A method for testing the reliability of steel reinforcement

Abstract

The application provides a steel reinforcing reliability test method, including the following steps: S1: providing a flexible circuit board with steel reinforcing, and measuring the initial steel reinforcing ground resistance; S2: after the flexible circuit board is subjected to moisture removal and baking, a SMT reflow temperature curve is simulated to perform reflow processing, the flexible circuit board after reflow is obtained, and the steel reinforcing ground resistance of the flexible circuit board after reflow is measured; S3: the flexible circuit board after reflow is placed in 95 DEG C hot water for soaking treatment, and after soaking is completed, the surface moisture is removed, and the flexible circuit board after hot water soaking treatment is obtained; S4: the steel reinforcing ground resistance of the flexible circuit board after hot water soaking treatment is measured, and the steel reinforcing ground resistance reliability is verified. The method has the advantages of short test period, and can accurately distinguish the two failure modes of the reduction of the conductive path caused by the moisture absorption of the steel reinforcing conductive adhesive and the reduction of the grounding area caused by the interface debonding.

Description

Technical Field

[0001] This application relates to flexible printed circuit board testing technology, and more specifically, to a method for testing the reliability of steel reinforcement. Background Technology

[0002] In the field of flexible printed circuit board (FPCB) manufacturing, steel reinforcement, as a key process material, serves the dual function of enhancing the support strength of the component area and realizing electrostatic discharge. The reliability of the grounding resistance of the steel reinforcement directly affects the electrical performance stability of the product in actual use. Traditional verification methods rely on accelerated testing in high-temperature and high-humidity environments. For example, the Heat Soak test requires 500 hours at 65 degrees Celsius and 90% relative humidity, or thermal cycling tests simulate stress caused by differences in the thermal expansion coefficients of materials. While such tests can reflect long-term environmental effects, they have significant drawbacks: firstly, the excessively long testing cycle leads to low efficiency in product development and quality verification; secondly, when increasing temperature conditions to shorten testing time, conventional environmental testing equipment struggles to meet extreme temperature requirements, and excessively high temperatures may induce side reactions that do not occur during normal use, interfering with the accurate identification of failure modes. The failure mechanism of the grounding resistance of steel reinforcement mainly involves the reduction of conductive paths due to moisture absorption by the conductive adhesive of the steel reinforcement or the reduction of grounding area caused by interface debonding. Traditional environmental testing cannot specifically distinguish between these two failure modes, nor can it fully simulate moisture absorption behavior within a reasonable timeframe. In existing technologies, the moisture absorption process dominated by water vapor diffusion is slow, which severely restricts the timeliness of reliability verification. The industry urgently needs a test scheme that can quickly simulate the moisture absorption effect and avoid additional side reactions in order to solve the technical bottleneck caused by long-term reliance on time-consuming environmental tests.

[0003] To address the aforementioned problems, existing technologies urgently need improvement. Summary of the Invention

[0004] The purpose of this application is to provide a reliability testing method for steel reinforcement, which significantly shortens the testing cycle and can accurately distinguish between two failure modes: the reduction of conductive paths caused by moisture absorption of the conductive adhesive in the steel reinforcement and the reduction of grounding area caused by interface debonding.

[0005] This application provides a method for testing the reliability of steel reinforcement, the technical solution of which is as follows: Includes the following steps: S1: Provide a flexible circuit board with steel reinforcement and measure the initial steel reinforcement grounding resistance. S2: After dehumidifying and baking the flexible circuit board, set the SMT reflow temperature profile for reflow treatment to obtain the reflowed flexible circuit board, and measure the steel reinforcement grounding resistance of the reflowed flexible circuit board. S3: Immerse the reflowed flexible circuit board in 95℃ hot water. After immersion, remove it and remove the surface moisture to obtain the flexible circuit board after hot water immersion treatment. S4: Measure the steel reinforcement grounding resistance of the flexible circuit board after hot water immersion treatment to verify the reliability of the steel reinforcement grounding resistance.

[0006] Furthermore, this application also proposes that the flexible circuit board includes at least a flexible circuit board substrate, flexible circuit board L1 surface lines on the flexible circuit board substrate, protective adhesive covering the flexible circuit board substrate and the flexible circuit board L1 surface lines, a protective film covering the protective adhesive, steel reinforcing steel, and steel reinforcing conductive adhesive for connecting the protective film, the flexible circuit board L1 surface lines, the flexible circuit board substrate and the steel reinforcing steel.

[0007] Furthermore, this application also proposes that the steel reinforcing material is steel with nickel electroplated or chemically deposited on the outer surface, and one side of it is covered with a steel reinforcing conductive adhesive with polyurethane as the substrate and filled with metal conductive particles.

[0008] Furthermore, this application also proposes to perform tensile tests on the steel reinforcing material in steps S1, S2, and S4.

[0009] Furthermore, this application also proposes that in step S1, the initial steel reinforcement grounding resistance is measured by a four-terminal electrical testing device without sudden current change, and the maximum current is 20mA. Flexible circuit boards with resistance values ​​less than a first predefined threshold proceed to step S2.

[0010] Furthermore, this application also proposes that in step S2, the dehumidification baking conditions are baking at 120°C for 2 hours, the reflux treatment is reflux twice with a simulated 260°C reflux curve, and the time from the end of baking to the completion of reflux is controlled within 24 hours.

[0011] Furthermore, this application also proposes that the tensile test be conducted in a direction perpendicular to the plane of the steel reinforcing steel, at a test speed of 57 mm / min, and the tensile force per unit width is obtained by calculating the average load and dividing it by the width of the steel reinforcing steel.

[0012] Furthermore, this application also proposes that steps S1, S2, and S4 further include setting a minimum unit width tensile force value. When the calculated unit width tensile force value is less than the predefined minimum unit width tensile force value, the flexible circuit board in steps S1 and S2 is marked as a defective product and does not proceed to the next step. In step S4, the flexible circuit board is directly marked as a defective product.

[0013] Furthermore, this application also proposes that step S3 specifically involves immersing the reflowed flexible circuit board in hot water at 95°C for 4 hours, then removing it and wiping off the surface moisture to obtain a flexible circuit board treated with hot water immersion.

[0014] Furthermore, this application also proposes that step S4 includes: measuring the steel reinforcement grounding resistance of the flexible circuit board after hot water immersion treatment using a four-terminal electrical testing device without sudden current change, with an upper limit of 5Ω for the resistance value; If the resistance of all flexible circuit boards is ≤5Ω, then randomly select some flexible circuit boards to measure and record the tensile strength data of the steel reinforcing steel. If there are flexible circuit boards with a resistance value > 5Ω, then these flexible circuit boards are marked as defective, and the tensile strength data of the steel reinforcing steel is measured for all defective products. Reliability is determined based on the measurement results: If the resistance of some parts is greater than 5Ω, and the tensile strength of the steel reinforcement after hot water immersion treatment is not significantly reduced compared with that after reflow, it is determined that the steel reinforcement has failed. The main failure mode is that the conductive adhesive of the steel reinforcement absorbs moisture, resulting in a reduction of the conductive path. If the resistance of some parts is greater than 5Ω, and the tensile strength of the steel reinforcement after hot water immersion is significantly lower than that after reflow, it is determined that the steel reinforcement has failed. The main failure mode is that the conductive adhesive of the steel reinforcement absorbs moisture and debonds, which induces partial delamination and reduces the grounding area. If all resistance values ​​are ≤5Ω, and the tensile strength of the steel reinforcement after hot water immersion treatment is greater than the minimum unit width tensile strength, then the steel reinforcement is deemed reliable and effective.

[0015] As can be seen from the above, the steel reinforcement reliability testing method provided in this application effectively replaces the traditional high temperature and high humidity environment test by rapidly simulating the moisture absorption effect through hot water immersion and combining it with the dual verification mechanism of grounding resistance and tensile force test. This significantly shortens the test cycle and accurately identifies two failure modes: moisture absorption and interface delamination of the conductive adhesive in the steel reinforcement. This method has the advantages of significantly shortening the test cycle and accurately distinguishing between two failure modes: the reduction of conductive path caused by moisture absorption of the conductive adhesive in the steel reinforcement and the reduction of grounding area caused by interface delamination. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of a steel reinforcement reliability testing method provided in this application.

