Method for improving board explosion of FPC (Flexible Printed Circuit) high-multilayer board

Abstract

The invention discloses an FPC high multilayer board explosion improvement method, and relates to the technical field of FPC manufacturing. The method comprises the steps of cover film type selection, cover film pretreatment, pure glue type selection, substrate type selection, combination pretreatment, interlayer structure design, steel sheet treatment, lamination process optimization and the like, wherein the stripping force gt is selected; the thickness of the cover film is 1.0 kg / cm < 2 >, and a pretreatment mode with preferential medium roughening is adopted; the method comprises the following steps: selecting anti-ion pure glue, and carrying out medium coarsening or brownification pretreatment; frosted steel sheets without holes and thick steel sheet glue are selected, and the steel sheets are subjected to plasma treatment; and the pressing parameters are optimized, and a fast pressing / pressure transmitting mode is adapted. Through multi-dimensional collaborative design, interlayer bonding force is improved, stress concentration and moisture absorption risks are reduced, the FPC high multilayer board passes double 85 + Reflow and PCT strict tests, the problem that in the prior art, an FPC multilayer board is high in board explosion rate is solved, and the method is particularly suitable for four or more layers of multilayer boards and is high in practicability.

Description

Technical Field

[0001] This invention relates to the technical field of FPC manufacturing, and in particular to a method for improving the cracking of FPC multilayer boards. Background Technology

[0002] As electronic devices become thinner, lighter, and more high-performance, the application of FPCs is becoming increasingly widespread, especially in fields such as display modules. Customers are constantly raising their reliability requirements for high-multilayer (four layers and above) FPCs. Some customers are requesting an unconventional test item where FPCs must pass the dual 85 test and then immediately undergo reflow soldering within 5 minutes. The dual 85 test is used to test the reliability of FPC multilayer boards in high-temperature and high-humidity environments. The test conditions are to place the FPC multilayer board in an environment of 85°C and 85% relative humidity for 5 hours (some extended tests are 12 hours), which is an accelerated aging test for FPC multilayer boards. Within 5 minutes of the double 85 test, the FPC undergoes a reflow test to simulate extreme scenarios throughout the entire process, from storage (high temperature and humidity) to assembly (high temperature reflow soldering). This verifies the structural stability of the FPC under extreme high temperatures while in a hygroscopic state. High temperatures soften the interlayer adhesive layers of the FPC, and the moisture generated by moisture absorption vaporizes and expands. If the interlayer bonding is insufficient, it can directly lead to delamination, blistering, or board bursting. Therefore, the reflow test is a core test for evaluating the FPC's resistance to the combined damage caused by thermal stress and moisture absorption. During the test, the FPC undergoes high-temperature reflow soldering in a humid state, making it highly susceptible to delamination and board bursting due to insufficient interlayer bonding, stress concentration, or moisture absorption, which seriously affects product yield and reliability.

[0003] In existing technologies, the problem of FPC multilayer board explosion is mainly concentrated in high multilayer boards with four or more layers. As the more layers there are, the greater the stress and the higher the risk of moisture absorption and explosion. At the same time, the peel strength of traditional cover films is low, the perforated design of steel sheets is prone to moisture absorption, and improper pretreatment methods and unreasonable lamination process parameters can also exacerbate the explosion problem. Existing technologies lack a systematic and comprehensive solution to improve the explosion of FPC multilayer boards and cannot meet the stringent testing requirements of customers.

[0004] In view of this, the present invention addresses the numerous shortcomings and inconveniences caused by the board bursting problem in existing FPC multilayer boards, and has been developed through in-depth research, improvement and trial production. Summary of the Invention

[0005] The purpose of this invention is to improve the technical problem of high failure rate of FPC high multilayer boards in the prior art, and to provide an improvement method for FPC high multilayer boards that can improve the interlayer bonding force of FPC and avoid stress concentration leading to delamination failure, so that FPC high multilayer boards can meet the test conditions of passing Reflow directly within 5 minutes after the double 85 test.

