Automobile flexible printed circuit board based on solderable silver paste and preparation method thereof

Abstract

The application provides a flexible printed circuit board prepared based on a weldable silver paste and a preparation method, and relates to the technical field of flexible printed circuit boards.The flexible printed circuit board comprises a substrate shielding composite layer, a weldable silver paste layer is arranged on the substrate shielding composite layer, a protective layer is arranged on the weldable silver paste layer, a pad area and a non-pad area are divided on the protective layer, the pad area forms a weak curing structure after light-heat synchronous curing, and the pad area is exposed after weak alkali treatment, and the non-pad area forms a complete curing structure;the weldable silver paste system is used to improve the line conductivity, the silver surface is directly exposed, the weak alkali treatment breaks through the weldability bottleneck of a traditional metal plating layer, the screen printing process is used to replace multi-step plating, and the production process is greatly simplified.Compared with an aluminum plating process, a copper plating process and a gold plating process, the application has significant advantages in conductivity, weldability and process simplification.

Description

Technical Field

[0001] This invention relates to the field of flexible printed circuit board technology, and more particularly to automotive flexible printed circuit boards based on solderable silver paste and their preparation method. Background Technology

[0002] With the rapid development of modern electronic technology, especially in fields such as automotive electronics, consumer electronics, smart wearable devices, and industrial control, flexible printed circuit boards (FPCs) have become an important platform for connecting electronic components. FPCs are characterized by their flexibility, light weight, and high space utilization, and are widely used in various high-tech devices. Traditional FPCs typically use metal plating (such as copper, gold, or aluminum) as the conductive layer, but with the increasing demand for high-performance, low-cost, and highly reliable flexible circuit boards, existing technologies face many challenges.

[0003] For example, in traditional FPC production, commonly used metal plating technologies include copper plating, gold plating, and aluminum plating. While these metal platings can provide a certain degree of conductivity, their conductivity is often limited by the metal material itself. Especially in high-frequency, high-temperature, and long-term operating environments, the conductivity of traditional metal platings is prone to degradation, leading to reduced circuit efficiency.

[0004] Traditional metal plating (such as gold plating, copper plating, etc.) usually has good conductivity, but its solderability is poor. Especially for some high-frequency circuits and electronic components in high-temperature environments, traditional metal plating may have problems with weak soldering. This leads to the need to rely on additional soldering fluxes and auxiliary materials during the circuit board production process, which increases production costs and process complexity.

[0005] Traditional FPC production processes typically require multiple plating steps, such as copper plating, gold plating, and aluminum plating. These steps are not only complex but also require high-precision equipment and long production cycles. Furthermore, traditional gold plating processes require particularly high-temperature and high-pressure conditions, resulting in relatively high production costs and energy consumption, leading to a cumbersome and expensive production process.

[0006] Therefore, this invention proposes a flexible printed circuit board for automobiles based on solderable silver paste and its preparation method to solve the problems existing in the prior art. Summary of the Invention

[0007] To address the aforementioned problems, the present invention aims to propose a flexible printed circuit board for automobiles based on solderable silver paste and its preparation method, which solves many defects in traditional technologies and provides significant advantages in terms of conductivity, solderability, and simplified manufacturing process.

[0008] To achieve the objectives of this invention, the invention is implemented through the following technical solution: a flexible printed circuit board for automobiles based on solderable silver paste, comprising a substrate shielding composite layer, a solderable silver paste layer on the substrate shielding composite layer, a protective layer on the solderable silver paste layer, and a pad area and a non-pad area on the protective layer. The pad area forms a weakly cured structure after photothermal synchronous curing, and then exposes the pads after weak alkali treatment. The non-pad area forms a fully cured structure.

[0009] A further improvement is that the substrate shielding composite layer is composed of a polyimide film matrix, conductive carbon black uniformly dispersed in the matrix, and an organosilicon modifier coated on the surface of the polyimide film matrix. The conductive carbon black accounts for 12-15% of the total mass of the substrate shielding composite layer, the organosilicon modifier accounts for 0.4-0.6% of the total mass of the substrate shielding composite layer, and the remainder is the polyimide film matrix.

