Micro gradient dispensing and pre-stress release curing process of flexible circuit board, module and electronic equipment

Abstract

The application discloses a micro-gradient dispensing and prestress release curing process of a flexible circuit board, a module and electronic equipment. The process comprises: performing bottom dispensing on a binding area of the flexible circuit board, and performing ultraviolet pre-curing on the bottom glue; performing upper dispensing on the bottom glue to form a protection dam with an asymmetric slope structure in a cross section; and bending the flexible circuit board to a preset transition radius, and performing stepwise heating and curing in a bent state. The asymmetric slope structure formed by the double-layer process can smoothly disperse the shear stress of the binding area; meanwhile, the residual internal stress generated by the glue cross-linking shrinkage is offset in advance through the heating and curing in the pre-bent state. The process effectively solves the problems of line easy breakage and interface easy peeling of the flexible circuit board caused by stress concentration and curing deformation under a very small bending radius, and improves the pull-out resistance and dynamic bending life of the product.

Description

Technical Field

[0001] This invention relates to the field of flexible printed circuit board packaging and assembly technology, specifically to a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, modules, and electronic devices. Background Technology

[0002] As flexible electronic devices become thinner and lighter with higher screen-to-body ratios, the bending radius of flexible circuit boards in display modules is constantly decreasing. Conventional flexible circuit board packaging processes typically involve applying adhesive evenly in a single application and then thermally curing it in a flat state.

[0003] When facing applications with extremely small bending radii, existing technologies have room for further optimization: First, under extremely small curvature, the tensile and compressive stress distribution on the inner and outer sides of flexible circuit boards differs significantly. Conventionally uniformly applied encapsulating adhesives are unable to smoothly disperse the shear stress in the bonding area, easily leading to fatigue fracture during long-term bending operations. Second, when adhesives crosslink from liquid to solid in a flat state, there is volume shrinkage. This shrinkage deformation introduces residual internal stress on the surface of the flexible substrate, making the module prone to interface peeling during subsequent dynamic bending. In addition, the narrow module space has a low tolerance for dispensing accuracy, and conventional processes pose an engineering risk of adhesive overflow contaminating the display area during wetting and curing. Summary of the Invention

[0004] The present invention aims to provide a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, as well as modules and electronic devices, to solve the technical problems of easy circuit breakage and easy interface peeling caused by uneven stress distribution and curing shrinkage deformation under extremely small bending radii in existing conventional dispensing and curing processes.

[0005] To solve the above-mentioned technical problems, the present invention provides a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, comprising the following steps: S1, applying a bottom layer of adhesive to the bonding area of ​​the flexible circuit board to form a bottom layer colloid; S2, pre-curing the bottom layer colloid under ultraviolet light; S3, applying a top layer of adhesive on the bottom layer colloid to form a protective dam, wherein the cross-section of the protective dam has an asymmetrical slope structure; S4, bending the flexible circuit board to a preset transition radius, and performing stepped heating thermosetting while maintaining the bent state.

[0006] Preferably, in step S1, the viscosity of the adhesive used for the bottom layer dispensing is 3500 cps to 5000 cps; and the dispensing pressure of the bottom layer dispensing is 0.10 MPa to 0.20 MPa.

[0007] Preferably, in step S2, the ultraviolet pre-curing uses an ultraviolet light source with a wavelength of 365nm and a radiation energy of 600mJ / cm² to 1000mJ / cm².

[0008] Preferably, in step S3, the dispensing pressure of the upper layer adhesive is 0.25 MPa to 0.35 MPa; the residence time of the upper layer adhesive near the stress concentration area of ​​the bonding region is longer than the residence time of the adhesive in other areas, so that the cross-section of the protective dam is asymmetrical trapezoidal or arc-shaped.

[0009] Preferably, in step S4, the preset transition radius is 1.5 mm to 2.0 mm.

[0010] Preferably, in step S4, the stepped heating thermosetting specifically includes: a first stage: holding at 55°C to 60°C for 15 to 20 minutes; and a second stage: heating to 85°C to 90°C and holding at 85°C to 90°C for 15 to 20 minutes.

[0011] Preferably, before step S1, an assembly and pretreatment step is included: controlling the step difference of the bonding area of ​​the flexible circuit board to be less than or equal to 0.05 mm; cleaning the bonding area using a plasma device for 8 to 15 seconds.

