A method for manufacturing a tantalum capacitor electrode and a tantalum capacitor

Abstract

The application provides a preparation method of a tantalum capacitor electrode, the tantalum capacitor comprising a capacitor element, a positive internal connecting piece, a negative internal connecting piece and a packaging body covering the above components, the preparation method comprising the following steps: polishing the end face of the packaging body exposed with the positive internal connecting piece and the negative internal connecting piece to make the roughness Ra of the end face reach 0.8-3.2 μm to increase the surface friction; uniformly coating the end face of the polished packaging body with conductive paste with a thickness of 5-20 μm, wherein the conductive paste comprises a mixture of metal conductive filler and resin binder; drying the packaging body coated with the conductive paste in an environment with a temperature of 80-150 ℃ to make the conductive paste completely solidify and tightly combine with the end face of the packaging body; and placing the solidified packaging body as a cathode into an electroplating solution containing electrode material to electrochemically deposit, thereby forming an outer electrode layer, wherein the outer electrode layer is electrically connected with the end face of the positive internal connecting piece and the negative internal connecting piece.

Description

Technical Field

[0001] This application relates to the field of tantalum capacitors, and more particularly to a method for preparing tantalum capacitor electrodes. Background Technology

[0002] Tantalum capacitors, as highly reliable passive electronic components, possess outstanding advantages such as small size, high capacitance density, good temperature stability, and long lifespan. They are widely used in fields with stringent performance requirements, such as military electronics, 5G communications, AI servers, new energy vehicles, and medical equipment. With the miniaturization and high integration of electronic systems, higher demands are being placed on the capacitance density, equivalent series resistance (ESR), and reliability of tantalum capacitors.

[0003] The external electrodes of existing tantalum chip capacitors are typically fabricated using a lead frame bending process: a pre-formed metal lead frame is connected to the capacitor element and then packaged. After removing excess lead frame material, the exposed lead frame terminals are bent and bonded to the surface of the package to form the external electrodes. This process has the following inherent drawbacks: The frame terminals need to meet the bending strength requirements, and the thickness is usually 0.1 to 0.3 mm. They occupy a lot of packaging space, which limits the volume ratio of capacitor components and makes it difficult to further increase the capacitance density. The frame terminals after bending have residual stress, which can easily separate from the package during temperature cycling due to the difference in thermal expansion coefficients, resulting in a decrease in sealing performance. Moisture intrusion can cause capacitor performance degradation or even failure. The bent terminals are mechanically bonded to the surface of the package, resulting in high contact resistance and requiring a large amount of silver paste to fill the gaps, which increases production costs.

[0004] To address the aforementioned issues, researchers in this field have begun exploring the use of electrochemical deposition to directly form external electrodes on the surface of the package. However, since tantalum capacitor packages are typically made of epoxy molding compound, an insulating material, direct electrochemical deposition on its surface is difficult. Furthermore, the adhesion between the deposited layer and the insulating package is poor, leading to easy detachment and insufficient electrode reliability. Currently, there is no mature process solution capable of enabling large-scale mass production of electrochemically deposited external electrodes for tantalum capacitors. Summary of the Invention

[0005] The purpose of this application is to overcome the shortcomings of the prior art and provide a method for preparing tantalum capacitor electrodes, which solves the technical problems of the inability to directly electrochemically deposit electrodes on the surface of epoxy encapsulation and the poor adhesion of the deposited layer. While improving the capacitance density of tantalum capacitors, it also reduces the equivalent series resistance and improves product reliability.

