A double silver paste printed light-emitting keyboard module and a preparation method thereof

Abstract

The application provides a light-emitting keyboard module of a double-silver paste printed circuit and a preparation method thereof. The module comprises a flexible substrate, a stress buffer light shielding layer, a silver paste circuit layer, an insulating protective layer, a reflective ink layer and a MiniLED array. The silver paste circuit layer comprises a conductive circuit area formed by a first silver paste and a pad and gold finger plug area formed by a second silver paste. The conductive circuit area further comprises a plurality of power supply branches corresponding to the pads, and the circuit resistances of different power supply branches are differentially set. The application realizes double-silver paste partition printing and step-by-step curing, so that the conductive circuit area has bending flexibility, and the pad and gold finger plug area has wear resistance. Meanwhile, the whole MiniLED array cooperates with the differential resistance control of the power supply branches, so that the working current tends to be consistent, and the brightness uniformity is greatly improved. In addition, through the synergistic effect of the substrate pre-shrinking treatment, standing balance, back stress buffer layer and step-by-step curing process, the size deviation problem of multi-layer printing and curing is solved.

Description

Technical Field

[0001] This invention relates to the field of keyboard backlight module technology, and in particular to a light-emitting keyboard module with dual silver paste printed circuits and its preparation method. Background Technology

[0002] Backlit keyboard modules are widely used in laptop, tablet, external keyboard, and other portable electronic devices. A typical backlit keyboard module includes a light guide plate, a reflective film, a flexible circuit board, and LED light sources. Existing backlit keyboard modules often use a single-sided or center-mounted LED light source, with the light diffused through the light guide plate before reaching the key areas. This concentrated light source layout causes the light to gradually attenuate as it travels within the light guide plate, easily resulting in uneven brightness—higher brightness near the light source and lower brightness further away.

[0003] In terms of circuit implementation, existing keyboard backlight circuits typically employ flexible circuit boards or silver paste printed circuits. While flexible circuit boards are mature and reliable, they are more expensive and require additional bonding and assembly processes. Although silver paste printed circuits can reduce costs, conventional silver paste, after multiple printing and baking processes on flexible substrates, is susceptible to thermal shrinkage of the substrate, silver paste curing shrinkage, and stress from multiple layers of ink. This can easily lead to misalignment of circuit and pad positions, making it difficult to meet high-precision mounting requirements. Furthermore, conventional silver paste cannot simultaneously provide the bending flexibility required for conductive circuit areas and the high hardness and wear resistance required for gold finger areas. If flexible silver paste is used for the entire circuit, the gold finger insertion and extraction resistance will be insufficient; if high-hardness silver paste is used for the entire circuit, the bending reliability of the main circuit will decrease.

[0004] Therefore, it is necessary to provide a backlit keyboard module and its manufacturing method that can simultaneously improve dimensional stability, circuit flexibility, local wear resistance and pluggability, MiniLED mounting reliability and backlight uniformity. Summary of the Invention

[0005] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a light-emitting keyboard module with dual silver paste printed circuits and its preparation method, to solve the following technical problems: the light-emitting module with silver paste printed circuits on a flexible substrate has poor dimensional stability during the multilayer printing and curing process, and the positions of the pads and gold fingers are prone to displacement, making it difficult to meet the high-precision mounting requirements of MiniLEDs; a single silver paste cannot simultaneously achieve the bending flexibility required for the conductive circuit area and the high hardness and wear resistance required for the gold finger area; the concentrated light source layout leads to uneven brightness, with higher brightness near the light source and lower brightness far from the light source; and the substrate and ink reflectivity are limited, and the light efficiency utilization needs to be improved.

[0006] To achieve the above and other related objectives, the present invention provides the following technical solutions: In a first aspect, the present invention provides a backlit keyboard module with dual silver paste printed circuits, comprising a flexible substrate, a stress-buffered light-shielding layer, a silver paste circuit layer, an insulating protective layer, a reflective ink layer, and a MiniLED array. The stress-buffering light-shielding layer is disposed on the back side of the flexible substrate to disperse the shrinkage stress generated by the front printing and curing and to block light leakage from the back side. The silver paste circuit layer is disposed on the front side of the flexible substrate. The silver paste circuit layer includes a conductive circuit area formed by a first silver paste and a pad and gold finger insertion area formed by a second silver paste. After the first silver paste is cured, it forms a flexible conductive circuit. After the second silver paste is cured, it forms a wear-resistant conductive area with a hardness of not less than 3H. The insulating protective layer covers at least a portion of the conductive circuit area and avoids the solder pads and the gold finger insertion / removal area; The reflective ink layer is disposed on the insulating protective layer and avoids the solder pads and the gold finger insertion / removal area; The MiniLED array includes multiple MiniLED beads arranged in an array, and the MiniLED beads are respectively soldered onto the pads.

[0007] Furthermore, the conductive circuit area includes a main circuit and multiple power supply branches corresponding to the pads. Both ends of each power supply branch are connected to the main circuit (forming an electrical connection), so that each power supply branch and the main circuit form an independent current loop. At the same time, the line width, line length, and / or printing thickness of different power supply branches are compensated, so that the line resistance of different power supply branches is set differently, so that the operating current of different power supply branches tends to be consistent, and each MiniLED bead receives a uniform driving current.

[0008] Furthermore, the power supply branch near the power supply end increases the line resistance by at least one of the following methods: reducing line width, adding serpentine compensation segments, and increasing local equivalent series resistance; the power supply branch far from the power supply end reduces the line resistance by at least one of the following methods: increasing line width, reducing winding paths, and increasing printing thickness. This compensated equivalent series resistance design ensures that the operating current of the power supply branches near and far from the power supply end is consistent under the rated input voltage, thereby guaranteeing the brightness uniformity of the MiniLED array and avoiding dark areas and light spots generated by traditional centralized light source layouts.

[0009] Furthermore, the MiniLED array includes multiple MiniLED beads arranged at intervals along the length and width directions. Compared to traditional single-sided or center-mounted lighting methods, the full-surface array distribution provides a more uniform light source distribution, effectively reducing brightness gradients caused by light propagation attenuation.

[0010] Furthermore, the flexible substrate is a white PET substrate with a thickness of 0.05mm to 0.10mm, a whiteness of not less than 100, and a reflectivity of not less than 95%, serving as the load-bearing substrate and reflective base layer of the entire module.

[0011] Furthermore, the stress-buffering light-shielding layer includes at least one layer of black ink, preferably at least two layers of black ink, for dispersing the shrinkage stress generated by the front printing curing and blocking light leakage from the back.

[0012] Preferably, the stress-buffering light-shielding layer includes a first black ink layer and a second black ink layer sequentially disposed on the back side of the flexible substrate. Both the first black ink layer and the second black ink layer are continuous film layers, used to disperse the shrinkage stress generated during the multilayer printing and curing process, and to provide back-side compensation for the unilateral shrinkage stress generated by the silver paste circuit layer and reflective ink layer on the front side of the flexible substrate.

[0013] Furthermore, the average particle size D50 of the first silver powder in the first silver paste is no greater than 1.5 micrometers, the sheet resistance of the conductive circuit area after curing is less than 8.5 mΩ / □ / 25.4 μm, and it has good bending flexibility. The average particle size D50 of the second silver powder in the second silver paste is 8 to 12 micrometers, the surface hardness of the solder pads and the gold finger insertion / removal area after curing is no less than 3H, the sheet resistance is also less than 8.5 mΩ / □ / 25.4 μm, and it can withstand more than 300 repeated insertions and removals without failure.