[0012] Preferably, an anti-overflow treatment step is included before step S1. The anti-overflow treatment step is selected from one of micro-heating treatment or local damping surface construction treatment. The micro-heating treatment is to heat the bonding area using an infrared heating source to make the surface temperature reach 30°C to 45°C. The local damping surface construction treatment is to form a hydrophobic isolation band in the non-sealing area around the bonding area by plasma irradiation.

[0013] The second aspect of this solution discloses a flexible circuit board module prepared using the process described in any one of the above-mentioned methods, wherein the bonding area of ​​the flexible circuit board module has a protective dam with an asymmetrical slope structure in its cross-section.

[0014] The third aspect of this solution discloses an electronic device comprising the aforementioned flexible circuit board module.

[0015] Beneficial effects

[0016] Smooth transmission of mechanical stress: This invention utilizes a two-layer process combining "bottom-layer penetrating adhesive" and "upper-layer gradient damming" to form a protective dam with an asymmetrical slope structure. This asymmetrical morphology can smoothly disperse the shear stress concentrated in the bonding area to the flexible extension area, adapting to dynamic deformation under extreme bending conditions of 0.45mm and below, thus improving bending life.

[0017] Residual internal stress offsetting: This invention changes the traditional flat curing mode, adopting a stepped heating thermosetting process while maintaining the folded state. By causing the colloid to undergo macromolecular chain rearrangement and cross-linking under a deformation state with a specific transition radius, some of the deformation internal stress generated during the final folding assembly is offset in advance, thereby improving the tensile strength of the interface.

[0018] Enhanced interfacial bonding strength and reliability: The anti-overflow treatment of this invention (micro-heating or local damping surface construction) physically blocks the spread of adhesive while achieving full wetting of micro-gaps, ensuring product yield; the specific plasma pretreatment time and power combination ensures the interfacial bonding strength between adhesives of specific viscosities and flexible materials. Detailed Implementation

[0019] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and comparative examples.

[0020] The "protective dam" mentioned in this invention specifically refers to a cured colloid with an asymmetrical slope structure (such as trapezoidal or arc-shaped) formed by stacking on top of the bottom colloid through an upper layer dispensing process, which is used for structural support and stress buffering.

[0021] The "preset transition radius" mentioned in this invention refers to an intermediate radius of curvature during the thermosetting stage, in which the flexible circuit board is pre-bent and held by a fixture. This radius of curvature is between the flat state and the ultimate bending radius of the final product.

[0022] Adhesive: Commercially available modified acrylate-epoxy dual-curing adhesive, such as Henkel Loctite 3128 or a self-prepared equivalent system.

[0023] Plasma equipment: Commercially available atmospheric pressure wide-width plasma cleaners, such as Shengding Precision SP-PT 10.

[0024] Test standards: Pull-out force test is performed in accordance with IPC-TM-650 2.4.9 standard; roller test is performed in accordance with IEC62715-6-1 standard, with the limit bending state set at 0.45mmMAX for dynamic fatigue test.

[0025] In the following embodiments, the step difference between the backlight LED side and the LCD TFT side is controlled to be 0.05mm or less.

[0026] Example 1

[0027] This embodiment provides a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, specifically including the following steps: S0, cleaning the bonding area of ​​the flexible circuit board using a plasma device with a cleaning power of 300W and a cleaning time of 12s; S1, applying a base layer of adhesive to the tiny gaps in the bonding area using an adhesive with a viscosity of 4000cps, a dispensing pressure of 0.15MPa, and a dispensing speed of 30mm / s, forming a base layer adhesive with a width of 0.4mm; S2, pre-curing the base layer adhesive using ultraviolet light with a wavelength of 365nm. The light source irradiates the substrate with a radiation energy of 800 mJ / cm². S3: Apply upper layer adhesive along the same path to the bottom layer colloid, with an adhesive pressure of 0.35 MPa. Increase the dwell time by 0.2 seconds near the stress concentration point of the bonding area to form a protective dam with an overall adhesive width of 1.0 mm and an asymmetrical trapezoidal slope structure. S4: Bend the flexible circuit board to a preset transition radius of 1.5 mm. While maintaining the bend, place it in an oven for stepped temperature-curing: the first stage is held at 60°C for 15 minutes; the second stage is heated to 85°C and held for 20 minutes.