[0010] A further improvement is that the solderable silver paste layer comprises the following materials in parts by weight: 50-55 parts of nano silver powder, 15-20 parts of micron-sized silver flakes, 3-5 parts of nickel-coated graphene, 2-3 parts of silicon carbide nanosheets, 1-2 parts of microencapsulated Sn42Bi58 alloy, 10-15 parts of UV-curable acrylic resin, 6-8 parts of butyl carbitol acetate, and 0.5-0.8 parts of polycarboxylate dispersant.

[0011] A further improvement is that the protective layer is composed of photosensitive polyurethane and 1-hydroxycyclohexylphenyl ketone, accounting for 1-2% of the mass of the photosensitive polyurethane.

[0012] Further improvements include: the thickness of the substrate shielding composite layer is 125±5μm, the thickness of the solder pad area in the solderable silver paste layer is 18±1μm, the thickness of the non-solder pad area is 12±1μm, and the thickness of the protective layer is 8~10μm.

[0013] A further improvement is made in that the specific method of the weak alkaline treatment is: spraying with 5% Na2CO3 solution to expose the pad area, wherein the contact angle of the exposed silver surface is ≤15°.

[0014] The method for fabricating a flexible printed circuit board for automobiles based on solderable silver paste includes the following steps:

[0015] Step 1: Laser activation of the substrate

[0016] The substrate shielding composite layer is surface treated with an ultraviolet laser with a wavelength of 355nm, wherein the ultraviolet laser power density is 35J / cm²±5%, and the positioning marks are ablated in the preset pad area.

[0017] Step 2: Printing the silver paste layer

[0018] The solderable silver paste layer is printed onto the substrate shielding composite layer using screen printing. The pad area uses a 400-mesh mesh and the non-pad area uses a 200-mesh mesh. During the printing process, a DC electric field of 15kV / cm is applied and maintained for 10±2s to induce the silicon carbide nanosheets in the solderable silver paste layer to achieve Z-axis vertical arrangement.

[0019] Step 3: Photothermal Simultaneous Curing

[0020] The protective layer is applied onto the solderable silver paste layer by roller coating. Then, the protective layer is cured by a combination of 395nm ultraviolet light irradiation and 120℃ infrared heating. Afterward, the weakly cured layer is removed by spraying with 5% Na2CO3 solution to expose the solder pads.

[0021] A further improvement is made in step three, by using a mask to block the non-pad area, the ultraviolet radiation dose is controlled so that the dose received by the pad area is lower than the critical curing dose.

[0022] The beneficial effects of this invention are as follows:

[0023] (1) This invention achieves a fundamental improvement in the conductivity of circuits through a solderable silver paste system. The silver paste uses conductive components such as nano-silver powder and micron-sized silver flakes, which greatly improves the conductivity of the entire circuit. Especially under high frequency and long-term operation, it can maintain more stable electrical performance. Compared with traditional metal plating (such as gold plating, copper plating, aluminum plating, etc.), the silver paste system has lower resistance and higher conductivity, which helps to improve the working efficiency of the circuit board and extend its service life.

[0024] (2) This invention achieves solderability by directly exposing the silver surface of the solderable silver paste layer, breaking through the bottleneck of solderability of traditional metal plating (such as copper plating or gold plating) without relying on other metal plating.

[0025] (3) The present invention adopts screen printing process, which only requires a few processes such as "laser activation, silver paste printing, protective layer coating, photothermal curing and weak alkali treatment". The overall process is simpler than the traditional gold plating process, greatly simplifying the production process. Compared with aluminum plating, copper plating and gold plating processes, it has significant advantages in conductivity, solderability and process simplification, and is especially suitable for automotive electronics to meet the requirements of high reliability, high flexibility and low cost FPC. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the preparation method steps of the present invention. Detailed Implementation

[0027] To enhance understanding of the present invention, the present invention will be further described in detail below with reference to embodiments. These embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0028] Example 1