[0017] Figure 2 This is a schematic diagram of a steel reinforcement structure provided in this application.

[0018] Figure 3 This is a schematic diagram illustrating the conductive principle of steel reinforcement provided in this application.

[0019] Figure 4 A schematic diagram of a double-layer plate provided in this application.

[0020] Figure 5 This application provides a schematic diagram of measuring the steel reinforcement grounding resistance of a flexible circuit board using a four-terminal electrical testing device.

[0021] Figure 6 A schematic diagram of the moisture absorption and weight gain curve of a steel reinforcing material provided in this application.

[0022] Figure 7 This is a schematic diagram illustrating a failure mode of conductive adhesive provided in this application.

[0023] Figure 8 This is a schematic diagram illustrating a defect caused by an air bubble, as provided in this application. Detailed Implementation

[0024] The technical solutions of this application will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of this application, and not all embodiments. The components of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0025] It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. Furthermore, in the description of this application, terms such as "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0026] Traditional reliability testing methods for steel reinforcement in flexible printed circuit boards, such as the Heat Soak environmental moisture absorption test, typically require a cycle of up to 500 hours, resulting in low testing efficiency and failing to meet the needs of rapid verification. Furthermore, existing methods struggle to effectively distinguish between reduced conductive paths and delamination when determining the failure of the steel reinforcement grounding resistance, thus limiting the development of targeted improvement measures.

[0027] This application proposes a method for testing the reliability of steel reinforcement, including the following steps: S1: Provide a flexible circuit board with steel reinforcement and measure the initial steel reinforcement grounding resistance. S2: After dehumidifying and baking the flexible circuit board, set the SMT reflow temperature profile for reflow treatment to obtain the reflowed flexible circuit board, and measure the steel reinforcement grounding resistance of the reflowed flexible circuit board. S3: Immerse the reflowed flexible circuit board in 95℃ hot water. After immersion, remove it and remove the surface moisture to obtain the flexible circuit board after hot water immersion treatment. S4: Measure the steel reinforcement grounding resistance of the flexible circuit board after hot water immersion treatment to verify the reliability of the steel reinforcement grounding resistance.

[0028] For ease of understanding, the following explains some key terms in this embodiment: Grounding resistance, which represents the resistance between the steel reinforcement and the grounding path of the flexible circuit board, is used to evaluate the grounding connection. SMT reflow temperature profile, which simulates the specific temperature change process experienced during surface mount technology (SMT) soldering, is used to evaluate the performance of the product under actual production thermal stress.

[0029] Reflow processing refers to the process of heating and cooling flexible circuit boards according to a preset SMT reflow temperature profile to simulate the impact of the actual production environment on the materials.

[0030] Common steel reinforcement structures include Figure 2 As shown in the diagram (1. Front protective film PI 2. Front protective film adhesive 3. Flexible circuit board L1 side circuit 4. Flexible circuit board PI substrate 5. Flexible circuit board L2 side circuit 6. Back protective film adhesive 7. Back protective film PI 8. Steel reinforcing material 9. Steel reinforcing conductive adhesive), this is a sheet of nickel-plated steel reinforcing material with an isotropic steel reinforcing conductive adhesive of the same size attached to one side, used to bond the steel reinforcing material and the flexible circuit board. The steel reinforcing conductive adhesive is composed of a high-molecular polymer as the base material, doped with conductive metal particles in different proportions. Figure 3 As shown, initially, the volume ratio of metal particles to polymer resin did not reach the "throughflow threshold," so it was non-conductive or exhibited low conductivity. When the steel reinforcement was attached to the grounding area of ​​the flexible circuit board and subjected to hot pressing and curing, the shrinkage of the conductive adhesive in the steel reinforcement reduced the distance between the metal particles, even causing them to come into contact with each other, establishing numerous conductive channels in multiple directions. The more conductive channels formed, the lower the grounding resistance.

[0031] Although both the steel reinforcement grounding resistance and the reliability of the circuit board can be verified under high temperature and humidity conditions, their failure mechanisms differ and require different approaches in analysis. The main failure modes of circuit boards are roughly as follows: 1. Delamination caused by temperature and humidity changes; 2. Aging of the substrate after moisture absorption; 3. Dendritic growth caused by moisture-driven electrochemical migration. All three affect the electrical function of the circuit board by damaging its insulation.

[0032] The influence of environmental conditions on the resistance of steel reinforcement differs slightly from the above model, but can generally be categorized into three types: 1. Weakening the bonding force between the steel reinforcement, the conductive adhesive, and the flexible circuit board, inducing debonding and peeling, reducing the contact area between interfaces and increasing resistance; 2. Oxidation and corrosion of the steel; 3. Reactions involving the conductive adhesive (such as degradation, plasticization, expansion, etc.), which increase resistance by increasing the bulk resistance or interrupting the original conductive particle pathways.

[0033] Since the resistance of the steel reinforcing material itself is generally around 5-10 mΩ and is relatively stable in the environment, its impact on the resistance is negligible. Therefore, the influence of the steel reinforcing material on the resistance will not be discussed here. Taking PU-based steel-reinforcing conductive adhesive as an example, its failure modes are twofold: 1. Moisture absorption and debonding of conductive adhesive in steel reinforcement and differences in interlayer expansion coefficients cause interface bubbles or delamination, resulting in a reduction in grounding area; 2. The original conductive particle pathway is disrupted due to moisture absorption and expansion.

[0034] Having understood the above failure modes, we can design analytical methods to investigate each one systematically. For failure mode 1, we can confirm it through tensile testing of the steel reinforcement and cross-section analysis. If there is no significant difference in the tensile strength of the steel reinforcement before and after the environmental changes, or if no bubbles or delamination are observed in the cross-section, this failure mode can be ruled out. For failure mode 2, since it is mainly affected by the moisture in the conductive adhesive of the steel reinforcement, further research on the material's moisture absorption behavior is needed.

[0035] This embodiment provides a method for testing the reliability of steel reinforcement, the specific steps of which are as follows: In step S1, a flexible circuit board with steel reinforcement is first provided. This flexible circuit board can be made of various substrates, such as a polyimide film, on which conductive lines can be integrated. The steel reinforcement can be made of stainless steel or other suitable metal materials and is connected to the flexible circuit board by adhesive or conductive glue. Subsequently, the initial grounding resistance of the provided flexible circuit board with the steel reinforcement is measured. This measurement can be performed by connecting probes to the grounding terminals of the steel reinforcement and the flexible circuit board and reading the value using a resistance meter. For example, a common digital multimeter or a two-terminal resistance meter can be used to obtain the reference grounding resistance of the steel reinforcement before it is subjected to any environmental stress.

[0036] In step S2, the flexible circuit board is subjected to dehumidification baking. Dehumidification baking can be performed by placing the flexible circuit board in a heating oven, for example, baking at a temperature of 80°C to 100°C for several hours, to remove any initial moisture that may be present in the material. After baking, the flexible circuit board is reflowed to simulate the SMT reflow temperature profile. This reflow process can be performed by passing the flexible circuit board through a reflow oven, for example, setting a single reflow process with a peak temperature of 240°C to 250°C. After the reflow process, a reflowed flexible circuit board is obtained. Subsequently, the steel reinforcement grounding resistance of the reflowed flexible circuit board is measured again. The measurement method can be similar to that in step S1, using a resistance meter.

[0037] In step S3, the reflowed flexible circuit board is immersed in 95°C hot water. This immersion process can be performed by completely submerging the flexible circuit board in a container filled with 95°C hot water, for example, for approximately 2 to 3 hours. After immersion, the flexible circuit board is removed from the hot water, and its surface moisture is removed. Surface moisture removal can be achieved by natural air drying, wiping with an absorbent cloth, or blowing with a low-pressure airflow. After this treatment, the flexible circuit board obtained after hot water immersion has sufficient moisture absorption in its internal materials, especially the steel-reinforcing conductive adhesive.

