the picosecond level are approximately cold processing due to their fast processing speed and have no obvious thermal impact on the material. When the laser pulse hits the metal / ceramic surface, the pulse energy is first absorbed by electrons through the inverse bremsstrahlung mechanism, and the electrons absorb energy and quickly become hot electrons. The interaction time between the ultrashort pulse of the laser and the material is very short and close to the electron-phonon coupling time. In this way, the laser pulse has ended before heat diffusion occurs. Therefore, ultrafast laser processing has no heat accumulation and material melting, approximating "cold processing".

[0037] In addition, the ultrafast laser processing uses a real-time focus tracking system (Z-axis direction compensation accuracy 0.5 μm). The processing energy density is 3 - 6 J / cm 3 for alloy cover plates and 2 - 5 J / cm 3 for aluminum oxide ceramic cover plates. The laser processing path uses a spiral progressive scan with a coverage rate of over 90%. During processing, nitrogen / argon is used for assistance, and the gas flow rate is 10 - 15 L / min. As Figure 1As shown in the left figure, after multiple transverse and longitudinal laser scanning treatments on different cover plates, Figure 1 the "protrusion-valley" shaped surface microstructure shown in the right figure (as shown in the right figure in Figure 1 and the enlarged figure at the bottom) is obtained. The annular region of the outer circle of the cover plate with the microstructure is the same size as the subsequently pre-set solder ring. At this time, the depth of the "valley" of the microstructure is controlled at 30 - 50 μm, the width of the "protrusion" is controlled at 30 - 75 μm, that is, the aspect ratio of the microstructure is 1:1.5, and the distance between two "protrusions" is controlled at 1 - 1.5 times the depth of the "valley". As shown in Figure 2 (upper), in the top view of the structure of the surface processed area and the unprocessed area of the pre-set cover plate, 1 is the microstructure area after laser processing on the cover plate, and 2 is the area without laser processing.

[0038] For the surface treatment of the cover plate after laser processing, ultrasonic cleaning is carried out with anhydrous ethanol at 40 kHz / 80 W for 10 min, and then mixed plasma treatment with argon / oxygen (4:1) is carried out for 90 - 120 s to obtain a cover plate with uniform initial surface roughness.

[0039] After the above cleaning, nickel plating is first carried out on the microstructure on the surface of the cover plate by pulsed magnetron sputtering (HiPIMS) or electrochemical deposition (ECD), and then gold plating is carried out. Among them, HiPIMS and ECD are applicable to microstructures with an aspect ratio < 5:1, and ECD is applicable to microstructures with 5:1 ≤ aspect ratio < 10:1. It should be noted that both technologies can be used for the construction of the coating. When the cover plate is a ceramic material, the ceramic copper plating layer is controlled at 2 - 3 μm (compared with metal substrates such as molybdenum copper alloy, the ceramic material needs to be copper-plated first), the nickel plating layer is controlled at 2 - 6 μm, and the gold plating layer is controlled at 1 - 3 μm.

[0040] The schematic diagram of the packaging structure of the cover plate and the substrate is as shown in Figure 2 (lower). There is a microstructure 4 formed after laser processing on the cover plate 3, and there is a pre-set solder ring layer 5 on the microstructure 4 (the coating is omitted).

[0041] The cross-sectional structure of the prefabricated cover plate is as shown in Figure 3 . Its main body is composed of a cover plate substrate 7. The surface of the substrate is formed with a microstructure functional layer 8 by ultrafast laser processing. On it are successively a nickel plating layer 9 and a gold plating layer 10, and a pre-set gold-tin solder ring 11 is integrated on the top, thus forming a complete packaging interface.