[0014] Further, the first silver paste comprises, by weight, 65-78 parts of first silver powder, 8-15 parts of organic resin, 12-28 parts of solvent, and 0.2-2 parts of additives. The D50 of the first silver powder in the first silver paste is not greater than 1.5 μm, and the sheet resistance of the cured first silver paste is less than 8.5 mΩ / □ / 25.4 μm. After the first silver paste is cured, the resistance change rate of the conductive circuit area after bending 100 times with a bending radius of 5 mm is not greater than 10%. The ultra-fine particle size of the first silver powder can achieve a minimum line width of 0.12 mm to 0.20 mm, meeting the requirements of high-density narrow-bezel wiring. The number of contact points between particles is greatly increased, the contact resistance is reduced, and the conductivity consistency is better.

[0015] Further, the second silver paste comprises, by weight, 75-88 parts silver powder, 5-12 parts organic resin, 8-20 parts solvent, and 0.2-2 parts additives. The silver powder includes the second silver powder, and the average particle size D50 of the second silver powder is 8μm-12μm. By adjusting the ratio of additives and resin, the second silver powder with a relatively large particle size results in a dense silver layer after curing with a surface hardness greater than or equal to 3H (measured by the pencil hardness test method), exhibiting excellent scratch resistance and wear resistance.

[0016] Furthermore, the silver powder also includes a third silver powder with a D50 of 0.5μm to 2μm as an interstitial component to improve the density and conductivity stability of the conductive layer after curing. The third silver powder accounts for 5wt% to 20wt% of the total silver powder in the second silver paste, preferably 8wt% to 15wt%.

[0017] Furthermore, the silver paste circuit layer is formed using a step-by-step curing process, which includes low-temperature pre-curing and high-temperature final curing. The low-temperature pre-curing temperature is 115℃~135℃, and the time is 15min~30min. The high-temperature final curing temperature is 150℃~160℃, and the time is 30min~60min. Step-by-step curing can reduce the internal stress concentration caused by rapid solvent evaporation, and reduce the warping of the flexible substrate and the offset of the silver paste pattern.

[0018] Furthermore, the reflective ink layer includes at least one white ink layer to increase the reflectivity around the MiniLED array and reduce light absorption by the underlying material. The white reflective ink layer and the white PET substrate together form a high-reflectivity light environment, allowing more light emitted by the MiniLEDs to be reflected to the light-emitting side, thereby improving the keyboard backlight brightness and light efficiency.

[0019] Furthermore, the reflective ink layer includes a first white ink layer and a second white ink layer sequentially disposed on the insulating protective layer, with the first and second white ink layers having cutouts at the locations of the solder pads and gold finger insertion / removal areas. The reflective ink layer and the flexible substrate together constitute a high-reflectivity light efficiency system with a reflectivity of not less than 95%.

[0020] Furthermore, the MiniLED lamp beads are soldered onto the pads using low-temperature lead-free solder paste, which is Sn-Bi-Ag based low-temperature lead-free solder paste with a melting point of 138℃~143℃.

[0021] A second aspect of the present invention provides a method for preparing a light-emitting keyboard module with dual silver paste printed circuitry, comprising the following steps: S1. Provide a flexible substrate and pre-shrink the flexible substrate; S2. A stress-buffered light-shielding layer is formed on the back side of the flexible substrate; S3. A conductive line area is printed on the front side of the flexible substrate using a first silver paste, and then pre-cured at low temperature. S4. On the front side of the flexible substrate, the pads and the gold finger insertion area are aligned and printed using a second silver paste, and low-temperature pre-curing is performed to partially overlap the pads, the gold finger insertion area and the conductive line area to form an electrical connection. S5. The conductive circuit area formed by the first silver paste in step S3 and the pad and gold finger insertion area formed by the second silver paste in step S4 are subjected to high-temperature final curing to obtain the silver paste circuit layer. S6. An insulating protective layer is formed on the silver paste circuit layer, and the insulating protective layer avoids the pads and gold finger insertion / removal areas. S7. A reflective ink layer is formed on the insulating protective layer, and the reflective ink layer avoids the solder pads and gold finger insertion / removal areas; S8. Print low-melting-point solder onto the pads, mount the MiniLED beads onto the corresponding pads, and perform low-temperature reflow soldering.

[0022] Furthermore, in step S1, the flexible substrate is a white PET substrate with a thickness of 0.05mm to 0.10mm, a whiteness of not less than 100, and a reflectivity of not less than 95%, serving as the load-bearing substrate and reflective base layer of the entire module.

[0023] Further, in step S1, the pre-shrinking treatment temperature is 150℃~170℃, and the time is 50min~90min. After the pre-shrinking treatment, the flexible substrate is left to stand for more than 6 hours, and the critical dimension printing passes are completed within 48 hours. The pre-shrinking treatment allows the thermal shrinkage stress of the flexible substrate to be released in advance during subsequent printing, baking, and reflow soldering processes. The static balancing allows the thermal shrinkage stress and moisture absorption state of the substrate to stabilize. The static balancing before the critical dimension printing passes and the control of the time window further ensure the accuracy and stability of the pad positions.

[0024] Further, in step S2, the stress buffer light-shielding layer includes at least one black ink layer, which is formed by printing with flexible low-shrinkage black ink. The thickness of a single layer is 4μm~6μm. Each layer is baked and cured after printing. The baking temperature is 115℃~135℃ and the baking time is 15min~30min.

[0025] Preferably, the stress-buffering light-shielding layer includes a first black ink layer and a second black ink layer sequentially disposed on the back side of the flexible substrate, both of which are continuous film layers. The two black ink layers continuously cover most of the back side of the flexible substrate, serving two purposes: firstly, to block light leakage from the back side; and secondly, to form a back-facing stress-buffering structure, dispersing the shrinkage stress generated during the curing of the multi-layer printed structure on the front side, reducing substrate warping, and ensuring the positional stability of the pads and gold finger insertion / removal areas.

[0026] Furthermore, in steps S3 and S4, the low-temperature pre-curing temperature is 115℃~135℃, and the time is 15min~30min. During the low-temperature pre-curing process, the solvent in the silver paste slowly evaporates, the resin system initially cures, and the silver powder particles form a continuously overlapping conductive network, reducing the internal stress of the film layer caused by a one-time rapid heating.

[0027] Furthermore, in step S4, when using the second silver paste to align and print the pads and gold finger insertion / removal areas, the alignment accuracy is controlled within ±0.05mm using a visual recognition system, with the Mark point or pad as a reference. This alignment printing strategy, based on the Mark point or pad, ensures the accuracy of the pad position, avoids the influence of main circuit printing deformation on the pad position, and improves the reliability of MiniLED chip mounting.

[0028] Furthermore, the thickness of the conductive circuit area formed after curing is between 6 and 10 μm; the thickness of the pad and gold finger insertion area formed after curing is between 10 and 14 μm.

[0029] Furthermore, in step S4, the solder pads are printed above the corresponding power supply branch, and the two form an electrical connection; the gold finger plug-in area is connected to the corresponding main line to form an electrical connection.

[0030] Further, in step S5, the high-temperature final curing temperature is 150℃~160℃, and the time is 30min~60min. By combining low-temperature pre-curing and high-temperature final curing in a stepwise curing process, the solvent evaporation, resin cross-linking, and conductive network stabilization processes in the silver paste layer proceed gradually, reducing internal stress concentration and minimizing warpage of the flexible substrate and silver paste pattern shift. The stepwise curing process is one of the key processes for improving dimensional stability in this invention. Compared to one-time high-temperature curing, stepwise curing can significantly reduce the accumulation of thermal stress in the PET substrate and silver paste layer, improving the positional accuracy and flatness of the pads and gold finger areas.