[0038] In step S4, the grounding resistance of the steel reinforcement on the flexible circuit board after hot water immersion is measured. This measurement can be performed using a resistance meter, similar to steps S1 and S2. After measurement, the reliability of the steel reinforcement grounding resistance is verified based on the obtained data. For example, a simple upper limit can be set, such as 10Ω. If all measured values ​​are below this upper limit, the steel reinforcement is considered reliable; if any value exceeds this upper limit, the reliability is considered unreliable. This step aims to quickly evaluate the electrical performance of the steel reinforcement under simulated long-term high temperature and high humidity conditions.

[0039] To verify the reliability of a material in a short period of time, accelerated environmental testing is necessary. The settings for accelerated conditions under common high-temperature and high-humidity environments are typically calculated using the Hallberg-Peck model, a model specifically designed to describe the accelerated aging process of materials or products under the dual stresses of temperature and humidity. The acceleration factor (AF) is linearly related to the reaction rate; therefore, by calculating the relative acceleration factor of the operating conditions compared to the preset conditions, the equivalent reaction time can be estimated. The calculation formula is as follows:

[0040] Where π T π is the temperature factor. RH For humidity factor, π V E is the volume factor. aThe activation energy is given by k, and the Boltzmann constant is given by T. The temperature is represented by R. H Represents relative humidity, n represents the humidity acceleration constant (usually 2-3, here it is 2), the subsuffix U represents conventional parameters, and the subsuffix S represents the parameters used in the test. Since there is no significant volume change, exp[β*(V S- V u This value is 1. When using it, a suitable set of temperature and relative humidity conditions can be found for the test through the target acceleration factor.

[0041] Table 1

[0042] Table 1 shows the theoretical reaction time under the two test conditions compared to the Heat Soak test (65℃, 90%RH, 500H). It can be seen that to compress the original 500-hour test to 24 hours or even a few hours, the test conditions, especially the temperature conditions, are extremely stringent, to the point that conventional environmental chambers cannot meet the requirements (HIRAYAMA PC-422R9 high-accelerated life test chamber, with a maximum set temperature of 121℃). Furthermore, excessively high temperatures may cause side reactions in the product that would not occur under normal operating conditions, interfering with the determination of failure modes and deviating from the original purpose of environmental accelerated testing.

[0043] Therefore, the drawback of traditional environmental testing is that it takes too long (500 hours). Even with accelerated testing, the humidity absorption of the environment is limited by the saturation moisture content of the air, so the only way to speed up the test is to continuously increase the temperature. However, for excessively high temperatures, conventional equipment cannot meet the requirements, and side reactions that would not occur at the original temperature may also arise.

[0044] This method significantly shortens the reliability verification cycle of steel reinforcement by introducing 95℃ hot water immersion treatment, compressing the traditional environmental moisture absorption test, which takes hundreds of hours, into a few hours. By measuring and comparing the grounding resistance values ​​of the steel reinforcement after initial, reflow, and hot water immersion treatment, the electrical performance of the steel reinforcement in high temperature and high humidity environments can be quickly evaluated, thereby effectively improving the efficiency and timeliness of reliability testing of flexible circuit board steel reinforcement.

[0045] In some of the solutions mentioned above in this application, a steel reinforcement reliability testing method was proposed. However, in practical applications, the structure of flexible circuit boards is complex and diverse. If the flexible circuit board is tested in a general way, it is impossible to clarify the specific composition of the test object in terms of physical structure. This results in the lack of specificity of the test method and makes it difficult to accurately assess the failure risk of each layer of materials and connection interfaces in high temperature and high humidity environments.

[0046] This application further proposes that the flexible circuit board includes at least a flexible circuit board substrate, flexible circuit board L1 surface lines on the flexible circuit board substrate, protective adhesive covering the flexible circuit board substrate and the flexible circuit board L1 surface lines, a protective film covering the protective adhesive, steel reinforcing steel, and steel reinforcing conductive adhesive for connecting the protective film, the flexible circuit board L1 surface lines, the flexible circuit board substrate and the steel reinforcing steel.

[0047] In some of the solutions described above in this application, grounding is achieved by connecting the steel reinforcing steel to the flexible circuit board using steel-reinforcing conductive adhesive. However, in practical applications, the interfacial bonding force between the steel reinforcing steel and the conductive adhesive, as well as the distribution of conductive particles within the conductive adhesive, directly determine the stability of the grounding resistance. If the surface treatment of the steel reinforcing steel is inadequate or the substrate and filler of the conductive adhesive are mismatched, interface debonding or breakage of the conductive path can easily occur after experiencing high temperature and high humidity or reflow environments, leading to grounding failure.

[0048] This application further proposes that the steel reinforcing material is a steel material with nickel electroplated or chemically deposited on the outer surface, and one side of it is covered with a steel reinforcing conductive adhesive with polyurethane as the base and filled with metal conductive particles.

[0049] In some of the solutions mentioned above in this application, a method for verifying reliability is proposed by measuring the grounding resistance of the steel reinforcement. However, in the actual reliability assessment process, relying solely on the grounding resistance as a single electrical indicator is insufficient to fully reflect the stability of the physical bonding state between the steel reinforcement and the flexible circuit board. Especially when facing physical failure modes such as debonding or peeling caused by insufficient interface bonding, simple resistance monitoring may not be able to detect potential structural risks in time, resulting in blind spots in reliability assessment.

[0050] This application further proposes a method for testing the reliability of steel reinforcement, in which tensile tests are performed on the steel reinforcement material in steps S1, S2, and S4.

[0051] Tensile testing is a physical and mechanical testing method used to evaluate the bonding strength between materials or components. Its core lies in applying tensile loads and measuring the deformation and failure behavior of materials under stress, thereby quantifying their resistance to separation. In steel reinforcement reliability testing, tensile testing of the steel reinforcement aims to directly evaluate the interfacial bonding strength between the steel reinforcement and the flexible circuit board, in order to identify and quantify potential failure modes such as physical debonding and peeling. This test can be implemented in several ways, including but not limited to: One method is peel force testing. By peeling the steel reinforcement from the flexible circuit board at a specific angle and speed, and recording the force required during peeling in real time, the interfacial peel strength data is obtained. This method effectively reflects the toughness and stability of the interface under continuous peel stress. Another method is shear force testing. By applying a shear force parallel to the interface between the steel reinforcement and the flexible circuit board, the force required for separation under shear is measured. This method can evaluate the bonding performance of the interface under lateral stress and is of great significance for determining the shear strength of conductive adhesives and their adhesion to the substrate.

[0052] By introducing tensile testing of the steel reinforcement in key steps S1, S2, and S4 of the steel reinforcement reliability testing method, this application achieves a physical quantitative assessment of the bonding strength between the steel reinforcement and the flexible circuit board. Tensile tests are performed in the initial state (S1), after SMT reflow treatment (S2), and after accelerated aging in a humid and hot environment (hot water immersion treatment, S4), enabling dynamic monitoring of the changing trend of the interfacial bonding force between the steel reinforcement and the flexible circuit board. This allows the testing method to go beyond a single electrical performance indicator (grounding resistance) and simultaneously consider the physical structural integrity. Specifically, tensile testing can directly reveal physical failure modes such as debonding, peeling, or delamination caused by conductive adhesive aging, thermal stress, or hygroscopic expansion. These failure modes may not initially manifest as a significant increase in grounding resistance, but they indicate potential structural risks. Through tensile testing, these physical defects can be detected and quantified in a timely manner, providing more comprehensive reliability assessment data and effectively avoiding the blind spots that may arise from relying solely on electrical resistance assessments, ensuring the long-term physical stability of the steel reinforcement under different operating conditions and service conditions.

