High-density circuit layout process for flexible circuit board

Abstract

The invention relates to the technical field of flexible circuit boards, and discloses a high-density circuit layout process for a flexible circuit board. The novel flexible circuit board high-density circuit layout process comprises the following steps: S1, FPC cutting; s2, drilling holes; s3, surface treatment of the copper foil; s4, hole metallization is carried out; s5, making a pattern; s6, pattern electroplating and etching; s7, selection of a covering layer, S8, hot air leveling and gold plating, and S9, appearance processing. According to the flexible circuit board produced through the method, the problem that a circuit is broken due to stress concentration can be effectively prevented, cracks can be effectively prevented from being generated between the circuit and a bonding pad, and the reliability of the flexible circuit board is improved. According to the method, the flexibility of a dry film after the flexible circuit board is vacuumized well and photo-polymerized is poorer than that of an unpolymerized dry film, falling of the dry film and a base material caused by bending of a plate is avoided during operation, and bending or clamping of the flexible circuit board caused by the spraying pressure of a developing solution and the influence of cold and hot air can be avoided by adopting a rigid plate for traction during developing.

Description

Technical Field

[0001] The invention belongs to the technical field of flexible circuit boards, and in particular relates to a high-density circuit layout process for flexible circuit boards. Background Art

[0002] As more and more mobile phones adopt folding and curved structures, flexible circuit boards are also increasingly being used. According to the bonding method between the substrate and copper platinum, flexible circuit boards can be divided into two types: flexible boards with glue and flexible boards without glue. The price of flexible boards without glue is much higher than that of flexible boards with glue, but its flexibility, the bonding strength between copper foil and substrate, the flatness of pads and other parameters are also better than those of flexible boards with glue. Therefore, it is generally only used in those occasions with very high requirements. Due to its high price, most of the flexible boards currently used in the market are still flexible boards with glue. Since flexible boards are mainly used in occasions that require bending, if the circuit layout design or process is unreasonable, defects such as microcracks and open welding are prone to occur; therefore, in view of the current situation, it is now necessary to improve it. Summary of the invention

[0003] In view of the above situation, in order to overcome the defects of the prior art, the present invention provides a high-density circuit layout process for a flexible circuit board, which effectively solves the problem that if the circuit layout design or process of the flexible circuit board is unreasonable, defects such as micro cracks and open welding are easily generated.

[0004] To achieve the above object, the present invention provides the following technical solution: a high-density circuit layout process for a flexible circuit board, comprising the following steps:

[0005] S1: FPC cutting: Except for some materials, the materials used for flexible circuit boards are basically in roll form. Since not all processes must be processed using the tape process, some processes must be cut into sheets for processing, such as the drilling of metallized holes in double-sided flexible circuit boards. Currently, drilling can only be done in sheet form. Therefore, the first process of double-sided flexible circuit boards is cutting. Flexible copper-clad laminates have extremely poor tolerance to external forces and are easily damaged. If they are damaged during cutting, the qualified rate of subsequent processes will be seriously affected;

[0006] S2: Drilling: The drilling equipment for flexible circuit boards is the same as that for rigid printed circuit boards, but the drilling conditions are slightly different. Since the adhesive used in FPC boards is soft and easy to stick to the knife, it is recommended to use FPC special tools for drilling, or use new drill bits (the number of holes drilled is less than 1000), and optimize the drilling parameters at the same time to reduce drilling grease or burrs. The drilling of flexible circuit boards, cover layers and reinforcement layers is basically the same. Since the copper clad laminate and the cover layer are both thin, multiple overlapping holes can be drilled when drilling (according to the thickness of the board, the copper clad laminate can be stacked about 7-10 pieces and the cover layer can be stacked 10-15 pieces). The upper pad can use aluminum pads or epoxy yellow pads, and the lower pad can use epoxy yellow pads. The processing conditions for drilling, milling the outer shape of the cover film and reinforcement board are basically the same, but because the adhesive used in the flexible printed circuit board material is soft, it is very easy to adhere to the drill bit, and the drill bit status needs to be frequently inspected, and the drill bit speed should be appropriately increased;

