A process for anti-aging treatment of the flexible area of ​​a rigid-flex PCB

Abstract

This invention discloses an anti-aging treatment process for the flexible area of ​​a rigid-flex PCB, comprising the following steps: Step 1: Pretreatment of the flexible PCB substrate; Step 2: Composite protection of the copper circuit surface; Step 3: Stress buffering treatment of the rigid-flex transition zone of the rigid-flex PCB; Step 4: Coating protection and edge sealing; Step 5: Layered temperature-controlled annealing and curing. This invention, through collaborative innovation across the entire process, precise parameter control, and material optimization, constructs a complete anti-aging system encompassing "materials—process—environment—verification," comprehensively solving the core defects of traditional processes such as poor anti-aging effect, low batch stability, short bending fatigue life, and moisture penetration. It significantly improves the anti-aging performance and operational reliability of the flexible area of ​​the rigid-flex PCB, while also considering industrial applicability and cost controllability. It can be widely applied in fields with high requirements for the anti-aging performance of flexible PCBs, such as automotive electronics, aerospace electronics, and industrial control equipment, and has significant practical value and promotional significance.

Description

Technical Field

[0001] This invention relates to the field of circuit board manufacturing technology, specifically to an anti-aging treatment process for the flexible area of ​​a rigid-flex PCB. Background Technology

[0002] Rigid-flex circuit boards combine the structural stability of rigid boards with the flexibility of flexible boards, making them widely used in various precision electronic devices. The flexible board area uses polyimide (PI) as the core substrate, combined with copper foil to form conductive lines. However, during long-term use, they are susceptible to aging failures such as substrate yellowing, circuit oxidation, interface peeling, and circuit breakage due to factors such as heat and oxygen, humidity and heat, bending fatigue, copper ion migration (CAF), and chemical corrosion. This severely affects the service life and operational reliability of the rigid-flex board.

[0003] In existing technologies, anti-aging treatment of flexible printed circuit boards (FPCBs) often employs single-stage protection, such as only chemically immersing copper traces in nickel plating, simply coating the FPCB surface with ordinary conformal coating, or only optimizing the simple structural design of the rigid-flexible transition zone. Each process step is operated independently, lacking coordination, and the process parameters are vague, with insufficient material selection, resulting in poor anti-aging effects. Traditional processes have the following shortcomings: insufficient substrate pretreatment adhesion, easily leading to the peeling of protective and adhesive layers; single-layer copper trace protection, with limited resistance to copper ion migration and oxidation; significant stress concentration in the rigid-flexible transition zone, resulting in short bending fatigue life; poor coating sealing effect, allowing moisture penetration and aging; unreasonable annealing process, limiting the optimization of copper foil fatigue resistance; and lack of comprehensive material and environmental control, resulting in poor batch stability of finished products. Summary of the Invention

[0004] The purpose of this invention is to provide an anti-aging treatment process for the flexible area of ​​a rigid-flex PCB. Through collaborative innovation throughout the entire process, precise parameter control, and material optimization, this process solves the problems of poor anti-aging effect, low batch stability, and short bending fatigue life of traditional processes, thereby comprehensively improving the anti-aging performance of the flexible area and addressing the issues raised in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: an anti-aging treatment process for the flexible area of ​​a rigid-flex PCB, comprising the following steps: Step 1: Pretreatment of flexible PCB substrate. A combination of plasma cleaning and silane coupling agent modification is used to remove the weak boundary layer on the surface of the flexible PCB PI substrate and introduce active groups. Step 2: Composite protection of copper circuit surface. First, chemically deposit nickel-gold to form a basic protective layer, and then coat with a fluorinated imidazole antioxidant to form a self-assembled protective film. Step 3: Stress buffering treatment for the rigid-flexible transition zone of the flexible board, using a gradual window design and adding an elastic buffer layer filled with nano-SiO2. Step 4: Apply protective coating and edge sealing. Selectively spray a modified silicone rubber three-proof coating on the flexible board area, and at the same time use epoxy sealant to fully wrap and seal the edges of the flexible board and the rigid-flexible transition area. Step 5: Layered temperature-controlled annealing and curing. The flexible board area is annealed in sections to complete the anti-aging process. Through the synergistic effect of the above steps, it resists aging failure caused by heat and oxygen, damp heat, bending fatigue and copper ion migration.