[0029] according to Figure 1 As shown, this embodiment proposes a flexible printed circuit board for automobiles based on solderable silver paste and its fabrication method. The board includes a substrate shielding composite layer, a solderable silver paste layer on the substrate shielding composite layer, and a protective layer on the solderable silver paste layer. The protective layer is divided into pad areas and non-pad areas. The pad areas are photothermally cured to form a weakly cured structure, and then exposed after a weak alkali treatment. The non-pad areas form a fully cured structure. This invention is mainly applied to automotive dashboards. Accordingly, the pad areas are areas on the protective layer specifically designed for connection and soldering with electronic components within the automotive dashboard. Therefore, this embodiment employs a self-exposing design, providing good solder wettability. This process ensures reliable soldering connections between the pad areas and electronic components within the automotive dashboard, and maintains a low contact angle (≤15°), thereby improving soldering stability and electrical performance. The non-pad area is another part of the protective layer. It does not require soldering and mainly serves to protect the silver paste layer and maintain structural stability. Since the display, sensor and other components in the instrument panel are often subjected to high temperature, strong light, vibration and environmental changes, the protective layer of the non-pad area can effectively isolate the influence of external factors on the circuit board and extend the service life of the entire circuit board.

[0030] The substrate shielding composite layer provides the necessary electromagnetic shielding effect for flexible circuit boards and ensures the reliability of the material in high-temperature and high-vibration environments such as automotive dashboards. It consists of a polyimide film matrix (possessing excellent high-temperature resistance, electrical insulation, and mechanical strength, especially suitable for automotive electronic systems operating in high-temperature and vibration environments), conductive carbon black uniformly dispersed in the matrix (which improves electromagnetic shielding effectiveness and conductivity; by being uniformly dispersed in the polyimide matrix, the conductive carbon black effectively prevents external electromagnetic interference), and an organosilicon modifier coated on the surface of the polyimide film matrix (used to improve the surface properties of the polyimide film, resulting in better wettability and adhesion). The conductive carbon black accounts for 15% of the total mass of the substrate shielding composite layer, the organosilicon modifier accounts for 0.6%, and the remainder is the polyimide film matrix. The thickness of the substrate shielding composite layer is 125±5μm, ensuring both electromagnetic shielding effectiveness and maintaining the material's flexibility to adapt to applications requiring a certain degree of bending, such as automotive dashboards.

[0031] The solderable silver paste layer is composed of solderable silver paste, which includes the following materials by weight: 55 parts of nano-silver powder (40nm), 20 parts of micron-sized silver flakes (2μm), 5 parts of nickel-coated graphene (particle size ≤50nm), 3 parts of silicon carbide nanosheets (thickness ≤5nm), 2 parts of microencapsulated Sn42Bi58 alloy (particle size ≤10μm, lowering the melting point to 135℃), 15 parts of UV-curable acrylic resin (viscosity ≤5000mPa·s, improving printability), 8 parts of butyl carbitol acetate (low-volatile solvent, reducing printing cracks), and 0.8 parts of polycarboxylate dispersant (solving the problem of silver powder agglomeration). The thickness of the solder pad area in the solderable silver paste layer is 18±1μm, and the thickness of the non-solder pad area is 12±1μm. The nano-silver powder can form good electrical connections at relatively low sintering temperatures, making it suitable for high-frequency and high-temperature applications. The micron-sized silver flakes enhance the thermal stability and mechanical strength of the silver paste. The introduction of nickel-coated graphene provides extremely high electrical and thermal conductivity. The nickel coating enhances the bonding between graphene and silver paste and improves oxidation resistance, preventing oxidation of the silver paste in high-temperature environments. Silicon carbide nanosheets improve the reliability of silver paste in automotive electronics applications, especially maintaining its stability under long-term exposure to high temperatures. Microencapsulated Sn42Bi58 alloy, a low-melting-point alloy, lowers the soldering temperature of the silver paste (to 135°C). Microencapsulation technology allows for better control of alloy release, preventing premature reactions and helping to reduce thermal stress on the substrate during soldering, thus improving soldering quality and reliability. UV-curable acrylic resin forms a hard protective film through UV curing, improving the printability and curing speed of the silver paste. Butyl carbitol acetate effectively reduces cracking problems in the silver paste during printing. Polycarboxylate dispersants help to uniformly disperse silver powder, preventing aggregation or precipitation in the silver paste.