[0042] 2) Vacuum hot pressing reflux pre-melting solder ring

[0043] The gold-tin alloy ingot obtained by melting and casting in a vacuum induction melting furnace is rolled into thin sheet materials by a multi-roll rolling mill, and the thin sheet materials are cold-stamped to obtain gold-tin ring solders (solder rings). The length and width dimensions of the solder rings are the same as the microstructural area of the cover plate, and the ratio of the depth of the microstructural "valley" to the thickness of the solder ring is controlled at 1:1.5 to 1.8, that is, the thickness of the solder ring is controlled at 45 to 90 μm to ensure the welding effect in the subsequent reflow pre-placement process.

[0044] The gold-tin solder ring can be pre-placed on the cover plate by the method of pre-melting. On the one hand, it can simplify the process of pre-placing the cover plate and substrate packaging. On the other hand, when the pre-placed cover plate and substrate are subjected to secondary reflow packaging, since the gold-tin solder has formed a reliable metallurgical bond with the gold-plated layer of the cover plate during pre-melting (primary reflow), the overflow and climbing phenomenon of the solder layer during secondary reflow will be suppressed, and precise packaging can be achieved. Among them, the pre-melting process is as follows: Under a vacuum of 10 -3 Pa, the cover plate and the solder ring are subjected to thermal compression reflow soldering treatment. The heating rate in the first stage is controlled at 30 °C / min to 150 °C and then held for 1.5 min, and then heated to 300 °C at a rate of 50 °C / min and start to hold. The holding time is synchronized with the pressurization time. At this time, thermal pressure is applied, the pressure is controlled at 0.3 to 0.5 MPa, and the time is controlled at 30 to 90 s. Subsequently, heating and pressurization are stopped, and the temperature is rapidly cooled to 100 °C at a cooling rate of 50 °C / min and then cooled with the furnace to finally obtain the cover plate with the pre-placed gold-tin solder ring.

[0045] The present invention will be further described in detail below through comparative examples and examples.

[0046] Comparative Example 1

[0047] Preferably, Mo60Cu40 molybdenum-copper alloy is used as the cover plate material and machined into a cover plate with dimensions of 10*4*0.7 mm and rounded corners with a radius of 0.25 mm at the four corners. The material of the solder ring is selected as Au20Sn (Sn content 20 ± 0.5 wt%). After the gold-tin alloy is purified, melted, rough-rolled and fine-rolled, it is stamped into a 60-μm-thick sheet-shaped solder ring. The size of the solder ring is adapted to the cover plate ( Figure 2 in Region 1), the size of its outer frame in the x-y plane is the same as that of the cover plate (10*4 mm), the inner frame size is 9*3 mm, the four corners are rounded corners with a radius of 0.25 mm, and the difference between the inner and outer frames (frame width) is controlled at 1 mm. The thickness of the solder sheet is: 60 μm. After the molybdenum-copper cover plate undergoes a cleaning process, surface metallization treatment is completed by a multi-step electroplating process: First, a nickel diffusion barrier layer (thickness 4.5 μm ± 0.5 μm) is deposited, and then a gold antioxidant layer (thickness 1.5 μm ± 0.2 μm) is covered to form a composite coating system.

[0048] Protective gas spot welding process: Under the protection of 50 L / min nitrogen, the spot welding of the cover plate and the solder ring is pre-set using a common laser. The laser energy is 80 mJ, and the spot welding temperature is controlled at 300 ± 5 °C to complete the spot welding, thereby forming a cover plate with a common pre-set AuSn solder ring.

[0049] The shear strength and 1.2 - 1.8 m drop test are carried out on the pre-set cover plate, and the pre-set cover plate is subjected to secondary reflow soldering to test its spreading area. At this time, the shear strength is 34.55 MPa, it passes the 1.2 m drop test but fails the 1.8 m drop test, and the spreading area after secondary reflow is 118.67%, exceeding 100%. This shows that the strength of the common pre-set cover plate is low, and there are phenomena of solder overflow and climbing on the cover, indicating low welding precision.