[0006] Preferably, the plasma cleaning parameters in step one are: radio frequency power 280-320W, vacuum degree 1900-2100mTorr, and cleaning time 8-12min; the silane coupling agent modification treatment is as follows: the pretreated PI substrate is immersed in a silane coupling agent solution with a mass concentration of 1.5-2.5% for 15-25min, and then dried at 110-130℃ for 30-40min.

[0007] Preferably, in step two, the Ni layer thickness of the electroless nickel plating is 3.5-4.5 μm, and the Au layer thickness is 0.06-0.09 μm; the fluorinated imidazole antioxidant is 5224FIM, the coating thickness is 0.8-1.2 μm, and after coating, it is kept at 90-110℃ for 20-30 min to form a uniform and dense self-assembled protective film with a corrosion inhibition efficiency of not less than 94%.

[0008] Preferably, in step two, 0.4-0.6 ppm of thiourea inhibitor is added to the chemically deposited copper wire to reduce copper ion activity and inhibit copper ion migration (CAF). At the same time, a two-stage browning treatment is performed before nickel-gold plating, and the micro-etching amount is controlled at 1.2-1.5 μm to improve the adhesion between the protective layer and the copper circuit.

[0009] Preferably, in step three, the transition length of the gradual window opening is 6-7 mm, and the transition angle is ≥45°; the thickness of the elastic buffer layer filled with nano-SiO2 is 25-35 μm, wherein the particle size of nano-SiO2 is 50-100 nm, the filling amount is 8-12% of the total mass of the elastic adhesive layer, and the elastic modulus of the elastic adhesive layer is 1.5-2.5 GPa.

[0010] Preferably, in step three, a U-shaped serpentine buffer section is added to the rigid-flexible transition zone. The length of the buffer section is 3.5-4.5 times the bending radius of the flexible board, and the width of the serpentine line is consistent with the width of the flexible board line, so as to avoid stress concentration in the transition zone.

[0011] Preferably, in step four, the modified silicone rubber conformal coating contains 10-15% nano-zinc oxide anti-aging agent, has a coating thickness of 15-25 μm, a curing temperature of 140-160℃, a curing time of 40-60 min, a coating temperature resistance range of -60℃ to 220℃, and a bending resistance of not less than 10 times.6 Second-rate.

[0012] Preferably, the epoxy sealant in step four is a low-hygroscopic modified epoxy sealant with a moisture absorption rate of <0.8% and a sealing thickness of 10-15μm. It adopts a UV curing method after dispensing, with a curing time of 8-12s. After curing, there is no excess adhesive or bubbles, which prevents moisture from penetrating along the edge of the flexible board.

[0013] Preferably, the segmented annealing process in step five is as follows: the first stage is held at 120-130℃ for 1.5-2.5 hours, the second stage is held at 150-170℃ for 2.5-3.5 hours, the third stage is held at 180-200℃ for 0.5-1.5 hours, and after annealing, it is naturally cooled to room temperature, and the fatigue resistance of the copper foil is improved by no less than 65%.

[0014] Preferably, the flexible circuit board substrate is made of high heat-resistant PI film with Tg≥260℃ and thermal decomposition temperature≥500℃; the copper foil is rolled annealed copper (RA copper) with a thickness of 12-35μm and thickness uniformity of ±3μm; the entire process is carried out in a dust-free and low-humidity environment, and the finished product is dehumidified by vacuum baking at 100-120℃ for 2-4 hours. After aging verification at high temperature and high humidity (85℃ / 85%RH) for 1000 hours, the insulation resistance remains stable, and there is no copper migration or interface peeling.

