Terminal electrode, preparation method thereof and method for detecting bonding strength of bottom silver layer and flexible layer of terminal electrode

Abstract

The invention discloses a terminal electrode, a preparation method thereof and a method for detecting the bonding strength of a bottom silver layer and a flexible layer of the terminal electrode, and relates to the field of electrodes, preparation methods thereof and bonding strength detection methods, and the preparation method of the terminal electrode comprises the following steps: coating a ceramic substrate with the bottom silver layer; sand blasting is conducted on the surface of the bottom silver layer so that the surface of the bottom silver layer can be roughened; coating flexible slurry on the surface of the roughened bottom silver layer; and curing the coated flexible slurry to obtain the flexible layer, wherein the curing temperature is 180-220 DEG C. Sand blasting is carried out on the surface of the bottom silver layer, the surface roughness of the bottom silver layer is improved, the contact area between the flexible layer and the bottom silver layer is increased, meanwhile, the characteristic that wettability is weakened is made up by adjusting the curing temperature, and therefore interface bonding becomes good, and the adhesive force is improved.

Description

Technical Field

[0001] The invention relates to the field of electrodes and preparation methods thereof and bonding strength detection methods, and in particular to a terminal electrode and a preparation method thereof, and a method for detecting the bonding strength between a bottom silver layer and a flexible layer of the terminal electrode. Background Art

[0002] Flexible terminal electrode multilayer ceramic capacitors are based on the traditional three-layer electrode. A flexible electrode layer composed of resin-conductive filler is added between the bottom silver layer and the nickel-plated layer of the terminal electrode, forming a four-layer flexible electrode layer. The function of the flexible terminal electrode is to absorb the heat and mechanical stress generated by the capacitor during welding and use, thereby reducing the risk of cracking of the capacitor ceramic body. The flexible product structure is a three-layer structure with an additional flexible electrode layer on the surface of the bottom silver layer after sintering. During the coating process of the flexible resin layer, the key to ensuring good contact between the flexible layer and the bottom silver layer is to completely wet the bottom metal layer.

[0003] The resin layer formed by the flexible terminal electrode slurry mainly plays a role similar to a spring in the MLCC chip. It can withstand high external impact and external stretching. It can still maintain the good electrical properties of the MLCC under certain deformation conditions, thereby preventing the chip capacitor from breaking and failing due to mechanical stress. In the process of studying flexible terminal electrode multilayer ceramic capacitors, it was found that insufficient or excessive adhesion of the flexible layer may lead to failure. At present, the research method used is to analyze the manufactured MLCC through grinding, observe the interface structure between the flexible layer and the bottom silver layer, and combine the test results of the bending / end face bonding strength to determine the influence of the flexible layer adhesion on the MLCC failure mode. Mechanical polishing is generally used for grinding analysis. Laboratories with the necessary conditions can use ion polishing or FIB sectioning to obtain the interface structure. Use a stereo microscope and a metallographic microscope to perform structural inspection on the cross section of the MLCC, and use a scanning electron microscope (SEM) and a focused ion beam (FIB) to analyze the interface structure of the flexible end electrode. The board bending test bends the MLCC to different depths, and uses appearance inspection and in-situ capacitance testing methods. When there is visible damage on the appearance of the capacitor or the in-situ capacitance attenuation reaches 5%, the sample is judged to have failed.

[0004] Flexible slurry is mainly composed of metallic silver, organic conductive resin, organic solvent, etc. Due to the characteristics of the material, its bonding with the ceramic substrate is mainly achieved through the "wetting-contraction-embedded" method of the flexible slurry, that is, van der Waals force or hydrogen bond, and the adhesion is relatively weak. By optimizing the curing conditions of the flexible layer, such as increasing the curing temperature and extending the curing time, the adhesion between the flexible layer and the bottom silver electrode can be enhanced. The traditional inspection method is to conduct a 4mm bending test on the product to evaluate the performance of the flexible terminal electrode MLCC under external stress, especially focusing on the capacitance change of the capacitor and the potential damage mode of the internal structure. However, this kind of detection method can only qualitatively judge the quality of the results of the finished product by observing the interface. It is difficult for scientific researchers to study the performance of the flexible slurry surface clearly, and it is also difficult to determine the influence of different production processes (whether sandblasting, curing temperature) on the strength of the bonding force. Therefore, it is difficult to improve the bonding strength between the bottom silver layer and the flexible layer through the regulation of the production process. This situation restricts the production process from finding suitable process parameters. Wettability is one of the important properties of material surface, which is characterized by static contact angle. The factors affecting wettability are mainly the chemical composition and microstructure of the material surface. The wettability of the material surface can be changed mainly through surface modification and surface micro-modeling. Summary of the invention

[0005] The object of the present invention is to provide a terminal electrode and a preparation method thereof and a method for detecting the bonding strength between the bottom silver layer and the flexible layer of the terminal electrode, so as to solve the problem of weak bonding strength between the bottom silver layer and the flexible layer of the terminal electrode in the prior art.