[0031] Further, in step S6, the insulating protective layer covers at least a portion of the conductive circuit area and avoids the solder pads and the gold finger insertion / removal area. The insulating protective layer primarily serves to cover and initially fill the gaps, preventing exposed circuitry and electrostatic discharge (ESD) damage. The insulating protective layer is printed with insulating ink, requiring a resting time of at least 6 hours before printing and operation within 48 hours. After printing, it undergoes baking curing at a temperature of 115-135°C for 15-30 minutes. The thickness of the insulating protective layer is 8-15 μm.

[0032] Further, in step S7, the reflective ink layer includes at least one white ink layer, which is cut out at the locations of the solder pads and the gold finger insertion / removal areas. The thickness of a single white ink layer is 4~6μm.

[0033] Preferably, the reflective ink layer comprises a first white ink layer and a second white ink layer sequentially printed on an insulating protective layer. Each layer is baked and cured after printing at a temperature of 115°C to 135°C for 15 to 30 minutes, avoiding the pads and gold fingers. This ensures that the white reflective ink layer and the white PET substrate together form a high-reflectivity light efficiency system with a reflectivity greater than or equal to 95%. The double-layer white reflective ink further increases the reflective layer thickness, ensuring complete coverage of any gaps that may leak light. Together with the white PET substrate, it forms a closed high-reflectivity cavity, maximizing the reflection of light emitted by the MiniLED towards the light-emitting surface, thereby improving light efficiency and overall brightness.

[0034] Further, in step S8, the roll material is slit into individual products, and the individual products are fixed onto an SMT heat sink fixture. Solder is printed using a stainless steel stencil and solder paste printing equipment.

[0035] Further, in step S8, the low-melting-point solder is a low-temperature lead-free solder paste, specifically a Sn-Bi-Ag based low-temperature lead-free solder paste with a melting point of 138℃~143℃. After low-temperature reflow soldering, the thickness of the resulting solder paste layer is 8~15μm.

[0036] Furthermore, in step S8, using the pads or Mark points as visual recognition references, low-temperature lead-free solder paste is precisely printed onto the pads; the MiniLED chips are then precisely placed onto the corresponding pads using the placement machine's vision system for positioning and correction. This solder paste printing and MiniLED mounting process, using pads or Mark points as visual recognition references, combined with the aforementioned high-precision alignment printing of the pad positions, ensures that the alignment accuracy between the MiniLED and the pads reaches within ±0.05mm, improving mounting yield and connection reliability.

[0037] Furthermore, multiple MiniLED beads are arranged at intervals along the length and width directions to form a MiniLED array. The MiniLED array consists of 4 to 7 rows, with a total of 47 to 99 beads. Compared to traditional single-sided or center-mounted lighting methods, this full-surface array distribution provides a more uniform light source distribution, effectively reducing brightness gradients caused by light propagation attenuation.

[0038] Further, in step S8, the temperature curve of the low-temperature reflow soldering includes a preheating stage, a uniform heating stage, a reflow stage, and a cooling stage; the preheating stage temperature is 25℃~100℃, the heating rate is 1℃ / s~3℃ / s, and the preheating time is 25~75 seconds; the uniform heating stage temperature is 100℃~120℃, and the time is 60 seconds~120 seconds; the reflow stage temperature is higher than 143℃, the time is 30 seconds~90 seconds, the peak temperature is 150℃~160℃, and the time from the start point to the peak value is 180 seconds~300 seconds; the cooling stage cooling rate is 1℃ / s~5℃ / s. The low-temperature reflow soldering temperature profile is adapted to the melting point characteristics of Sn-Bi-Ag-based low-temperature lead-free solder paste, with the peak temperature controlled at 150℃~160℃, which is significantly lower than the reflow peak temperature of traditional Sn-Ag-Cu-based solder paste. This avoids thermal deformation of the PET substrate at high temperatures, while ensuring a reliable metallurgical connection between the MiniLED and the pad, improving connection strength and heat dissipation performance.

[0039] Furthermore, the process includes step S9: using an automatic dispensing device, applying UV protective adhesive around the MiniLED beads, and then curing it in a UV curing oven after dispensing. The UV curing energy is 2000mJ / cm² to 4000mJ / cm², forming a UV protective adhesive layer.

[0040] Furthermore, the process includes step S10: applying a rated voltage to the backlit keyboard module and detecting the operating current of each power supply branch and the brightness of each key area. The operating current deviation of each power supply branch is no greater than 10%, and the brightness uniformity is no less than 90%. Full current inspection and illumination testing are key steps in quality control. By detecting the operating current deviation and brightness uniformity, the effectiveness of the differentiated circuit resistor design and the uniform light emission effect of the entire MiniLED array are ensured.

[0041] As described above, the light-emitting keyboard module with dual silver paste printed circuitry and its preparation method of the present invention have the following beneficial effects: 1. This invention significantly reduces dimensional deviations during multi-layer printing and curing by combining pre-shrinking treatment of the white PET substrate with a double-layer black ink stress buffer layer on the back and a step-by-step curing process for the silver paste. The pad position accuracy reaches ±0.05mm, and warpage is reduced to ±0.2mm, meeting the high-precision mounting requirements of MiniLEDs and improving mounting yield. The pre-shrinking treatment releases the thermal shrinkage stress of the PET substrate during subsequent printing, baking, and reflow soldering. The double-layer black ink stress buffer layer on the back disperses the shrinkage stress from the multi-layer printing and curing on the front through a back-side stress compensation mechanism. The step-by-step curing process allows for the gradual evaporation of solvents, resin cross-linking, and stabilization of the conductive network in the silver paste layer. These three processes work synergistically to confine deformation to localized micro-regions, ensuring the dimensional stability of the macroscopic graphic and pad positions.

[0042] 2. This invention solves the contradiction of simultaneously achieving flexibility and wear resistance by using two types of silver paste for partitioned printing. The first silver paste is used for the main circuit, and the second silver paste is used for the solder pads and the gold finger insertion contact area. The resistance change rate after bending of the main circuit is no more than 10%, preferably controllable within 5%; the surface hardness of the gold finger is no less than 3H, and the contact resistance increases by less than 10% after 300 insertions and removals. The first silver paste uses ultra-fine silver powder, which has good bending flexibility after curing, adapting to repeated pressing and assembly deformation of the keyboard module during use; the second silver paste uses larger silver powder, and by adjusting the ratio of additives and resin, the silver layer is dense and has high surface hardness after curing, with excellent scratch resistance and wear resistance, and can withstand more than 300 repeated insertions and removals without failure.

[0043] 3. This invention utilizes the gradation and combination of second and third silver powders in the second silver paste. The larger-particle-size second silver powder (D50 of 8 to 12 micrometers) forms the main conductive framework with high hardness and wear resistance, while the smaller-particle-size third silver powder (D50 of 0.5 to 2 micrometers) fills the gaps between the second silver powders. This improves the density and conductivity stability of the pads and connection areas while avoiding excessively high proportions of fine silver powder, which could lead to increased silver paste viscosity, decreased printing flowability, or insufficient hardness after curing. By controlling the proportion of the third silver powder to 5 wt% to 20 wt% of the total silver powder, high hardness (not less than 3H) is maintained while reducing the contact resistance change rate; after 300 insertion and removal cycles of the gold fingers, the contact resistance change rate is no more than 20%.

[0044] 4. This invention, through the combination of MiniLED array and differentiated line resistance control, ensures that the operating current deviation of each MiniLED's power supply branch is less than 10%, and the brightness uniformity is greater than or equal to 90%, solving the problem of uneven brightness in traditional single-sided or center-mounted lighting layouts. Compared to traditional centralized light source layouts, the entire array distribution is more uniform, effectively reducing brightness gradients caused by light propagation attenuation. Differentiated line resistance control adjusts the line width, line length, or printing thickness of different power supply branches, ensuring that branches closer to the power supply end have higher compensation resistance and branches farther from the power supply end have lower line resistance. This makes the operating current of power supply branches near and far from the power supply end tend to be consistent under rated input voltage, ensuring the brightness uniformity of the entire MiniLED array and avoiding dark areas and light spots produced by traditional centralized light source layouts.