[0006] To achieve the above objectives, the solution of the present invention is: A method for improving the cracking of FPC multilayer boards includes the following steps: S1. Cover film selection: The cover film is selected with a peel strength greater than 1.0 kg / cm², and has excellent high temperature resistance, moisture resistance and interlayer bonding stability. The cover film can withstand the reflow soldering high temperature of 265℃~270℃ for 10 seconds within 5 minutes after the double 85 test and repeat it 5 times without interlayer separation. At the same time, the thermal expansion coefficient of the cover film is matched with the thermal expansion coefficient of the FPC substrate and copper foil, and the difference is controlled within 5 ppm / ℃. S2. Pretreatment of the cover film: The cover film selected in step 1 is subjected to surface pretreatment. The pretreatment is any one or any combination of roughening, micro-etching or sandblasting on the adhesive layer side. The roughening treatment controls the copper etching amount between 0.8μm and 1.2μm, so that the adhesive layer surface of the cover film forms a uniformly distributed micro-rough structure. After the pretreatment, the peel force between the cover film and the substrate is greater than 0.9kg / cm², and the peel force variation coefficient is controlled within 5%. S3. Selection of pure adhesive: Anti-ion pure adhesive is selected. The moisture absorption rate of the anti-ion pure adhesive is controlled below 0.1% after 5 hours of double 85 test, and the glass transition temperature of the anti-ion pure adhesive is higher than 200℃. S4. Substrate selection: The selected substrate should have a moisture absorption rate below 0.2% after 5 hours of double 85 testing and a thermal decomposition temperature above 300℃. For multilayer boards with four or more layers, the interlayer adhesive layer of the substrate should be a modified epoxy resin adhesive with high bonding strength, and the adhesive layer thickness should be controlled between 80μm and 100μm. At the same time, the thermal expansion coefficient of the substrate should be compatible with the thermal expansion coefficient of the cover film and copper foil. S5. Pre-treatment before assembly: Pre-treatment is performed on the substrate or copper foil. The treatment method includes any one of medium roughening, browning, or micro-etching. After pre-treatment, it is combined with pure adhesive. The peel strength of the combined pure adhesive is greater than 1.0 kg, and the peel strength decrease rate does not exceed 8% after 12 hours of double 85 testing. S6. Interlayer structure design: Multilayer boards with four or more layers adopt an interlayer structure with thin adhesive in the middle layer and thick adhesive on the outer layer. The adhesive thickness of the middle layer is 80μm~100μm and the adhesive thickness of the outer layer is 120μm~150μm. S7. Optimization of pressing process: Set the pressing parameters as follows: pressing pressure 90-130KG, pressing time 90-150S, pressing temperature 180℃, and the pressure fluctuation range during the pressing process should not exceed ±5KG.

[0007] Furthermore, the FPC high multilayer board includes steel sheets. Before the pressing process optimization in step S7, step S8, steel sheet treatment, is also included: the steel sheet is a frosted, non-perforated steel sheet with a thickness of 0.1mm~0.3mm and a hardness of HV200~HV250. The steel sheet is subjected to plasma pretreatment to introduce active groups on the surface of the steel sheet through plasma treatment. A steel sheet adhesive with a thickness of 150μm~200μm is selected, and the elastic modulus of the steel sheet adhesive is controlled at 1.5GPa~2.5GPa.

[0008] Furthermore, the edges of the steel sheet adopt a rounded transition design with a rounded corner radius of 0.5mm to 1mm; the bonding area between the steel sheet and the FPC adopts a combination of local dispensing and overall pressing. High-strength adhesive is pre-applied to the edges and key stress points in the center of the steel sheet, with a dispensing diameter of 1mm to 2mm and a dispensing spacing of 4mm to 6mm, and then overall pressing is performed to form a locally reinforced bond between the steel sheet and the cover film.

[0009] Furthermore, the high-FPC multilayer board includes a steel sheet with multiple blind holes that do not penetrate the steel sheet. The inner wall of the blind holes is coated with a hydrophobic coating. The hydrophobic coating is made of polytetrafluoroethylene or fluorine-modified resin material, with a coating thickness of 5μm to 10μm and a contact angle greater than 110°.

[0010] Furthermore, the FPC wiring uses a grid-like copper layer distribution with a grid spacing of 1mm to 2mm and a grid line width of 0.1mm to 0.2mm.

[0011] Furthermore, the present invention also includes step S9, which follows step S7, and slow cooling treatment after pressing: after pressing, the FPC high multilayer board is placed in a slow cooling furnace for gradual cooling at a cooling rate of 5℃ / min to 8℃ / min, gradually cooling from 180℃ to room temperature, and the temperature difference during the slow cooling process is controlled within ±3℃.

[0012] Furthermore, the present invention also includes step S10, post-processing and enhanced testing: after reflow soldering, a vacuum degassing process is added, and the layers are treated at 200℃ and -0.08MPa for 30 minutes to remove trace amounts of water vapor and gas remaining between the layers; online detection of interlayer bonding force is added, and after the pressing process, 100% of each batch of high-FPC multilayer boards are inspected using an ultrasonic scanning microscope with a detection accuracy of 5μm; 5% of each batch of high-FPC multilayer boards are sampled for accelerated aging testing, and the high-FPC multilayer boards are subjected to 120 hours of double 85 testing followed by 10 reflow soldering cycles to verify the reliability of the high-FPC multilayer boards after long-term use. The qualified standard is no board explosion or delamination, and the peel strength reduction rate is ≤10%.