[0053] In some of the embodiments described above in this application, the initial steel reinforcement grounding resistance is measured to assess reliability. However, in actual measurement, if the current of the test equipment is too large, it may cause electrical breakdown or thermal damage to the tiny conductive structure of the flexible circuit board, resulting in unexpected resistance changes introduced into the measurement itself. This makes it impossible to accurately screen out qualified products that meet the requirements of subsequent tests, thus affecting the accuracy of subsequent reliability verification.

[0054] This application further proposes that, in step S1, the initial steel reinforcement grounding resistance is measured by a four-terminal electrical testing device without sudden current change, with a maximum current of 20mA. Flexible circuit boards with resistance values ​​less than a first predefined threshold proceed to step S2.

[0055] Specifically, the four-terminal electrical testing device without sudden current is a high-precision resistance measuring device. By separating the current path and the voltage measurement path, it effectively eliminates the interference of the test lead resistance and contact resistance on the measurement results, thereby ensuring the accuracy and reliability of the measurement results.

[0056] Furthermore, the "first predefined threshold" is a pre-set upper limit for resistance value, used to screen flexible circuit boards whose initial steel reinforcement grounding resistance value is qualified. This threshold can be set in the testing software. When the measurement result exceeds this threshold, the system automatically marks it as unqualified and prevents it from proceeding to the next step; alternatively, the operator can manually judge whether it meets the threshold requirement based on the results displayed by the measuring equipment and perform corresponding sorting operations. According to practice in the field, the upper limit specification for grounding resistance value of finished flexible circuit boards is usually 0.5Ω, therefore, the first predefined threshold is preferably 0.5Ω. Only flexible circuit boards with resistance values ​​less than this first predefined threshold are allowed to proceed to the subsequent step S2.

[0057] In some of the solutions described above in this application, dehumidification baking and reflow treatment of flexible circuit boards are proposed to simulate SMT process. However, in actual production, if the time interval between baking and reflow is not properly controlled, or the process parameters of baking and reflow are not set reasonably, the flexible circuit board may reabsorb moisture before entering the reflow stage, or the potential failure mode of the steel reinforcing conductive adhesive may not be effectively induced due to inaccurate simulation of the reflow temperature profile, thereby affecting the accuracy and consistency of subsequent reliability testing.

[0058] This application further proposes that in the above-mentioned steel reinforcement reliability test method, the dehumidification baking conditions in step S2 are 120°C baking for 2 hours, the reflow treatment is to simulate a 260°C reflow curve and perform two reflows, and the time from the end of baking to the completion of reflow is controlled within 24 hours.

[0059] Specifically, dehumidification baking aims to remove moisture from the interior and surface of the flexible circuit board (PCB) to prevent defects such as delamination and blistering caused by moisture vaporization and expansion during subsequent high-temperature processing. It also ensures that the material is tested under controlled dry conditions, thereby improving the accuracy and consistency of test results. This can be achieved by placing the PCB in a constant-temperature oven for heating or by using a vacuum oven for low-temperature, low-pressure baking. Setting the temperature to 120℃ and the baking time to 2 hours ensures that the moisture inside the PCB can fully evaporate and be removed, achieving the desired dryness. These temperature and time parameters were determined based on experimental verification of the PCB material properties and moisture diffusion patterns, effectively and thoroughly removing moisture while avoiding thermal damage to the PCB.

[0060] Reflow processing aims to simulate the high-temperature thermal shock process experienced by flexible printed circuit boards (FPCBs) during actual surface mount technology (SMT) manufacturing, inducing potential failure modes of the steel reinforcing conductive adhesive and related interfaces. Reflow processing is typically performed in a reflow oven, subjecting the FPCB to preheating, isothermal treatment, reflow, and cooling stages within a pre-defined temperature profile. Simulating a 260°C reflow profile aims to realistically reproduce the highest thermal stress applied to the FPCB during the SMT process, fully exposing performance changes in the steel reinforcing conductive adhesive at high temperatures, such as changes in conductive particle pathways or weakening of interfacial adhesion. Performing two reflow processes simulates multiple reflow scenarios that may occur in actual production, further intensifying thermal stress and thus providing a more comprehensive assessment of the reliability of the steel reinforcement under extreme conditions.

[0061] The time from the end of baking to the completion of reflow is controlled within 24 hours. This aims to strictly limit the exposure time of flexible printed circuit boards (FPCBs) after dehumidification baking and before reflow treatment, preventing them from reabsorbing moisture from the ambient air. FPCB materials have a certain degree of hygroscopicity; if the exposure time is too long, they will reabsorb moisture, significantly reducing the effectiveness of dehumidification baking and consequently affecting the effectiveness of reflow treatment and the accuracy of subsequent reliability testing. This time limit ensures that the test samples remain in a controlled dry state when entering the reflow stage.

[0062] In some of the solutions mentioned above in this application, tensile testing of steel reinforcing materials is proposed to evaluate their bonding strength. However, in actual operation, due to the lack of unified and standardized testing specifications, different testers or equipment have differences in the angle, speed and load calculation methods when applying tensile force, resulting in a lack of comparability of the obtained tensile force data. This makes it difficult to accurately reflect the true bonding state between the steel reinforcing conductive adhesive, the steel reinforcing material and the flexible circuit board, thus affecting the accurate determination of the reliability failure mode of the steel reinforcement.

[0063] This application further proposes that the above-mentioned tensile test is carried out in a direction perpendicular to the plane of the steel reinforcing steel, with a test speed of 57 mm / min, and the tensile force per unit width is obtained by calculating the average load and dividing it by the width of the steel reinforcing steel.

[0064] In some of the solutions mentioned above in this application, a method for evaluating the reliability of steel reinforcement through tensile testing was proposed. However, in the actual testing process, there is a lack of a clear set of judgment criteria to distinguish between qualified and defective products, which makes it impossible to remove unqualified flexible circuit boards in a timely manner in the early stages of the testing process, thereby affecting the accuracy and efficiency of subsequent reliability verification.

[0065] This application further proposes that in steps S1, S2, and S4 of the above-mentioned steel reinforcement reliability test method, a minimum unit width tensile force value is set. When the calculated unit width tensile force value is less than the predefined minimum unit width tensile force value, the flexible circuit board in steps S1 and S2 is marked as a defective product and does not proceed to the next step. In step S4, the flexible circuit board is directly marked as a defective product.

[0066] Specifically, setting a minimum tensile force per unit width refers to pre-determining a mechanical performance index to measure whether the bond strength between the steel reinforcement and the flexible circuit board meets minimum requirements. This minimum tensile force per unit width can be determined based on product design specifications, material performance data, or through statistical analysis of a large number of qualified samples to set a lower limit that ensures long-term product reliability. Alternatively, based on specific application environments or customer requirements for product durability, the minimum bond strength to be achieved during the testing phase can be derived through accelerated aging tests.

[0067] The calculated tensile force per unit width is less than the predefined minimum tensile force per unit width, which serves as a quantitative standard for determining whether a product is qualified. By comparing the actual measured tensile force per unit width of the steel reinforcement with the preset minimum tensile force per unit width, the bonding performance between the steel reinforcement and the flexible circuit board can be objectively evaluated. This comparison process is usually completed automatically by testing equipment or supporting software to ensure the accuracy and consistency of the judgment. Alternatively, test data can be manually checked against standard values ​​for secondary confirmation or manual intervention in specific circumstances.

[0068] The flexible printed circuit boards (FPCBs) in steps S1 and S2 are marked as defective and do not proceed to the next step. This mechanism aims to perform quality screening early in the testing process. If the bonding strength of the steel reinforcement fails to meet the standard in the initial state of the FPCB (step S1) or after simulated SMT reflow (step S2), it indicates a potential reliability risk. For FPCBs deemed defective, various handling methods can be adopted, such as removing them from the test batch, physically isolating and marking them, or recording their non-conforming status in the production management system to avoid unnecessary subsequent testing resource investment. This helps improve testing efficiency and ensures that only products with basic mechanical properties can proceed to the more stringent reliability verification stages.