[0045] 5. This invention utilizes a high-reflectivity white PET substrate combined with a double-layer white reflective ink layer to construct a high-reflectivity light efficiency system with a reflectivity greater than or equal to 95%. Compared to conventional materials, this significantly improves average brightness and maximizes light utilization efficiency. The white PET substrate itself has high reflectivity characteristics of 100% whiteness and a reflectivity greater than or equal to 95%, serving as the reflective base layer. The double-layer white reflective ink layer further increases the thickness of the reflective layer, ensuring complete coverage of any gaps that may leak light. Together with the white PET substrate, it forms a closed high-reflectivity cavity, reflecting the light emitted by the MiniLED to the light-emitting surface to the maximum extent, reducing light absorption by the underlying material, and significantly improving light efficiency and overall brightness.

[0046] 6. This invention employs a low-temperature lead-free solder paste reflow soldering process, controlling the peak temperature to 150℃ to 160℃. This avoids thermal deformation of the PET substrate, improves the connection strength and heat dissipation performance between the MiniLED and the pads, and replaces conventional conductive adhesive mounting methods, thereby enhancing the long-term reliability of the luminous keyboard module. The melting point of the Sn-Bi-Ag series low-temperature lead-free solder paste is 138℃ to 143℃, significantly lower than that of traditional Sn-Ag-Cu series solder paste. The reflow soldering peak temperature can be controlled at 150℃ to 160℃, far below the glass transition temperature and heat distortion temperature of the PET substrate, preventing thermal deformation, dimensional shrinkage, or warping of the PET substrate at high temperatures. Simultaneously, the metallurgical connection formed by solder paste soldering, compared to the physical adhesion of conductive adhesive, has higher connection strength, lower contact resistance, and better heat dissipation performance, which is beneficial for improving the long-term reliability and luminous stability of the MiniLED. Attached Figure Description

[0047] Figure 1 This is a cross-sectional schematic diagram of the backlit keyboard module with dual silver paste printed circuits disclosed in an embodiment of the present invention.

[0048] Figure 2 This is a front view of the backlit keyboard module with dual silver paste printed circuits disclosed in an embodiment of the present invention.

[0049] Figure 3 for Figure 2 Enlarged diagram of point A in the middle.

[0050] Figure 4 This is a photograph of the backlit keyboard module with dual silver paste printed circuitry disclosed in an embodiment of the present invention.

[0051] Figure 5 This is a photograph of the backlit keyboard module with dual silver paste printed circuits disclosed in this embodiment of the invention after being powered on.

[0052] Figure 6 This is a schematic diagram of the temperature curve parameters for low-temperature reflow soldering disclosed in an embodiment of the present invention.

[0053] Component designation explanation: 1. Flexible substrate; 2. Stress-buffered light-shielding layer; 21. First black ink layer; 22. Second black ink layer; 3. Silver paste circuit layer; 31. Main circuit; 32. Solder pad; 33. Gold finger insertion / removal area; 34. Power supply branch; 4. Insulation protection layer; 5. Reflective ink layer; 51. First white ink layer; 52. Second white ink layer; 6. MiniLED lamp bead; 7. Solder layer; 8. Front black ink layer. Detailed Implementation

[0054] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0055] Example 1 like Figure 1 and Figure 2 As shown, this embodiment provides a backlit keyboard module with dual silver paste printed circuits, including a flexible substrate 1, a stress buffer light-shielding layer 2, a silver paste circuit layer 3, an insulating protective layer 4, a reflective ink layer 5, MiniLED beads 6, and a solder layer 7.

[0056] Flexible substrate 1 is a white PET substrate with a thickness of 0.05mm to 0.10mm, a whiteness of not less than 100, and an average reflectivity of not less than 95% in the visible light band. Flexible substrate 1 serves as the load-bearing base for each functional layer and also as a reflective base layer. The white PET substrate undergoes pre-shrinking treatment before printing.

[0057] A stress-buffered light-shielding layer 2 is disposed on the back side of the flexible substrate 1, comprising a first black ink layer 21 and a second black ink layer 22 disposed sequentially. Both the first black ink layer 21 and the second black ink layer 22 are formed by printing with flexible low-shrinkage black ink, with a single layer thickness of 4 to 8 micrometers. The two black ink layers continuously cover most of the back side of the flexible substrate 1, serving two purposes: firstly, to block light leakage from the back side, and secondly, to form a back stress-buffered structure, reducing warping and positional drift caused by the curing shrinkage of the silver paste circuit layer 3, the insulating protective layer 4, and the reflective ink layer 5 on the front side of the flexible substrate 1.

[0058] The silver paste circuit layer 3 is disposed on the front side of the flexible substrate 1, including a conductive circuit area, pads 32, gold finger insertion / removal area 33, and power supply branch 34. The conductive circuit area includes a main circuit 31 and a power supply branch 34, and the conductive circuit area is formed by printing with a first silver paste; the pads 32 and the gold finger insertion / removal area 33 are formed by printing with a second silver paste.

[0059] The first silver paste comprises, by weight, 65-78 parts of first silver powder, 8-15 parts of organic resin, 12-28 parts of solvent, and 0.2-2 parts of additives. The average particle size D50 of the first silver powder is not greater than 1.5 μm. The first silver paste is used to form the main circuit 31 and power supply branch 34 in the conductive circuit area, and the sheet resistance after curing is less than 8.5 mΩ / □ / 25.4 μm. After curing, the main circuit formed by the first silver paste has a resistance change rate of not more than 10% after bending 100 times with a bending radius of 5 mm, preferably less than 5%, exhibiting good bending flexibility. The first silver paste A forms a stable conductive network during the low-temperature pre-curing stage through slow solvent evaporation, initial resin curing, and continuous overlapping between silver powder particles.

[0060] The second silver paste comprises, by weight, 75-88 parts silver powder, 5-12 parts organic resin, 8-20 parts solvent, and 0.2-2 parts additives. The silver powder consists of second and third silver powders, wherein the average particle size D50 of the second silver powder is 8μm-12μm, and the average particle size D50 of the third silver powder is 0.5μm-2μm; the third silver powder accounts for 12wt% of the total silver powder. The third silver powder acts as an interstitial component, filling the spaces between the second silver powders to improve the density and conductivity stability of the conductive area after the second silver paste has cured. The sheet resistance of the cured second silver paste is less than 8.5mΩ / □ / 25.4μm, and the surface hardness measured by the pencil hardness test method is not less than 3H. After 300 insertion and removal tests, the gold finger insertion and removal area 33 showed no exposure, breakage, or open circuit, and the contact resistance change rate was not greater than 10%, indicating it can withstand more than 300 repeated insertion and removal cycles without failure.

[0061] The organic resin includes one or more of polyester resin, polyurethane resin, acrylic resin, epoxy resin, phenolic resin, vinyl chloride resin, ethylene-vinyl acetate copolymer, and polyamide resin; the solvent includes one or more of diethylene glycol butyl ether acetate, diethylene glycol butyl ether, dipropylene glycol methyl ether, dipropylene glycol methyl ether acetate, propylene glycol methyl ether acetate, ethylene glycol butyl ether, cyclohexanone, isophorone, terpineol, and butyl acetate; the additives include one or more of dispersants, leveling agents, curing agents, coupling agents, defoamers, thixotropic agents, adhesion promoters, wetting agents, anti-settling agents, and curing accelerators.