[0028] Example 2

[0029] This embodiment provides a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, specifically including the following steps: S0, cleaning the bonding area of ​​the flexible circuit board using a plasma device with a cleaning power of 300W and a cleaning time of 8s; S1, applying a base layer of adhesive to the tiny gaps in the bonding area using an adhesive with a viscosity of 4000cps, a dispensing pressure of 0.15MPa, and a dispensing speed of 30mm / s, forming a base layer adhesive with a width of 0.4mm; S2, pre-curing the base layer adhesive using ultraviolet light with a wavelength of 365nm. Source irradiation, radiation energy is 800mJ / cm²; S3, apply upper layer adhesive on the bottom layer adhesive along the same path, the adhesive pressure is 0.35MPa, and increase the residence time by 0.2s near the stress concentration area of ​​the bonding area to form a protective dam with an overall adhesive width of 1.0mm and an asymmetrical trapezoidal slope structure; S4, bend the flexible circuit board to a preset transition radius of 1.5mm, and place it in an oven for stepped heating and heat curing while maintaining the bent state: the first stage is to keep it at 60℃ for 15 minutes; the second stage is to heat it to 85℃ and keep it at 20 minutes.

[0030] Example 3

[0031] This embodiment provides a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, specifically including the following steps: S0, cleaning the bonding area of ​​the flexible circuit board using a plasma device with a cleaning power of 300W and a cleaning time of 15s; S1, applying a base layer of adhesive to the tiny gaps in the bonding area using an adhesive with a viscosity of 4000cps, a dispensing pressure of 0.15MPa, and a dispensing speed of 30mm / s, forming a base layer adhesive with a width of 0.4mm; S2, pre-curing the base layer adhesive using ultraviolet light with a wavelength of 365nm. The light source irradiates the substrate with a radiation energy of 800 mJ / cm². S3: Apply upper layer adhesive along the same path to the bottom layer colloid, with an adhesive pressure of 0.35 MPa. Increase the dwell time by 0.2 seconds near the stress concentration point of the bonding area to form a protective dam with an overall adhesive width of 1.0 mm and an asymmetrical trapezoidal slope structure. S4: Bend the flexible circuit board to a preset transition radius of 1.5 mm. While maintaining the bend, place it in an oven for stepped temperature-curing: the first stage is held at 60°C for 15 minutes; the second stage is heated to 85°C and held for 20 minutes.

[0032] Example 4

[0033] This embodiment provides a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, specifically including the following steps: S0, cleaning the bonding area of ​​the flexible circuit board using a plasma device with a cleaning power of 300W and a cleaning time of 12s; S1, applying a base layer of adhesive to the tiny gaps in the bonding area using an adhesive with a viscosity of 3500cps, a dispensing pressure of 0.15MPa, and a dispensing speed of 30mm / s, forming a base layer adhesive with a width of 0.4mm; S2, pre-curing the base layer adhesive using ultraviolet light irradiation with a wavelength of 365nm. The radiation energy is 800 mJ / cm²; S3, apply the upper layer of adhesive (using the same viscosity adhesive) along the same path on the bottom layer colloid, with an application pressure of 0.35 MPa, and increase the dwell time by 0.2 s at the stress concentration point near the bonding area to form a protective dam with an overall adhesive width of 1.0 mm and an asymmetrical trapezoidal slope structure; S4, bend the flexible circuit board to a preset transition radius of 1.5 mm, and place it in an oven for stepped heating and heat curing while maintaining the bent state: the first stage is to keep it at 60°C for 15 minutes; the second stage is to heat it to 85°C and keep it at 85°C for 20 minutes.

[0034] Example 5

[0035] This embodiment provides a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, specifically including the following steps: S0, cleaning the bonding area of ​​the flexible circuit board using a plasma device with a cleaning power of 300W and a cleaning time of 12s; S1, applying a base layer of adhesive to the tiny gaps in the bonding area using an adhesive with a viscosity of 5000cps, a dispensing pressure of 0.15MPa, and a dispensing speed of 30mm / s, forming a base layer adhesive with a width of 0.4mm; S2, pre-curing the base layer adhesive using ultraviolet light irradiation with a wavelength of 365nm. The radiation energy is 800 mJ / cm²; S3, apply the upper layer of adhesive (using the same viscosity adhesive) along the same path on the bottom layer colloid, with an application pressure of 0.35 MPa, and increase the dwell time by 0.2 s at the stress concentration point near the bonding area to form a protective dam with an overall adhesive width of 1.0 mm and an asymmetrical trapezoidal slope structure; S4, bend the flexible circuit board to a preset transition radius of 1.5 mm, and place it in an oven for stepped heating and heat curing while maintaining the bent state: the first stage is to keep it at 60°C for 15 minutes; the second stage is to heat it to 85°C and keep it at 85°C for 20 minutes.