[0015] Compared with the prior art, the beneficial effects of the present invention are: This invention constructs a complete anti-aging system encompassing "materials—process—environment—verification" through collaborative innovation across the entire process, precise parameter control, and material optimization. It comprehensively solves the core defects of traditional processes, such as poor anti-aging effects, low batch stability, short bending fatigue life, and moisture penetration. It significantly improves the anti-aging performance and operational reliability of the flexible circuit board area in rigid-flex circuit boards, while also considering industrial applicability and cost controllability. It can be widely applied in fields with high requirements for the anti-aging performance of flexible circuit boards, such as automotive electronics, aerospace electronics, and industrial control equipment, and has significant practical value and promotional significance.

[0016] The process improvements of this invention are specifically reflected in the following aspects: Significantly improved substrate adhesion: Composite pretreatment enhances the surface activity of PI substrate, resulting in stronger adhesion of subsequent adhesive and protective layers, reducing the risk of interface peeling, and solving the pain point of insufficient adhesion in traditional pretreatment. Upgraded copper migration and oxidation resistance: The dual action of composite protective film and thiourea inhibitor results in a corrosion inhibition efficiency of ≥94%, effectively inhibiting CAF failure. The copper circuit's oxidation and corrosion resistance is superior to traditional single protection. Significantly extended bending fatigue life: Nano-SiO2 buffer layer and optimized buffer structure, combined with segmented annealing, improve the fatigue resistance of copper foil by ≥65% and increase the number of bending cycles by ≥10. 6Secondly, it adapts to dynamic bending scenarios and solves the problem of easy breakage of traditional flexible circuit boards; Enhanced environmental adaptability: The modified three-proof coating has an expanded temperature resistance range of -60℃ to 220℃, and the low moisture absorption sealant (moisture absorption rate <0.8%) fully covers the surface, improving the ability to resist heat, oxygen, humidity, and salt spray, making it suitable for harsh working environments. Higher process stability: Layered temperature control, precise material parameter limitation, and full-process environmental control ensure that the insulation resistance of the finished product is stable after aging verification (85℃ / 85%RH, 1000h), with no copper migration or interface peeling, solving the problems of poor stability and large batch differences in traditional processes. Balancing compatibility and practicality: The innovative process does not require major modifications to existing production equipment and can be directly adapted to traditional rigid-flex PCB production lines, balancing innovation with industrial feasibility and controllable costs. Attached Figure Description

[0017] Figure 1 This is a flowchart of the process of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] This invention provides, for example Figure 1 The process for anti-aging treatment of the flexible area of ​​a rigid-flex PCB, as shown, includes the following steps; Step 1: Pretreatment of flexible board substrate. A combination of plasma cleaning and silane coupling agent modification is used to remove the weak boundary layer on the surface of the flexible board PI substrate and introduce active groups, laying the foundation for the bonding of subsequent steps. Step 2: Composite protection of copper circuit surface. First, a basic protective layer is formed by chemical electroless nickel-gold plating to block the corrosion of copper circuit by oxygen and moisture. Then, a fluorinated imidazole antioxidant is coated to form a self-assembled protective film, which further enhances the antioxidant and corrosion resistance. Step 3: Stress buffering treatment in the rigid-flexible transition zone. A gradual window design is adopted to reduce stress abrupt changes. At the same time, an elastic buffer layer filled with nano-SiO2 is added to disperse stress concentration during the bending process. Step 4: Apply protective coating and edge sealing. Selectively spray a modified silicone rubber three-proof coating on the flexible board area to achieve comprehensive protection. At the same time, use epoxy sealant to fully wrap and seal the edges of the flexible board and the rigid-flexible transition area to block water vapor penetration. Step 5: Layered temperature-controlled annealing and curing. The flexible circuit board area is annealed in stages to optimize the copper foil grain structure, improve fatigue resistance, and complete the entire anti-aging process.

[0020] Through the synergistic effect of the above steps, it can effectively resist various aging failures caused by heat and oxygen, damp heat, bending fatigue and copper ion migration. Compared with traditional single-stage protection processes, the anti-aging reliability is significantly improved.

