Method for preventing micro-deformation of backlight FPC of LCM (Liquid Crystal Module)

Abstract

The invention relates to the technical field of manufacturing of electronic components, and discloses an LCM (liquid crystal module) backlight FPC (flexible printed circuit) micro-deformation prevention method, which comprises the following specific steps: S1, implementing large-area copper laying design on an element surface of a flexible printed circuit board, and covering a main area of the element surface with uniformly distributed copper foils to form a continuous conductive layer with consistent thickness, while the conductive layer bears electrical connection of the LED light source, the surface reflection characteristic of the conductive layer is optimized, and the phenomenon of uneven light reflection caused by surface fluctuation is reduced. Through the large-area copper laying design of the element surface and the S-shaped isolation interval layout, thermal stress is effectively guided to be symmetrically distributed in multiple directions and offset each other, plane micro-deformation of the FPC under thermal circulation is remarkably restrained, the deformation quantity is controlled within 50 micrometers, the problems of stress concentration and deformation amplification caused by linear intervals in the prior art are solved, and the reliability of the FPC is improved. And long-term stability and optical performance of the backlight module are ensured.

Description

Technical Field

[0001] This invention relates to the field of electronic component manufacturing technology, specifically to a method for preventing micro-deformation of LCM module backlight FPC. Background Technology

[0002] With the continuous evolution of LCD technology, LCM modules are rapidly developing towards thinner, lighter, and borderless designs, placing higher demands on the design precision and stability of backlight systems. As a core component of LCD panels, the performance of the backlight module directly affects display uniformity and visual experience. Among them, the flexible printed circuit board (FPC) for backlighting carries the LED light source and realizes electrical connections, making its structural stability particularly critical. Within a confined space, the FPC must withstand the heat generated by the continuous operation of high-density LEDs for extended periods. The thermal expansion of materials and structural stress can easily cause minute deformations, which are then optically amplified through the diffuser plate, leading to display defects such as lamp holes, mura, or light leakage, seriously damaging product quality and user trust.

[0003] Backlight FPCs typically employ a slender structural layout with LEDs evenly distributed on them. While the double-sided copper plating design facilitates conductivity and heat dissipation, uneven copper foil distribution can lead to asymmetric stress due to differences in local thermal expansion coefficients under thermal cycling or ambient temperature changes, inducing micro-deformation of the planar surface. Although this deformation is minute, it is particularly noticeable on medium to large-sized panels due to the amplification effect of the optical path, manifesting as uneven brightness distribution and failing to meet the stringent standards for display uniformity required by high-end brands.

[0004] In existing technologies, conventional FPC designs often employ partial copper plating or linear spacing on the component surface. After coating the copper foil surface with white solder resist, microscopic undulations are easily formed, which not only exacerbate uneven light reflection but also make it difficult to release stress under thermal shock because the stress is concentrated at the linear boundaries, resulting in uncontrollable overall deformation. Even if some solutions attempt to improve heat distribution by uniformly plating copper, the stress direction at the regional boundaries is consistent, which cannot effectively counteract the deformation trend. Summary of the Invention

[0005] (a) Technical problems to be solved

[0006] To address the shortcomings of existing technologies, this invention provides a method for preventing micro-deformation of LCM module backlight FPC, which solves the problem of "difficulty in controlling overall deformation" in the aforementioned background technologies.

[0007] (II) Technical Solution

[0008] To achieve the above objectives, the present invention provides a method for preventing micro-deformation of LCM module backlight FPC, comprising the following specific steps:

[0009] S1: A large-area copper plating design is implemented on the component side of the flexible printed circuit board. The main area of ​​the component side is covered by uniformly distributed copper foil to form a continuous and uniformly thick conductive layer. While carrying the electrical connection of the LED light source, the surface reflection characteristics of this conductive layer are optimized to reduce the uneven light reflection caused by surface undulations.

[0010] S2: An isolation interval is set between adjacent large copper-plated areas. The isolation interval adopts a continuous arc structure with central symmetry, namely an S-shaped layout. During the thermal expansion of the material, the continuous arc structure can guide the stress distribution along the arc tangent direction, forming a multi-directional symmetrical stress field, so that the stress components in different directions cancel each other out, thereby suppressing the generation of planar micro-deformation.

[0011] S3: Perform thermodynamic performance verification on the flexible printed circuit board with completed copper-lay design. By simulating the thermal cycling conditions generated by changes in ambient temperature and continuous operation of LEDs, detect the deformation of the circuit board in the Y direction to ensure that the deformation amplitude is controlled within the tolerance range of the optical diffuser plate.