[0036] Example 6

[0037] This embodiment provides a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, specifically including the following steps: S0, cleaning the bonding area of ​​the flexible circuit board using a plasma device with a cleaning power of 300W and a cleaning time of 12s; S1, applying a base layer of adhesive to the tiny gaps in the bonding area using an adhesive with a viscosity of 4000cps, a dispensing pressure of 0.10MPa, and a dispensing speed of 30mm / s, forming a base layer adhesive with a width of 0.4mm; S2, pre-curing the base layer adhesive using ultraviolet light with a wavelength of 365nm. The light source irradiates the substrate with a radiation energy of 800 mJ / cm². S3: Apply upper layer adhesive along the same path to the bottom layer colloid, with an adhesive pressure of 0.35 MPa. Increase the dwell time by 0.2 seconds near the stress concentration point of the bonding area to form a protective dam with an overall adhesive width of 1.0 mm and an asymmetrical trapezoidal slope structure. S4: Bend the flexible circuit board to a preset transition radius of 1.5 mm. While maintaining the bend, place it in an oven for stepped temperature-curing: the first stage is held at 60°C for 15 minutes; the second stage is heated to 85°C and held for 20 minutes.

[0038] Example 7

[0039] This embodiment provides a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, specifically including the following steps: S0, cleaning the bonding area of ​​the flexible circuit board using a plasma device with a cleaning power of 300W and a cleaning time of 12s; S1, applying a base layer of adhesive to the tiny gaps in the bonding area using an adhesive with a viscosity of 4000cps, a dispensing pressure of 0.20MPa, and a dispensing speed of 30mm / s, forming a base layer adhesive with a width of 0.4mm; S2, pre-curing the base layer adhesive using ultraviolet light with a wavelength of 365nm. The light source irradiates the substrate with a radiation energy of 800 mJ / cm². S3: Apply upper layer adhesive along the same path to the bottom layer colloid, with an adhesive pressure of 0.35 MPa. Increase the dwell time by 0.2 seconds near the stress concentration point of the bonding area to form a protective dam with an overall adhesive width of 1.0 mm and an asymmetrical trapezoidal slope structure. S4: Bend the flexible circuit board to a preset transition radius of 1.5 mm. While maintaining the bend, place it in an oven for stepped temperature-curing: the first stage is held at 60°C for 15 minutes; the second stage is heated to 85°C and held for 20 minutes.

[0040] Example 8

[0041] This embodiment provides a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, specifically including the following steps: S0, cleaning the bonding area of ​​the flexible circuit board using a plasma device with a cleaning power of 300W and a cleaning time of 12s; S1, applying a base layer of adhesive to the tiny gaps in the bonding area using an adhesive with a viscosity of 4000cps, a dispensing pressure of 0.15MPa, and a dispensing speed of 30mm / s, forming a base layer adhesive with a width of 0.4mm; S2, pre-curing the base layer adhesive using ultraviolet light with a wavelength of 365nm. The light source irradiates the substrate with a radiation energy of 800 mJ / cm². S3: Apply upper layer adhesive along the same path to the bottom layer colloid, with an application pressure of 0.35 MPa. Increase the dwell time by 0.2 seconds near the stress concentration point of the bonding area to form a protective dam with an overall adhesive width of 1.0 mm and an asymmetrical trapezoidal slope structure. S4: Bend the flexible circuit board to a preset transition radius of 2.0 mm. While maintaining the bend, place it in an oven for stepped temperature-curing: the first stage involves holding at 60°C for 15 minutes; the second stage involves raising the temperature to 85°C and holding for 20 minutes.

[0042] Example 9

[0043] This embodiment provides a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, specifically including the following steps: S0, cleaning the bonding area of ​​the flexible circuit board using a plasma device with a cleaning power of 300W and a cleaning time of 12s; S1, applying a base layer of adhesive to the tiny gaps in the bonding area using an adhesive with a viscosity of 4000cps, a dispensing pressure of 0.15MPa, and a dispensing speed of 30mm / s, forming a base layer adhesive with a width of 0.4mm; S2, pre-curing the base layer adhesive using ultraviolet light with a wavelength of 365nm. The light source irradiates the substrate with a radiation energy of 800 mJ / cm². S3: Apply upper layer adhesive along the same path to the bottom layer colloid, with an adhesive pressure of 0.35 MPa. Increase the dwell time by 0.2 seconds near the stress concentration point of the bonding area to form a protective dam with an overall adhesive width of 1.0 mm and an asymmetrical trapezoidal slope structure. S4: Bend the flexible circuit board to a preset transition radius of 1.5 mm. While maintaining the bend, place it in an oven for stepped temperature-curing: the first stage is held at 55°C for 20 minutes; the second stage is heated to 90°C and held for 15 minutes.