[0006] A method for preparing a tantalum capacitor electrode: The tantalum capacitor includes a capacitor element, a positive electrode inner connector, a negative electrode inner connector, and a package covering the above components. The end faces of the positive electrode inner connector and the negative electrode inner connector are exposed at the end face of the package. The manufacturing method includes the following steps: The end faces and bottom faces of the package body that expose the positive and negative inner connectors are polished to achieve an end face roughness Ra of 0.8 μm to 3.2 μm. A conductive paste with a thickness of 5μm to 20μm is uniformly coated on the end face of the polished package. The conductive paste contains a mixture of metal conductive filler and resin binder. The package coated with conductive paste is dried to allow the conductive paste to fully cure and bond tightly to the end face of the package. The cured encapsulated body is placed in an electroplating solution containing electrode material as a cathode for electrochemical deposition to form an outer electrode layer. The outer electrode layer is electrically connected to the end faces of the positive electrode inner connector and the negative electrode inner connector.

[0007] A method for preparing a tantalum capacitor includes the method for preparing the tantalum capacitor electrode described above.

[0008] The technical advantage of this application lies in the following: by polishing the package, coating it with a conductive paste, and then electrochemically depositing the electrode material onto the surface of the conductive paste, the resulting external electrode is more firmly bonded to the package and less prone to detachment. Attached Figure Description

[0009] Figure 1 This is a schematic diagram of a capacitor. Figure 2 This is a schematic diagram of the packaged body after polishing, where the shaded area represents the polished area; Figure 3 This is a schematic diagram of the package body of this application after being coated with conductive paste, wherein the shaded area represents the area coated with conductive paste; Figure 4 This is a schematic diagram of the encapsulation after electrochemical deposition, with the shaded area representing the electrochemical deposition area; In the diagram: 1. Capacitor element; 2. Positive electrode internal connector; 3. Negative electrode internal connector; 4. Package body. Detailed Implementation

[0010] The embodiments of the technical solution of this application will be described in detail below. The following embodiments are only used to illustrate the technical solution of this application more clearly, and are therefore only examples, and should not be used to limit the scope of protection of this application.

[0011] A method for preparing a tantalum capacitor electrode: Please refer to Figure 1The tantalum capacitor includes a capacitor element 1, a positive electrode inner connector 2, a negative electrode inner connector 3, and a package 4 covering the above components. The end faces of the positive electrode inner connector 2 and the negative electrode inner connector 3 are exposed at the end face of the package 4. The manufacturing method includes the following steps: Please refer to Figure 2 The end face and bottom face of the package 4, which expose the positive electrode inner connector 2 and the negative electrode inner connector 3, are polished to make the surface roughness Ra reach 0.8μm to 3.2μm. In a preferred embodiment, the polished area can be the electrochemical deposition area or the surface where the electrochemical deposition is located.

[0012] The roughness of the 0.8–3.2 μm micro-pits formed by roughening in this application is not an isolated parameter, but is linked to the subsequent curing of the slurry: on the one hand, the microstructure improves the wettability of the slurry, allowing the slurry to fully penetrate into the pits; on the other hand, the volume shrinkage of the slurry during curing can be constrained by the pit structure, thereby forming a stronger mechanical anchoring.

[0013] Please refer to Figure 2 A conductive paste with a thickness of 5μm to 20μm is uniformly coated on the end face of the polished package body 4. The conductive paste contains a mixture of metal conductive filler and resin binder. The conductive paste with a thickness of 5–20 μm in this application has two functions: Firstly, as a base layer for electroplating, it can ensure uniform current distribution, prevent pinholes or uneven thickness in the plating layer, and improve deposition efficiency.

[0014] Secondly, as a thermal expansion buffer layer, it can match the difference in thermal expansion coefficients between the epoxy encapsulation and the copper plating, thereby reducing interfacial stress during temperature cycling and improving reliability.

[0015] The thickness range of the conductive paste is specifically optimized; too thin a paste will result in insufficient conductivity, while too thick a paste will increase stress, making it impossible to achieve a balance between the two functions.

[0016] The package coated with conductive paste is dried to allow the conductive paste to fully cure and bond tightly to the end face of the package. Please refer to Figure 3 The cured encapsulated body 4 is placed in an electroplating solution containing electrode material as a cathode for electrochemical deposition to form an outer electrode layer. The outer electrode layer is electrically connected to the end faces of the positive electrode inner connector 2 and the negative electrode inner connector 3.

