A rapid screening method for crack resistance of potting materials

Abstract

A kind of quick screening method of potting material crack resistance belongs to the technical field of material performance and reliability evaluation, and the method comprises the following steps: step 1: preparing sample, preparing a screening appliance, which has pouring groove, and the middle part of the bottom surface of pouring groove is vertically provided with embedded part of rigid structure, embedded part has at least one sharp corner structure, the screening appliance is preheated to predetermined temperature, the liquid potting material to be evaluated is poured into pouring groove, and the inner side wall of pouring groove and potting material are mutually adhered;Step 2: solidification;Step 3: induce cracking test, after solidification is completed, at least one rapid cooling treatment is carried out on screening appliance and sample;Step 4: result observation and evaluation, whether radial radial crack is generated at sharp corner of embedded part is observed by visual or microscopic observation, and "cracking" or "not cracking" is used as the most direct screening basis.The scheme can more quickly, low cost and accurately screen the potting material of I type cracking failure.

Description

Technical Field

[0001] This invention belongs to the field of material performance and reliability evaluation technology, and in particular relates to a rapid screening method for the crack resistance of potting materials. Background Technology

[0002] Epoxy resins, polyurethanes, and other potting materials are widely used for the encapsulation and protection of electronic and electrical equipment and components due to their excellent insulation, sealing, and mechanical properties. In high-reliability fields such as power electronics and aerospace, the crack resistance of potting materials directly determines the long-term reliability of the entire module.

[0003] The core mechanism of potting cracking lies in the residual stress generated by the curing shrinkage of the material, and the superposition of thermal stress caused by the mismatch in the coefficients of thermal expansion between the material and the encapsulated components, such as metal, under temperature changes. Studies have shown that this stress is particularly concentrated at the interface, which easily leads to interface debonding or bulk cracking, mainly of the type I, i.e., opening type.

[0004] To address the aforementioned problems, existing technology research and development mainly focuses on two directions: One direction is to improve the intrinsic properties of potting materials. This involves developing potting materials with high toughness and low coefficient of thermal expansion by adding toughening agents, modified curing agents, and specific fillers. However, the industry currently lacks efficient and standardized evaluation methods for the crack resistance of potting materials, which restricts the speed of material research and development iteration and application verification.

[0005] The second approach involves evaluating and testing the crack resistance of potting materials. A common method is to embed a metal insert within the potting material and cure it. The insert can be a stainless steel gasket, screw, or a metal block with sharp edges. The cured potting compound is then placed in an environment with varying temperatures. The difference in thermal expansion and contraction between the metal block and the potting material during these temperature changes causes stress, which can lead to cracking of the potting material. The crack resistance of the potting material is then assessed by comparing the temperature range at which cracking occurs or the number of temperature cycles. This evaluation method, which involves encapsulating metal inserts with potting material and conducting temperature tests, has the following shortcomings: First, the insert structure is complex, and its stress state is multidimensional and mixed, making it difficult to accurately correlate with specific Type I cracking failure modes in actual products, thus limiting the guiding significance of the evaluation results; Second, actual electronic potting cracking problems often involve the potting material being tightly wrapped by a rigid shell or mold, and the constraint effect formed by the rigid shell is significantly greater than that of the potting material encapsulating the metal insert, while the latter evaluation method does not adequately simulate the constraint effect of such strong structures; In addition, the potting material encapsulating the metal insert usually requires dozens to hundreds of cycles in a wide temperature range, such as -55℃ to 150℃, before cracking occurs, with a testing cycle of several weeks, resulting in low efficiency and high cost.

[0006] Therefore, there is an urgent need to develop a rapid, low-cost method for screening the crack resistance of potting materials with failure modes that are highly correlated with real-world scenarios, in order to accelerate the selection and development of high-reliability potting materials. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a rapid screening method for the crack resistance of potting materials, which can more quickly, at a lower cost, and accurately screen potting materials that fail with type I cracking.