[0013] Furthermore, the cover film in step S1 includes at least a PI film layer and an adhesive layer. The thickness of the PI film layer is 25μm~50μm, and its surface is pretreated by plasma to introduce active groups such as hydroxyl and carboxyl groups to enhance the interfacial bonding force with the adhesive layer. The adhesive layer is used to achieve a firm bond with the substrate and copper foil. The thickness of the adhesive layer is 15um-25um. The adhesive layer uses a modified epoxy resin with high bonding strength as the base adhesive, and adds 5%~8% isocyanate curing agent and 2%~3% silane coupling agent. At the same time, 3%~5% nano calcium carbonate filler is mixed in to increase the initial peel force to more than 1.2kg / cm². During the curing process of the adhesive layer, the degree of crosslinking is ≥85%, forming a dense three-dimensional network structure. The molecular weight distribution of the adhesive layer is between 1000 and 3000, and the molecular weight distribution width (PDI) is ≤2.5.

[0014] Furthermore, after the PI film layer and the adhesive layer are combined, the edges are sealed with edge-sealing adhesive with a width of 0.5mm~1mm; the cover film is designed with a surface protective coating on the PI film layer. The surface protective coating is a fluorine-modified resin coating or a silicone coating with a thickness of 2um-5um. The surface protective coating forms a hydrophobic barrier with a contact angle ≥110°.

[0015] Furthermore, after the pretreatment of the cover film in step S2, a vacuum drying process is added, drying at 120℃ and -0.09MPa for 60min~90min; after the combined pretreatment in step S5, nitrogen baking for 2H or plasma cleaning is used. When using plasma cleaning, the processing power is controlled at 100W~150W and the processing time is 30s~60s. For better bonding between the pure adhesive and the substrate, a combined pretreatment method of plasma and micro-etching is adopted. First, the surface oil and oxide layer are removed by plasma cleaning, and then light micro-etching is performed with a micro-etching amount of 0.5μm~0.8μm.

[0016] By adopting the above solution, this invention addresses the core causes of FPC multilayer boards bursting—insufficient bonding strength, stress concentration, and moisture absorption—through a systematic optimization of the entire process, including material selection, pretreatment, structural design, and process parameters. This solves the problem of existing technologies being unable to handle multiple testing conditions (dual 85+Reflow, PCT) and multilayer FPC bursting issues. The materials used in this invention, such as the cover film, pure adhesive, and steel sheet, are readily available. Pretreatment and lamination processes can be implemented using existing production equipment without large-scale modifications. Test verification shows that this method can effectively reduce the bursting rate of multilayer FPC boards, meeting stringent customer testing requirements. It is particularly suitable for FPC products with high multilayer and high reliability requirements, further reducing the bursting rate to below 0.1%, meeting more demanding application scenarios (such as automotive electronics and industrial control equipment). Detailed Implementation

[0017] To further explain the technical solution of the present invention, the present invention will be described in detail below through specific embodiments.

[0018] This invention discloses a method for improving the cracking of FPC multilayer boards, which mainly includes the following steps: S1. Selection of Covering Membrane: The cover film is selected with a peel strength greater than 1.0 kg / cm², possessing excellent high temperature resistance, moisture resistance, and interlayer bonding stability. After the double 85 test, the cover film can withstand a reflow soldering temperature of 265℃~270℃ for 10 seconds within 5 minutes and repeat 5 times without interlayer separation. At the same time, the thermal expansion coefficient of the cover film matches that of the FPC substrate and copper foil, with the difference controlled within 5 ppm / ℃, in order to reduce the interlayer stress caused by the difference in thermal expansion and contraction during temperature cycling and avoid board explosion caused by stress accumulation. The cover film can be selected from commercially available products that meet the requirements for testing, or it can be improved based on existing cover films.

[0019] The cover film includes at least a PI film layer and an adhesive layer. The cover film provides mechanical support, high temperature resistance and insulation. The present invention selects a high temperature resistant PI (polyimide) film with a thickness of 25μm~50μm. Its surface is pretreated by plasma to introduce active groups such as hydroxyl and carboxyl groups to improve the interfacial bonding force with the adhesive layer.