[0062] The flexible substrate 1 also has a front-side black ink layer 8, which is located below the gold finger insertion / removal area 33 on the front side of the flexible substrate 1. This layer is used to cover any white edges that may appear around the gold finger area, improving the uniformity of the product's appearance. The front-side black ink layer 8 is an optional structure.

[0063] The insulating protective layer 4 covers the conductive circuit area and avoids the solder pads 32 and the gold finger insertion / removal area 33. The insulating protective layer 4 is used to prevent exposed circuits, electrostatic discharge, and short circuits between adjacent circuits.

[0064] A reflective ink layer 5 is disposed on the insulating protective layer 4, avoiding the solder pads 32 and the gold finger insertion / removal area 33. The reflective ink layer 5 includes a first white ink layer 51 and a second white ink layer 52 disposed sequentially. The first white ink layer 51 and the second white ink layer 52 together form a high reflectivity light efficiency environment, improving the reflection utilization rate of the light emitted by the MiniLED lamp bead 6 to the light-emitting side. The white reflective ink layer 5 and the white flexible substrate 1 together constitute a high reflectivity light efficiency system with a reflectivity greater than or equal to 95%, allowing more light emitted by the MiniLED lamp bead 6 to be reflected to the light-emitting side, maximizing light utilization efficiency.

[0065] MiniLED chips 6 are soldered onto pads 32 via solder layer 7. Solder layer 7 is formed by reflow of Sn-Bi-Ag low-temperature lead-free solder paste, which has a melting point of 138°C to 143°C. The MiniLED chips 6 are arranged at intervals along the length and width directions to form a MiniLED array, which can be set to 4 to 7 rows, with a total number of 47 to 99 chips.

[0066] like Figure 2 and Figure 3 As shown, the conductive circuit area includes a main circuit 31 and multiple power supply branches 34. Both ends of each power supply branch 34 are connected to the main circuit 31 (forming an electrical connection), so that each power supply branch 34 and the main circuit 31 form an independent current loop. Each pad 32 is set on the corresponding power supply branch 34. The line resistance of different power supply branches 34 is set differently to make the current of each power supply branch 34 tend to be consistent, ensuring the brightness uniformity of each MiniLED bead.

[0067] The linewidth, line length, and / or printing thickness of different power supply branches 34 are compensated based on the difference in path voltage drop from the power supply end. Power supply branches 34 closer to the power supply end have their compensation resistance increased by reducing linewidth, adding serpentine compensation segments, and / or increasing local equivalent series resistance; power supply branches 34 farther from the power supply end have their line resistance reduced by increasing linewidth, reducing winding paths, and / or increasing printing thickness. Therefore, the operating current deviation of different power supply branches 34 under rated input voltage is no more than 10%, ensuring the brightness uniformity of the entire MiniLED array and avoiding dark areas and light spots caused by traditional centralized light source layouts.

[0068] It is understandable that differential resistance control is not limited to adjusting line width and line length. It can also be achieved by adjusting the printing thickness from 5 to 15 micrometers per print, adding local resistive elements such as carbon paste resistors, thick film resistors, or using multi-layer printing with double layers of silver paste.

[0069] It is understandable that the stress-buffering light-blocking ink layer is not limited to two layers of black ink, but can also be a single layer of black ink with a thickness of 8 to 15 micrometers or three layers of black ink with a total thickness of 15 to 25 micrometers, and the thickness of the stress-buffering layer can be adjusted according to the thickness of the substrate and the number of printing layers on the front side.

[0070] It is understandable that the white reflective ink layer is not limited to two layers of white ink, but can also be a single layer of white ink with a thickness of 10 to 20 micrometers or three layers of white ink with a total thickness of 20 to 30 micrometers.

[0071] Example 2 This embodiment provides a method for preparing a backlit keyboard module with dual silver paste printed circuits as described in Embodiment 1, including the following steps.

[0072] S1. Provide a flexible substrate 1, and pre-shrink the flexible substrate 1: Flexible substrate 1 is a white PET substrate. It undergoes pre-shrinking treatment by baking at 150℃~170℃ for 50min~90min. After pre-shrinking, flexible substrate 1 is placed in a clean environment at 20℃~30℃ and relative humidity 40%~60% for at least 6 hours, and the printing process must be completed within 48 hours. (If the corresponding printing pass is not performed within 48 hours, the pre-shrinking or resting equilibration process must be repeated.) S2. A stress-buffered light-shielding layer 2 is formed on the back side of the flexible substrate 1: A first black ink layer 21 is printed on the back side of the flexible substrate 1. After printing, it is baked at 115°C to 135°C for 15 to 30 minutes to cure it into a film. Subsequently, a second black ink layer 22 is printed on the first black ink layer 21 and baked at 115°C to 135°C for 15 to 30 minutes, so that the first black ink layer 21 and the second black ink layer 22 together form a stress-buffered light-shielding layer 2.

[0073] S3. The conductive circuit area is printed using the first silver paste and pre-cured at low temperature: A first silver paste is printed onto the front side of the flexible substrate 1 using a high-precision screen printing process, forming a conductive circuit area (including the main circuit 31, power supply branch circuit 34, and some connecting circuits). After printing, it is pre-cured at a low temperature of 115℃~135℃ for 15~30 minutes. During the low-temperature pre-curing process, the solvent in the first silver paste slowly evaporates, the resin system initially solidifies, and the first silver powder particles form a continuously contacting conductive network, thereby reducing the internal stress of the film layer caused by a one-time rapid temperature rise.

[0074] S4. Use the second silver paste to align and print the solder pads 32 and the gold finger insertion / removal area 33, and perform low-temperature pre-curing: Using the pattern formed by the first silver paste, the mark points formed in the previous process, the substrate positioning holes, or independent visual positioning marks as alignment references, the second silver paste is aligned and printed on the front side of the flexible substrate 1 to form pads 32 and gold finger insertion / removal areas 33. A localized area of ​​the second silver paste is directly printed above the conductive line area formed by the first silver paste, causing the pads 32 to partially overlap with the gold finger insertion / removal areas 33 and the conductive line area, forming an electrical connection. The alignment accuracy after printing the second silver paste is controlled within ±0.05mm. After printing, a low-temperature pre-curing process is performed at 115℃~135℃ for 15~30 minutes.

[0075] S5. Perform high-temperature final curing: The conductive circuit area formed by the first silver paste and the pads 32 and gold finger insertion / removal areas 33 formed by the second silver paste are subjected to high-temperature final curing at a temperature of 150℃~160℃ for 30min~60min to obtain the silver paste circuit layer 3. This step-by-step curing method, combining low-temperature pre-curing and high-temperature final curing, allows the solvent evaporation, resin cross-linking, and conductive network stabilization processes in the silver paste layer to proceed gradually, thereby reducing internal stress concentration and minimizing warping of the flexible substrate 1 and silver paste pattern shift.

[0076] S6. Forming an insulating protective layer 4: Insulating ink is printed on the silver paste circuit layer 3 to form an insulating protective layer 4. The insulating protective layer 4 covers the conductive circuit area and avoids the solder pads 32 and the gold finger insertion / removal area 33. Before printing the insulating ink, the flexible substrate 1 is allowed to stand for more than 6 hours and the operation is carried out within 48 hours; after printing, it is baked at 115~135℃ for 15~30 minutes.

[0077] S7, Forming reflective ink layer 5: A first white ink layer 51 is printed on the insulating protective layer 4, ensuring that the first white ink layer 51 avoids the pads 32 and the gold finger insertion / removal area 33. After printing, it is baked at 115~135℃ for 15~30 minutes. Subsequently, a second white ink layer 52 is printed on the first white ink layer 51, also ensuring that the second white ink layer 52 avoids the pads 32 and the gold finger insertion / removal area 33. After printing, it is baked at 115~135℃ for 15~30 minutes. The first white ink layer 51 and the second white ink layer 52 together form a white reflective ink layer 5.