[0007] S3: Surface treatment of copper foil: The surface of the flexible circuit board needs to be cleaned before hole metallization to improve the bonding strength between the hole metallization coating and the substrate. At the same time, effective treatment of the copper surface before the hole metallization and the pattern imaging is also very necessary to enhance the adhesion strength between copper and the anti-corrosion layer. The surface of the FPC board is not cleaned before the hole metallization, and the hole plating may be delaminated and blistered: the copper surface is not cleaned and roughened enough before the pattern imaging, and the bonding with the anti-corrosion agent is not strong. It is almost impossible to make fine circuit patterns, and it is easy to seep plating in the subsequent electroplating process. This causes uneven line edges or even short circuits, affecting the pattern accuracy;

[0008] S4: Hole metallization: Before hole metallization of flexible multilayer boards, degreasing is required. There are two commonly used degreasing processes: one is alkaline potassium permanganate solution treatment, and the other is plasma treatment. Because acrylic adhesives have poor alkali resistance, alkaline potassium permanganate process is only suitable for epoxy adhesives. Therefore, plasma treatment process is often used for flexible circuit boards. It can not only effectively remove grease, but also etch the insulating medium, so that the inner copper layer forms a reliable three-dimensional structure, enhance the bonding force of the hole wall plating, and use a hole inspection mirror to observe the state of the hole wall. After plasma treatment, the complete copper ring is clearly protruding, ensuring that the hole wall is clean and free of drilling dirt. When hole metallization is performed, the contact area is increased and the plating bonding is better;

[0009] S5: Graphics production: The graphic transfer of flexible circuit boards is not much different from that of ordinary printed boards. The commonly used anti-corrosion layers are liquid resists and dry films. When applying the liquid anti-corrosion layer, static dust and floating objects should be avoided from being adsorbed into the anti-corrosion layer. In addition, the viscosity of the anti-corrosion agent and the time and temperature of the baking board should be selected to ensure that the adhesion between the anti-corrosion agent and the copper plate reaches the maximum and remains dry, so as to prevent the base plate from sticking during the image transfer, resulting in incomplete graphics or uneven edges.

[0010] S6: Graphic plating and etching: The biggest feature of flexible circuit boards is flexibility. Therefore, when plating, avoid excessively thick or rough coating. The thicker the line, the worse the flexibility. Rough coating affects the appearance of the graphics. When making flexible circuit boards, it is necessary to analyze and adjust the main salt and additive content of the electroplating solution, select the appropriate current density and electroplating time. The coating thickness is generally 15 to 20um. In order to ensure reliability, the thickness of the locking layer of military flexible circuit boards is generally greater than 25um. When clamping, the characteristics of the flexible circuit board should be considered, and the frame should be used to strengthen the support of the flexible circuit board. If the board area is too large, the circuit board will be damaged during the electroplating. During the copper plating process, the air stirring should be weakened or turned off, and only the circulation filtration and cathode movement should be used to prevent the board from wrinkling. The film removal process of the flexible circuit board should be strictly controlled. The alkali resistance of polyimide is poor, and the alkali resistance of acrylic glue is even worse. Therefore, when using a strong alkaline solution to remove the film, the temperature, concentration and film removal time of the film removal solution must be strictly controlled. After the film is removed, the residual liquid on the board surface and the hole should be washed with clean water immediately. The production of flexible circuit boards has higher requirements for the etching machine. Before etching, the etching machine must be cleaned first, and the crystals on the transmission roller must be cleaned to avoid residual crystals scratching the pattern: the cleaning nozzle ensures the uniformity of the spray corrosion;