[0008] In order to achieve the objective of this invention, the following solution is proposed: A rapid screening method for the crack resistance of potting materials includes the following steps: Step 1: Prepare a sample. Prepare a screening device with a filling tank. A rigid embedded part is vertically provided in the middle of the bottom surface of the filling tank. The embedded part has at least one sharp corner structure. Preheat the screening device to a predetermined temperature. Fill the liquid potting material to be evaluated into the filling tank. The inner sidewall of the filling tank and the potting material are adhered to each other. Step 2: Curing, which allows the potting material in the injection tank to form a solid sample; Step 3: Induced cracking test. After curing, the screening equipment and the sample are subjected to at least one rapid cooling treatment. Step 4: Result observation and evaluation. Visually or microscopically observe whether radial cracks are generated at the sharp corners of the insert. "Cracked" or "not cracked" is the most direct screening criterion.

[0009] The beneficial effects of this invention are as follows: 1. The stress mode is purer and more correlated. The strong constraint of the injection tank simulates the rigid constraint of the device shell. The debonding of the insert focuses the sharp corner, creating and amplifying the type I stress concentration. This makes the failure mode highly consistent with the typical cracking mode of products such as dry capacitors. The evaluation results are more direct and instructive. 2. The rapid screening feature is more prominent. This solution greatly improves the rigor and sensitivity of the test, so that most materials will crack in a identifiable way during the first cooling process, shortening the evaluation cycle from the traditional days / weeks to hours, and achieving true rapid screening. Attached Figure Description

[0010] The accompanying drawings described herein are merely illustrative of selected embodiments, not all possible implementations, and are not intended to limit the scope of the invention.

[0011] Figure 1 A flowchart illustrating the screening method of this application is shown.

[0012] Figure 2 An exploded view of the structure of the screening device of this application is shown.

[0013] Figure 3 A schematic diagram of the screening device of this application is shown.

[0014] Figure 4 A schematic diagram showing the state of the screening device after the potting material has been poured in.

[0015] Figure 5 A cross-sectional view of the screening device of this application after the potting material has been poured in is shown.

[0016] The markings in the figure are: insert-1, base plate-2, positioning hole-21, rigid frame-3, and sample-4. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the implementation methods of the present invention will be described in detail below with reference to the accompanying drawings. However, the embodiments described in this invention are only some embodiments of the present invention, and not all embodiments.

[0018] like Figure 1 As shown, a rapid screening method for the crack resistance of potting materials includes the following steps: Step 1: Prepare sample 4, such as Figures 2 to 5 As shown, a screening device is prepared, which has an upward-opening filling tank. A rigid insert 1 is vertically provided in the center of the bottom surface of the filling tank, and the insert 1 has at least one sharp corner structure. Specifically, the edge of the sharp corner structure is perpendicular to the bottom surface of the filling tank. The screening device is preheated to a predetermined temperature, which is usually the recommended operating temperature of the potting material, i.e., the temperature at which the potting material flows during filling. The potting material is usually a resin, such as UV glue, epoxy resin, or polyurethane resin. The liquid potting material to be evaluated is poured into the filling tank, and the inner wall of the filling tank adheres to the potting material.

[0019] Preferably, the potting material needs to be vacuum degassed before injection to avoid affecting the screening results.

[0020] Preferably, the insert 1 can be designed as a triangular prism, a square prism, or something similar. Figures 2 to 4 The elliptical cylinder shown can have at least one sharp corner on the outside of the insert 1, or both ends of the elliptical cylinder can be set as sharp corners.

[0021] More preferably, the insert 1 is a long strip structure with a length-to-width ratio of more than 10, used to induce type I stress concentration.

[0022] Step 2: Curing. The potting material in the injection tank forms a solid sample 4. The potting material in the injection tank is cured according to the specified curing process for different potting materials. Different types of resins, such as UV adhesives, epoxy resins, and silicone resins, have significant differences in curing methods, performance, and applicable scenarios. UV adhesives: These are single-component systems that can be rapidly cured within seconds to one minute by irradiation with ultraviolet light of a specific wavelength (365nm-405nm), making them suitable for automated production lines and scenarios requiring rapid positioning. Epoxy resins: These are typically two-component AB adhesives that cure through a chemical cross-linking reaction after the main agent and curing agent are mixed. They require several hours to 24 hours to fully cure at room temperature, making them suitable for structural bonding scenarios requiring ultra-high strength, high temperature resistance, and chemical corrosion resistance. Polyurethane resins offer a variety of curing methods, with two-component reactive curing being the most mainstream. This method involves mixing and reacting a component containing isocyanate groups (-NCO) with a polyol component containing hydroxyl groups (-OH) to generate urethane bonds for cross-linking and curing. It is suitable for electronic potting compounds, coatings, elastomers, and other fields, and features strong formulation designability and a wide range of performance adjustment.