[0078] S8, Low-temperature solder printing, MiniLED mounting and low-temperature reflow soldering: The roll material is slit into individual products, and a resistance tester is used to perform a full inspection of each MiniLED power supply branch. According to the product design values, the branch resistance deviation is controlled within ±0.5 ohms.

[0079] The single-piece product is fixed onto an SMT heat sink. Using a stainless steel stencil and solder paste printing equipment, and with pad 32 or a mark point as the visual identification reference, low-temperature lead-free solder paste is printed onto pad 32. Then, a pick-and-place machine picks up the MiniLED chips 6. Positioning and correction are performed using a vision system, and the MiniLED chips 6 are placed onto the corresponding pads 32. The product with the MiniLED chips 6 is then sent to a reflow oven. The preferred low-temperature lead-free solder paste is Sn42Bi57Ag1.

[0080] The temperature profile for low-temperature reflow soldering is as follows: Figure 6 As shown, the process includes a preheating stage, a uniform heating stage, a reflow stage, and a cooling stage. The preheating stage temperature is 25℃~100℃, the heating rate is 1℃ / s~3℃ / s, and the preheating time is 25~75 seconds. The uniform heating stage temperature is 100℃~120℃, and the time is 60 seconds~120 seconds. The reflow stage temperature is above 143℃, the time is 30 seconds~90 seconds, the peak temperature is 150℃~160℃, and the time from the start to the peak temperature is 180 seconds~300 seconds. The cooling stage cooling rate is 1℃ / s~5℃ / s. This low-temperature reflow soldering process ensures a reliable connection between the MiniLED chip 6 and the pad 32 formed by the second silver paste, while preventing excessive thermal deformation of the flexible substrate 1.

[0081] S9. Dispensing protection and curing: Using automated dispensing equipment, UV protective adhesive is applied around the MiniLED beads 6, with the pads or mark points as positioning references. After dispensing, the adhesive is cured in a UV curing oven with a UV energy of 2000mJ / cm² to 4000mJ / cm², forming a UV protective adhesive layer.

[0082] S10, Light-up Detection: Apply the rated voltage to the backlit keyboard module and test the operating current of each power supply branch 34 and the brightness of each key area. The operating current deviation of each power supply branch should not exceed 10%, and the brightness uniformity should not be less than 90%.

[0083] This embodiment solves the problem of dimensional offset in multi-layer printing and curing by synergistic effects of substrate pre-shrinking treatment, static balancing, back stress buffer layer and step-by-step curing process. The pad position accuracy reaches ±0.05mm, which meets the high-precision mounting requirements of MiniLED.

[0084] By adopting dual silver paste partition printing and step-by-step curing, the contradiction of single silver paste being unable to balance flexibility and wear resistance is solved. The resistance change rate after the main circuit is bent is less than 5%, the gold fingers can withstand more than 300 insertion and removal cycles, and the contact resistance increases by less than 10%.

[0085] By employing a MiniLED full-area array combined with differentiated resistor control in the power supply branch, the problem of uneven brightness caused by traditional single-sided or center-mounted lighting is solved, with a working current deviation of less than 10% and brightness uniformity of no less than 90%. By using a high-reflectivity white PET substrate combined with a double-layer white reflective ink layer, brightness is increased by more than 60%.

[0086] By using low-temperature lead-free solder paste reflow soldering, the problems of thermal deformation and connection reliability of PET substrate are solved. The peak temperature is controlled at 150℃ to 160℃ to avoid substrate damage and improve the connection strength and heat dissipation performance between MiniLED and pad.

[0087] Example 3 This embodiment provides a backlit keyboard module with dual silver paste printed circuits and its preparation method. Compared with Embodiments 1 and 2, the only difference is: The thickness of the white PET substrate is 0.075mm, the pre-shrinking temperature is 160℃, and the pre-shrinking time is 60 minutes.

[0088] The first silver paste comprises, by weight, 72 parts silver powder, 10 parts organic resin, 17 parts solvent, and 1 part additive. The second silver paste comprises, by weight, 82 parts silver powder, 8 parts organic resin, 9 parts solvent, and 1 part additive. The silver powder includes second and third silver powders. The average particle size D50 of the second silver powder is 8 to 12 micrometers, and the average particle size D50 of the third silver powder is 0.5 to 2 micrometers; the third silver powder accounts for 12 wt% of the total silver powder.

[0089] The low-temperature pre-curing temperature in steps S3 and S4 is 125℃, and the time is 20 minutes. The high-temperature final curing temperature in step S5 is 155℃, and the time is 45 minutes.

[0090] Example 4 This embodiment provides a backlit keyboard module with dual silver paste printed circuits and its preparation method. Compared with Embodiment 3, the only difference is: The white PET substrate has a thickness of 0.05 mm, a pre-shrinking temperature of 155℃, and a pre-shrinking time of 70 minutes. The first silver paste, by weight, comprises 68 parts silver powder, 12 parts organic resin, 19 parts solvent, and 1 part additive. The second silver paste, by weight, comprises 78 parts silver powder, 10 parts organic resin, 11 parts solvent, and 1 part additive.

[0091] The low-temperature pre-curing temperature is 120℃, and the time is 25 minutes. The high-temperature final curing temperature is 150℃, and the time is 60 minutes. Considering the tendency of ultra-thin 0.05mm substrates to warp, the pre-shrinking time is extended to 70 minutes, and the pre-shrinking temperature is reduced to 155℃, allowing for more complete release of thermal shrinkage stress in the substrate.

[0092] The silver paste formulation was adjusted to increase the resin content, with 12 parts of the first silver paste resin and 10 parts of the second silver paste resin, to enhance the adhesion between the silver paste layer and the ultra-thin substrate. The high-temperature final curing time was extended to 60 minutes to ensure full curing of the silver paste layer. Through a double-layer stress buffer layer on the back and a step-by-step curing process, the warpage of the ultra-thin substrate was controlled within ±0.3mm.

[0093] The pad positioning accuracy still reaches ±0.05mm. This embodiment is suitable for ultra-thin keyboard backlight modules with higher requirements for overall thickness, with an overall thickness of less than 0.5mm. While ensuring dimensional accuracy and mounting reliability, the module thickness is further reduced.

[0094] Example 5 This embodiment provides a backlit keyboard module with dual silver paste printed circuits and its preparation method. Compared with Embodiment 3, the only difference is: The white PET substrate has a thickness of 0.10 mm, a pre-shrinking temperature of 165℃, and a pre-shrinking time of 50 minutes. The first silver paste A, by weight, comprises 76 parts silver powder, 9 parts organic resin, 14 parts solvent, and 1 part additive. The second silver paste B, by weight, comprises 86 parts silver powder, 6 parts organic resin, 7 parts solvent, and 1 part additive.

[0095] The low-temperature pre-curing temperature is 130℃, and the time is 15 minutes. The high-temperature final curing temperature is 160℃, and the time is 30 minutes. Considering the characteristics of a 0.10mm thick substrate with good support but large heat capacity, the pre-shrinking temperature is increased to 165℃, and the pre-shrinking time is shortened to 50 minutes to accelerate the release of thermal shrinkage stress in the substrate.

[0096] The silver paste formulation was adjusted to increase the silver powder content, with the first silver paste containing 76 parts silver powder and the second silver paste containing 86 parts silver powder. The resin and solvent content were reduced to improve conductivity and curing speed. The low-temperature pre-curing temperature was increased to 130℃, and the high-temperature final curing time was shortened to 30 minutes to adapt to the thermal conductivity characteristics of thick substrates.