[0044] Example 10

[0045] This embodiment provides a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, specifically including the following steps: S0, cleaning the bonding area of ​​the flexible circuit board using a plasma device with a cleaning power of 300W and a cleaning time of 12s; S1, applying a base layer of adhesive to the tiny gaps in the bonding area using an adhesive with a viscosity of 4000cps, a dispensing pressure of 0.15MPa, and a dispensing speed of 30mm / s, forming a base layer adhesive with a width of 0.4mm; S2, pre-curing the base layer adhesive using ultraviolet light with a wavelength of 365nm. The light source irradiates the substrate with a radiation energy of 600 mJ / cm². S3: Apply upper layer adhesive along the same path to the bottom layer colloid, with an adhesive pressure of 0.35 MPa. Increase the dwell time by 0.2 seconds near the stress concentration point of the bonding area to form a protective dam with an overall adhesive width of 1.0 mm and an asymmetrical trapezoidal slope structure. S4: Bend the flexible circuit board to a preset transition radius of 1.5 mm. While maintaining the bend, place it in an oven for stepped temperature-curing: the first stage is held at 60°C for 15 minutes; the second stage is heated to 85°C and held for 20 minutes.

[0046] Example 11

[0047] This embodiment provides a micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, specifically including the following steps: S0, cleaning the bonding area of ​​the flexible circuit board using a plasma device with a cleaning power of 300W and a cleaning time of 12s; S1, applying a base layer of adhesive to the tiny gaps in the bonding area using an adhesive with a viscosity of 4000cps, a dispensing pressure of 0.15MPa, and a dispensing speed of 30mm / s, forming a base layer adhesive with a width of 0.4mm; S2, pre-curing the base layer adhesive using ultraviolet light with a wavelength of 365nm. Source irradiation, radiation energy is 1000mJ / cm²; S3, apply upper layer adhesive on the bottom layer adhesive along the same path, the adhesive pressure is 0.35MPa, and increase the residence time by 0.2s near the stress concentration area of ​​the bonding area to form a protective dam with an overall adhesive width of 1.0mm and an asymmetrical trapezoidal slope structure; S4, bend the flexible circuit board to a preset transition radius of 1.5mm, and place it in an oven for stepped heating and heat curing while maintaining the bent state: the first stage is to keep it at 60℃ for 15 minutes; the second stage is to heat it to 85℃ and keep it at 20 minutes.

[0048] The following comparative examples are all based on Example 1:

[0049] Comparative Example 1

[0050] The only difference between this comparative example and Example 1 is that the double-layer gradient dispensing process in steps S1 to S3 is omitted and replaced with a single dispensing at a dispensing pressure of 0.25 MPa.

[0051] Comparative Example 2

[0052] The only difference between this comparative example and Example 1 is that the step-by-step curing process of "bending the flexible circuit board to a preset transition radius and keeping it bent" in step S4 is omitted. Instead, the flexible circuit board is kept flat and directly placed in an oven for single-stage heat curing at 85°C for 35 minutes.

[0053] Comparative Example 3

[0054] The only difference between this comparative example and Example 1 is that the viscosity of the adhesive used in steps S1 and S3 is replaced with 2000 cps.

[0055] Comparative Example 4

[0056] The only difference between this comparative example and Example 1 is that the pre-processing step S0 is omitted.

[0057] Comparative Example 5

[0058] The only difference between this comparative example and Example 1 is that the bonding area structure is not flush, and the step difference between the backlight LED side and the LCD TFT side is 0.25mm.

[0059] The flexible circuit board modules prepared in Examples 1-11 and Comparative Examples 1-5 were evaluated for performance under the aforementioned test standards at a 0.45mm extreme bending state. The detailed results are recorded in Table 1.