[0012] S4: Integrate the flexible printed circuit board with anti-micro-deformation design into the LCM backlight module, assemble it with the optical film group and LED light source array, and verify the uniformity of backlight brightness through actual display tests to ensure that there are no lamp eye, mura or light leakage defects.

[0013] Preferably, in S1, the copper foil coverage of the large-area copper-plated design reaches more than 90% of the usable area of ​​the component surface, the copper foil thickness is controlled between 18 micrometers and 35 micrometers, the surface flatness tolerance is less than or equal to 5 micrometers, and the copper foil edge is rounded with a radius of not less than 0.1 millimeters.

[0014] Preferably, in step S1, the copper foil surface is coated with white solder resist ink, the reflectivity of which is greater than or equal to 85%, the thickness uniformity deviation is controlled within ±2 micrometers, and the difference between its coefficient of thermal expansion and that of the substrate polyimide is less than 10%.

[0015] Preferably, the width of the S-shaped interval in S2 is designed to be 0.3 mm to 0.8 mm, the radius of curvature of the arc is 1 mm to 3 mm, the spacing between adjacent S-shaped units is consistent, and the overall layout presents a periodic symmetrical distribution along the length of the flexible printed circuit board.

[0016] Preferably, the angle between the axis of symmetry of the S-shaped interval in S2 and the length direction of the flexible printed circuit board is 45 degrees to 90 degrees.

[0017] Preferably, the thermodynamic performance verification in S3 includes high-temperature aging test and low-temperature impact test. The high-temperature test temperature is 85 degrees Celsius, the low-temperature test temperature is -40 degrees Celsius, the number of cycles is greater than or equal to 1000, and the deformation detection uses a laser displacement sensor with a measurement accuracy of ±1 micrometer.

[0018] Preferably, the deformation control standard in S3 is that after thermal cycling test, the maximum deformation of the flexible printed circuit board in the Y direction is less than or equal to 50 micrometers, and the deformation distribution uniformity index is greater than 95%.

[0019] Preferably, the method further includes implementing complementary copper pouring on the non-component side of the flexible printed circuit board. The copper pouring pattern corresponds to the S-shaped interval area on the component side, forming a symmetrical heat conduction path and further balancing the thermal stress distribution of the overall structure.

[0020] Preferably, the copper foil coverage of the complementary copper plating is controlled between 30% and 50%, the thickness is consistent with that of the copper foil on the component surface, and electrical connection is achieved with the copper foil on the component surface through vias, with the diameter of the vias designed to be between 0.2 mm and 0.3 mm.

[0021] Preferably, the substrate of the flexible printed circuit board is a polyimide film with a thickness of 25 micrometers to 50 micrometers, a glass transition temperature greater than 200 degrees Celsius, and a long-term operating temperature range covering -55 degrees Celsius to 125 degrees Celsius. The LED light source adopts a flip-chip structure with a size of 0.4 mm x 0.2 mm. The soldering method is low-temperature solder paste reflow soldering, with the peak temperature controlled within 240 degrees Celsius. After soldering, the minimum distance between the center of the LED and the edge of the copper-filled area is not less than 0.15 mm.

[0022] (III) Beneficial Effects

[0023] This invention provides a method for preventing micro-deformation of LCM module backlight FPC. It has the following beneficial effects:

[0024] (1) This invention effectively guides thermal stress to be distributed symmetrically in multiple directions and cancels each other out through the large-area copper-plated design of the component surface and the S-shaped isolation interval layout. It significantly suppresses the planar micro-deformation of FPC under thermal cycling, and controls the deformation within 50 micrometers. This solves the problem of stress concentration and deformation amplification caused by straight intervals in the prior art, and ensures the long-term stability and optical performance of the backlight module.

[0025] (2) This invention optimizes the coating and thickness uniformity of white solder resist ink on the copper foil surface, increases the light reflectivity to over 85%, eliminates the uneven light reflection caused by micro-surface undulations, and combines complementary double-sided copper pouring to form a symmetrical heat conduction path, further balancing the thermal stress distribution, improving the LED heat conduction efficiency, and avoiding the defects of thermal resistance gradient and uneven brightness under the existing local copper pouring design.

[0026] (3) Through thermodynamic performance verification and integrated testing, the present invention achieves a backlight brightness uniformity of greater than or equal to 85%, without display defects such as lamp eye, Mura or light leakage, meets the stringent requirements of high-end LCM modules for display uniformity, overcomes the optical magnification effect induced by thermal expansion differences in the prior art, and improves product quality and user experience. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the method flow of the present invention;

[0028] Figure 2 This is a schematic diagram of the thermodynamic performance verification process of the present invention. Detailed Implementation

[0029] 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.