[0020] The adhesive layer is used to achieve a strong bond with the substrate and copper foil. The thickness of the adhesive layer is 15um-25um, ensuring that the adhesive layer can completely fill the tiny gaps between the PI film layer and the substrate after lamination, forming a gapless bonding interface, while avoiding the accumulation of internal stress caused by an excessively thick adhesive layer. The adhesive layer of this invention uses a high-bonding-strength modified epoxy resin as the base adhesive, adding 5%~8% isocyanate curing agent and 2%~3% silane coupling agent. Through cross-linking reaction, the chemical bonding force between the adhesive layer and the substrate and copper foil can be improved. Simultaneously, 3%~5% nano-calcium carbonate filler is mixed in to increase the mechanical interlocking force between the adhesive layer and the contact surface, raising the initial peel force to over 1.2kg / cm². During the curing process, the degree of cross-linking of the adhesive layer is ≥85%, forming a dense three-dimensional network structure, reducing the diffusion coefficient of moisture in the adhesive layer, and avoiding the increase in internal pressure caused by moisture accumulation. The molecular weight distribution of the adhesive layer is between 1000 and 3000, and the molecular weight distribution width (PDI) is ≤2.5. This ensures that the adhesive layer can fully wet the contact surface during the pressing process and form a dense adhesive layer, avoiding insufficient wetting due to excessively large molecular weight or insufficient strength due to excessively small molecular weight.

[0021] After the PI film layer and adhesive layer are laminated, the edges are sealed with edge-sealing adhesive with a width of 0.5mm~1mm to prevent moisture penetration from the edges. A surface protective coating can be designed on the PI film layer of the cover film. This surface protective coating can be a fluorine-modified resin coating or a silicone coating, with a thickness of 2um-5um. The surface protective coating forms a hydrophobic barrier with a contact angle ≥110°, reducing moisture adsorption and improving the cover film's high-temperature resistance, moisture resistance, and anti-adhesion properties. The cover film can also further incorporate flexible elastomers such as nitrile rubber or polyether urethane into the adhesive layer to improve the toughness and elastic modulus of the adhesive layer. During temperature changes, this allows for the absorption of some thermal expansion stress through elastic deformation, preventing interlayer separation caused by stress concentration.

[0022] The overall coefficient of thermal expansion (CTE) of the cover film is between 10 ppm / ℃ and 18 ppm / ℃ (25℃ to 200℃), with a CTE difference of ≤5 ppm / ℃ between the cover film and the FPC substrate and copper foil, reducing interlayer stress during temperature cycling. Because the PI film layer has a high CTE, to control the overall CTE of the cover film within 10 ppm / ℃ to 18 ppm / ℃, this invention adds 15% to 20% of low-CTE fillers (such as alumina or silicon carbide, CTE ≤5 ppm / ℃) to the adhesive layer. This neutralizes the high expansion characteristics of the PI film layer, ensuring a CTE difference of ≤5 ppm / ℃ between the cover film and the FPC substrate and copper foil. This results in low interlayer stress during temperature changes, effectively preventing delamination and warping caused by differences in thermal expansion and contraction.

[0023] The cover film adopts a three-layer composite structure design consisting of a surface protective coating, a PI film layer, and an adhesive layer. The layers are tightly bonded, eliminating the risk of interlayer peeling. The adhesive layer has a high degree of cross-linking and a dense structure, which not only prevents moisture penetration but also resists chemical degradation at high temperatures. Its performance degrades slowly during long-term use, making it suitable for high-reliability FPC products (such as automotive electronics and display modules).

[0024] S2. Pretreatment of the covering membrane: The selected cover film in step 1 is subjected to surface pretreatment. Pretreatment involves any one or a combination of roughening, micro-etching, or sandblasting on the adhesive layer side. The roughening treatment controls the copper etching amount between 0.8μm and 1.2μm to form a uniformly distributed micro-rough structure on the adhesive layer surface of the cover film, increasing the contact area and mechanical interlocking force between the cover film and the substrate. At the same time, it removes the oxide layer, oil, and other impurities on the adhesive layer surface of the cover film, ensuring that a stable bonding interface can be formed between the cover film and the substrate. The peel force between the cover film and the substrate after pretreatment should be greater than 0.9kg / cm², and the peel force variation coefficient should be controlled within 5% to ensure the uniformity of the bonding force.

[0025] S3, Pure Rubber Selection: Antiionic pure adhesive is selected, which possesses low moisture absorption, high temperature resistance, and excellent bonding performance. Its moisture absorption rate is controlled below 0.1% after 5 hours of double 85 testing. Simultaneously, the glass transition temperature of this antiionic pure adhesive is above 200℃, maintaining stable bonding strength under the high-temperature environment of reflow soldering. Existing commercially available products meeting the above functions can be directly selected as the antiionic pure adhesive, or pure adhesives that meet the above conditions can be selected after minor process adjustments.