[0017] As a preferred technical solution of this application, in a preferred embodiment, the grinding process can be carried out by any one of sandblasting, mechanical grinding or plasma etching, which can achieve the required surface roughness.

[0018] In a preferred embodiment, the metallic conductive filler is at least one of gold powder, palladium powder, silver powder, copper powder, nickel powder, or silver-coated copper powder or silver-coated nickel powder, and the metallic conductive filler accounts for 60% to 90% of the mass of the conductive slurry.

[0019] The silver-coated copper powder refers to a core-shell structure powder with copper as the core and silver as the outer coating. Similarly, the silver-coated nickel powder refers to a core-shell structure powder with nickel as the core and silver as the outer coating.

[0020] In a preferred embodiment, the metallic conductive filler is a mixture of silver powder and copper powder, with a mass ratio of silver powder to copper powder of 1:1 to 3:1, which can reduce material costs while ensuring conductivity.

[0021] In a preferred embodiment, the resin adhesive is any one of epoxy resin, acrylic resin, or phenolic resin.

[0022] In a preferred embodiment, the electroplating solution is any one of nickel salt electroplating solution, tin salt electroplating solution, gold salt electroplating solution or nickel-tin alloy electroplating solution, and different electrode materials can be selected according to the product application scenario.

[0023] In a preferred embodiment, the current density of the electrochemical deposition is 0.5 A / dm³. 2 ~3A / dm 2 The time is 10 min to 60 min.

[0024] The following will describe the proposed solution in conjunction with embodiments and comparative examples.

[0025] Example 1 This embodiment describes a method for fabricating electrodes for tantalum capacitors with a 7343 package size, including the following steps: The tantalum capacitor packaging strip, after being encapsulated and having its lead frame excess material removed, is placed in a sandblasting equipment. The end face and bottom face with exposed positive and negative internal connectors are sandblasted and polished using 120-mesh white corundum abrasive, with a sandblasting pressure of 0.2 MPa. The surface roughness Ra of the treated end face is 1.6 μm. A silver-copper mixed conductive paste was coated on the end face of the polished package using a screen printing process. The mass ratio of silver powder to copper powder was 2:1, and the paste thickness was 10μm. Place the encapsulation strip coated with the slurry in a hot air circulating oven and dry it at 120°C for 20 minutes to allow the conductive slurry to fully cure. The cured encapsulation strip was placed as the cathode in a nickel sulfamate electroplating solution, with the current density controlled at 1 A / dm³. 2 Electroplating time is 30 minutes to form a nickel external electrode layer with a thickness of 50 μm.

[0026] Example 2 This embodiment describes a method for fabricating electrodes for a tantalum capacitor suitable for a 6032 package size, including the following steps: The end face and bottom face of the package were treated by mechanical grinding using 1500-mesh silicon carbide sandpaper, and the roughness Ra of the rear face was 0.8μm. A pure silver conductive paste was applied to the polished end face using a spraying process, with a paste thickness of 5μm. Place the encapsulated body in an 80℃ oven for 30 minutes to dry, allowing the slurry to solidify; The sulfate tin plating process is used, with a current density of 0.5 A / dm³. 2 The deposition time was 10 min, forming a tin external electrode layer with a thickness of 20 μm.

[0027] Example 3 This embodiment describes an electrode fabrication method suitable for automotive electronic-grade tantalum capacitors, comprising the following steps: The end face of the package was processed by plasma etching with an etching power of 300W and a processing time of 5min, resulting in an end face roughness Ra of 3.2μm. The coating is made of nickel powder conductive paste with a thickness of 20μm, and the resin binder is high-temperature resistant phenolic resin. Dry at 150℃ for 10 minutes to allow the slurry to fully solidify; The process employs a nickel-tin alloy electroplating technique with a current density of 3A / dm². 2 The deposition time was 60 min, forming a nickel-tin alloy external electrode layer with a thickness of 100 μm.