[0030] Please see Figure 1 - Figure 2 This invention provides a method for preventing micro-deformation of LCM module backlight FPC, comprising the following steps:

[0031] S1: A large-area copper plating design is implemented on the component side of the flexible printed circuit board. Uniformly distributed copper foil covers the main area of ​​the component side, forming a continuous and uniformly thick conductive layer. This conductive layer not only carries the electrical connection of the LED light source, but its surface reflection characteristics are also optimized to reduce uneven light reflection caused by surface undulations. Specifically:

[0032] The S1's large-area copper plating design achieves a copper foil coverage of over 90% of the usable area on the component surface. The copper foil thickness is controlled between 18 and 35 micrometers, with a surface flatness tolerance of less than or equal to 5 micrometers. Furthermore, the copper foil edges are rounded with a radius of at least 0.1 millimeters. This continuity of the conductive layer ensures uniform current distribution, preventing Joule heat concentration caused by excessively high local current density. Simultaneously, its consistent thickness ensures the homogeneity of the heat conduction path, preventing thermal resistance gradients caused by thickness differences. The rounded corners effectively eliminate sharp geometric features, preventing cracks or warping induced by stress concentration at right-angled edges during thermal cycling. In addition, the copper foil surface in S1 is coated with white solder resist ink. The reflectivity of this ink is greater than or equal to 85%, the thickness uniformity deviation is controlled within ±2 micrometers, and the difference between its thermal expansion coefficient and the substrate polyimide is less than 10%. The high reflectivity of the white solder resist ink ensures effective reflection of backlight and improves light utilization efficiency. The strict control of its thickness uniformity deviation avoids micro-surface undulations caused by coating thickness fluctuations, thereby eliminating the resulting local light intensity differences. The matching design of the thermal expansion coefficients of the ink and the substrate makes their deformation trends tend to be consistent during temperature changes, reducing the risk of interface peeling or micro-cracks and ensuring long-term reliability.

[0033] S2: An isolation interval is set between adjacent large copper-plated areas. This isolation interval adopts a continuous arc-shaped structure with central symmetry, i.e., an S-shaped layout. During the thermal expansion of the material, this structure can guide the stress distribution along the arc tangent direction, forming a multi-directional symmetrical stress field, so that the stress components in different directions cancel each other out, thereby suppressing the generation of planar micro-deformation. Specifically:

[0034] The width of the S-shaped spacing in S2 is designed to be 0.3 mm to 0.8 mm, and the radius of curvature ranges from 1 mm to 3 mm. The spacing between adjacent S-shaped units remains consistent, and the overall layout exhibits a periodic symmetrical distribution along the length of the flexible printed circuit board. The width of the S-shaped spacing determines the effective cross-sectional area of ​​the stress relief channel; if it is too narrow, the stress conduction capacity is insufficient, while if it is too wide, the electrical and thermal integrity of the copper-plated area is weakened. The width range of 0.3 mm to 0.8 mm has been experimentally verified as the optimal balance point. The choice of the radius of curvature directly affects the smoothness and turning efficiency of the stress flow lines. A radius of curvature of 1 mm to 3 mm can ensure structural compactness while effectively guiding stress. The stress is dispersed along the tangential direction to avoid stress accumulation at inflection points. The consistency of the spacing between adjacent S-shaped units ensures a uniform distribution of the stress field along the entire length of the flexible printed circuit board. The periodic symmetrical layout enhances the mechanical balance of the overall structure. In addition, the angle between the axis of symmetry of the S-shaped interval in S2 and the length direction of the flexible printed circuit board is 45 degrees to 90 degrees. This angle configuration ensures that the stress components along the arc tangential direction can achieve optimal mutual cancellation under thermal stress. When the angle is 90 degrees, the S-shaped structure is perpendicular to the length direction and mainly cancels the transverse stress. When the angle is 45 degrees, the stress components are effectively decomposed in both the longitudinal and transverse directions, which is suitable for anisotropic thermal expansion scenarios.