[0026] S4. Substrate selection: The FPC substrate is selected with low moisture absorption and high thermal stability. The moisture absorption rate of the substrate is controlled below 0.2% after 5 hours of double 85 testing, and the thermal decomposition temperature is above 300℃. It maintains stable physical and chemical properties under high temperature and high humidity and reflow soldering high temperature environments. The number of layers of the substrate is determined according to the application requirements of the FPC multilayer board, including double-sided boards, three-layer boards and multilayer boards with four or more layers. For multilayer boards with four or more layers, the interlayer adhesive layer of the substrate must be a modified epoxy resin adhesive with high bonding strength, and the adhesive layer thickness is controlled between 80μm and 100μm to ensure strong interlayer bonding. At the same time, the thermal expansion coefficient of the substrate must be coordinated with the thermal expansion coefficient of the cover film and copper foil to reduce interlayer stress.

[0027] S5. Pre-processing for assembly: The substrate or copper foil undergoes pretreatment, including any one of medium roughening, browning, or micro-etching. After pretreatment, it is combined with pure adhesive. The peel strength of the combined pure adhesive is greater than 1.0 kg, and the peel strength decrease rate does not exceed 8% after 12 hours of double 85 testing. After combination, nitrogen baking for 2 hours or plasma cleaning can be used. When using plasma cleaning, the processing power is controlled at 100W~150W and the processing time is 30s~60s to further remove impurities and oxide layers from the surface of the pure adhesive and improve the adhesion between the pure adhesive and the substrate or copper foil. The bonding surface between the pure adhesive and the substrate is better achieved using a "plasma + micro-etching composite pretreatment" process. First, plasma cleaning removes surface oil and oxide layers, followed by light micro-etching with a depth of 0.5μm~0.8μm. This improves surface roughness while avoiding substrate damage caused by excessive etching. The peel force between the bonded surface and the pure adhesive is 30%~40% higher than that of a single pretreatment. After the pretreatment of the cover film, a vacuum drying process is added, drying at 120℃ and -0.09MPa for 60min~90min to thoroughly remove moisture and residual gas adsorbed during the pretreatment process. This prevents moisture and gas from expanding due to heat during subsequent pressing and reflow soldering, which could lead to interlayer bubbles and board bursting.

[0028] S6. Interlayer Structure Design: For multilayer boards with four or more layers, a differentiated interlayer structure design of "thin adhesive in the middle layer + thick adhesive in the outer layer" is adopted. The thickness of the middle layer adhesive is 80μm~100μm to ensure the bonding accuracy between layers and the signal transmission performance. The thickness of the outer layer adhesive is 120μm~150μm to enhance the buffer performance and impact resistance of the outer layer structure and effectively absorb external thermal stress and mechanical stress. Stress relief grooves are added to the large solid copper area of ​​the FPC multilayer board. The width of the stress relief grooves is 0.2mm~0.3mm and the spacing is 5mm~8mm. The grooves are filled with flexible adhesive with an elastic modulus of 0.5GPa~1.0GPa. The flexible adhesive can absorb the thermal expansion stress of the copper layer when the temperature changes, and avoid the thermal expansion of the copper layer from pulling on adjacent layers and causing board explosion.

[0029] S7. Optimization of the lamination process: The pressing parameters are set as follows: pressing pressure 90-130KG, pressing time 90-150S, and pressing temperature 180℃. During the pressing process, the pressure is kept uniform, with fluctuations not exceeding ±5KG, ensuring full adhesion between the cover film, adhesive, and substrate, without gaps or incomplete pressing. A "segmented pressing process" is adopted, with the pressing of high-multilayer boards performed in three stages: the first stage (80℃, 50KG) preheating for 30s, the second stage (150℃, 110KG) initial curing for 60s, and the third stage (180℃, 130KG) complete curing for 40s. This gradually releases interlayer gas and stress, avoiding interlayer separation caused by a single high-pressure, high-temperature process.

[0030] If the FPC multilayer board contains steel sheets, the process also includes step S8, steel sheet processing: The steel sheet is preferably a frosted, non-perforated steel sheet with a thickness ranging from 0.1mm to 0.3mm and a hardness controlled between HV200 and HV250. This ensures sufficient support strength while providing a certain degree of toughness, preventing mechanical damage to the cover film due to excessive hardness. The steel sheet undergoes plasma pretreatment to introduce active groups onto its surface, enhancing the chemical bond between the steel sheet and the adhesive. A thick steel sheet adhesive with a thickness of 150μm to 200μm is selected, possessing good elasticity and cushioning properties, with an elastic modulus controlled between 1.5GPa and 2.5GPa. This effectively absorbs the thermal and mechanical stresses generated during reflow soldering, preventing peeling and delamination between the steel sheet and the cover film due to stress concentration.