[0032] The protective layer consists of photosensitive polyurethane and 2% (by mass) of 1-hydroxycyclohexylphenyl ketone. The protective layer is 10 μm thick. Photosensitive polyurethane is a material with good mechanical strength, chemical resistance, and abrasion resistance. Its photosensitive properties allow it to cure rapidly under ultraviolet light, forming a hard protective film that effectively protects the silver paste layer and the substrate surface. 1-hydroxycyclohexylphenyl ketone is a common ultraviolet photoinitiator that promotes the curing of photosensitive polyurethane under ultraviolet light. It accelerates the cross-linking reaction of the polyurethane, enabling it to quickly form a robust protective layer. The protective layer effectively protects the silver paste layer from chemicals, moisture, and oxidation, making it particularly suitable for use in automotive environments, preventing performance degradation caused by high humidity, high temperature, or chemical contamination.

[0033] The specific method of weak alkaline treatment is as follows: The pad area is exposed by spraying with a 5% Na2CO3 solution, with the contact angle of the exposed silver surface ≤15°. The main purpose of the weak alkaline treatment is to chemically remove excess protective layer from the pad area, thereby exposing the silver surface (available for soldering the silver paste layer). This provides sufficient preparation for subsequent soldering, ensuring the reliability of the solder joints. A contact angle ≤15° indicates excellent wettability of the silver surface, which helps to form good contact and reliable solder joints during the soldering process. Furthermore, after spraying, the area must be rinsed with deionized water to ensure complete removal of the solution and prevent residual solution from negatively impacting the circuit board. Regarding the contact angle, a lower contact angle means that the solder can better wet the pad area, reducing defects such as bubbles and cold solder joints during the soldering process, and ensuring stable electrical connections.

[0034] The method for fabricating a flexible printed circuit board for automobiles based on solderable silver paste includes the following steps:

[0035] Step 1: Laser activation of the substrate

[0036] The aim is to activate the substrate surface through laser surface treatment, enhancing the adhesion between the silver paste layer and the substrate, and ensuring good adhesion of the silver paste layer in subsequent printing. Specifically, a 355nm ultraviolet laser is used to treat the substrate shielding composite layer, with a laser power density of 35J / cm²±5%. Simultaneously, positioning marks are ablated in pre-defined pad areas (during laser activation, specific pad areas are ablated to form positioning marks, providing precise pad positions for subsequent silver paste layer printing. Laser ablation ensures that the silver paste layer in the pad areas is accurately printed in the predetermined positions, avoiding errors). After this laser treatment, active groups suitable for silver paste adhesion are formed on the substrate surface, and the precise markings in the pad areas ensure high accuracy in subsequent silver paste layer printing.

[0037] Step 2: Printing the silver paste layer

[0038] The aim is to print a solderable silver paste layer onto an activated substrate shielding composite layer using screen printing, ensuring good printing quality and electrical performance of the silver paste layer in both the solder pad and non-solder pad areas. Specifically, a solderable silver paste layer (pre-mixed to obtain a solderable silver paste) is printed onto the substrate shielding composite layer using screen printing. The solder pad area uses a 400-mesh screen (this mesh size is suitable for printing thicker silver paste layers), with a pressure of 0.45 MPa, resulting in a thickness of 18 ± 1 μm. The non-solder pad area uses a 200-mesh screen, with a pressure of 0.35 MPa, resulting in a thickness of 12 ± 1 μm. During the printing process, a 15 kV / cm DC electric field is simultaneously applied and maintained for 10 ± 2 s, inducing the silicon carbide nanosheets within the solderable silver paste layer to achieve a Z-axis vertical alignment.