[0011] S7: Selection of covering layer: One of the biggest differences between flexible circuit boards and rigid boards is the covering layer. The insulating protective layer of the flexible circuit can be made of covering film and printed liquid flexible ink. The screen-printed liquid covering layer ink has low material cost and low mass production cost, so it is widely used in automobiles and some civilian products. However, due to the disadvantages of hardness and poor bending resistance of flexible ink, liquid ink is rarely used for boards that need to be bent multiple times and high-density fine circuit products. Instead, a covering film of the same material as the base film is used. There is a release film (or paper) on the adhesive film of the covering film. The semi-cured epoxy resin adhesive will gradually solidify at room temperature, so it should be stored in a low-temperature refrigerated storage (stored at about 5 degrees before use). The improper storage environment will shorten the service life of the covering layer. At the end of its service life, the fluidity of the adhesive is very small. If the lamination temperature and pressure are not high, a covering film that fills the gaps in the pattern and has high bonding strength cannot be obtained. Before pasting the covering layer, the circuit surface should be cleaned to remove surface pollution and oxidation. After removing the release film from the covering film with window holes, it should be pasted on the etched substrate. Use butanone to stick the edge of the pattern to prevent position deviation during lamination. After tearing off the release film, the covering layer has no supporting strength and has many window holes. Be careful when positioning it with the substrate. Do not force the film to be stretched or make wrinkles on the film, otherwise it will cause misalignment or uneven appearance.

[0012] S8: Hot air leveling and gold plating: Due to the high water absorption of polyimide, it should be pre-baked in a 120C oven for 2 to 4 hours before hot air leveling to prevent the metallized holes from breaking or the inner layer from stratifying and bubbling when subjected to thermal shock. The printed circuit board should be hot-air leveled immediately after baking to prevent the board from absorbing moisture again. It is recommended to use a rigid frame to reinforce the flexible circuit board during hot air leveling, and adjust the process parameters at the same time: shorten the lead-tin immersion time (from 5 seconds to 2 to 3 seconds) and reduce the front and rear air knife pressure (from 4MPa to 25Mpa). In order to avoid the flexible circuit board from being wrinkled or torn after heat treatment and improve the qualified rate of finished products, chemical nickel / gold plating process is also used;

[0013] S9: Shape processing: Most of the processing of holes and shapes of flexible printed circuit boards is done by punching, but it is not the only method. Depending on the situation, various methods or combinations of methods can be used for processing.

[0014] Preferably, in step S1, the lead and the pad are chamfered to prevent the occurrence of microcracks. If the pad strength is required to be very high or the component on the pad is subjected to stress during operation, the pad should be designed to be reinforced. In order to avoid stress concentration, the lead should avoid right-angle corners and should use arc-shaped corners.

[0015] Preferably, in step S5, when designing for SMT, the length direction of the component should avoid the bending direction of the flexible board; large QFP or BGA should be attached with a reinforcing plate on the back of the flexible board or glued under the IC, the reinforcing plate is made of FR4, the thickness is greater than 0.2 mm, and the area is 0.5 mm larger than the outer edge of the component, two positioning holes are made near the edge of the flexible board, and two patch identification pads are also needed, with a diameter of 1 mm, at least 3.5 mm away from other pads, gold-plated or tin-plated on the surface, and good flatness. If the large flexible board is composed of many small boards, a ring-shaped (inner diameter 3.2 mm) bad circuit board identification pad should be made on each small board. If this small circuit board is damaged, this identification pad can be painted black with a black marker to avoid subsequent operations. The minimum distance between the component and the edge of the circuit board is to avoid 2.5 mm, and the minimum spacing between components is 0.5 mm. There should be no vias under the component because the flux will flow through the vias and cause pollution.

[0016] Preferably, in step S4, the hole metallization process needs to pay special attention to adjusting the degreasing station: because the adhesive of the flexible circuit board has poor alkali resistance, the alkaline degreasing process (temperature of about 60°C) usually used in the production of printed circuit boards is not suitable for flexible circuit boards. Therefore, it is recommended to use an acidic degreasing adjustment process, and use a fast and highly active chemical copper plating solution to reduce the contact time of acrylic resin and polyimide with the alkaline copper plating solution to prevent the swelling of the flexible material caused by a long reaction time, and avoid excessive speed causing hole voids and poor mechanical properties of the coating.