[0069] In step S4, the flexible circuit board is directly marked as a defective product. After undergoing simulated high temperature and high humidity environment (hot water immersion treatment), if the bonding strength of the steel reinforcement drops below the preset minimum tensile strength per unit width, it indicates a defect in its durability under long-term humid and hot conditions. In this case, regardless of its electrical performance, it should be judged as a defective product to ensure the long-term reliability of the product in actual use. For the defective products judged in step S4, in addition to marking and recording, comprehensive analysis can be conducted in conjunction with other test data (such as grounding resistance) to further investigate the causes of failure and provide more comprehensive information for product improvement.

[0070] In some of the embodiments described above in this application, a hot water immersion treatment was proposed to simulate the moisture absorption behavior of flexible circuit board steel reinforcement in a high temperature and high humidity environment. However, in actual operation, there is a lack of clear quantitative limitations on the specific process parameters of immersion, such as temperature, time and subsequent treatment methods, which leads to inconsistent moisture absorption under different operating environments. It is difficult to ensure the repeatability and reliability of the test results, and thus it is impossible to accurately evaluate the performance of the conductive adhesive for steel reinforcement under a specific moisture absorption.

[0071] This application further proposes that step S3 specifically involves immersing the reflowed flexible circuit board in hot water at 95°C for 4 hours, then removing it and wiping off the surface moisture to obtain a flexible circuit board after hot water immersion treatment.

[0072] Specifically, the core of "immersion in 95℃ hot water" is to provide a controlled high-temperature water environment to accelerate the moisture absorption process of the steel-reinforcing conductive adhesive in the flexible circuit board.

[0073] Through the above technical solution, this application provides a standardized environmental simulation method for the reliability testing of steel reinforcements by specifying the immersion temperature, immersion time, and subsequent surface treatment steps. Immersing the reflowed flexible circuit board in 95℃ hot water utilizes the physical properties of water bath heating to ensure the conductive adhesive of the steel reinforcement comes into full contact with moisture under controlled high temperature, thereby accelerating the material's moisture absorption process. Setting an immersion time of 4 hours is an optimized choice based on the material's moisture absorption kinetics, ensuring the conductive adhesive reaches the expected moisture saturation state, thus effectively simulating the aging effect under long-term high temperature and high humidity conditions. The step of wiping the surface dry after removal aims to eliminate the interference of residual surface moisture on subsequent grounding resistance measurements, ensuring the accuracy of the measurement data. Through this series of rigorous process constraints, this solution achieves rapid and controllable simulation of the moisture absorption behavior of the conductive adhesive in steel reinforcements, providing a solid data foundation for subsequent verification of the reliability of the steel reinforcement grounding resistance value, and effectively solving the problem of reliability judgment deviation caused by inconsistent test conditions. This standardized processing procedure works in conjunction with other steps in the aforementioned steel reinforcement reliability test method to ensure that the measurement of the grounding resistance of the steel reinforcement of the flexible circuit board after hot water immersion is based on a stable, repeatable, and representative moisture absorption state. This improves the accuracy and reliability of the entire test method, making the verification of the reliability of the steel reinforcement grounding resistance more scientific and effective.

[0074] In some of the solutions mentioned above in this application, a method is proposed to simulate a high temperature and high humidity environment by soaking in hot water to verify the reliability of steel reinforcement. However, in actual testing, it is often difficult to accurately distinguish the specific failure mechanism that causes abnormal resistance by measuring the grounding resistance value alone. This makes it impossible to make targeted process improvements and lacks a standardized judgment logic to comprehensively evaluate the reliability status of steel reinforcement after moisture absorption.

[0075] This application further proposes that step S4 above specifically includes: measuring the steel reinforcement grounding resistance of the flexible circuit board after hot water immersion treatment using a four-terminal electrical testing device without sudden current change, with an upper limit of 5Ω; if the resistance of all flexible circuit boards is ≤5Ω, then randomly select a portion of the flexible circuit boards to measure and record the tensile strength data of the steel reinforcement; if there are flexible circuit boards with a resistance >5Ω, then these flexible circuit boards are marked as defective, and the tensile strength data of the steel reinforcement is measured for all defective products; reliability is determined based on the measurement results: if some resistances are >5Ω, and the hot water immersion treatment is complete, the resistance of the flexible circuit boards is determined. If the tensile strength of the steel reinforcement after soaking is not significantly reduced compared to that after reflow, it is determined that the steel reinforcement has failed. The main failure mode is that the conductive adhesive of the steel reinforcement absorbs moisture, resulting in a reduction of the conductive path. If some resistance values ​​are >5Ω, and the tensile strength of the steel reinforcement after hot water soaking is significantly lower than that after reflow, it is determined that the steel reinforcement has failed. The main failure mode is that the conductive adhesive of the steel reinforcement absorbs moisture and debonds, inducing partial delamination and reducing the grounding area. If all resistance values ​​are ≤5Ω, and the tensile strength of the steel reinforcement after hot water soaking is greater than the minimum unit width tensile strength value, it is determined that the steel reinforcement is reliable.

[0076] Specifically, when measuring the grounding resistance of the steel reinforcement of a flexible circuit board after hot water immersion, a four-terminal electrical testing device with no sudden current was used. This device connects to both ends of the device under test via four independent leads. Two leads are used to apply the test current, and the other two are used to measure the voltage, effectively eliminating the influence of test lead resistance and contact resistance on the measurement results and ensuring the accuracy of the grounding resistance value. Simultaneously, the design without sudden current prevents secondary damage to the conductive path inside the conductive adhesive of the steel reinforcement that may have been damaged by a sudden large current during the measurement process, ensuring the authenticity of the test results. In actual operation, the test current of this device is usually controlled at a low level, for example, a maximum current of 20mA or less, to further protect the test sample. After the measurement is completed, the upper limit of the resistance value is set to 5Ω. Any flexible circuit board exceeding this limit is considered a potential failure sample for further in-depth analysis. Besides using a four-terminal electrical testing device, a high-precision digital multimeter with a dedicated test fixture can also be used for measurement, but the stability and safety of the test current must be ensured, and any existing contact resistance must be effectively calibrated or compensated.

[0077] When assessing the reliability of flexible printed circuit boards (FPCBs) after hot water immersion treatment, initial screening is performed based on grounding resistance. If the grounding resistance of the steel reinforcements on all FPCBs meets the requirement of ≤5Ω, it indicates good electrical performance. Based on this, to further verify whether the mechanical bonding force between the steel reinforcements and the FPCBs is equally reliable, a random sample of FPCBs is selected for tensile testing of the steel reinforcements, and the results are recorded. The tensile test is typically conducted perpendicular to the plane of the steel reinforcement at a speed of 57 mm / min. The tensile force per unit width is obtained by calculating the average load and dividing it by the width of the steel reinforcement. This sampling test aims to evaluate the overall mechanical reliability of a batch of products at a reasonable cost and efficiency. In addition to random sampling, statistical principles can be used to set specific confidence levels and error ranges, calculating the minimum required sample size to ensure that the sampling results are sufficiently representative of the overall batch.

[0078] On the other hand, if during the measurement process, some flexible circuit boards are found to have a steel reinforcement grounding resistance >5Ω, these substandard flexible circuit boards will be marked as defective. For these defective boards, to further investigate the cause of failure, tensile strength measurements of the steel reinforcement will be performed on all defective boards. This targeted and comprehensive testing aims to ensure that no mechanical defects that could lead to electrical performance failure are overlooked. In addition to tensile testing, these defective boards with abnormal resistance values ​​can undergo more refined microscopic analysis, such as through cross-section observation and scanning electron microscopy (SEM), to directly observe the internal structure of the conductive adhesive in the steel reinforcement, the interface bonding state, and the presence of defects such as delamination and bubbles, thus providing more intuitive evidence for failure mode diagnosis.