[0060] Table 1: Process parameters and performance test results of the examples and comparative examples

[0061] Group Core Single Variable (Comparative Example 1) Drum test life (times) Pull-out force (N) Objective morphology and mechanism verification conclusions Example 1 - 150,000 (No disconnections) 19.5 The dam's cross-sectional structure is intact and its buffer function is normal. Example 2 Plasma time: 8s 120,000 (No disconnections) 17.2 The interface activation is basically up to standard, and the functions are qualified. Example 3 Plasma time: 15s 130,000 (No disconnections) 18.8 The interface is fully activated and the functions are satisfactory. Example 4 Adhesive viscosity: 3500 cps 110,000 (No disconnections) 16.5 The height of the protective dam has been slightly reduced, but remains within acceptable tolerances. Example 5 Adhesive viscosity: 5000 cps 115,000 (No disconnections) 17 The permeability of the gaps is slightly reduced, but the functionality is satisfactory. Example 6 Base pressure: 0.10 MPa 120,000 (No disconnections) 17.8 The grouting effect meets the technical requirements. Example 7 Base pressure: 0.20 MPa 130,000 (No disconnections) 18.5 The grout is well filled and free of air bubbles. Example 8 Transition radius: 2.0 mm 130,000 (No disconnections) 18.1 The prestress release function is operating normally. Example 9 Thermosetting: 55℃ / 20min + 90℃ / 15min 125,000 (No broken lines) 17.9 Complete cross-linking of polymer chains Example 10 UV energy: 600 mJ / cm² 115,000 (No disconnections) 16.8 Surface initial curing function qualified Example 11 UV energy: 1000 mJ / cm² 120,000 (No disconnections) 17.4 Surface drying effect meets standards, and structural support is stable. Comparative Example 1 Cancel double-layer gradient dispensing 40,000 (Line broken) 11 Microcracks have appeared in the circuit. Conventional single-application adhesive cannot create an asymmetric stress buffer slope, leading to localized shear fatigue. Comparative Example 2 Cancel pre-bending curing (flat curing) 30,000 (colloidal cracking) 12.5 Flattened single-stage curing introduces residual shrinkage stress, making it unable to adapt to extreme dynamic bending. Comparative Example 3 Adhesive viscosity: 2000 cps - (Test terminated early) 5.0 Excess glue contamination; loose structure, loss of mechanical protective function. Comparative Example 4 Cancel plasma pretreatment 15,000 (Large-area degumming) 6.5 Without surface activation, the interfacial bonding between the adhesive and the flexible material is lost. Comparative Example 5 Structural step difference: 0.25 mm 10,000 (stress fracture) 4.2 The imbalance in colloidal distribution underscores the necessity of flush design (step difference ≤ 0.05 mm) for micro-gradient molding.

[0062] At a maximum bending radius of 0.45 mm, the FPC joint experiences significant cyclic shear stress. Comparative Example 1 uses a single dispensing method, resulting in a symmetrical or irregularly distributed adhesive cross-section, which fails to effectively unload specific stress concentration points (such as the TFT side edge), leading to circuit fatigue fracture after 40,000 cycles. Examples 1-11, however, utilize a double-layer damming process combining bottom-layer penetration and top-layer retention to create an asymmetrical gradient slope. This geometry mechanically smooths the stress transfer, effectively transmitting shear stress concentrated on the pads to the flexible extension area, thus achieving 150,000 cycles without breakage.

[0063] The formation of the aforementioned asymmetric slope has strict material rheological prerequisites. When the adhesive viscosity is below the lower limit (Comparative Example 3, 2000 cps), the adhesive collapses and overflows before UV pre-curing (surface drying) due to excessive fluidity and slow thixotropic recovery. This not only contaminates the display area but also prevents the adhesive from stacking and forming on the Z-axis, completely losing its physical structural support function as a "protective dam," thus resulting in premature peeling during drop tests.

[0064] During the transition of polymer adhesives from a liquid to a cross-linked solid state, volume shrinkage is inevitable. In Comparative Example 2, when cured in a flat state, the polymer network locks in this residual shrinkage stress at a 0-degree angle (flat). When it is forcibly assembled to a 0.45mm state, the residual curing stress inside the material and the externally imposed bending physical stress combine, exceeding the yield limit of the interface, resulting in colloid cracking after 30,000 cycles.