[0021] To ensure the stability and consistency of substrate pretreatment results and avoid batch variations, the specific process parameters for plasma cleaning and silane coupling agent modification are precisely defined: Plasma cleaning parameters: RF power controlled at 280-320W, vacuum degree maintained at 1900-2100mTorr, cleaning time at 8-12min. This parameter range can effectively remove oil, impurities and weak boundary layers from the surface of PI substrate, while avoiding damage to the substrate due to over-cleaning. Silane coupling agent modification treatment: Immerse the pretreated PI substrate in a 1.5-2.5% (w / w) silane coupling agent solution for 15-25 minutes to allow the silane coupling agent to fully adhere to the substrate surface and introduce active groups. Then dry at 110-130℃ for 30-40 minutes to ensure that the silane coupling agent is fully bonded to the substrate and to improve the adhesion of the subsequent protective layer and adhesive layer.

[0022] This parameter limitation solves the problems of ambiguity and poor controllability of traditional preprocessing parameters, ensuring the stability of preprocessing results and effectively reducing the risk of subsequent interface stripping.

[0023] To enhance the oxidation resistance, corrosion resistance, and copper ion migration resistance of copper circuits, the materials and parameters of electroless nickel-gold plating and self-assembled protective films were precisely defined: Chemical plating nickel-gold parameters: Ni layer thickness is controlled at 3.5-4.5μm, which can effectively block oxygen and moisture from corroding copper circuits; Au layer thickness is controlled at 0.06-0.09μm, which can ensure good solderability and further improve corrosion resistance. Self-assembled protective film: A fluoroimidazole antioxidant (specifically model 5224FIM) is used, with a coating thickness of 0.8-1.2μm. After coating, the film is kept at 90-110℃ for 20-30 minutes to allow the antioxidant to form a uniform and dense self-assembled protective film on the copper circuit surface. Tests show that the corrosion inhibition efficiency of this protective film is not less than 94%, which can significantly improve the oxidation and corrosion resistance of copper circuits, while inhibiting copper ion migration.

[0024] This limitation overcomes the shortcomings of traditional copper circuit protection, which only specifies the thickness of the base layer and lacks specific antioxidant models and parameters, making the copper circuit protection effect more stable and reliable.

[0025] Adding 0.4-0.6 ppm of thiourea inhibitor to the chemically deposited copper wire can form a stable complex with copper ions, effectively reducing the activity of copper ions and specifically inhibiting copper ion migration (CAF) failure, thus solving the short circuit problem caused by copper ion migration in traditional processes. Before immersion nickel-gold plating, a two-stage browning treatment is performed, with the micro-etching amount controlled at 1.2-1.5μm. Through the two-stage browning treatment, a uniform micro-rough surface can be formed on the copper circuit surface, which further enhances the adhesion between the immersion nickel-gold protective layer and the copper circuit, prevents the protective layer from falling off, and enhances the oxidation resistance of the copper circuit.

[0026] To effectively disperse stress concentration in the rigid-flexible transition zone and prevent circuit breakage during bending, the key parameters of the gradient windowing and the nano-SiO2 buffer layer are precisely defined: Gradual window opening: The transition length is controlled at 6-7mm and the transition angle is ≥45°. This design can avoid right-angle stress abruptness in the transition zone and reduce stress concentration. The elastic buffer layer filled with nano-SiO2 has a thickness of 25-35 μm, with nano-SiO2 particles having a size of 50-100 nm. The filling amount is 8-12% of the total mass of the elastic adhesive layer, and the elastic modulus of the elastic adhesive layer is controlled between 1.5-2.5 GPa. This parameter range ensures that the elasticity and strength of the buffer layer are well-matched, effectively dispersing bending stress while ensuring the structural stability of the transition zone, thus avoiding protective failure caused by the buffer layer being too soft or too hard.

[0027] The rigid-flexible transition zone also features a U-shaped serpentine buffer section. The length of the buffer section is 3.5-4.5 times the bending radius of the flexible circuit board, and the width of the serpentine line is consistent with the width of the circuit board. This design allows the stress generated during bending to be fully released through the serpentine structure, forming a double buffering effect with the nano-SiO2 buffer layer. This effectively avoids circuit damage caused by stress concentration in the transition zone, adapts to dynamic bending scenarios, and significantly extends the bending fatigue life of the flexible circuit board.