[0028] The following are comparative examples to be used in conjunction with this application.

[0029] Comparative Example 1: The surface roughening step was omitted, and the same conductive paste was directly coated on the end face of the original smooth package. The remaining process parameters were completely consistent with those of Example 1.

[0030] Comparative Example 2: After surface roughening, Ra is 6.3 μm (exceeding the range of 0.8 to 3.2 μm in this application), and the remaining process parameters are completely consistent with those of Example 1.

[0031] Comparative Example 3: The conductive paste coating thickness was 3 μm (lower than the 5 μm lower limit of this application), and the remaining process parameters were completely consistent with those of Example 1.

[0032] Comparative Example 4: The conductive paste coating thickness was 30 μm (higher than the 20 μm upper limit of this application), and the remaining process parameters were completely consistent with those of Example 1.

[0033] Comparative Example 5: The conductive paste contains 50% metal filler (lower than the 60% lower limit of this application), and uses the same silver-copper mixed filler (mass ratio 2:1). The remaining process parameters are completely consistent with those of Example 1.

[0034] Comparative Example 6: The conductive paste contains 95% metal filler (higher than the 90% upper limit of this application), and uses the same silver-copper mixed filler (mass ratio 2:1). The remaining process parameters are completely consistent with those of Example 1.

[0035] The tantalum capacitors prepared in each embodiment and comparative example were tested, and the results are shown in Table 1.

[0036] Table 1: As shown in Table 1, the electrode bonding force of all three examples is ≥5.8 N / mm, which is significantly higher than that of the comparative examples. The capacity and ESR of Example 1 are significantly improved compared with those of Comparative Examples 1 to 6. The coating uniformity deviation is ≤0.6 μm, and the temperature cycling pass rate is ≥98.7%, which meets the requirements of industrial and automotive applications.

[0037] After omitting the roughening step in Comparative Example 1, the bonding force was only 1.5 N / mm, and the temperature cycling pass rate plummeted to 62.1%, proving that surface roughening is a necessary prerequisite for ensuring interfacial bonding. After the process parameters of Comparative Examples 2 to 4 exceeded the limits of this application, all performances decreased by 15% to 40%, verifying the rationality and optimality of the parameter range of this application.

[0038] When the proportion of metal filler in Comparative Examples 5 and 6 deviated from the range of 60% to 90%, either the conductivity was insufficient, resulting in limited improvement in electrical performance, or the adhesion was insufficient, resulting in decreased reliability. This proves that the slurry formulation of this application is the optimal choice that takes into account conductivity, bonding strength, and cost.

[0039] The following will provide a detailed explanation of the synergistic mechanism among roughness, conductive paste thickness, and metal filler ratio in this application, in conjunction with embodiments and comparative examples.

[0040] First, there is interfacial synergy between roughness and slurry thickness.

[0041] Micro-pit structures formed by surface roughness Ra of 0.8–3.2 μm typically have a pit depth of 2–5 times the Ra value, or approximately 1.6–16 μm. This size range forms a depth matching relationship with the coating thickness of conductive paste, which is 5–20 μm: when the paste thickness is approximately 1.2–3 times the pit depth, the paste can fully penetrate to the bottom of the pit under capillary force, while forming a continuous cover layer at the top of the pit. If the roughness is too low (Ra < 0.8 μm), the pit depth is insufficient to generate effective mechanical interlocking, and the paste layer and the encapsulated body are only physically bonded; if the roughness is too high (Ra > 3.2 μm), the pit depth may exceed the paste thickness, resulting in discontinuity of the paste at the top of the pit, uneven current distribution during electroplating, and the formation of pinholes or uneven plating thickness.

[0042] As shown in Comparative Example 2, when the roughness Ra exceeds the range of this application and reaches 6.3 μm, the bonding force drops from 6.2 N / mm in Example 1 to 3.8 N / mm, and the temperature cycling pass rate drops from 99.2% to 85.3%. This is because the excessively deep pits cause discontinuity in the slurry layer, which impairs the uniformity of the electroplated layer and the quality of the interface bonding.