[0006] The present invention is implemented by the following technical solutions: First, an embodiment of the present invention provides a method for preparing a terminal electrode, comprising the following steps:

[0007] coating a bottom silver layer on a ceramic substrate;

[0008] Sandblasting the surface of the bottom silver layer to roughen the surface of the bottom silver layer;

[0009] Coating a flexible paste on the roughened surface of the bottom silver layer;

[0010] The coated flexible slurry is cured to obtain a flexible layer, and the curing temperature is 180°C to 220°C.

[0011] Optionally, coating the bottom silver layer includes coating Ag paste on the ceramic substrate, and then combining the Ag paste with the ceramic substrate by sintering to form the bottom silver layer, and the sintering temperature is 600-900°C.

[0012] In order to better solve the above problem, an embodiment of the present invention further provides a terminal electrode, which is prepared by the above preparation method, and the contact angle between the bottom silver layer and the flexible layer is 11.59° to 14.59°.

[0013] In order to better implement and adjust the above scheme, an embodiment of the present invention also provides a method for detecting the bonding strength between the bottom silver layer and the flexible layer of the terminal electrode. The testing method includes characterizing the contact angle of a ceramic substrate coated with a terminal electrode, and the terminal electrode includes the terminal electrode prepared by the above preparation method.

[0014] Optionally, it also includes S4: product verification; the product verification includes subjecting the product prepared by S2 to a bending test to verify the relationship between wettability, the bonding force between the bottom silver layer and the flexible layer and the capacitor failure mode, and to determine whether the contact angle result obtained by the S3 test is credible.

[0015] Optionally, the preparation of the terminal electrode includes at least two preparation methods to obtain at least two terminal electrodes, and the preparation difference of the two terminal electrodes includes at least one difference in the flexible slurry formula, the treatment method of the bottom silver layer surface before coating the flexible slurry, and the flexible slurry curing temperature.

[0016] Optionally, the surface treatment of the bottom silver layer before applying the flexible slurry includes sandblasting the surface of the bottom silver layer before applying the flexible slurry and not sandblasting the surface of the bottom silver layer before applying the flexible slurry.

[0017] Optionally, the preparation of the ceramic substrate includes adding electronic ceramic powder to an organic adhesive system to cast it into a ceramic diaphragm, wherein the organic adhesive includes an organic solvent, a resin and a plasticizer, and the weight fractions are 50wt% to 70wt%, 18wt% to 25wt% and 8wt% to 15wt% respectively.

[0018] Optionally, the organic solvent includes toluene and ethanol, the resin includes polyvinyl butyral, and the plasticizer includes dioctyl phthalate.

[0019] Compared with the prior art, the embodiments of the present invention have the following advantages and beneficial effects:

[0020] 1. In the embodiment of the present invention, sandblasting is performed on the surface of the bottom silver layer to increase the surface roughness of the bottom silver layer so as to increase the contact area between the flexible layer and the bottom silver. However, the wettability is reduced. The weakened wettability is compensated by adjusting the curing temperature, thereby improving the interface bonding and enhancing its adhesion. As the curing temperature increases, the wettability increases. However, too high a curing temperature will cause the destruction of the molecular structure of the material, making it impossible to produce a curing effect. In the embodiment of the present invention, the curing temperature is strictly controlled to ensure that the material performance reaches the optimal state.

[0021] 2. The terminal electrode provided in the embodiment of the present invention sets the contact angle between the bottom silver layer and the flexible layer to 11.59° to 14.59°, thereby achieving a more suitable bonding strength between the bottom silver layer and the resin layer.