[0031] Optimization of the connection structure between steel sheet and FPC: The edges of the frosted, non-perforated steel sheet feature a rounded transition design with a radius of 0.5mm to 1mm, replacing the traditional right-angled edges. This avoids sharp angles causing stress concentration on the cover film, reducing the risk of the cover film cracking in stress concentration areas. The bonding area between the steel sheet and the FPC uses a combination of "local dispensing + overall pressing". High-strength adhesive is pre-applied to key stress points such as the edges and center of the steel sheet, with a dispensing diameter of 1mm to 2mm and a dispensing spacing of 4mm to 6mm. Then, overall pressing is performed to form a locally reinforced bond between the steel sheet and the cover film, improving the bonding strength in key areas and preventing the steel sheet from peeling off from the cover film.

[0032] If FPC multilayer boards require steel sheet holes due to special needs such as heat dissipation, a blind via + hydrophobic coating design is adopted. The blind via does not penetrate the steel sheet, preventing the cover film from being directly exposed to the external environment and reducing moisture adsorption channels. The inner wall of the via is coated with a hydrophobic coating made of polytetrafluoroethylene or fluorinated modified resin material, with a coating thickness of 5μm~10μm, giving the inner wall of the via excellent hydrophobic properties and a contact angle greater than 110°, effectively reducing the adsorption and residue of moisture in the via. When wiring, avoid continuous distribution of large areas of copper layers, and use a grid-like copper layer instead, with a grid spacing of 1mm~2mm and a grid line width of 0.1mm~0.2mm. This ensures both conductivity and heat dissipation performance, reduces the stress generated by the overall thermal expansion of the copper layer, and improves interlayer permeability, facilitating the discharge of interlayer gas during the lamination process and reducing moisture residue.

[0033] It also includes step S9, slow cooling treatment after pressing: After pressing, the product is placed in a slow cooling furnace for gradual cooling at a rate of 5℃ / min to 8℃ / min, gradually cooling from 180℃ to room temperature. This replaces direct room temperature cooling, reducing thermal stress caused by sudden temperature changes and avoiding the risk of decreased interlayer bonding and board bursting due to thermal stress. During the slow cooling process, the furnace environment must be kept clean and free of airflow disturbances to ensure uniform cooling of all areas of the product, with temperature differences controlled within ±3℃, thus ensuring consistent performance of all parts of the product.

[0034] It also includes step S10, post-processing and detection enhancement: A vacuum degassing process is added after reflow soldering, where the product is treated at 200℃ and -0.08MPa for 30 minutes to remove any trace amounts of moisture and gas that may remain between the layers, further reducing the risk of board breakage during subsequent use. Online interlayer bonding strength detection is also added. After the lamination process, each batch of products is 100% inspected using an ultrasonic scanning microscope (SAM) with a detection accuracy of 5μm. This can identify tiny gaps and areas of poor pressure between layers, allowing for the timely removal of defective products and preventing them from flowing into subsequent processes. Simultaneously, 5% of each batch of products is randomly selected for accelerated aging testing. Under the stringent conditions of 120 hours of double 85 testing followed by 10 reflow soldering cycles, the reliability of the product after long-term use is verified. The pass criteria are no board breakage or delamination, and a peel strength reduction rate ≤10%.

[0035] This invention improves the performance of multi-layer FPC boards by employing multi-dimensional collaborative methods, primarily as follows: Using a cover film with a peel strength >1.0 kg / cm² is significantly better than that of an ion migration resistant cover film (>0.7 kg / cm²), which can improve interlayer adhesion, resist thermal stress during reflow soldering, and reduce the risk of interlayer separation.

[0036] Roughening pretreatment of the cover film can increase the surface roughness, while roughening or browning pretreatment of pure adhesive can optimize the interfacial bonding state and improve the peel force. Among them, the peel force of the same cover film after medium roughening treatment is greater than that of micro-etching and sandblasting, and the peel force of pure adhesive after medium roughening or browning treatment is greater than that of micro-etching, thus ensuring the stability of interlayer bonding.

[0037] The use of non-perforated steel sheets prevents the cover film from absorbing moisture through the vents in high-temperature and high-humidity environments. Plasma pretreatment enhances the bonding strength between the steel sheet and the adhesive. Thick steel sheet adhesive can buffer thermal stress and reduce stress concentration that could lead to sheet bursting.

[0038] The improved multilayer FPC was tested and found to maintain structural stability under high temperature, high pressure and high humidity conditions, improving reliability in extreme environments. It can meet the stringent requirements of customers to pass the Reflow test after 5-12 hours of double 85 testing, and the product qualification rate is significantly improved.

[0039] After pressing, a "slow cooling treatment" is added: After pressing, the product is placed in a slow cooling furnace and gradually cooled from 180°C to room temperature (cooling rate 5°C / min~8°C / min), which replaces direct room temperature cooling and reduces the thermal stress caused by sudden temperature changes.