[0017] Preferably, in step S5, special attention should be paid to the dry film laminating, because the copper-clad laminate is relatively soft, and a thin plate laminating machine should be used to ensure that the dry film is closely attached to the substrate. The exposure parameters are the same as those of conventional printed circuit boards, but the vacuum absorption may be poor due to the thinness and unevenness of the board. For this reason, a scraper can be used to drive away the air to ensure that the flexible circuit board has good vacuum absorption. The dry film after photopolymerization is less flexible than the unpolymerized dry film. During operation, the board should be bent to avoid the dry film and the substrate from falling off. It is recommended to use a rigid board for traction during development, which can avoid the spray pressure of the developer and the influence of hot and cold winds that cause the flexible circuit board to bend or jam.

[0018] Preferably, in step S6, in order to master the spraying time, firstly, a curve diagram of the spraying time and the etching speed should be prepared, and then the spraying time should be selected according to the required etching amount. Generally, the copper at the edge of the board will be etched off first during corrosion, and local uncleanness can be solved by point spraying. During operation, it is necessary to avoid over-corrosion or under-corrosion of the lines, otherwise the neatness of the edges of the wires will be poor. The substrate after corrosion is softer, so it is recommended to use a hard board to prevent the board from being stuck or folded.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention can effectively prevent the problem of stress concentration and breakage of the circuit by producing the flexible circuit board by the above method, and can effectively prevent cracks from occurring between the circuit and the pad;

[0020] 2. The flexible circuit board produced by the above method can ensure that the flexible circuit board has good vacuum absorption. The dry film after photopolymerization is less flexible than the unpolymerized dry film. During operation, the board should be bent to avoid the dry film and the substrate from falling off. It is recommended to use a rigid board for traction during development, so as to avoid the spray pressure of the developer and the influence of hot and cold winds that cause the flexible circuit board to bend or jam.

[0021] 3. The flexible circuit board produced by the above method of the present invention has the advantages of reliable performance and good appearance. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0023] In the attached picture:

[0024] Figure 1 It is a flow chart of the present invention; DETAILED DESCRIPTION

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0026] See also Figure 1 The present invention provides a high-density circuit layout process for a flexible circuit board, comprising the following steps:

[0027] S1: FPC cutting: Except for some materials, the materials used for flexible circuit boards are basically in roll form. Since not all processes must be processed using the tape process, some processes must be cut into sheets for processing, such as the drilling of metallized holes in double-sided flexible circuit boards. Currently, drilling can only be done in sheet form. Therefore, the first process of double-sided flexible circuit boards is cutting. Flexible copper-clad laminates have extremely poor tolerance to external forces and are easily damaged. If they are damaged during cutting, the qualified rate of subsequent processes will be seriously affected;

[0028] S2: Drilling: The drilling equipment for flexible circuit boards is the same as that for rigid printed circuit boards, but the drilling conditions are slightly different. Since the adhesive used in FPC boards is soft and easy to stick to the knife, it is recommended to use FPC special tools for drilling, or use new drill bits (the number of holes drilled is less than 1000), and optimize the drilling parameters at the same time to reduce drilling grease or burrs. The drilling of flexible circuit boards, cover layers and reinforcement layers is basically the same. Since the copper clad laminate and the cover layer are both thin, multiple overlapping holes can be drilled when drilling (according to the thickness of the board, the copper clad laminate can be stacked about 7-10 pieces and the cover layer can be stacked 10-15 pieces). The upper pad can use aluminum pads or epoxy yellow pads, and the lower pad can use epoxy yellow pads. The processing conditions for drilling, milling the outer shape of the cover film and reinforcement board are basically the same, but because the adhesive used in the flexible printed circuit board material is soft, it is very easy to adhere to the drill bit, and the drill bit status needs to be frequently inspected, and the drill bit speed should be appropriately increased;