[0079] The final reliability assessment is based on a comprehensive analysis of the grounding resistance and the tensile test results of the steel reinforcing material. Specifically, it includes the following three scenarios: In the first scenario, if the grounding resistance of some flexible circuit boards is greater than 5Ω, but the tensile strength of the steel reinforcement after hot water immersion treatment does not decrease significantly compared to that after reflow, it is determined that the steel reinforcement has failed. The main failure mode is that the conductive adhesive of the steel reinforcement absorbs moisture, leading to a reduction in conductive pathways. This indicates that the mechanical bonding force between the steel reinforcement and the flexible circuit board is still acceptable, but the conductive particle pathways inside the conductive adhesive are damaged due to moisture absorption, expansion, or degradation, resulting in a decrease in conductivity.

[0080] In the second scenario, if the grounding resistance of some flexible circuit boards is greater than 5Ω, and the tensile strength of the steel reinforcement after hot water immersion is significantly lower than after reflow, then the steel reinforcement is considered to have failed. The main failure mode is that the conductive adhesive of the steel reinforcement absorbs moisture and debonds, leading to partial delamination and a reduction in the grounding area. This indicates that not only does the conductivity decrease, but the mechanical bonding force between the steel reinforcement and the flexible circuit board is also significantly weakened. This is mainly attributed to the moisture absorption and expansion of the conductive adhesive in the steel reinforcement, leading to interface debonding or delamination, thereby reducing the effective grounding contact area.

[0081] In the third scenario, if the grounding resistance of all flexible circuit boards is ≤5Ω, and the tensile strength of the steel reinforcement after hot water immersion treatment is greater than the minimum tensile strength per unit width, then the steel reinforcement is deemed reliable. This indicates that after being treated in a simulated high-temperature and high-humidity environment, the product's electrical and mechanical properties meet the requirements, demonstrating good reliability. In practical applications, the "significance" of the tensile strength reduction can be further refined, for example, by setting a specific percentage threshold, to improve the objectivity of the judgment.

[0082] Through the above technical solution, this application establishes a comprehensive judgment logic based on the combination of grounding resistance measurement and tensile testing, achieving accurate diagnosis of the reliability failure mode of steel reinforcement. Utilizing a four-terminal electrical testing device with no sudden current to measure the grounding resistance after hot water immersion ensures accurate data acquisition under low current conditions, avoiding secondary damage to the conductive path during testing. By setting a resistance upper limit of 5Ω, potential failure samples can be quickly screened. Based on this, tensile testing of the steel reinforcement is introduced as an auxiliary verification method. By comparing the changes in tensile data before and after hot water immersion, the physical nature of the failure can be effectively distinguished. When the resistance is abnormal but the tensile force does not change significantly, it indicates that the interfacial bonding is still acceptable, and the failure originates from the breakage of the conductive particle pathways within the conductive adhesive. This provides a clear direction for subsequent optimization of the conductive adhesive formulation or adjustment of the conductive particle distribution. When the resistance is abnormal and the tensile force decreases significantly, it indicates that debonding or delamination has occurred, pointing to insufficient interfacial bonding, thus guiding process engineers to improve reliability through surface cleaning or improving interfacial energy. By logically combining resistance and tensile strength determination, we can not only make a final and valid conclusion on the reliability of steel reinforcement, but also transform the originally ambiguous test results into actionable process improvement strategies through failure mode classification. This significantly improves the quality control capability of flexible circuit board steel reinforcement process while ensuring testing efficiency.

[0083] In one specific embodiment, the following steps are included: Step 1: Select a finished flexible circuit board with steel reinforcement design and requirements for controlling the grounding resistance of the steel reinforcement as the verification object for the reliability of the steel reinforcement. A double-layer board is used as an example here. Figure 4As shown, in practical applications, the number of flexible circuit board layers, the internal circuit design, and the size of the grounding opening of the steel reinforcement are not limited. The steel reinforcement type is a steel reinforcement with an outer surface electroplated nickel / chemically deposited nickel, one side of which is covered with a layer of PU-based conductive adhesive filled with metal conductive particles. The grounding resistance value of the steel reinforcement of all finished flexible circuit boards is measured and recorded using a four-terminal electrical testing device with no sudden current (20mA and below). Boards with a resistance value within 0.5Ω are selected and recorded as finished flexible circuit boards. The tensile strength data of the steel reinforcement is measured for at least 5 pieces from this batch of finished flexible circuit boards and recorded as the tensile strength of the steel reinforcement in the finished flexible circuit board. The tensile strength of the steel reinforcement must be ensured to be above 0.056 kgf / mm; otherwise, it indicates that the steel reinforcement has failed in the finished flexible circuit board process, requiring improvement of the steel reinforcement process for the flexible circuit board, and further testing is not necessary at this time.

[0084] Step 2: Bake the finished flexible circuit board from Step 1 in a 120℃ oven for 2 hours to remove any moisture before reflow. Simulate the temperature profile of the SMT reflow process and reflow the flexible circuit board twice (this test uses 260℃ reflow twice as an example). Note that the time from baking to reflow completion should be controlled within 24 hours. Use a four-terminal electrical testing device without sudden current changes to measure the grounding resistance of all steel reinforcements. Select boards with a resistance of less than 5Ω and record them as finished SMT products. Take at least 5 pieces of this batch of reflowed flexible circuit boards and measure the tensile strength of the steel reinforcement. Record this as the tensile strength of the steel reinforcement in the finished SMT product. The tensile strength of the steel reinforcement must be ensured to be above 0.056 kgf / mm. Otherwise, it indicates that the steel reinforcement has failed in the SMT process, and improvements need to be made to the steel reinforcement process and SMT environmental control. There is no need to conduct further testing at this time.

[0085] Step 3: This is a critical processing step. Take at least 45 SMT finished products from Step 2 and immerse them in 95℃ hot water for 4 hours using a water bath heating method. After immersion, remove them, wipe off the surface moisture, and record this as the flexible circuit board after hot water immersion treatment. The reason for choosing the temperature of 95℃ is due to equipment limitations. Although the boiling point of water is 100℃, the actual maximum stable temperature of a common open-type water bath is about 95℃ due to water evaporation. The time of 4 hours is chosen because experimental tests showed that the water absorption of the steel reinforcing conductive adhesive reaches its upper limit at around 4 hours, and the properties of the steel reinforcing material after 4 hours of treatment are similar to those of the product after absorbing moisture from the environment.

[0086] Step 4: Use a four-terminal electrical testing device without sudden current changes to measure and record the grounding resistance of the steel reinforcement of the flexible circuit board after the hot water immersion treatment in Step 3. The upper limit of the resistance value is 5Ω. If the resistance values ​​are all within 5Ω, randomly select 5 flexible circuit boards to measure and record the tensile strength data of the steel reinforcement. If some flexible circuit boards have a resistance greater than 5Ω, the non-compliant flexible circuit boards are defective products after the hot water immersion treatment. Take all the defective products after the hot water immersion treatment to measure and record the tensile strength data of the steel reinforcement.

[0087] Step 5: Regarding the steel reinforcement grounding resistance value in the above steps, the processing method in Step 3 is similar to the effect of the 65℃ / 90%RH 500H environment on the steel reinforcement grounding resistance value. Therefore, the results of the steel reinforcement grounding resistance value and the tensile strength data of the steel reinforcement in Step 4 can be used to directly replace the original 500H test results.

[0088] The following different failure modes may be observed, and their corresponding analyses are as follows: 1. If the resistance exceeds 5Ω and the tensile strength of the steel reinforcement in the defective product after hot water immersion is not significantly different from that of the finished SMT product, it indicates that the steel reinforcement has failed. The main failure mode is the reduction of conductive paths due to moisture absorption by the conductive adhesive of the steel reinforcement. The main countermeasures are to maximize the conductive paths formed by conductive particles between the steel reinforcement and the flexible circuit board before and after moisture absorption. One approach is to increase the conductive particle content per unit volume, such as by using a steel reinforcement conductive adhesive with a higher conductive particle content and better particle morphology / distribution. Another approach is to shorten the distance between the steel reinforcement and the grounding pad (reducing the number of conductive particles needed to form a conductive path), such as by increasing the fast pressing time of the steel reinforcement and local copper plating in the grounding area.