[0097] After the second silver paste cures, its hardness is further increased to greater than 3.5H, and the gold finger insertion and removal life is greater than 500 cycles. This embodiment is suitable for keyboard backlight modules with higher requirements for overall support and insertion and removal reliability, using a thicker white PET substrate in conjunction with a high-hardness second silver paste.

[0098] It improves the structural stability and durability of the module during assembly and repeated insertion and removal processes, making it suitable for industrial or high-frequency applications.

[0099] Example 6 This embodiment is a data embodiment of light efficacy comparison test. The test objects include Comparative Example 1 and Embodiment 3 of the present invention.

[0100] The only difference between Comparative Example 1 and Example 3 is that the reflective ink layer consists of only a single layer of white ink, and the power supply branch adopts a design with equal line width and equal thickness.

[0101] The test environment was a darkroom with an input voltage of 5V. A luminance meter was used to measure the brightness of the backlighting at each key position on the keyboard, with units of cd / m². The brightness values ​​of each standard key were recorded. Brightness uniformity was calculated as: minimum brightness / maximum brightness × 100%. The test results for Comparative Example 1 are shown in Table 1, and the test data for Example 3 are shown in Table 2.

[0102] Table 1. Brightness test data of the backlit keyboard module in Comparative Example 1

[0103] Table 2. Brightness test data of the backlit keyboard module in Example 3

[0104] The test data from Comparative Example 1 shows that the lowest brightness of the Esc button near the power source is 8.52 cd / m². 2 The F7 button, located near the power supply, has a maximum brightness of 10.91 cd / m². 2 Average brightness 10.12 cd / m² 2 The brightness uniformity is 78%.

[0105] Test data from Example 3 shows that the lowest brightness of the Esc button near the power source is 15.75 cd / m². 2 The maximum brightness of the F7 button near the power supply is 16.65 cd / m². 2 Average brightness 16.42 cd / m² 2 The brightness uniformity is 94.6%.

[0106] This embodiment demonstrates that a balance between high brightness and high uniformity can be achieved through the synergistic effect of differentiated resistance control and a high reflectivity system.

[0107] Example 7 This embodiment is an example of comparative test data on dimensional stability. The test subjects include comparative examples 2-4 and embodiment 3 of the present invention.

[0108] The only difference between Comparative Example 2 and Example 3 is that the white PET substrate of Comparative Example 2 was not pre-shrinked.

[0109] The only difference between Comparative Example 3 and Example 3 is that the stress-buffering light-shielding ink layer on the back of the white PET substrate in Comparative Example 3 is only provided with one layer of black ink.

[0110] The only difference between Comparative Example 4 and Example 3 is that the white PET substrate of Comparative Example 4 does not have a stress-buffered light-shielding ink layer on the back.

[0111] A high-precision image measuring instrument was used to measure the offset of the pad center position after multiple printing, baking, and low-temperature reflow soldering processes. The maximum offset in the X and Y directions, as well as the warpage, relative to the design position were recorded.

[0112] The test results of Example 3 show that the maximum offset of the pad position in the X direction is 0.03mm and the maximum offset in the Y direction is 0.02mm; the maximum warpage of the substrate is 0.2mm, which meets the requirements of MiniLED high-precision mounting; in the sample mounting verification, the mounting yield is greater than 98%.

[0113] The test results of Comparative Example 2 without pre-shrinkage treatment show that the maximum offset of the pad position in the X direction is 0.18mm and the maximum offset in the Y direction is 0.12mm; the maximum warpage of the substrate is 0.8mm; the MiniLED mounting deviation increases, and the risk of local cold solder joints increases.

[0114] The test results of the single-layer stress buffer layer scheme in Comparative Example 3 show that the maximum offset of the pad position in the X direction is 0.10mm and the maximum offset in the Y direction is 0.06mm; the maximum warpage of the substrate is 0.4mm, the substrate warps on one side, and the flatness of the gold finger area decreases.

[0115] The test results of the stress-free buffer layer scheme in Comparative Example 4 show that the maximum offset of the pad position in the X direction is 0.15mm and the maximum offset in the Y direction is 0.10mm; the maximum warpage of the substrate is 0.6mm, the substrate warps on one side, and the flatness of the gold finger area decreases.

[0116] This embodiment demonstrates that, through the synergistic effect of substrate pre-shrinking treatment, static setting, and a double-layer stress buffer layer on the back side, the pad position offset is reduced to less than 0.03 mm, and the warpage is reduced to 0.2 mm, proving the significant effect of pre-shrinking treatment, static setting, and stress buffer layer in improving dimensional stability.

[0117] Example 8 This embodiment is a comparative test data example of silver paste performance. The test subjects include comparative examples 5-6 and embodiment 3 of the present invention.

[0118] Comparative Example 5 is a single flexible silver paste solution. The only difference from Example 3 is that the entire silver paste circuit layer is formed by flexible formation of first silver paste ultrafine silver powder with a D50 of less than or equal to 1.5 micrometers.

[0119] Comparative Example 6 is a single high-hardness silver paste solution. The only difference from Example 3 is that the silver paste circuit layer is formed entirely using a second silver paste with larger silver powder D50 of 8 to 12 micrometers and a hardness greater than or equal to 3H.

[0120] The test methods included a bending test, in which the conductive circuit area was bent 180 degrees 100 times, and the rate of change in resistance after bending was measured. An insertion and removal test was performed, in which the gold finger insertion and removal area was repeatedly inserted and removed 300 times, and the rate of change in contact resistance after insertion and removal was measured. A hardness test was performed, measuring the surface hardness of the gold finger insertion and removal contact area after curing. The test results are shown in Table 3.

[0121] Table 3. Reliability Comparison Test Results of Dual Silver Paste Functional Zones

[0122] The test results of the single flexible silver paste solution in Comparative Example 5 show that the resistance change rate after bending the main circuit is less than 5%, which is excellent; the surface hardness of the gold finger is H~2H, which is poor; and the contact resistance increases by more than 50% after 300 insertions and removals, which is also poor. The main circuit has good flexibility, but the wear resistance of the gold finger is insufficient.

[0123] The test results of the single high-hardness silver paste solution in Comparative Example 6 show that the resistance change rate after the main circuit is bent is greater than 15%, which is poor; the surface hardness of the gold finger is 3.5H, which is excellent; the contact resistance increase after 300 insertions and removals is less than 10%, which is excellent; the gold finger has good wear resistance, but the main circuit is not flexible enough.

[0124] The test results of the dual silver paste partitioning scheme in Embodiment 3 of this invention show that the resistance change rate after bending the main circuit is less than 5%, which is excellent. The surface hardness of the gold fingers is 3H, which is excellent. After 300 insertions and removals, the increase in contact resistance is less than 10%, which is excellent. The flexibility of the main circuit and the wear resistance of the gold fingers are both taken into account.

[0125] This embodiment demonstrates that by using two types of silver paste for partitioned printing—the first silver paste for the main circuitry and the second for the solder pads and gold fingers—the contradiction of balancing flexibility and wear resistance, which is difficult to achieve with a single silver paste, is resolved. The resistance change rate after bending of the main circuitry is less than 5%, and the contact resistance increase after 300 insertions and removals of the gold fingers is less than 10%, proving the significant effect of dual-silver paste partitioning and step-by-step curing in achieving a balance of performance.

[0126] Example 9 This embodiment is a comparative test data embodiment for power supply branches. The test objects include Comparative Example 7 and Embodiment 3 of the present invention.

[0127] Compared with Example 3, Comparative Example 7 differs only in that the power supply branch adopts a design with equal line width and thickness, and is not differentiated according to the difference in path voltage drop.