[0023] Step 3: Induced cracking test. After curing, the screening device and sample 4 are subjected to at least one rapid cooling treatment. During this process, the potting material shrinks, but is constrained by the rigid bonding with the frame. At the same time, a significant stress concentration area is generated at the tip of the insert. During the test, the screening device and sample 4 are placed in a temperature-controlled environment and the temperature of the screening device and sample 4 is controlled. Specifically, a temperature-controlled chamber can be used to create a temperature-controlled environment.

[0024] The specific testing process can be implemented in the following three ways: Method 1: Cool the cured sample 4 to room temperature and maintain it for a predetermined time, perform a single cooling test, and then observe. Many materials with insufficient crack resistance will crack at this step and can be immediately screened out. Method 2: If sample 4 does not crack when cooled to room temperature, a gradient test is performed, with each fixed temperature step being a gradual decrease in temperature and the same holding time for each step. For example, each temperature is decreased by 10°C and held for 30 minutes each time. Observe whether cracking occurs. This way, the critical point of the material's crack resistance at low temperatures can be accurately measured.

[0025] Method 3: If sample 4 does not crack after cooling to a predetermined temperature, such as the minimum design temperature, a high-low temperature cycling test is performed, for example, cycling between -40°C and +85°C at a rate of 5°C / min for 30 minutes. This method is suitable for some high-performance potting materials and can serve as a basis for durability evaluation. Semi-quantitative / quantitative evaluation: Multiple cycles can be performed, recording the number of cycles at which a visible crack first appears or measuring the crack length.

[0026] Step 4: Result observation and evaluation. Visually or microscopically observe whether radial cracks are generated at the sharp corners of the insert. "Cracked" or "not cracked" is the most direct screening criterion.

[0027] As a preferred method, in addition to visual inspection and crack length testing, digital image correlation (DIC) technology can also be used to monitor the strain field evolution in the sharp corner area of ​​the insert during the cooling process, so as to predict the cracking tendency earlier.

[0028] In practice, an aluminum alloy frame with roughened inner walls can be used, and an aluminum alloy base plate can be placed at its bottom to form a casting groove. An elliptical aluminum alloy sheet with a major axis of 40mm and a minor axis of 3mm is placed vertically in the casting groove, and a silicone release agent is applied to the surface of the aluminum alloy sheet. The epoxy resin to be tested is poured in, and after curing, it is placed in an oven. The oven is closed from 80℃, and the sample 4 is allowed to cool to room temperature with the oven. After removal, it is observed whether cracks appear at both ends of the aluminum alloy sheet. If cracks appear, it indicates that the material has not passed this rapid screening.

[0029] Preferred, such as Figure 2 , Figure 3 As shown, the injection tank is assembled from a base plate 2 and a rigid frame 3. Specifically, the rigid frame 3 is made of rigid metal, such as aluminum alloy, copper alloy or stainless steel, preferably aluminum alloy, which not only has better heat conduction effect but is also easy to process. The rigid frame 3 is located on the top surface of the base plate 2, thereby forming the injection tank. The top surface of the base plate 2 is provided with positioning holes 21 for installing the insert 1. This structural design facilitates the quick removal of the sample 4 after the test and the cleaning of the inside of the injection tank for the next test.

[0030] Further preferably, silicone rubber is applied to the joints between the insert 1, the base plate 2 and the rigid frame 3 to ensure that the internal shape of the injection groove is regular and to prevent the injection material from extending into the assembly gap.

[0031] As a preferred option, the screening device can be extended into a quantitative evaluation platform. The screening device can not only be used for qualitative screening of pass-through types, but also accurately record crack initiation temperature, propagation rate and energy release by integrating digital image correlation technology, providing a standardized platform for quantitative comparison and in-depth research of material properties.