[0040] Enhanced Testing and Verification: Expanding testing conditions from "meeting customer requirements" to "exceeding customer expectations," in addition to the dual 85+ Reflow and PCT tests, we have added "temperature cycling test" (-40℃~125℃, 100 cycles) and "damp heat cycling test" (40℃ / 90%RH16h→125℃ / 5%RH8h, 50 cycles) to simulate the reliability of the product under extreme usage environments and identify potential board failure risks in advance.

[0041] Add "interlayer stress test": use strain gauges attached to the inner layer of FPC to monitor the interlayer stress changes in real time during reflow soldering, and optimize structural design and process parameters (target: maximum interlayer stress ≤ 50MPa).

[0042] For products that experienced board breakage during testing, a combination of "slice analysis + Fourier transform infrared spectroscopy (FTIR) + thermogravimetric analysis (TGA)" was used: slice analysis was used to observe the location and morphology of interlayer separation, FTIR was used to detect whether the adhesive layer had degraded, and TGA was used to test the moisture absorption rate and thermal stability of the adhesive layer to accurately locate the root cause of board breakage (such as adhesive layer degradation, moisture residue, stress concentration).

[0043] Batch consistency verification: 5% of each batch of products are sampled for "accelerated aging test" (120 hours of double 85 test followed by 10 Reflow cycles) to verify the reliability of the product after long-term use and ensure the consistency of performance between batches (passing standard: no board bursting, delamination, peel force reduction rate ≤10%).

[0044] The above embodiments are not intended to limit the product form and style of the present invention. Any appropriate changes or modifications made by those skilled in the art should be considered as not departing from the patent scope of the present invention.

Claims

1. A method for improving the bursting effect of FPC multilayer boards, characterized in that, Includes the following steps: S1. Cover film selection: The cover film is selected with a peel strength greater than 1.0 kg / cm², and has excellent high temperature resistance, moisture resistance and interlayer bonding stability. The cover film can withstand the reflow soldering high temperature of 265℃~270℃ for 10 seconds within 5 minutes after the double 85 test and repeat it 5 times without interlayer separation. At the same time, the thermal expansion coefficient of the cover film is matched with the thermal expansion coefficient of the FPC substrate and copper foil, and the difference is controlled within 5 ppm / ℃. S2. Pretreatment of the cover film: The cover film selected in step 1 is subjected to surface pretreatment. The pretreatment is any one or any combination of roughening, micro-etching or sandblasting on the adhesive layer side. The roughening treatment controls the copper etching amount between 0.8μm and 1.2μm, so that the adhesive layer surface of the cover film forms a uniformly distributed micro-rough structure. After the pretreatment, the peel force between the cover film and the substrate is greater than 0.9kg / cm², and the peel force variation coefficient is controlled within 5%. S3. Selection of pure adhesive: Anti-ion pure adhesive is selected. The moisture absorption rate of the anti-ion pure adhesive is controlled below 0.1% after 5 hours of double 85 test, and the glass transition temperature of the anti-ion pure adhesive is higher than 200℃. S4. Substrate selection: The selected substrate should have a moisture absorption rate below 0.2% after 5 hours of double 85 testing and a thermal decomposition temperature above 300℃. For multilayer boards with four or more layers, the interlayer adhesive layer of the substrate should be a modified epoxy resin adhesive with high bonding strength, and the adhesive layer thickness should be controlled between 80μm and 100μm. At the same time, the thermal expansion coefficient of the substrate should be compatible with the thermal expansion coefficient of the cover film and copper foil. S5. Pre-treatment before assembly: Pre-treatment is performed on the substrate or copper foil. The treatment method includes any one of medium roughening, browning, or micro-etching. After pre-treatment, it is combined with pure adhesive. The peel strength of the combined pure adhesive is greater than 1.0 kg, and the peel strength decrease rate does not exceed 8% after 12 hours of double 85 testing. S6. Interlayer structure design: Multilayer boards with four or more layers adopt an interlayer structure with thin adhesive in the middle layer and thick adhesive on the outer layer. The adhesive thickness of the middle layer is 80μm~100μm and the adhesive thickness of the outer layer is 120μm~150μm. S7. Optimization of pressing process: Set the pressing parameters as follows: pressing pressure 90-130KG, pressing time 90-150S, pressing temperature 180℃, and the pressure fluctuation range during the pressing process should not exceed ±5KG.

2. The method for improving FPC multilayer board bursting as described in claim 1, characterized in that: The FPC high-multilayer board includes steel sheets. Before the pressing process optimization in step S7, step S8, steel sheet treatment, is also included: the steel sheet is a frosted, non-perforated steel sheet with a thickness of 0.1mm to 0.3mm and a hardness of HV200 to HV250. The steel sheet is subjected to plasma pretreatment to introduce active groups on the surface of the steel sheet. A steel sheet adhesive with a thickness of 150μm to 200μm is selected, and the elastic modulus of the steel sheet adhesive is controlled at 1.5GPa to 2.5GPa.