[0128] The test environment was a darkroom with an input voltage of 5V. A luminance meter was used to measure the brightness of the backlighting at each key position on the keyboard, with units of cd / m². The brightness values ​​of each standard key were recorded. The test results for Comparative Example 7 are shown in Table 4.

[0129] Table 4. Brightness test data of the backlit keyboard module in Comparative Example 7

[0130] During the power-on test, the current of the power supply branch near the power supply end in Comparative Example 7 was too high, while the current of the power supply branch far from the power supply end was too low. The deviation of the branch operating current reached 26.8%, which caused the brightness uniformity of the keyboard area to drop to about 80%. Bright spots appeared in the area near the power supply end, and dark areas appeared in the area far from the power supply end.

[0131] In Example 3, by compensating for the line width, line length and / or printing thickness of different MiniLED power supply branches, the difference in line voltage drop on different power supply paths is compensated, the branch operating current deviation is reduced to 8.2%, and the brightness uniformity is improved to 94.6%.

[0132] The above results demonstrate that the compensated power supply branch design of the present invention can effectively improve the current consistency and lamp brightness uniformity of the entire MiniLED array, and reduce the local bright spots and far-end dark areas that are easily generated under the traditional equal linewidth branch design.

[0133] In summary, this invention, by employing dual-silver-paste partitioned printing and step-by-step curing, resolves the contradiction between flexibility and wear resistance inherent in single-silver-paste applications. The conductive circuit area exhibits bending flexibility, while the pads and gold finger insertion / removal areas possess wear resistance. Simultaneously, the full-surface MiniLED array eliminates the uneven brightness issues associated with traditional single-sided or center-mounted LED displays. Combined with differentiated resistance control in the power supply branches, the operating current deviation is reduced to less than 10%, significantly improving brightness uniformity. Furthermore, the synergistic effect of substrate pre-shrinking treatment, static balancing, a back-side stress buffer layer, and step-by-step curing processes resolves the issue of dimensional misalignment during multi-layer printing and curing, ensuring precise pad placement to meet the high-precision mounting requirements of MiniLEDs. Therefore, this invention effectively overcomes the various shortcomings of existing technologies and possesses high industrial applicability.

[0134] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A backlit keyboard module with dual silver paste printed circuitry, characterized in that, include: Flexible substrate; A stress-reducing light-shielding layer is disposed on the back side of the flexible substrate; A silver paste circuit layer is disposed on the front side of the flexible substrate. The silver paste circuit layer includes a conductive circuit area formed by a first silver paste and a pad and gold finger insertion area formed by a second silver paste. After the first silver paste is cured, it forms a flexible conductive circuit. After the second silver paste is cured, it forms a wear-resistant conductive area with a hardness of not less than 3H. The conductive circuit area includes a main circuit and multiple power supply branches corresponding to the pads. The circuit resistance of different power supply branches is set differently so that the operating current of different power supply branches tends to be consistent. An insulating protective layer covers at least a portion of the conductive circuit area and avoids the solder pads and the gold finger insertion / removal area; A reflective ink layer is disposed on the insulating protective layer and avoids the solder pads and the gold finger insertion / removal area; The MiniLED array includes multiple MiniLED beads arranged in an array, each of which is soldered onto a pad.

2. The backlit keyboard module with dual silver paste printed circuitry according to claim 1, characterized in that, The power supply branch near the power supply end increases the line resistance by at least one of the following methods: reducing the line width, adding serpentine compensation segments, and increasing the local equivalent series resistance; the power supply branch far from the power supply end reduces the line resistance by at least one of the following methods: increasing the line width, reducing the winding path, and increasing the printing thickness.

3. The backlit keyboard module with dual silver paste printed circuitry according to claim 1, characterized in that, The first silver paste contains silver powder with an average particle size D50 of no more than 1.5 micrometers. After curing, the first silver paste forms a flexible conductive circuit with a sheet resistance of less than 8.5 mΩ / □ / 25.4 μm and good bending flexibility. The second silver paste contains silver powder with an average particle size D50 of 8 to 12 micrometers. After curing, the second silver paste forms a wear-resistant conductive area with a hardness of no less than 3H and a sheet resistance of less than 8.5 mΩ / □ / 25.4 μm.

4. The backlit keyboard module with dual silver paste printed circuitry according to claim 3, characterized in that, The first silver paste comprises, by weight, 65-78 parts of first silver powder, 8-15 parts of organic resin, 12-28 parts of solvent, and 0.2-2 parts of additives, wherein the D50 of the first silver powder in the first silver paste is not greater than 1.5 μm; the second silver paste comprises, by weight, 75-88 parts of silver powder, 5-12 parts of organic resin, 8-20 parts of solvent, and 0.2-2 parts of additives, wherein the silver powder includes the second silver powder, and the average particle size D50 of the second silver powder is 8 μm-12 μm.

5. The backlit keyboard module with dual silver paste printed circuitry according to claim 4, characterized in that, The second silver paste also includes a third silver powder with a D50 of 0.5μm to 2μm, wherein the third silver powder accounts for 5wt% to 20wt% of the total silver powder in the second silver paste.

6. A method for preparing a backlit keyboard module with dual silver paste printed circuitry, characterized in that, Includes the following steps: S1. Provide a flexible substrate and pre-shrink the flexible substrate; S2. A stress-buffered light-shielding layer is formed on the back side of the flexible substrate; S3. A conductive line area is printed on the front side of the flexible substrate using a first silver paste, and then pre-cured at low temperature. S4. On the front side of the flexible substrate, the pads and the gold finger insertion area are aligned and printed using a second silver paste, and low-temperature pre-curing is performed to partially overlap the pads, the gold finger insertion area and the conductive line area to form an electrical connection. S5. The conductive circuit area formed by the first silver paste in step S3 and the pad and gold finger insertion area formed by the second silver paste in step S4 are subjected to high-temperature final curing to obtain a silver paste circuit layer; after the first silver paste is cured, a flexible conductive circuit is formed, and after the second silver paste is cured, a wear-resistant conductive area with a hardness of not less than 3H is formed. S6. An insulating protective layer is formed on the silver paste circuit layer, and the insulating protective layer avoids the pads and gold finger insertion / removal areas. S7. A reflective ink layer is formed on the insulating protective layer, and the reflective ink layer avoids the solder pads and gold finger insertion / removal areas; S8. Print low-melting-point solder onto the pads, mount the MiniLED beads onto the corresponding pads, and perform low-temperature reflow soldering.

7. The preparation method according to claim 6, characterized in that, In step S1, the temperature of the pre-shrinking treatment is 150℃~170℃, and the time is 50min~90min.

8. The preparation method according to claim 6, characterized in that, In steps S3 and S4, the low-temperature pre-curing temperature is 115℃~135℃ and the time is 15min~30min; in step S5, the high-temperature final curing temperature is 150℃~160℃ and the time is 30min~60min.

9. The preparation method according to claim 6, characterized in that, In step S8, the low-melting-point solder is a low-temperature lead-free solder paste, which is a Sn-Bi-Ag-based low-temperature lead-free solder paste with a melting point of 138℃~143℃.

10. The preparation method according to claim 6, characterized in that, In step S8, the temperature curve of the low-temperature reflow soldering includes a preheating stage, a homogenizing stage, a reflow stage, and a cooling stage; the preheating stage temperature is 25℃~100℃, the heating rate is 1℃ / s~3℃ / s, and the preheating time is 25~75 seconds; the homogenizing stage temperature is 100℃~120℃, and the time is 60 seconds~120 seconds; the reflow stage temperature is higher than 143℃, the time is 30 seconds~90 seconds, the peak temperature is 150℃~160℃, and the time from the start point to the peak value is 180 seconds~300 seconds; the cooling stage cooling rate is 1℃ / s~5℃ / s.

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