[0050] Example 1

[0051] Preferably, Mo60Cu40 molybdenum-copper alloy is used as the cover plate material, and it is machined into a cover plate. The cover plate size is 10 * 4 * 0.7 mm, and the four corners are rounded with a radius of 0.25 mm. The parameters of the picosecond laser are selected as follows: pulse width 10 ps, wavelength 1064 nm, repetition frequency 500 kHz, and energy density 5 J / cm 3 , and a segmented processing strategy is adopted. The single laser scanning speed is 300 mm / s, the processing depth per time is 5 μm, the distance between two "protrusions" is controlled at 1.2 times the depth of the "valley", about 50 μm; the width of the "protrusion" is controlled at 1.5 times the depth, about 60 μm; a total of 8 times of processing are completed to reach a depth of 40 μm, and the inclination angle of the protrusion wall is controlled > 85°. The construction of the in-line "protrusion" micro-structure is completed first horizontally and then vertically.

[0052] Laser post-cleaning: Hydrofluoric acid (2% concentration) ultrasonic cleaning and plasma cleaning are used to remove the recast contamination layer splashed by the laser. After the molybdenum-copper cover plate undergoes the cleaning process, surface metallization treatment is completed using a magnetron sputtering multi-stage electroplating process: First, a nickel diffusion barrier layer (thickness 4.5 μm ± 0.5 μm) is deposited, and then a gold anti-oxidation surface layer (thickness 1.5 μm ± 0.2 μm) is covered to form a composite coating system.

[0053] Vacuum hot pressing reflow pre-melting solder ring:

[0054] The solder ring material is Au20Sn (Sn content 20 ± 0.5 wt%). After the gold-tin alloy is purified, melted, rough-rolled and fine-rolled, it is stamped into a 60-μm-thick thin solder ring. The solder ring size is adapted to the cover plate. Its outer frame x-y plane size (10 * 4 mm) is the same as that of the cover plate, the inner frame size is 9 * 3 mm, the four corners are rounded with a radius of 0.25 mm, the difference between the inner and outer frames (frame width) is controlled at 1 mm, and the solder sheet thickness is 1.5 times the groove depth: 60 μm.

[0055] Gradient temperature rise welding is carried out under a vacuum degree of 10⁻³ Pa. The heating rate in the first stage is controlled at 30 °C / min until it reaches 150 °C and then is held for 1.5 min. Then it is heated at a rate of 50 °C / min to 300 °C and starts to be held. At this time, hot pressing is applied, the pressure is controlled at 0.3 MPa, and the time is controlled at 60 s. Subsequently, heating and pressing are stopped, and it is rapidly cooled at a cooling rate of 50 °C / min to 100 °C and then cooled in the furnace to obtain the cover plate with a pre-placed gold-tin solder ring.

[0056] The shear strength and 1.2 - 1.8 m drop test are carried out on the cover plate with a pre-placed gold-tin solder ring obtained in Example 1. The pre-placed cover plate is subjected to secondary reflow soldering, and its spreading area is tested. At this time, the shear strength is 52.76 MPa, it passes the 1.8 m drop test, and the spreading area after secondary reflow is 97.89%. It shows that the cover plate with a pre-placed gold-tin solder ring in Example 1 has extremely high strength, high assembly accuracy, and less solder overflow, and can complete high-reliability and high-precision packaging.

[0057] Example 2:

[0058] Aluminum oxide ceramic (Al₂O₃ 96%) is preferably used as the cover plate material and is machined into a cover plate. The cover plate size is 10 * 4 * 0.7 mm, and the four corners are rounded with a radius of 0.25 mm. The picosecond laser parameters are selected as follows: pulse width 300 fs, wavelength 515 nm, repetition frequency 200 kHz. The anisotropic processing path is adopted: first, transverse scanning (speed 800 mm / s) is carried out, and then longitudinal trimming (speed 1200 mm / s) is carried out. A total of 5 scans are performed, the single-pass processing depth is 6 μm, the distance between two "protrusions" is controlled at 1 times the depth of the "valley", about 30 μm; the width of a single "protrusion" is controlled at 1.33 times the depth, about 40 μm; a total of 30 μm of depth is completed in 5 passes of processing, and the inclination angle of the protrusion wall is controlled > 85°. The construction of the aligned "protrusion" micro-structure is completed first transversely and then longitudinally.