[0043] Secondly, the curing shrinkage of the slurry works in synergy with the mechanical anchoring of the pit structure.

[0044] During the drying and curing process of the conductive paste at 80℃ to 150℃, the resin binder undergoes a cross-linking reaction, resulting in a volume shrinkage rate of approximately 3% to 8%. On smooth surfaces, this shrinkage generates tensile stress perpendicular to the interface between the paste and the encapsulation, leading to microcracks or even delamination at the interface. However, on surfaces with a roughness Ra of 0.8 to 3.2 μm, the paste penetrates into the pits, forming a three-dimensional anchoring structure: the sidewalls of the pits provide normal constraint forces to the shrinking paste, transforming the volume shrinkage from "interfacial peeling force" to "sidewall extrusion force," upgrading the interface between the paste and the encapsulation from purely physical adhesion to mechanical interlocking. This transformation is the main reason why the bonding force jumps from 1.5 N / mm (Comparative Example 1, smooth surface) to 6.2 N / mm (Example 1, Ra = 1.6 μm).

[0045] Then, the proportion of metal filler has a dual effect on the conductive network and stress transmission.

[0046] Metal fillers, comprising 60% to 90% of the conductive paste, form a conductive permeation network. This network performs two functions: firstly, during electroplating, it acts as an electron conduction channel on the cathode surface, ensuring uniform current distribution across the encapsulation end face and preventing pinholes or uneven thickness in the plating layer, thus serving as a pre-plating layer; secondly, during temperature cycling, it acts as a stress transmission bridge, absorbing the thermal expansion difference between the epoxy encapsulation and the copper plating layer through the elastic deformation of the metal fillers, thus serving as a thermal expansion buffer.

[0047] When the filler content is below 60%, the conductive network is discontinuous, the electron conduction path is broken, the local current density is too high during electroplating, and pinholes appear in the plating layer. At the same time, the resin ratio is too high, the rigidity of the transition layer is insufficient, and the interfacial stress concentration during temperature cycling leads to delamination. As shown in Comparative Example 5 (filler content 50%), the ESR increases to 61.4 mΩ, and the temperature cycling pass rate drops to 81.2%. When the filler content is above 90%, the resin binder between metal particles is insufficient, and the interfacial adhesion between the conductive paste and the encapsulation substrate drops sharply. Although the conductivity is good, the mechanical fixing effect is lost. As shown in Comparative Example 6 (filler content 95%), the bonding force drops to 3.5 N / mm, and the temperature cycling pass rate drops to 88.7%.

[0048] Finally, the synergistic transfer path of roughness, slurry thickness, and filler ratio during temperature cycling.

[0049] Under temperature cycling conditions, the above three parameters form the following synergistic transmission path: the roughness Ra of 0.8 to 3.2 μm provides a micro-mechanical anchoring foundation, the slurry thickness of 5 to 20 μm forms a continuous and moderate transition layer thickness on the anchoring foundation, which ensures the integrity of the conductive network and controls the stress gradient, and the filler content of 60% to 90% constructs a microstructure with both electronic conduction and elastic deformation capabilities inside the transition layer. The three-layer mechanism progresses step by step from the interface (roughness) to the bulk layer (slurry thickness) and then to the microstructure (filler ratio), each step being indispensable: Without roughness, the slurry layer lacks an anchoring foundation, resulting in a bonding strength of only 1.5 N / mm (Comparative Example 1); when the slurry is too thin (3 μm), the anchored transition layer is insufficient to cover the top surface of the pit, causing the electroplated layer to directly contact the pit edge, leading to stress concentration and crack initiation, reducing the bonding strength to 3.2 N / mm (Comparative Example 3); when the slurry is too thick (30 μm), the mass of the transition layer increases, and the inertial stress during thermal cycling exceeds the constraint force of the anchoring structure, reducing the temperature cycling pass rate to 87.9% (Comparative Example 4); when the filler ratio deviates, the conductive network breaks or the binder phase is insufficient, leading to deterioration of electrical and mechanical properties, respectively (Comparative Examples 5-6). This synergistic transmission path explains why the parameter combination in this application can achieve a combined performance of bonding strength ≥ 5.8 N / mm and a temperature cycling pass rate ≥ 98.7%, while optimizing any single parameter alone cannot achieve the same effect.