[0039] Step 3: Photothermal Simultaneous Curing

[0040] The purpose is to ensure that the protective layer is uniformly cured on the surface of the solderable silver paste layer. Simultaneously, by adjusting the effects of ultraviolet light and infrared heating, the curing degree in the pad and non-pad areas is controlled, ensuring that the pad areas are suitable for subsequent soldering, while the non-pad areas are fully cured to provide adequate protection. The protective layer is applied to the solderable silver paste layer by roller coating, and then cured using a combination of 395nm ultraviolet light irradiation and 120℃ infrared heating. Specifically, 395nm ultraviolet light irradiation (1000mJ / cm²) and 120℃ infrared heating for 5 minutes are used. Because the silver layer is thicker in the pad area, the silver paste layer can reflect 85% of ultraviolet light, resulting in a lower effective ultraviolet irradiation intensity. This leads to weak curing of the protective layer in the pad area, while the silver layer in the non-pad area is thinner, and the silver paste layer can absorb 90% of ultraviolet light, allowing the ultraviolet energy to effectively act on the protective layer and promote its complete curing. Throughout the process, the UV radiation dose is controlled by using a mask to block non-pad areas, ensuring that the dose received by the pads is below the critical curing dose. For example, by using a mask to block non-pad areas, the UV radiation dose is controlled so that the dose received by the pads is ≤3000 mJ / cm² (below the critical curing dose of 15000 mJ / cm²), while the dose received by the non-pad areas is ≥24000 mJ / cm². Then, the weakly cured layer is removed by spraying with a 5% Na₂CO₃ solution, exposing the pads. This spraying process removes the weakly cured portion of the protective layer in the pad area, exposing the silver surface of the pads, ready for subsequent soldering.

[0041] Example 2

[0042] The difference between this embodiment and Embodiment 1 is that:

[0043] In the substrate shielding composite layer, conductive carbon black accounts for 12% of the total mass of the substrate shielding composite layer, organosilicon modifier accounts for 0.4% of the total mass of the substrate shielding composite layer, and the remainder is polyimide film matrix.

[0044] The solderable silver paste layer is composed of solderable silver paste, which includes the following materials by weight: 50 parts of nano silver powder (40nm), 15 parts of micron silver flakes (2μm), 3 parts of nickel-coated graphene (particle size ≤50nm), 2 parts of silicon carbide nanosheets (thickness ≤5nm), 1 part of microencapsulated Sn42Bi58 alloy (particle size ≤10μm, reducing melting point to 135℃), 10 parts of UV-curable acrylic resin (viscosity ≤5000mPa·s, improving printability), 6 parts of butyl carbitol acetate (low-volatile solvent, reducing printing cracks), and 0.5 parts of polycarboxylate dispersant (solving the problem of silver powder agglomeration).

[0045] The protective layer consists of photosensitive polyurethane and 1% (by mass) 1-hydroxycyclohexylphenyl ketone. The thickness of the protective layer is 8 μm.

[0046] The preparation method is the same as in Example 1.

[0047] Example 3

[0048] The difference between this embodiment and Embodiment 1 is that:

[0049] In the substrate shielding composite layer, conductive carbon black accounts for 14% of the total mass of the substrate shielding composite layer, organosilicon modifier accounts for 0.5% of the total mass of the substrate shielding composite layer, and the remainder is polyimide film matrix.

[0050] The solderable silver paste layer is composed of solderable silver paste, which includes the following materials by weight: 52 parts of nano silver powder (40nm), 18 parts of micron silver flakes (2μm), 4 parts of nickel-coated graphene (particle size ≤50nm), 3 parts of silicon carbide nanosheets (thickness ≤5nm), 2 parts of microencapsulated Sn42Bi58 alloy (particle size ≤10μm, reducing melting point to 135℃), 13 parts of UV-curable acrylic resin (viscosity ≤5000mPa·s, improving printability), 7 parts of butyl carbitol acetate (low-volatile solvent, reducing printing cracks), and 0.7 parts of polycarboxylate dispersant (solving the problem of silver powder agglomeration).

[0051] The protective layer consists of photosensitive polyurethane and 2% (by mass) 1-hydroxycyclohexylphenyl ketone. The thickness of the protective layer is 9 μm.

[0052] The preparation method is the same as in Example 1.