[0059] Laser post-cleaning: It is ultrasonically cleaned with anhydrous ethanol at 40 kHz / 80 W for 15 min, and then treated with a mixed plasma of argon / oxygen (4:1) for 90 s. In view of the non-conductive characteristics of the aluminum oxide ceramic matrix, a copper seed layer with a thickness of 3.0 ± 0.3 μm is deposited on the surface of the ceramic after cleaning treatment by magnetron sputtering; subsequently, gradient electroplating is carried out based on this seed layer: first, a nickel diffusion barrier layer is electroplated to 4.5 ± 0.2 μm, and then a gold antioxidant layer is electroplated to 1.5 ± 0.1 μm to form a metallized composite coating structure.

[0060] Vacuum hot pressing reflow pre-melting solder ring:

[0061] The welding ring is made of Au20Sn (Sn content: 20±0.5wt%). After purification, melting, rough rolling and fine rolling, the gold-tin alloy is stamped into a welding ring with a thickness of 50μm. The size of the welding ring (10*4mm) is adapted to the cover plate. The inner frame size is 9*3mm, and the four corners are rounded with a radius of 0.25mm. The difference between the inner and outer frames (frame width) is controlled to be 1mm, and the thickness of the solder sheet is 1.66 times the groove depth, which is 50μm.

[0062] Gradient temperature rise welding is carried out under a vacuum of 10-3Pa. In the first stage, the heating rate is controlled at 30℃ / min until 150℃, then keep warm for 1.5min, and then heat up to 300℃ at a rate of 50℃ / min and start to keep warm. At this time, apply hot pressing, the pressure is controlled at 0.5MPa, and the time is controlled at 60s. Then stop heating and pressing, and quickly cool down to 100℃ at a cooling rate of 50℃ / min and then cool with the furnace.

[0063] The shear strength and 1.2 - 1.8m drop test are carried out on the pre-placed gold-tin welding ring cover plate obtained in Example 2. The pre-placed cover plate is subjected to secondary reflow welding to test the spreading area of its solder sheet. At this time, the shear strength is 46.84MPa, and it passes the 1.8m drop test. After secondary reflow, the spreading area is 98.42%. It shows that the pre-placed gold-tin welding ring cover plate in Example 2 has high strength, high assembly accuracy, and extremely little solder overflow, and can achieve high-reliability and high-precision packaging.

[0064] From the results of the comparative examples and examples, the shear strength of the examples of the present invention is significantly higher than that of the comparative examples. This is because the post-welding interface of the micro-structure on the surface of the cover plate in the examples has a longer crack propagation path and greater fracture energy compared to the flat post-welding interface in the comparative examples. Therefore, it has a better blocking effect on the crack propagation and strengthens the post-welding interface. In addition, from the test results of the solder spreading area after secondary reflow, it can be seen that for the cover plate processed by laser in the examples of the present invention, the spreading area does not exceed 100% after secondary reflow, that is, there is no obvious solder overflow phenomenon. While for the unprocessed cover plate in the comparative example, the solder spreading area exceeds 100% after secondary reflow, resulting in serious overflow, which affects the subsequent packaging with the substrate. This is because: when the pre-placed cover plate is subjected to secondary reflow welding with the substrate, due to the existence of the micro-structure, the edge where the micro-structure area ( Figure 2 area 1 in it) contacts the unstructured area at the center of the cover plate ( Figure 2 area 2 in it) is the boundary where the specific surface area decreases extremely. The wettability of the solder in area 1 is much greater than that in area 2. Therefore, it hinders the flow and overflow of the liquid solder to area 2 and the climbing of the cover, ensuring the precision and airtightness of the packaging.