[0022] 3. In the embodiment of the present invention, the bottom silver layer is uniformly coated on the ceramic substrate, silver is burned, and the obtained burned silver surface is treated by sandblasting or not sandblasting and / or at least two flexible slurries of formula are selected for coating, and / or at least two curing temperatures are selected for curing, and the connection between the strength of the bonding between the flexible layer and the bottom silver layer and the failure mode is studied and analyzed through contact angle characterization. The characterization method of contact angle can not only quantitatively characterize, but also provide a guiding range for suitable process parameters for the production process. By accurately determining the wetting properties of the bottom silver layer and the flexible layer, it is helpful to study the relationship between the bonding strength of the bottom silver layer or the flexible layer, so as to judge the influence of adhesion on the failure mode of MLCC, thereby providing a range of application for the bonding strength between the bottom silver layer and the flexible layer and increasing the reliability of flexible MLCC. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0024] Figure 1 A graph showing the contact angle test results of the bottom silver layer after sandblasting provided by an embodiment of the present invention;

[0025] Figure 2 A graph showing the contact angle test results of the bottom silver layer without sandblasting provided in an embodiment of the present invention;

[0026] Figure 3 A scanning diagram of the interface between the bottom silver layer and the flexible layer provided in an embodiment of the present invention, wherein Figure 3 a corresponds to the result of sandblasting. Figure 3 b corresponds to the result without sandblasting;

[0027] Figure 4 The scanning diagram of the bonding state between the bottom silver layer and the flexible layer at different curing temperatures. Figure 4 a corresponds to a curing temperature of 180°C, Figure 4 b corresponds to a curing temperature of 200°C, Figure 4 c corresponds to a curing temperature of 220°C. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0029] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] An embodiment of the present invention provides a method for detecting the bonding strength between a bottom silver layer and a flexible layer of an end electrode, comprising the following steps:

[0031] (1) Preparation of ceramic substrate: electronic ceramic powder is added to an organic binder system to form a ceramic diaphragm, metal internal electrode slurry is printed on it by screen printing, and then the ceramic diaphragms are stacked and cut to form green bodies. The green bodies are then debonded and sintered at high temperature to form porcelain, thereby obtaining a ceramic substrate.

[0032] (2) Forming a bottom silver layer: The ceramic substrate is coated with Ag paste, and then the end paste physically bonded after the coating is tightly combined with the ceramic body through high-temperature sintering to form a bottom silver layer.

[0033] (3) Pre-treatment of the flexible layer: Sandblasting can be used to roughen the surface of the base silver and increase the contact area between the flexible layer and the base silver. Compressed air is used as the power to spray the material at high speed onto the surface of the workpiece to be processed. It should be noted that this step can be used or not for different flexible slurries.

[0034] (4) Coating flexible paste: Coat the flexible paste on the bottom silver layer.

[0035] (5) Curing: After the flexible layer is coated, the product is unloaded onto a setter plate, and then the setter plate is placed in an air debonding furnace for curing at a certain temperature and for a certain time.

[0036] (6) Contact angle test: Ceramic substrates provided with a bottom silver layer and a flexible layer were ground with different slurries, different surface treatment methods of the bottom silver layer before coating the flexible slurry (sandblasting or not), and different curing temperatures. The contact angle of the bottom silver layer and the flexible layer was tested using a scanning electron microscope based on the contact angle measuring instrument technology.

[0037] (7) Product verification: Batch produce products under the same process conditions as steps (1) to (6), and conduct a 4 mm bending test on the products to study the relationship between wettability, the bonding strength between the bottom silver layer and the flexible layer, and the capacitance failure mode, and determine whether the contact angle results are credible.

[0038] Example 1

[0039] The embodiment of the present invention provides a method for detecting the bonding strength between the bottom silver layer and the flexible layer of the terminal electrode, which specifically includes the following steps:

[0040] (1) Add electronic ceramic powder to an organic binder system and cast it into a ceramic diaphragm. The organic binder includes an organic solvent (50wt% to 70wt%), a resin (18wt% to 25wt%), and a plasticizer (8wt% to 15wt%). The organic solvent includes toluene and ethanol, the resin includes polyvinyl butyral (PVB), and the plasticizer includes dioctyl phthalate (DOP). After printing the metal inner electrode slurry on a 300-700 mesh screen, the ceramic diaphragm is staggered and stacked, cut to form a green body, and the green body is placed in a box for debonding (room temperature - 600°C), high temperature sintering (800-1200°C) to obtain a ceramic substrate.

[0041] (2) Forming a bottom silver layer: The ceramic substrate is coated with Ag paste, and then the end paste physically bonded after the coating is tightly combined with the ceramic body through high-temperature sintering (600-900°C) to form a bottom silver layer.