[0050] In summary, the surface roughness parameter, conductive slurry thickness parameter, and metal filler ratio parameter are not independent variables, but rather constitute a progressive synergistic system of "interface anchoring - bulk transition - micro-control". In Comparative Example 1, omitting the roughening step resulted in a bonding strength of only 1.5 N / mm and a sharp drop in the temperature cycling pass rate to 62.1%, proving that surface roughening is a necessary prerequisite for ensuring interfacial bonding. In Comparative Examples 2-4, when the process parameters exceeded the limits specified in this application, all performance characteristics decreased by 15%-40%, verifying the rationality of the parameter range in this application. In Comparative Examples 5-6, when the metal filler ratio deviated from the 60%-90% range, either insufficient conductivity led to limited improvement in electrical performance, or insufficient adhesion led to decreased reliability, proving that the slurry ratio in this application is the optimal choice balancing conductivity, bonding strength, and cost. The synergy of these three factors ensures that the external electrode formed by electrochemical deposition maintains good bonding strength and does not detach even after repeated thermal expansion and contraction.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for preparing a tantalum capacitor electrode, characterized in that, Includes the following steps: The end face and bottom face of the package (4) with the positive electrode internal connector (2) and the negative electrode internal connector (3) exposed are polished to make the surface roughness Ra reach 0.8μm~3.2μm; A conductive paste with a thickness of 5μm to 20μm is uniformly coated on the end face and bottom face of the polished package (4). The conductive paste contains a mixture of metal conductive filler and resin binder. The package (4) coated with conductive paste is dried to allow the conductive paste to solidify and bond with the package (4); The solidified encapsulated body (4) is placed in an electroplating solution containing electrode material as a cathode for electrochemical deposition to form an outer electrode layer. The outer electrode layer is electrically connected to the end faces of the positive electrode inner connector (2) and the negative electrode inner connector (3).

2. The method for preparing tantalum capacitor electrodes as described in claim 1, characterized in that, The polishing process can be carried out by any one of the following methods: sandblasting, mechanical grinding, or plasma etching.

3. The method for preparing the tantalum capacitor electrode as described in claim 1, characterized in that, The metallic conductive filler is at least one of gold powder, palladium powder, silver powder, copper powder, nickel powder, or silver-coated copper powder or silver-coated nickel powder, and the metallic conductive filler accounts for 60% to 90% of the mass of the conductive slurry.

4. The method for preparing the tantalum capacitor electrode as described in claim 3, characterized in that, The metallic conductive filler is a mixture of silver powder and copper powder, with a mass ratio of silver powder to copper powder of 1:1 to 3:

1.

5. The method for preparing the tantalum capacitor electrode as described in claim 1, characterized in that, The resin binder is any one of epoxy resin, acrylic resin, or phenolic resin.

6. The method for preparing the tantalum capacitor electrode as described in claim 1, characterized in that, The electroplating solution is any one of nickel salt electroplating solution, tin salt electroplating solution, gold salt electroplating solution, or nickel-tin alloy electroplating solution.

7. The method for preparing tantalum capacitor electrodes as described in claim 1, characterized in that, The current density of the electrochemical deposition is 0.5 A / dm³. 2 ~3A / dm 2 The time is 10 min to 60 min.

8. A method for preparing a tantalum capacitor, characterized in that, The method for preparing the tantalum capacitor electrode according to any one of claims 1 to 7.

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