[0065] In summary, the present invention can simplify the assembly process of the cover plate and the AuSn solder ring, improve the assembly accuracy, control the melting and spreading range of the AuSn solder ring on the cover plate during encapsulation, and inhibit the solder overflow and cover climbing during the encapsulation of the cover plate and the substrate.

Claims

1. A preparation method of a cover plate pre - placed with a gold - tin solder ring by ultrafast laser processing, comprising the following steps: S1. Constructing the surface microstructure of the cover plate by ultrafast laser: Select a cover plate with a specified size, process it with an ultrafast laser, construct a microstructure on the surface of the cover plate under a mixed protective gas of nitrogen / argon, clean the processed cover plate by ultrasonic and plasma cleaning processes, and then complete the surface metallization treatment by a multi - step plating process to form a composite coating; S2. Vacuum hot - pressing and re - flowing to pre - melt the solder ring: Stamp the gold - tin alloy into a sheet - shaped solder ring so that the size of the solder ring matches the cover plate, and then use the vacuum hot - pressing and re - flowing process to pre - melt the gold - tin solder ring on the cover plate with the microstructure obtained in step S1, thereby forming a cover plate with a pre - placed gold - tin solder ring.

2. The preparation method according to claim 1, characterized in that, In step 1, the cover plate material is selected from one of molybdenum - copper alloy, tungsten - copper alloy, kovar alloy or ceramic; in step 2, the material of the gold - tin solder ring is Au20Sn alloy foil.

3. The preparation method according to claim 1, wherein In step 1, the ultrafast laser is a femtosecond or picosecond pulsed laser, and the laser wavelength is selected according to different cover plate materials to be processed. For alloy materials, a wavelength of 1064 nm is selected, and for ceramic materials, a wavelength of 515 nm is selected, and the laser processing path is a spiral progressive scan.

4. The preparation method according to claim 1, wherein, In step 1, the shape of the microstructure is one of "valley - bump" shape, "thimble" shape or "wave" shape, the arrangement mode of the microstructure is one of array - type, horizontal or vertical arrangement type, circular arrangement type and random arrangement type, and the aspect ratio of the microstructure on the cover plate is less than 5:

1.

5. The preparation method according to claim 4, characterized in that, In step 1, for the "valley - bump" shaped microstructure, the depth of the valley is required to be 30 - 50 μm, the distance between two bumps is 30 - 75 μm, the width of the bump is 30 - 50 μm, and the inclination angle of the bump wall is required to be > 85°.

6. The preparation method according to claim 1, characterized in that, The ratio of the depth of the "valley" of the microstructure of the cover plate to the thickness of the solder ring is controlled at 1:1.5 - 1.

8.

7. The preparation method according to claim 1, characterized in that, The cover plate with the formed microstructure needs to be cleaned by ultrasonic and plasma compound cleaning. During ultrasonic cleaning, anhydrous ethanol or dilute hydrofluoric acid solution is used, and during plasma cleaning, argon and oxygen mixed in a ratio of 4:1 are used.

8. According to the preparation method described in claim 1, the multi - step plating process used for the cover plate after cleaning in step 1 is by magnetron sputtering or electro - chemical deposition; at this time, different coating preparation methods need to be selected according to the aspect ratio of the microstructure. When the aspect ratio < 5:1, magnetron sputtering plating or electro - chemical deposition is selected, and when 5:1 ≤ aspect ratio < 10:1, only electro - chemical deposition can be used.

9. The preparation method according to claim 1, wherein, In step 2, the difference between the inner and outer frames of the size of the solder ring is 1 - 2 mm, and the thickness of the solder ring is 1.2 - 1.5 times the depth of the constructed microstructure.

10. The preparation method according to claim 1, characterized in that, In the said step 2, the presetting process adopts vacuum thermal compression reflow soldering, the soldering vacuum degree is not less than 10 -3 Pa, and two-stage heating rates are set. It is kept warm in the peak reflow temperature zone and pressure is applied starting from this moment. The applied pressure is 0.3 - 0.5 MPa, and the cooling rate is the same as the heating rate.

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