[0028] To improve the anti-aging, temperature resistance, and bending resistance of the coating, the modified silicone rubber conformal coating underwent material optimization and parameter limitation: Adding 10-15% nano zinc oxide anti-aging agent to the modified silicone rubber conformal coating can help the nano zinc oxide absorb ultraviolet light, inhibit coating aging, and improve the coating's resistance to chemical corrosion. Coating parameters: Coating thickness 15-25μm, curing temperature 140-160℃, curing time 40-60min. Testing shows that the coating's temperature resistance range can be extended to -60℃~220℃, and its bending resistance is no less than 10 times. 6 Furthermore, it can be adapted to harsh scenarios such as wide temperature range and dynamic bending, and its environmental adaptability and durability are significantly improved compared to traditional three-proof coatings.

[0029] To prevent moisture from penetrating along the edges of the flexible plate, the materials and processes for edge sealing are optimized and limited: The sealant is a low-hygroscopic modified epoxy adhesive with a moisture absorption rate of <0.8%, which can effectively prevent moisture adsorption and penetration. The sealing thickness is controlled at 10-15μm, and UV curing is adopted after dispensing. The curing time is 8-12s. This curing method can quickly achieve sealing, improve production efficiency, and ensure that there is no excess adhesive or air bubbles after curing, forming a dense sealing layer that effectively blocks moisture from penetrating along the edge of the flexible board, solving the problems of poor sealing effect and aging failure caused by moisture penetration in traditional methods.

[0030] To optimize the grain structure of copper foil and improve its fatigue resistance, a three-stage layered temperature-controlled annealing process is adopted. The specific steps are as follows: First stage: Hold at 120-130℃ for 1.5-2.5 hours to remove residual moisture from the copper foil and substrate, and to prevent bubbles from forming during the annealing process; The second stage involves holding the copper foil at 150-170℃ for 2.5-3.5 hours to achieve initial reorganization of the copper foil grains and improve its toughness. The third stage: heat preservation at 180-200℃ for 0.5-1.5h to complete the full reorganization of copper foil grains and further optimize the mechanical properties of copper foil; After annealing, the copper foil is naturally cooled to room temperature. Tests show that this annealing process can improve the fatigue resistance of the copper foil by no less than 65%, while avoiding residual internal stress caused by single-temperature annealing, further improving the bending reliability of the flexible circuit board.

[0031] To ensure process stability and the anti-aging properties of the finished product, comprehensive optimization was carried out on material selection, production environment, and finished product processing: Material selection: The flexible circuit board substrate uses a high heat-resistant PI film with a Tg≥260℃ and a thermal decomposition temperature≥500℃, which can resist thermal and oxidative aging under high temperature environment; the copper foil uses rolled and annealed copper (RA copper) with a thickness of 12-35μm and a thickness uniformity of ±3μm, and its fatigue resistance is better than that of traditional electrolytic copper (ED copper). Production environment: The entire process is carried out in a dust-free and low-humidity environment to avoid the impact of dust and moisture on the process effect; Finished product processing: The finished product is vacuum baked at 100-120℃ for 2-4 hours to remove moisture and eliminate residual water vapor; Aging verification: The finished product must pass a high temperature and high humidity (85℃ / 85%RH) test for 1000 hours. After the test, the insulation resistance remains stable, with no copper migration or interface peeling, to ensure that the anti-aging performance of the finished product meets the standards. Example 1