[0053] Based on Example 1, comparisons were made with conventional aluminum plating, copper plating, and gold plating processes. For conductivity, sheet resistance (mΩ / sq) and volume resistivity (μΩ·cm) were targeted; for solderability, wetting force (mN / mm), zero-crossing time (s), and holding time (85℃ / 85%RH) were targeted. The results are shown in the table below:

[0054]

[0055] As shown in the table above, this invention achieves a fundamental improvement in circuit conductivity through a solderable silver paste system (nano-silver + micron-sized silver flakes). Furthermore, it overcomes the solderability bottleneck of traditional metal plating by directly exposing the silver surface and using a weak alkali treatment. Simultaneously, it simplifies the production process by replacing multi-step electroplating with screen printing. Compared to aluminum, copper, and gold plating processes, it offers significant advantages in conductivity, solderability, and process simplification, making it particularly suitable for the high reliability, high flexibility, and low-cost FPC requirements of automotive electronics.

[0056] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the present invention without departing from its framework and scope of application, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A flexible printed circuit board for automobiles based on solderable silver paste, characterized in that: The material includes a substrate shielding composite layer, on which a solderable silver paste layer is provided, and on which a protective layer is provided. The protective layer is divided into a pad area and a non-pad area. The pad area forms a weakly cured structure after photothermal synchronous curing, and then the pads are exposed after weak alkali treatment. The non-pad area forms a fully cured structure. The solderable silver paste layer comprises the following materials in parts by weight: 50-55 parts of nano silver powder, 15-20 parts of micron silver flakes, 3-5 parts of nickel-coated graphene, 2-3 parts of silicon carbide nanosheets, 1-2 parts of microencapsulated Sn42Bi58 alloy, 10-15 parts of UV-curable acrylic resin, 6-8 parts of butyl carbitol acetate, and 0.5-0.8 parts of polycarboxylate dispersant.

2. The automotive flexible printed circuit board based on solderable silver paste according to claim 1, characterized in that: The substrate shielding composite layer is composed of a polyimide film matrix, conductive carbon black uniformly dispersed in the matrix, and an organosilicon modifier coated on the surface of the polyimide film matrix. The conductive carbon black accounts for 12-15% of the total mass of the substrate shielding composite layer, the organosilicon modifier accounts for 0.4-0.6% of the total mass of the substrate shielding composite layer, and the remainder is the polyimide film matrix.

3. The automotive flexible printed circuit board based on solderable silver paste according to claim 1, characterized in that: The protective layer is composed of photosensitive polyurethane and 1-hydroxycyclohexylphenyl ketone, accounting for 1-2% of the mass of the photosensitive polyurethane.

4. The automotive flexible printed circuit board based on solderable silver paste according to claim 1, characterized in that: The thickness of the substrate shielding composite layer is 125±5μm, the thickness of the solder pad area in the solderable silver paste layer is 18±1μm, the thickness of the non-solder pad area is 12±1μm, and the thickness of the protective layer is 8~10μm.

5. The automotive flexible printed circuit board based on solderable silver paste according to claim 1, characterized in that: The specific method of the weak alkaline treatment is as follows: spray treatment with 5% Na2CO3 solution to expose the pad area, wherein the contact angle of the exposed silver surface is ≤15°.

6. The method for preparing a flexible automotive printed circuit board based on solderable silver paste as described in claim 1, characterized in that: Includes the following steps: Step 1: Laser activation of the substrate The substrate shielding composite layer is surface treated with an ultraviolet laser with a wavelength of 355nm, wherein the ultraviolet laser power density is 35J / cm²±5%, and the positioning marks are ablated in the preset pad area. Step 2: Printing the silver paste layer The solderable silver paste layer is printed onto the substrate shielding composite layer using screen printing. The pad area uses a 400-mesh mesh and the non-pad area uses a 200-mesh mesh. During the printing process, a DC electric field of 15kV / cm is applied and maintained for 10±2s to induce the silicon carbide nanosheets in the solderable silver paste layer to achieve Z-axis vertical arrangement. Step 3: Photothermal Simultaneous Curing The protective layer is applied onto the solderable silver paste layer by roller coating. Then, the protective layer is cured by a combination of 395nm ultraviolet light irradiation and 120℃ infrared heating. Afterward, the weakly cured layer is removed by spraying with 5% Na2CO3 solution to expose the solder pads.

7. The method for fabricating a flexible automotive printed circuit board based on solderable silver paste according to claim 6, characterized in that: In step three, the ultraviolet radiation dose is controlled by using a mask to block the non-pad area, so that the dose received by the pad area is lower than the critical curing dose.

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