[0042] (3) Pretreatment of the flexible layer: The test conditions are divided into two groups. One group is no sandblasting and no treatment of the end electrode; the other group is manual sandblasting. The sandblasting equipment is used to control the compressed air pressure to 0.01-0.5 MPa, and 100-1000 mesh white corundum sand is sprayed to roughen the end electrode. After treatment, ultrasonic cleaning is used to remove the spray material.

[0043] (4) Coating flexible slurry: Based on the key performance indicators of the flexible slurry and process test verification, the soft end silver paste provided by Dalian Overseas Huasheng Electronic Technology Co., Ltd. was selected to prepare the terminal electrode flexible layer.

[0044] (5) Curing: After the flexible layer is coated, the product is unloaded onto a support plate, and then the support plate is placed in an air debonding furnace. The curing temperature is selected at 180°C, 200°C, and 220°C, and the curing time is 1 hour.

[0045] (6) Contact angle test: The contact angle of the bottom silver layer and the flexible layer was tested using a scanning electron microscope based on the contact angle measuring instrument technology.

[0046] (7) Product verification: Batch produce products under the same process conditions as steps (1) to (6), conduct a 4mm bending test on the products, and evaluate the performance of the flexible terminal electrode MLCC under external stress, paying particular attention to the capacitance change of the capacitor and the potential damage mode of the internal structure. Use the method of appearance inspection and in-situ capacitance test to evaluate whether the capacitor has failed: when there is visible damage on the appearance of the capacitor or the capacitance attenuation of the in-situ test reaches 5%, the sample is judged to have failed. Through contact angle characterization, the relationship between the strength of the bonding between the flexible layer and the bottom silver layer and the failure mode is studied and analyzed.

[0047] Test results:

[0048] Regarding the effect of whether the pre-treatment of the flexible layer is performed on the bonding between the bottom silver layer and the flexible layer, the adhesion of the flexible layer and the bottom silver is mainly achieved through the "wetting-shrinkage-embedded" method of the flexible slurry. In order to ensure the interfacial adhesion between the flexible layer and the bottom silver layer, increasing the bonding area between it and the bottom silver layer is a more effective breakthrough point. At present, the surface of the bottom silver can be roughened by sandblasting to increase the contact area between the flexible layer and the bottom silver, but it will reduce its wettability. The statistics of whether sandblasting is performed before the flexible layer is coated, the changes in the contact angle caused by different curing temperatures, and the changes in the bonding state between the bottom silver layer and the flexible layer are shown in Table 1:

[0049] Table 1 Group test results

[0050]

[0051] From Table 1, we can see that under the process conditions of manual sandblasting and curing temperature of 200°C, the number of products with capacitance attenuation greater than 5% is 0, there is no internal damage, and the contact angle is between 11.59° and 14.59°. Ideally, the flexible electrode undergoes elastic deformation to cope with non-structural damage, thereby reducing the bending stress on the ceramic body during the bending of the plate. Cracks or interface peeling inside the flexible layer caused by greater stress may lead to a decrease in capacitance, but the consequences of this situation are relatively small compared to failure caused by ceramic rupture. Through preliminary experiments, guidance can be provided for the design and process optimization of flexible MLCCs, and the contact angle range can be given, which can also be used as a means of product inspection.

[0052] Figure 1 This is the contact angle test method for manual sandblasting products. Figure 2 The contact angle test method for the unblasted product is carried out in the same place with the help of SEM tools. From the contact angle results in Table 1, the contact angle of the sandblasted product is generally higher than that of the unblasted product at the same temperature. It can be seen from the SEM interface results that Figure 3 a interface is obviously better than Figure 3 b interface has more protrusions, Figure 3b The interface is smoother, and the interface of the product after sandblasting is rougher. Under the same other conditions, the sandblasting surface treatment method will increase the surface roughness and reduce the wetting performance to a certain extent.

[0053] From the contact angle range in Table 1, we can see that when the pre-treatment method of the flexible layer is the same, as the temperature increases, the contact angle decreases first and then increases. At 200°C, the contact angle is the smallest, the wetting performance is the best, and the performance of the product is also the best. From the SEM interface results, we can see that Figure 4 (a) The combination of the flexible layer and the bottom silver layer is better than Figure 4 The interfaces of (b) and (c) are obviously not tight. Figure 4 There is no significant difference in the interface tightness between (b) and (c), but Figure 4 (b) The silver is mostly round. Figure 4 (b) The silver is partially in strip form, which also affects its wettability. It can be seen that the sandblasting process can roughen the surface of the bottom silver layer, increase the contact area between the flexible layer and the bottom silver, and adjust the curing temperature to compensate for the weakened wettability, thereby improving the interface bonding and enhancing its adhesion.