[0032] A process for anti-aging treatment of the flexible area of ​​a rigid-flex circuit board includes the following steps: 1. Pretreatment of flexible PCB substrate: The substrate is treated with a combination of plasma cleaning and silane coupling agent modification. The plasma cleaning parameters are RF power 300W, vacuum degree 2000mTorr, and cleaning time 10min. The PI substrate is immersed in a 2.0% silane coupling agent solution for 20min and then dried at 120℃ for 35min. 2. Composite protection of copper circuit surface: First, chemical nickel-gold plating is performed with a Ni layer thickness of 4.0 μm and an Au layer thickness of 0.07 μm; 0.5 ppm thiourea inhibitor is added to the copper plating line, and a two-stage browning treatment is performed before nickel-gold plating with a micro-etching depth of 1.3 μm; then, 5224FIM fluorinated imidazole antioxidant is coated with a coating thickness of 1.0 μm and kept at 95℃ for 25 min; 3. Stress buffering treatment in the rigid-flexible transition zone: A gradual window design is adopted, with a transition length of 6.5mm and a transition angle of 45°; an elastic buffer layer filled with nano-SiO2 is added, with a thickness of 30μm, a nano-SiO2 particle size of 80nm, a filling amount of 10%, and an elastic modulus of 2.0GPa; at the same time, a U-shaped serpentine buffer section is added, with a length of 4.0 times the bending radius of the flexible board, and the width of the serpentine line is consistent with the width of the flexible board circuit. 4. Coating protection and edge sealing: Selectively spray modified silicone rubber conformal coating on the flexible board area, add 12% nano zinc oxide anti-aging agent, coating thickness 20μm, and cure at 150℃ for 50min; use low moisture absorption modified epoxy adhesive (moisture absorption rate 0.6%) to fully wrap and seal the edges of the flexible board and the rigid-flexible transition area, sealing thickness 12μm, and UV curing for 10s after dispensing; 5. Layered temperature-controlled annealing and curing: First stage: 125℃ for 2 hours; second stage: 160℃ for 3 hours; third stage: 190℃ for 1 hour; after annealing, allow to cool naturally to room temperature. 6. Finished product processing: The entire process is carried out in a dust-free and low-humidity environment. The finished product is vacuum baked at 110℃ for 3 hours to remove moisture and complete the anti-aging treatment. Example 2

[0033] A process for anti-aging treatment of the flexible area of ​​a rigid-flex circuit board includes the following steps: 1. Pretreatment of flexible PCB substrate: The substrate is treated with a combination of plasma cleaning and silane coupling agent modification. The plasma cleaning parameters are RF power 280W, vacuum degree 1900mTorr, and cleaning time 8min. The PI substrate is immersed in a 1.5% silane coupling agent solution for 15min and then dried at 110℃ for 30min. 2. Composite protection of copper circuit surface: First, chemical nickel-gold plating is performed with a Ni layer thickness of 3.5μm and an Au layer thickness of 0.06μm; 0.4ppm thiourea inhibitor is added to the copper plating line, and a two-stage browning treatment is performed before nickel-gold plating with a micro-etching depth of 1.2μm; then, 5224FIM fluorinated imidazole antioxidant is coated with a coating thickness of 0.8μm and kept at 90℃ for 20min; 3. Stress buffering treatment in the rigid-flexible transition zone: A gradual window design is adopted, with a transition length of 6mm and a transition angle of 50°; an elastic buffer layer filled with nano-SiO2 is added, with a thickness of 25μm, a nano-SiO2 particle size of 50nm, a filling amount of 8%, and an elastic modulus of 1.5GPa; at the same time, a U-shaped serpentine buffer section is added, with a length of 3.5 times the bending radius of the flexible board, and the width of the serpentine line is consistent with the width of the flexible board circuit. 4. Coating protection and edge sealing: Selectively spray modified silicone rubber conformal coating on the flexible board area, add 10% nano zinc oxide anti-aging agent, coating thickness 15μm, and cure at 140℃ for 40min; use low moisture absorption modified epoxy adhesive (moisture absorption rate 0.7%) to fully wrap and seal the edges of the flexible board and the rigid-flexible transition area, sealing thickness 10μm, and UV curing for 8s after dispensing; 5. Layered temperature-controlled annealing and curing: First stage: 120℃ for 1.5h, second stage: 150℃ for 2.5h, third stage: 180℃ for 0.5h, then allow to cool naturally to room temperature after annealing; 6. Finished product processing: The entire process is carried out in a dust-free and low-humidity environment. The finished product is vacuum baked at 100℃ for 2 hours to remove moisture and complete the anti-aging treatment. Example 3