[0035] S3: Perform thermodynamic performance verification on the flexible printed circuit board with completed copper-lay design. By simulating the thermal cycling conditions generated by changes in ambient temperature and continuous operation of LEDs, detect the deformation of the circuit board in the Y direction to ensure that the deformation amplitude is controlled within the tolerance range of the optical diffuser plate. Specifically:

[0036] Thermodynamic performance verification in S3 includes high-temperature aging testing and low-temperature impact testing. The high-temperature test temperature is 85 degrees Celsius, and the low-temperature test temperature is -40 degrees Celsius, with a cycle count of ≥1000. Deformation detection uses a laser displacement sensor with a measurement accuracy of ±1 micrometer. The high-temperature aging test simulates the long-term working state of the product in a high-temperature environment, while the low-temperature impact test examines the brittleness and shrinkage performance of the material at extreme low temperatures. The more than 1000 cycles are sufficient to cover the typical temperature fluctuations throughout the product's entire life cycle. With its non-contact and high-precision characteristics, the laser displacement sensor can capture the minute displacement changes of the flexible printed circuit board in the Y direction (i.e., perpendicular to the board surface) in real time and accurately.

[0037] Furthermore, the deformation control standard in S3 is that after thermal cycling test, the maximum deformation of the flexible printed circuit board in the Y direction is less than or equal to 50 micrometers, and the deformation distribution uniformity index is greater than 95%. The upper limit of 50 micrometers of deformation is derived from the optical tolerance of mainstream optical diffuser plates to ensure that any micro-deformation will not form visible defects on the final display screen. The deformation distribution uniformity index of greater than 95% requires that the deformation transitions smoothly across the entire board surface without any local abrupt changes, which is directly related to the uniformity of backlight brightness.

[0038] S4: The flexible printed circuit board with anti-micro-deformation design is integrated into the LCM backlight module and assembled with the optical film group and LED light source array. The backlight brightness uniformity is verified through actual display testing to ensure that there are no lamp eye, mura or light leakage defects. Specifically, the backlight brightness uniformity test in S4 is carried out using a genomic photometer with a measurement dot density of no less than 9 points per square centimeter. The brightness uniformity index is required to be greater than or equal to 85%, and there is no visible brightness gradient change. The high-density measurement dot matrix can accurately capture the subtle changes in the brightness field. The 85% uniformity index is the entry threshold for high-end display products, and the absence of visible brightness gradient change is the ultimate guarantee for the subjective feeling of the human eye.

[0039] Furthermore, the present invention also includes implementing complementary copper pouring on the non-component side of the flexible printed circuit board, wherein the copper pouring pattern corresponds to the S-shaped interval area on the component side, forming a symmetrical heat conduction path, further balancing the thermal stress distribution of the overall structure, specifically:

[0040] The copper foil coverage of complementary copper plating is controlled between 30% and 50%, with the thickness consistent with that of the component-side copper foil. Electrical connection is achieved through vias, with via diameters designed to be 0.2 mm to 0.3 mm. The complementary copper plating on the non-component side is not a simple replication of the component-side pattern, but rather an optimized design based on the heat flow direction and stress distribution. Its 30% to 50% coverage aims to provide sufficient thermal symmetry without excessively increasing material costs or affecting flexibility. The via design ensures the electrical connectivity of the copper foils on both sides, enabling them to form a complete low-impedance loop. Simultaneously, it serves as a vertical channel for heat conduction, accelerating the diffusion of heat from the component side to the non-component side.

[0041] This invention is applicable to elongated flexible printed circuit boards with a width of less than 10 mm, a length-to-width ratio greater than or equal to 5:1, and an LED light source spacing density of 1 to 3 per millimeter. Such elongated structures are extremely common in frameless LCM modules, and their high aspect ratio makes them particularly sensitive to micro-deformation. The substrate of the flexible printed circuit board uses a polyimide film with a thickness of 25 to 50 micrometers, a glass transition temperature greater than 200 degrees Celsius, and a long-term operating temperature range covering -55 degrees Celsius to 125 degrees Celsius. The LED light source adopts a flip-chip structure with a size of 0.4 mm by 0.2 mm. The soldering method is low-temperature solder paste reflow soldering, with the peak temperature controlled below 240 degrees Celsius. After soldering, the minimum distance between the center of the LED and the edge of the copper-filled area is not less than 0.15 mm. The flip-chip structure provides a good heat conduction path, the low-temperature soldering process avoids thermal damage to the flexible substrate, and the minimum distance setting between the LED and the edge of the copper-filled area prevents direct interference between the heat-affected zone of the soldering and the copper-filled structure, ensuring structural integrity.