3. The method for improving FPC multilayer board bursting as described in claim 2, characterized in that: The edges of the steel sheet are designed with a rounded transition, with a rounded corner radius of 0.5mm to 1mm. The bonding area between the steel sheet and the FPC adopts a combination of localized adhesive application and overall pressing. High-strength adhesive is pre-applied to the edges and key stress points in the center of the steel sheet, with an adhesive diameter of 1mm to 2mm and an adhesive spacing of 4mm to 6mm. Then, overall pressing is performed to form a locally reinforced bond between the steel sheet and the cover film.

4. The method for improving FPC multilayer board bursting as described in claim 1, characterized in that: The high-FPC multilayer board includes a steel sheet with multiple blind holes that do not penetrate the steel sheet. The inner wall of the blind holes is coated with a hydrophobic coating made of polytetrafluoroethylene or fluorine-modified resin material. The coating thickness is 5μm~10μm and the contact angle is greater than 110°.

5. The method for improving FPC multilayer board bursting as described in claim 1, characterized in that: When routing, a grid-like copper layer distribution is used, with a grid spacing of 1mm~2mm and a grid line width of 0.1mm~0.2mm.

6. The method for improving FPC multilayer board blowout as described in claim 1, characterized in that: It also includes step S9, which follows step S7, slow cooling treatment after pressing: After pressing, the FPC multilayer board is placed in a slow cooling furnace for gradual cooling at a rate of 5℃ / min to 8℃ / min, gradually cooling from 180℃ to room temperature, and the temperature difference during the slow cooling process is controlled within ±3℃.

7. The method for improving FPC multilayer board bursting as described in claim 6, characterized in that: It also includes step S10, post-processing and enhanced testing: after reflow soldering, a vacuum degassing process is added, and the layers are treated at 200℃ and -0.08MPa for 30 minutes to remove trace amounts of residual water vapor and gas between the layers; online detection of interlayer bonding force is added, and after the pressing process, 100% of each batch of high-FPC multilayer boards are inspected using an ultrasonic scanning microscope with a detection accuracy of 5μm; 5% of each batch of high-FPC multilayer boards are sampled for accelerated aging test, and the high-FPC multilayer boards are subjected to 120 hours of double 85 testing followed by 10 reflow soldering cycles to verify the reliability of the high-FPC multilayer boards after long-term use. The qualified standard is no board explosion or delamination, and the peel strength reduction rate is ≤10%.

8. The method for improving FPC multilayer board bursting as described in claim 1, characterized in that: The cover film in step S1 includes at least a PI film layer and an adhesive layer. The PI film layer has a thickness of 25μm to 50μm and its surface is pretreated by plasma to introduce active groups such as hydroxyl and carboxyl groups, thereby improving the interfacial bonding force with the adhesive layer. The adhesive layer is used to achieve a firm bond with the substrate and copper foil. The adhesive layer has a thickness of 15μm to 25μm. The adhesive layer uses a modified epoxy resin with high bonding strength as the base adhesive, and adds 5% to 8% isocyanate curing agent and 2% to 3% silane coupling agent. At the same time, 3% to 5% nano-calcium carbonate filler is mixed in to increase the initial peel force to more than 1.2kg / cm². During the curing process of the adhesive layer, the degree of crosslinking is ≥85%, forming a dense three-dimensional network structure. The molecular weight distribution of the adhesive layer is between 1000 and 3000, and the molecular weight distribution width (PDI) is ≤2.

5.

9. The method for improving FPC multilayer board bursting as described in claim 8, characterized in that: After the PI film layer and the adhesive layer are combined, the edges are sealed with edge sealant with a width of 0.5mm~1mm; the cover film is designed with a surface protective coating on the PI film layer. The surface protective coating is a fluorine-modified resin coating or a silicone coating with a thickness of 2um-5um. The surface protective coating forms a hydrophobic barrier with a contact angle ≥110°.

10. The method for improving FPC multilayer board bursting as described in claim 9, characterized in that: After the pretreatment of the cover film in step S2, a vacuum drying process is added, drying at 120℃ and -0.09MPa for 60min~90min; after the combined pretreatment in step S5, nitrogen baking for 2H or plasma cleaning is used. When using plasma cleaning, the processing power is controlled at 100W~150W and the processing time is 30s~60s. For better bonding between the pure adhesive and the substrate, a combined pretreatment method of plasma and micro-etching is adopted. First, the surface oil and oxide layer are removed by plasma cleaning, and then light micro-etching is performed with a micro-etching amount of 0.5μm~0.8μm.