[0034] A process for anti-aging treatment of the flexible area of ​​a rigid-flex circuit board includes the following steps: 1. Pretreatment of flexible PCB substrate: The substrate is treated with a combination of plasma cleaning and silane coupling agent modification. The plasma cleaning parameters are RF power 320W, vacuum degree 2100mTorr, and cleaning time 12min. The PI substrate is immersed in a 2.5% silane coupling agent solution for 25min and then dried at 130℃ for 40min. 2. Composite protection of copper circuit surface: First, chemical nickel-gold plating is performed, with a Ni layer thickness of 4.5μm and an Au layer thickness of 0.09μm; 0.6ppm thiourea inhibitor is added to the copper plating line, and a two-stage browning treatment is performed before nickel-gold plating, with a micro-etching amount of 1.5μm; then, 5224FIM fluorinated imidazole antioxidant is coated with a coating thickness of 1.2μm, and kept at 110℃ for 30min; 3. Stress buffering treatment in the rigid-flexible transition zone: A gradual window design is adopted, with a transition length of 7mm and a transition angle of 60°; an elastic buffer layer filled with nano-SiO2 is added, with a thickness of 35μm, a nano-SiO2 particle size of 100nm, a filling amount of 12%, and an elastic modulus of 2.5GPa; at the same time, a U-shaped serpentine buffer section is added, with a length of 4.5 times the bending radius of the flexible board, and the width of the serpentine line is consistent with the width of the flexible board circuit. 4. Coating protection and edge sealing: Selectively spray modified silicone rubber conformal coating on the flexible board area, add 15% nano zinc oxide anti-aging agent, coating thickness 25μm, and cure at 160℃ for 60min; use low moisture absorption modified epoxy adhesive (moisture absorption rate 0.5%) to fully wrap and seal the edges of the flexible board and the rigid-flexible transition area, sealing thickness 15μm, and UV curing for 12s after dispensing; 5. Layered temperature-controlled annealing and curing: First stage: 130℃ for 2.5h, second stage: 170℃ for 3.5h, third stage: 200℃ for 1.5h, then allow to cool naturally to room temperature after annealing; 6. Finished product processing: The entire process is carried out in a dust-free and low-humidity environment. The finished product is vacuum baked at 120℃ for 4 hours to remove moisture and complete the anti-aging treatment.

[0035]

[0036] The test results show that the finished products of Examples 1-3 of the present invention are significantly superior to those of traditional processes in terms of corrosion inhibition efficiency, copper foil fatigue resistance, bending resistance, temperature range, and high temperature and humidity stability. This fully demonstrates that the anti-aging performance of the process of the present invention is better and can effectively solve the shortcomings of the traditional process.

[0037] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for anti-aging treatment of the flexible area of ​​a rigid-flex PCB, characterized in that, Includes the following steps: Step 1: Pretreatment of flexible PCB substrate. A combination of plasma cleaning and silane coupling agent modification is used to remove the weak boundary layer on the surface of the flexible PCB PI substrate and introduce active groups. Step 2: Composite protection of copper circuit surface. First, chemically deposit nickel-gold to form a basic protective layer, and then coat with a fluorinated imidazole antioxidant to form a self-assembled protective film. Step 3: Stress buffering treatment for the rigid-flexible transition zone of the flexible board, using a gradual window design and adding an elastic buffer layer filled with nano-SiO2. Step 4: Apply protective coating and edge sealing. Selectively spray a modified silicone rubber three-proof coating on the flexible board area, and at the same time use epoxy sealant to fully wrap and seal the edges of the flexible board and the rigid-flexible transition area. Step 5: Layered temperature-controlled annealing and curing. The flexible board area is annealed in sections to complete the anti-aging process. Through the synergistic effect of the above steps, it resists aging failure caused by heat and oxygen, damp heat, bending fatigue and copper ion migration.

2. The anti-aging treatment process for the flexible area of ​​a rigid-flex PCB according to claim 1, characterized in that: The plasma cleaning parameters in step one are: radio frequency power 280-320W, vacuum degree 1900-2100mTorr, and cleaning time 8-12min; the silane coupling agent modification treatment is as follows: the pretreated PI substrate is immersed in a silane coupling agent solution with a mass concentration of 1.5-2.5% for 15-25min, and then dried at 110-130℃ for 30-40min.