[0089] 2. If the resistance exceeds 5Ω and the tensile strength of the steel reinforcement in the defective product after hot water immersion is lower than that of the finished SMT product, it indicates that the steel reinforcement has failed. The main failure mode is partial delamination caused by the conductive adhesive of the steel reinforcement absorbing moisture, leading to a reduction in the grounding area. The main improvement direction is to improve the bonding force between the interfaces. If delamination occurs between the conductive adhesive and the steel reinforcement, consider adding degreasing and ultrasonic water washing methods before bonding the steel reinforcement and conductive adhesive to improve surface bonding force, and strictly control the control time from cleaning to bonding. If delamination occurs between the conductive adhesive and the flexible circuit board, consider performing plasma cleaning on the flexible circuit board before bonding to increase surface energy. At the same time, consider increasing the bonding force by adjusting the material of the contact part between the flexible circuit board and the conductive adhesive (such as avoiding designs with strongly hydrophobic materials under the conductive adhesive).

[0090] 3. If all resistance values ​​are less than 5Ω, and the tensile strength of the steel reinforcement after immersion in hot water is greater than 0.056kgf / mm, it indicates that the steel reinforcement is reliable and effective, and there is no need to conduct a 500H moisture absorption test in the same environment.

[0091] In another specific embodiment, the present invention provides a test method for verifying the reliability of steel reinforcement under high temperature and high humidity conditions in a short period of time, and compares and verifies it with the original method. The implementation and verification process of this method includes the following steps: Step 1: Select a double-layer flexible circuit board with steel reinforcement design and steel reinforcement grounding resistance control requirements as the test object (this product has been proven in multiple tests to have a partial steel reinforcement grounding resistance greater than 5Ω after 500 hours at 65℃ / 90%RH). Both appearance and functional inspections are satisfactory. Here, we take a steel reinforcement grounding resistance upper limit of 0.5 ohms as an example (steel reinforcement grounding resistance control is commonly used in flexible circuit board products). Randomly select a certain number of samples. Figure 4 As shown. This example uses a double-layer board. In practical applications, the number of layers in the flexible circuit board, the internal circuit design, and the size of the grounding opening of the steel reinforcement are not limited. The steel reinforcement type is a steel reinforcement with an outer surface electroplated nickel / chemically deposited nickel, and one side is covered with a layer of conductive adhesive with a PU base and filled with metal conductive particles.

[0092] Step 2: Use a four-terminal electrical testing device to measure the grounding resistance of the steel reinforcement of the flexible circuit board, such as... Figure 5 As shown, note that a measurement mode with a current of 20mA or less should be selected. During measurement, an external oscilloscope should be used to monitor the measurement current to avoid sudden high current fluctuations in the measuring equipment that could temporarily polarize the steel reinforcement conductive adhesive and affect the resistance measurement data. Record the grounding resistance data of a certain number of flexible circuit boards with steel reinforcement. PCs with a resistance less than 0.5Ω are designated as finished flexible circuit boards.

[0093] Step 3: Use a tensile tester to measure the tensile strength of the steel reinforcing steel in at least 5 of the flexible circuit boards from Step 2. The specific method is as follows: Figure 4 First, prepare an FR4 board and wipe it clean with a lint-free cloth dampened with alcohol. Use a cotton swab to evenly apply adhesive (LT4204 as an example) to the surface of the steel reinforcement. Then, attach this surface to the FR4 board and press the sample firmly with the cotton swab. Let it sit for 10 minutes for the adhesive to cure. Fix the FR4 board in place. Clamp one end of the flexible circuit board with a tensile testing machine, set the speed to 57 mm / min, and perform a tensile test at an angle perpendicular to the plane of the steel reinforcement. Record the load-displacement curve for each piece. Calculate the average load from the beginning to the end of the curve, and divide it by the width of the steel reinforcement in the tensile direction to obtain the average tensile force. Set the lower limit to 0.056 kgf / mm.

[0094] Step 4: Bake the finished flexible circuit board from Step 2 in a 120℃ oven for 2 hours to remove moisture before reflow. Simulate the temperature profile of the SMT reflow process and reflow the finished flexible circuit board twice (this test uses 260℃ reflow twice as an example). After reflow, measure the same number of steel reinforcement grounding resistance and tensile force on the flexible circuit board, referring to the methods in Steps 2 and 3.

[0095] Step 5: Cut off the flexible circuit board portion from Step 4, leaving only the steel reinforcing steel end. Treat it using three methods: First, immerse it in 95℃ hot water for 12 hours, removing it every hour and quickly drying the surface moisture before weighing the average weight. Second, immerse it in 25℃ cold water for 24 hours, removing it every hour and quickly drying the surface moisture before weighing the average weight. Third, place the flexible circuit board in a 65℃ / 90%RH environment for 500 hours, removing it every 25 hours and weighing the average weight. This allows us to plot the moisture absorption and weight gain curves of the steel reinforcing steel under these three methods. By fitting the non-Fickian model of the moisture absorption behavior of polymer materials under specific conditions with the Washburn equation, the results are as follows... Figure 6 Comparing the moisture absorption curves under the three treatment methods reveals the following: Compared with hot water soaking treatment, the total moisture content of the product under high temperature and high humidity is relatively low, and the moisture absorption rate is generally slower. This is related to the different moisture absorption models in air and water discussed earlier. The water absorption rate of capillary action is several orders of magnitude faster than that of diffusion, and the activity of water molecules in water is also higher than that in air, resulting in a greater speed and a larger total amount of water absorbed. When the temperature rises above Tg (glass transition temperature), the moisture absorption rate under hot water immersion treatment is faster, and the final moisture absorption amount is higher. This can be explained by the fact that when the temperature exceeds Tg, the originally frozen segments of PU begin to relax, which increases the proportion of pores that absorb water under capillary action, thereby increasing the water absorption rate and also increasing the total space for accommodating water molecules.

[0096] Considering that prolonged hot water immersion, especially hot water immersion treatment, may cause side reactions such as hydrolysis and ion migration in flexible circuit boards, which may seriously interfere with the test results, we selected the shortest time for hot water immersion to approach the maximum water absorption capacity—4 hours—as the optimal hot water treatment time.

[0097] Step 6: Process the flexible circuit board from Step 4 according to the 4-hour hot water immersion method and the 500-hour method in a 65℃ / 90%RH environment, respectively. After the flexible circuit board has been reflowed, measure the steel reinforcement grounding resistance value according to the method in Step 2.

[0098] Step 7: Analyze the steel reinforcement grounding resistance values ​​of the finished flexible printed circuit board (FPCB), the reflowed FPCB, the 95℃ 4H hot water immersion treated product, and the 65℃ / 90%RH 500H treated product. The actual results show that the resistance of the finished FPCB is less than 0.5Ω, and the resistance of the reflowed FPCB is less than 5Ω, both within specifications. However, some pieces in both the hot water immersion treated and environmentally treated products have resistance values ​​exceeding 5Ω; these products are considered failed products after treatment. There is no significant difference in the steel reinforcement grounding resistance values ​​between the two treatment modes, indicating that the results are equivalent. Step 8: Take products treated with 95℃ 4H hot water immersion and 65℃ / 90%RH 500H treatment, and conduct tensile tests on products with steel reinforcement grounding resistance exceeding 5Ω (failed products). Compare the tensile strength of these products with that of the finished flexible circuit board and the reflowed flexible circuit board. If all four sets of data are greater than the lower limit of 0.056 kgf / mm, it indicates that the failed product did not experience failure mode 1, but rather failure mode 2. If the tensile strength under both treatment modes is significantly lower than that of the reflowed flexible circuit board, or the tensile strength value is lower than the lower limit of 0.056 kgf / mm, it indicates that failure mode 1 has occurred.