[0065] The embodiments of this invention (such as Embodiments 1 and 8) introduce a "preset transition radius (1.5-2.0 mm)". In this semi-bent state, a stepped heating process is performed (the first stage allows the molecular chains to rearrange with a certain degree of freedom under deformation stress, and the second stage achieves complete cross-linking), so that the final three-dimensional cross-linked network itself has a conformation that adapts to the bending shape. From a thermodynamic and polymer physics perspective, this process preemptively offsets most of the internal stress during the final molding of the product, stabilizing the pull-out force above 16.5 N.

[0066] Flexible substrates (such as polyimide) often exhibit chemical inertness. In Comparative Example 4, lacking plasma cleaning, the substrate surface free energy was lower than the surface tension of the adhesive, preventing the formation of effective chemical bonds or physical anchoring, resulting in a sharp drop in pull-out force to 6.5 N. The 8-15 s cleaning process specified in the examples precisely allowed for the formation of sufficient active groups (such as hydroxyl and carboxyl groups) on the substrate surface, ensuring that the peel strength met the requirements.

[0067] The physical step difference determines the continuity of the micro-gradient adhesive application. When the step difference is as high as 0.25mm (Comparative Example 5), the base adhesive is prone to voids or uneven thickness at the dead angles of the step difference. During dynamic bending, these geometric abrupt change points directly become stress concentration sources (crack initiation points), leading to breakage after 10,000 cycles.

[0068] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A micro-gradient dispensing and pre-stress release curing process for flexible circuit boards, characterized in that, Includes the following steps: S1. Apply adhesive to the bonding area of ​​the flexible circuit board to form a base adhesive. S2. Perform UV pre-curing on the underlying colloid; S3. Apply upper layer adhesive to the bottom layer of the colloid to form a protective dam. The cross-section of the protective dam has an asymmetrical slope structure. S4. Bend the flexible circuit board to a preset transition radius and perform stepped heating and thermosetting while maintaining the bent state.

2. The micro-gradient dispensing and pre-stress release curing process for flexible circuit boards according to claim 1, characterized in that, In step S1, the viscosity of the adhesive used for the bottom layer dispensing is 3500 cps to 5000 cps; the dispensing pressure of the bottom layer dispensing is 0.10 MPa to 0.20 MPa.

3. The micro-gradient dispensing and pre-stress release curing process for flexible circuit boards according to claim 1, characterized in that, In step S2, the ultraviolet pre-curing uses an ultraviolet light source with a wavelength of 365nm and a radiation energy of 600mJ / cm² to 1000mJ / cm².

4. The micro-gradient dispensing and pre-stress release curing process for flexible circuit boards according to claim 1, characterized in that, In step S3, the dispensing pressure of the upper layer adhesive is 0.25MPa to 0.35MPa; the residence time of the upper layer adhesive near the stress concentration area of ​​the bonding region is longer than the residence time of the adhesive in other areas, so that the cross-section of the protective dam is asymmetrical trapezoidal or arc-shaped.

5. The micro-gradient dispensing and pre-stress release curing process for flexible circuit boards according to claim 1, characterized in that, In step S4, the preset transition radius is 1.5mm to 2.0mm.

6. The micro-gradient dispensing and pre-stress release curing process for flexible circuit boards according to claim 1, characterized in that, In step S4, the stepped heating thermosetting specifically includes: the first stage: holding at 55°C to 60°C for 15 to 20 minutes; the second stage: heating to 85°C to 90°C and holding for 15 to 20 minutes.

7. The micro-gradient dispensing and pre-stress release curing process for flexible circuit boards according to claim 1, characterized in that, Before step S1, there are also assembly and pretreatment steps: controlling the step difference of the bonding area of ​​the flexible circuit board to be less than or equal to 0.05 mm; cleaning the bonding area with a plasma device for 8 to 15 seconds.

8. The micro-gradient dispensing and pre-stress release curing process for flexible circuit boards according to claim 1, characterized in that, Before step S1, an anti-overflow treatment step is also included, which is selected from one of micro-heating treatment or local damping surface construction treatment; the micro-heating treatment is to heat the bonding area using an infrared heating source to make the surface temperature reach 30°C to 45°C; the local damping surface construction treatment is to form a hydrophobic isolation band in the non-sealing area around the bonding area by plasma irradiation.

9. A flexible circuit board module prepared using the micro-gradient dispensing and pre-stress release curing process for flexible circuit boards as described in any one of claims 1 to 8, characterized in that, The bonding area of ​​the flexible circuit board module has a protective dam with an asymmetrical slope structure.

10. An electronic device, characterized in that, It includes the flexible circuit board module as described in claim 9.