3. The anti-aging treatment process for the flexible area of ​​a rigid-flex PCB according to claim 1, characterized in that: In step two, the Ni layer thickness of the electroless nickel plating is 3.5-4.5 μm, and the Au layer thickness is 0.06-0.09 μm. The fluorinated imidazole antioxidant is 5224FIM, with a coating thickness of 0.8-1.2 μm. After coating, the coating is kept at 90-110℃ for 20-30 min to form a uniform and dense self-assembled protective film with a corrosion inhibition efficiency of not less than 94%.

4. The anti-aging treatment process for the flexible area of ​​a rigid-flex PCB according to claim 1, characterized in that: In step two, 0.4-0.6 ppm of thiourea inhibitor is added to the chemically deposited copper wire to reduce the activity of copper ions and inhibit their migration. At the same time, a two-stage browning treatment is performed before nickel-gold plating, and the micro-etching amount is controlled at 1.2-1.5 μm to improve the adhesion between the protective layer and the copper circuit.

5. The anti-aging treatment process for the flexible area of ​​a rigid-flex PCB according to claim 1, characterized in that: In step three, the transition length of the gradual window opening is 6-7 mm, and the transition angle is ≥45°; the thickness of the elastic buffer layer filled with nano-SiO2 is 25-35 μm, wherein the particle size of nano-SiO2 is 50-100 nm, the filling amount is 8-12% of the total mass of the elastic adhesive layer, and the elastic modulus of the elastic adhesive layer is 1.5-2.5 GPa.

6. The anti-aging treatment process for the flexible area of ​​a rigid-flex PCB according to claim 1, characterized in that: In step three, a U-shaped serpentine buffer section is added to the rigid-flexible transition zone. The length of the buffer section is 3.5-4.5 times the bending radius of the flexible board, and the width of the serpentine line is consistent with the width of the flexible board line to avoid stress concentration in the transition zone.

7. The anti-aging treatment process for the flexible area of ​​a rigid-flex PCB according to claim 1, characterized in that: In step four, the modified silicone rubber conformal coating contains 10-15% nano-zinc oxide anti-aging agent. The coating thickness is 15-25 μm, the curing temperature is 140-160℃, the curing time is 40-60 min, the coating temperature range is -60℃ to 220℃, and the bending resistance is not less than 10 times. 6 Second-rate.

8. The anti-aging treatment process for the flexible area of ​​a rigid-flex PCB according to claim 1, characterized in that: The epoxy sealant used in step four is a low-hygroscopic modified epoxy sealant with a moisture absorption rate of <0.8% and a sealing thickness of 10-15μm. It adopts a UV curing method after dispensing, with a curing time of 8-12s. After curing, there is no excess adhesive or bubbles, which prevents moisture from penetrating along the edge of the flexible board.

9. The anti-aging treatment process for the flexible area of ​​a rigid-flex PCB according to claim 1, characterized in that: The segmented annealing process in step five is as follows: the first stage is held at 120-130℃ for 1.5-2.5 hours, the second stage is held at 150-170℃ for 2.5-3.5 hours, the third stage is held at 180-200℃ for 0.5-1.5 hours, and after annealing, it is naturally cooled to room temperature, and the fatigue resistance of the copper foil is improved by no less than 65%.

10. The anti-aging treatment process for the flexible area of ​​a rigid-flex PCB according to claim 1, characterized in that: The flexible circuit board substrate uses a high heat-resistant PI film with a Tg ≥ 260℃ and a thermal decomposition temperature ≥ 500℃; the copper foil uses rolled and annealed copper with a thickness of 12-35μm and a thickness uniformity of ±3μm; the entire process is carried out in a dust-free and low-humidity environment, and the finished product is dehumidified by vacuum baking at 100-120℃ for 2-4 hours. After aging verification at high temperature and high humidity for 1000 hours, the insulation resistance remains stable, and there is no copper migration or interface peeling.

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