[0032] Preferably, the inner wall of the injection tank is treated by sandblasting, chemical etching or coating with a strong coupling agent to ensure the adhesion strength between the potting material and the inner wall of the injection tank, thereby ensuring the constraint force on the outside of the sample 4 and transferring the shrinkage stress to the preset vulnerable interface, namely the sharp corner of the insert 1.

[0033] Preferably, the insert 1 is made of a rigid metal material, and its outer surface is coated with a release agent, such as silicone grease or Teflon coating, to ensure a completely "detachable" interface with the potting material, i.e., no adhesive force. This design makes the insert 1 a "free" obstacle in the potting body. When the potting material shrinks, the stress is highly concentrated at the sharp corner of the insert 1, i.e., at the point of minimum radius of curvature, in order to induce pure type I cracks.

[0034] Preferably, the bottom surface of the injection tank is a planar structure and is coated with a release agent, such as silicone grease or Teflon coating, to ensure that the cured sample 4 can be easily detached and is not constrained or adhered to by the bottom surface of the injection tank. This ensures that the sample 4 is only subjected to force within the system formed by the periphery of the injection tank and the insert 1, thereby improving the accuracy of the test.

[0035] The above description is merely a preferred embodiment of the present invention and is not intended to be the only or limiting of the invention. Those skilled in the art should understand that various changes or equivalent substitutions made to the present invention without departing from its scope are all within the protection scope of the present invention.

Claims

1. A rapid screening method for the crack resistance of potting materials, characterized in that, Includes the following steps: Step 1: Prepare sample (4), prepare a screening device with a filling tank, and a rigid embedded part (1) is vertically provided in the middle of the bottom surface of the filling tank. The embedded part (1) has at least one sharp corner structure. Preheat the screening device to a predetermined temperature, and pour the liquid potting material to be evaluated into the filling tank. The inner sidewall of the filling tank and the potting material are adhered to each other. Step 2: Curing, so that the potting material in the injection tank forms a solid sample (4). Step 3: Induced cracking test. After curing, the screening equipment and sample (4) are subjected to at least one rapid cooling treatment. Step 4: Result observation and evaluation. Visually or microscopically observe whether radial cracks are generated at the sharp corner of the insert (1).

2. The rapid screening method for the crack resistance of potting materials according to claim 1, characterized in that, The injection tank is assembled from a base plate (2) and a rigid frame (3). The rigid frame (3) is located on the top surface of the base plate (2). The top surface of the base plate (2) is provided with positioning holes (21) for installing the insert (1).

3. The rapid screening method for the crack resistance of potting materials according to claim 2, characterized in that, Apply silicone rubber to the joints between the insert (1), the base plate (2) and the rigid frame (3).

4. The rapid screening method for the crack resistance of potting materials according to claim 1, characterized in that, The inner wall of the injection tank is treated by sandblasting, chemical etching, or coating with a coupling agent.

5. The rapid screening method for the crack resistance of potting materials according to claim 1, characterized in that, The insert (1) is made of rigid metal material and its outer surface is coated with a release agent.

6. The rapid screening method for the crack resistance of potting materials according to claim 1, characterized in that, The bottom surface of the injection tank is flat and coated with a release agent.

7. The rapid screening method for the crack resistance of potting materials according to claim 1, characterized in that, In step 3, during the induced cracking test, the screening device and the sample (4) are placed in a temperature-controlled environment, and the temperature of the screening device and the sample (4) is controlled.

8. The rapid screening method for crack resistance of potting materials according to claim 7, characterized in that, In step 3, during the induced cracking test, the cured sample (4) was first cooled to room temperature and kept for a predetermined time before observation was performed.

9. The rapid screening method for the crack resistance of potting materials according to claim 8, characterized in that, If the sample (4) does not crack when cooled to room temperature, a gradient test is performed, with each fixed temperature as a step to gradually reduce the temperature, and each step is kept at the same temperature for the same amount of time.

10. The rapid screening method for crack resistance of potting materials according to claim 9, characterized in that, If the sample (4) does not crack when cooled to the predetermined temperature, a high and low temperature cycling test will be performed.

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