[0054] In general, the embodiments of the present invention utilize the wettability between the flexible layer and the bottom silver layer, and use the contact angle measurement method to obtain the connection between the contact angle and the strength of the bonding between the flexible layer and the bottom silver layer and the failure mode. This helps researchers / production personnel determine the influence of different process conditions (sandblasting, curing temperature, etc.) corresponding to different slurries on the strength of the bonding between the flexible layer and the bottom silver layer. On this basis, the influence of the strength of the bonding force and the failure mode of the flexible MLCC product is further determined, helping researchers to clarify the optimal range of the contact angle, that is, the product is not damaged; within what range, cracks or interface peeling inside the flexible layer may cause a decrease in capacitance, but no ceramic body rupture; beyond what range, the ceramic body will rupture and cause failure.

[0055] Through testing, it is found that when the contact angle is between 11.59° and 14.59°, the bonding strength between the bottom silver layer and the resin layer is appropriate. The contact angle is used to quantitatively characterize the bonding strength between layers using the wettability characteristics, which is used for guidance on design and process conditions and for product inspection criteria.

[0056] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a terminal electrode, characterized in that: The following steps are involved: coating a bottom silver layer on a ceramic substrate; Sandblasting the surface of the bottom silver layer to roughen the surface of the bottom silver layer; Coating a flexible paste on the roughened surface of the bottom silver layer; The coated flexible slurry is cured to obtain a flexible layer, and the curing temperature is 180°C to 220°C.

2. The method for preparing a terminal electrode according to claim 1, characterized in that: The coating of the bottom silver layer comprises coating Ag paste on the ceramic substrate, and then combining the Ag paste with the ceramic substrate through sintering to form the bottom silver layer, and the sintering temperature is 600-900°C.

3. A terminal electrode prepared according to claim 1 or 2, characterized in that: The contact angle between the bottom silver layer and the flexible layer is 11.59° to 14.59°.

4. A method for detecting the bonding strength between the bottom silver layer and the flexible layer of the terminal electrode, characterized in that: The test method comprises characterizing the contact angle of a ceramic substrate coated with a terminal electrode, wherein the terminal electrode comprises the terminal electrode prepared according to claim 1 or 2.

5. The method for detecting the bonding strength between the bottom silver layer and the flexible layer of the terminal electrode according to claim 4, characterized in that: The following steps are involved: S1: Preparation of ceramic substrate; S2: preparing terminal electrodes on the ceramic substrate; S3: Contact angle test.

6. A method for detecting the bonding strength between the bottom silver layer and the flexible layer of the terminal electrode according to claim 5, characterized in that: It also includes S4: product verification; the product verification includes subjecting the product prepared by S2 to a bending test to verify the relationship between wettability, the bonding strength between the bottom silver layer and the flexible layer, and the capacitor failure mode, and to determine whether the contact angle result obtained by the S3 test is credible.

7. A method for detecting the bonding strength between the bottom silver layer and the flexible layer of the terminal electrode according to claim 6, characterized in that: The preparation of the terminal electrode includes at least two preparation methods to obtain at least two terminal electrodes, and the preparation difference of the two terminal electrodes includes at least one difference in the flexible slurry formula, the treatment method of the bottom silver layer surface before coating the flexible slurry, and the flexible slurry curing temperature.

8. The method for detecting the bonding strength between the bottom silver layer and the flexible layer of the terminal electrode according to claim 7, characterized in that: The processing method of the bottom silver layer surface before coating the flexible slurry includes sandblasting the bottom silver layer surface before coating the flexible slurry and not sandblasting the bottom silver layer surface before coating the flexible slurry.

9. The method for detecting the bonding strength between the bottom silver layer and the flexible layer of the terminal electrode according to claim 6, characterized in that: The preparation of the ceramic substrate includes adding electronic ceramic powder into an organic adhesive system to cast a ceramic diaphragm, wherein the organic adhesive includes an organic solvent, a resin and a plasticizer, and the weight fractions are 50wt% to 70wt%, 18wt% to 25wt% and 8wt% to 15wt% respectively.

10. The method for detecting the bonding strength between the bottom silver layer and the flexible layer of the terminal electrode according to claim 6, characterized in that: The organic solvent includes toluene and ethanol, the resin includes polyvinyl butyral, and the plasticizer includes dioctyl phthalate.

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