[0042] 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 method for preventing micro-deformation of LCM module backlight FPC, characterized in that: The specific steps include the following: S1: A large-area copper plating design is implemented on the component side of the flexible printed circuit board. The main area of ​​the component side is covered by uniformly distributed copper foil to form a continuous and uniformly thick conductive layer. While carrying the electrical connection of the LED light source, the surface reflection characteristics of this conductive layer are optimized to reduce the uneven light reflection caused by surface undulations. S2: An isolation interval is set between adjacent large copper-plated areas. The isolation interval adopts a continuous arc structure with central symmetry, namely an S-shaped layout. During the thermal expansion of the material, the continuous arc structure can guide the stress distribution along the arc tangent direction, forming a multi-directional symmetrical stress field, so that the stress components in different directions cancel each other out, thereby suppressing the generation of planar micro-deformation. S3: Perform thermodynamic performance verification on the flexible printed circuit board with completed copper-lay design. By simulating the thermal cycling conditions generated by changes in ambient temperature and continuous operation of LEDs, detect the deformation of the circuit board in the Y direction to ensure that the deformation amplitude is controlled within the tolerance range of the optical diffuser plate. S4: Integrate the flexible printed circuit board with anti-micro-deformation design into the LCM backlight module, assemble it with the optical film group and LED light source array, and verify the uniformity of backlight brightness through actual display tests to ensure that there are no lamp eye, mura or light leakage defects.

2. The method for preventing micro-deformation of LCM module backlight FPC according to claim 1, characterized in that: The copper foil coverage of the large-area copper-clad design in S1 reaches more than 90% of the usable area of ​​the component surface. The copper foil thickness is controlled between 18 micrometers and 35 micrometers, the surface flatness tolerance is less than or equal to 5 micrometers, and the copper foil edges are rounded with a radius of not less than 0.1 millimeters.

3. The method for preventing micro-deformation of LCM module backlight FPC according to claim 1, characterized in that: In S1, the copper foil surface is coated with white solder resist ink. The reflectivity of the ink is greater than or equal to 85%, the thickness uniformity deviation is controlled within ±2 micrometers, and the difference between its thermal expansion coefficient and the substrate polyimide is less than 10%.

4. The method for preventing micro-deformation of LCM module backlight FPC according to claim 1, characterized in that: The width of the S-shaped interval in S2 is designed to be 0.3 mm to 0.8 mm, the radius of curvature of the arc is 1 mm to 3 mm, the spacing between adjacent S-shaped units is consistent, and the overall layout presents a periodic symmetrical distribution along the length of the flexible printed circuit board.

5. The method for preventing micro-deformation of LCM module backlight FPC according to claim 1, characterized in that: The angle between the axis of symmetry of the S-shaped interval in S2 and the length direction of the flexible printed circuit board is 45 degrees to 90 degrees.

6. The method for preventing micro-deformation of LCM module backlight FPC according to claim 1, characterized in that: The thermodynamic performance verification in S3 includes high-temperature aging test and low-temperature impact test. The high-temperature test temperature is 85 degrees Celsius, the low-temperature test temperature is -40 degrees Celsius, the number of cycles is greater than or equal to 1000, and the deformation detection uses a laser displacement sensor with a measurement accuracy of ±1 micrometer.

7. The method for preventing micro-deformation of LCM module backlight FPC according to claim 1, characterized in that: The deformation control standard in S3 is that after thermal cycling test, the maximum deformation of the flexible printed circuit board in the Y direction is less than or equal to 50 micrometers, and the deformation distribution uniformity index is greater than 95%.

8. The method for preventing micro-deformation of LCM module backlight FPC according to claim 1, characterized in that: It also includes implementing complementary copper pouring on the non-component side of the flexible printed circuit board. The copper pouring pattern corresponds to the S-shaped interval area on the component side, forming a symmetrical heat conduction path and further balancing the thermal stress distribution of the overall structure.

9. A method for preventing micro-deformation of LCM module backlight FPC according to claim 8, characterized in that: The copper foil coverage of the complementary copper plating is controlled between 30% and 50%, and the thickness is consistent with that of the copper foil on the component surface. It is electrically connected to the copper foil on the component surface through vias, with the diameter of the vias designed to be between 0.2 mm and 0.3 mm.

10. A method for preventing micro-deformation of LCM module backlight FPC according to claim 1, characterized in that: The substrate of the flexible printed circuit board is a polyimide film with a thickness of 25 to 50 micrometers, a glass transition temperature greater than 200 degrees Celsius, and a long-term operating temperature range covering -55 degrees Celsius to 125 degrees Celsius. The LED light source adopts a flip-chip structure with a size of 0.4 mm by 0.2 mm. The soldering method is low-temperature solder paste reflow soldering, with the peak temperature controlled within 240 degrees Celsius. After soldering, the minimum distance between the center of the LED and the edge of the copper-filled area is not less than 0.15 mm.

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