[0100] Failure Mode 1: Firstly, the plasticization of the steel-reinforcing conductive adhesive due to environmental temperature and humidity, and the stress generated at the interface between the steel, the steel-reinforcing conductive adhesive, and the flexible circuit board due to different coefficients of thermal / moisture expansion, weaken its bonding strength. Figure 7 (2) Step 1 a). Next, the moisture at the interface evaporates and forms small bubbles as the temperature increases. As the temperature rises further, the bubbles expand further under the action of vapor pressure. Figure 7 (2) Step 1 (b), finally the bubbles connect with each other and merge into a large bubble ( Figure 7 (2) Step 2), the large bubble reaches a new equilibrium between temperature, vapor pressure, and stress, and after returning to room temperature, the total delamination area is larger than that of the previous multiple small bubbles. Figure 7 (2) step3) greatly reduces the contact area between the steel reinforcing conductive adhesive and the substrate, and increases the resistance.

[0101] Failure mode 2 is used here Figure 8This illustrates that although a segment of the conductive particle string in the example diagram is separated by resin particles, the conductive path still exists because the volume ratio of metal particles to polymeric resin has reached the "throughflow threshold." First, the Tg (glass transition temperature) of common PU-based steel-reinforced conductive adhesive is around 50°C. When the temperature rises above 50°C, the PU polymer chains thaw, providing conditions for chain movement, volume expansion, and water molecule intrusion. Next, water molecules intrude into the gaps between resin particles and conductive particles, and the bubble size increases with temperature. There are two bubble growth modes: reversible and irreversible. The first occurs when moisture absorption is low; in this case, the bubbles are independent and do not merge after expansion. When the temperature returns to room temperature, the bubbles return to their original state without affecting the original conductive path. The second occurs when moisture absorption is high, and the bubble density is higher than when moisture absorption is low. Initially, each bubble undergoes reversible expansion (…). Figure 8 Step 1), at this point, some of the bubbles expand and fuse with the original defects in the steel reinforcing conductive adhesive to form large bubbles. Figure 8 Step 2), the large bubble merges with surrounding bubbles to form an even larger bubble, at which point the original conductive path is cut off by the merged large bubble. Figure 8 Step 3). At this point, if the temperature returns to room temperature, although the volume of the large bubble decreases with temperature, the open circuit still exists. Figure 8 Step 4, Case 1); even if the large bubble defect is repaired at this point due to polymer chain segment movement ( Figure 8 Step 4, Case 2.1, but due to slippage, the original conductive path was still misaligned, and the open circuit remained even after returning to room temperature. Figure 8 Step 4 (Case 2.2). At high temperatures, the polymer chain segments thaw, and the irreversible growth of bubbles after absorbing moisture breaks down the conductive particle strings that were originally formed by compression and solidification, resulting in increased resistance.

[0102] Based on the combined results of the 95℃ 4H hot water immersion treatment and the 65℃ / 90%RH 500H treatment, both showed a high degree of consistency in terms of grounding resistance, failure mode, and the amount of water absorption that determines the failure mode. Therefore, the 95℃ 4H hot water immersion method can be used to replace the 500H environmental moisture absorption test.

[0103] The above description is merely an embodiment of this application and is not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for testing the reliability of steel reinforcement, characterized in that, Includes the following steps: S1: Provide a flexible circuit board with steel reinforcement and measure the initial steel reinforcement grounding resistance. S2: After dehumidifying and baking the flexible circuit board, set the SMT reflow temperature for reflow treatment to obtain the reflowed flexible circuit board, and measure the steel reinforcement grounding resistance value of the reflowed flexible circuit board. S3: The reflowed flexible circuit board is immersed in 95°C hot water. After immersion, it is taken out and the surface moisture is removed to obtain the flexible circuit board after hot water immersion treatment. S4: Measure the steel reinforcement grounding resistance of the flexible circuit board after hot water immersion treatment to verify the reliability of the steel reinforcement grounding resistance.

2. The steel reinforcement reliability testing method according to claim 1, characterized in that, The flexible circuit board includes at least a flexible circuit board substrate, flexible circuit board L1 surface circuitry on the flexible circuit board substrate, protective adhesive covering the flexible circuit board substrate and the flexible circuit board L1 surface circuitry, a protective film covering the protective adhesive, steel reinforcing steel, and steel reinforcing conductive adhesive for connecting the protective film, the flexible circuit board L1 surface circuitry, the flexible circuit board substrate, and the steel reinforcing steel.

3. The steel reinforcement reliability testing method according to claim 2, characterized in that, The steel reinforcement material is a steel with nickel electroplated or chemically deposited on the outer surface, and one side is covered with a steel reinforcement conductive adhesive with a polyurethane base and filled with metal conductive particles.

4. The steel reinforcement reliability testing method according to claim 1, characterized in that, In steps S1, S2, and S4, a tensile test is performed on the steel reinforcing material.

5. The steel reinforcement reliability testing method according to claim 1, characterized in that, In step S1, the initial steel reinforcement grounding resistance is measured by a four-terminal electrical testing device with no sudden current change. The maximum current is 20mA. The flexible circuit board with a resistance value less than the first predefined threshold proceeds to step S2.

6. The steel reinforcement reliability testing method according to claim 1, characterized in that, In step S2, the dehumidification baking conditions are 120°C for 2 hours, and the reflow treatment is 260°C for two reflows, with the time from the end of baking to the completion of reflow controlled within 24 hours.

7. The steel reinforcement reliability testing method according to claim 4, characterized in that, The tensile test was conducted in a direction perpendicular to the plane of the steel reinforcing material, at a test speed of 57 mm / min. The average load was calculated and then divided by the width of the steel reinforcing material to obtain the tensile force per unit width.

8. The steel reinforcement reliability testing method according to claim 7, characterized in that, Steps S1, S2, and S4 also include setting a minimum unit width tensile force value. When the calculated unit width tensile force value is less than the predefined minimum unit width tensile force value, the flexible circuit board in steps S1 and S2 is marked as a defective product and does not proceed to the next step. In step S4, the flexible circuit board is directly marked as a defective product.

9. The steel reinforcement reliability testing method according to claim 1, characterized in that, Step S3 specifically involves immersing the reflowed flexible circuit board in hot water at 95°C for 4 hours, then removing it and wiping off the surface moisture to obtain the flexible circuit board after hot water immersion treatment.

10. The steel reinforcement reliability testing method according to claim 8, characterized in that, Step S4 includes: measuring the steel reinforcement grounding resistance of the flexible circuit board after hot water immersion treatment using a four-terminal electrical testing device without sudden current change, with an upper limit of 5Ω; If the resistance of all flexible circuit boards is ≤5Ω, then randomly select some flexible circuit boards to measure and record the tensile strength data of the steel reinforcing steel. If there are flexible circuit boards with a resistance value > 5Ω, then these flexible circuit boards are marked as defective, and the tensile strength data of the steel reinforcing steel is measured for all defective products. Reliability is determined based on the measurement results: If the resistance of some parts is greater than 5Ω, and the tensile strength of the steel reinforcement after hot water immersion treatment is not significantly reduced compared with that after reflow, it is determined that the steel reinforcement has failed. The main failure mode is that the conductive adhesive of the steel reinforcement absorbs moisture, resulting in a reduction of the conductive path. If the resistance of some parts is greater than 5Ω, and the tensile strength of the steel reinforcement after hot water immersion is significantly lower than that after reflow, it is determined that the steel reinforcement has failed. The main failure mode is that the conductive adhesive of the steel reinforcement absorbs moisture, which induces the delamination of some adhesive layers, resulting in a reduction of the grounding area. If all resistance values ​​are ≤5Ω, and the tensile strength of the steel reinforcement after hot water immersion treatment is greater than the minimum unit width tensile strength value, then the steel reinforcement is deemed reliable and effective.

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