[0029] S3: Surface treatment of copper foil: The surface of the flexible circuit board needs to be cleaned before hole metallization to improve the bonding strength between the hole metallization coating and the substrate. At the same time, effective treatment of the copper surface before the hole metallization and the pattern imaging is also very necessary to enhance the adhesion strength between copper and the anti-corrosion layer. The surface of the FPC board is not cleaned before the hole metallization, and the hole plating may be delaminated and blistered: the copper surface is not cleaned and roughened enough before the pattern imaging, and the bonding with the anti-corrosion agent is not strong. It is almost impossible to make fine circuit patterns, and it is easy to seep plating in the subsequent electroplating process. This causes uneven line edges or even short circuits, affecting the pattern accuracy;

[0030] S4: Hole metallization: Before hole metallization of flexible multilayer boards, degreasing is required. There are two commonly used degreasing processes: one is alkaline potassium permanganate solution treatment, and the other is plasma treatment. Because acrylic adhesives have poor alkali resistance, alkaline potassium permanganate process is only suitable for epoxy adhesives. Therefore, plasma treatment process is often used for flexible circuit boards. It can not only effectively remove grease, but also etch the insulating medium, so that the inner copper layer forms a reliable three-dimensional structure, enhance the bonding force of the hole wall plating, and use a hole inspection mirror to observe the state of the hole wall. After plasma treatment, the complete copper ring is clearly protruding, ensuring that the hole wall is clean and free of drilling dirt. When hole metallization is performed, the contact area is increased and the plating bonding is better;

[0031] S5: Graphics production: The graphic transfer of flexible circuit boards is not much different from that of ordinary printed boards. The commonly used anti-corrosion layers are liquid resists and dry films. When applying the liquid anti-corrosion layer, static dust and floating objects should be avoided from being adsorbed into the anti-corrosion layer. In addition, the viscosity of the anti-corrosion agent and the time and temperature of the baking board should be selected to ensure that the adhesion between the anti-corrosion agent and the copper plate reaches the maximum and remains dry, so as to prevent the base plate from sticking during the image transfer, resulting in incomplete graphics or uneven edges.

[0032] S6: Graphic plating and etching: The biggest feature of flexible circuit boards is flexibility. Therefore, when plating, avoid excessively thick or rough coating. The thicker the line, the worse the flexibility. Rough coating affects the appearance of the graphics. When making flexible circuit boards, it is necessary to analyze and adjust the main salt and additive content of the electroplating solution, select the appropriate current density and electroplating time. The coating thickness is generally 15 to 20um. In order to ensure reliability, the thickness of the locking layer of military flexible circuit boards is generally greater than 25um. When clamping, the characteristics of the flexible circuit board should be considered, and the frame should be used to strengthen the support of the flexible circuit board. If the board area is too large, the circuit board will be damaged during the electroplating. During the copper plating process, the air stirring should be weakened or turned off, and only the circulation filtration and cathode movement should be used to prevent the board from wrinkling. The film removal process of the flexible circuit board should be strictly controlled. The alkali resistance of polyimide is poor, and the alkali resistance of acrylic glue is even worse. Therefore, when using a strong alkaline solution to remove the film, the temperature, concentration and film removal time of the film removal solution must be strictly controlled. After the film is removed, the residual liquid on the board surface and the hole should be washed with clean water immediately. The production of flexible circuit boards has higher requirements for the etching machine. Before etching, the etching machine must be cleaned first, and the crystals on the transmission roller must be cleaned to avoid residual crystals scratching the pattern: the cleaning nozzle ensures the uniformity of the spray corrosion;

[0033] S7: Selection of covering layer: One of the biggest differences between flexible circuit boards and rigid boards is the covering layer. The insulating protective layer of the flexible circuit can be made of covering film and printed liquid flexible ink. The screen-printed liquid covering layer ink has low material cost and low mass production cost, so it is widely used in automobiles and some civilian products. However, due to the disadvantages of hardness and poor bending resistance of flexible ink, liquid ink is rarely used for boards that need to be bent multiple times and high-density fine circuit products. Instead, a covering film of the same material as the base film is used. There is a release film (or paper) on the adhesive film of the covering film. The semi-cured epoxy resin adhesive will gradually solidify at room temperature, so it should be stored in a low-temperature refrigerated storage (stored at about 5 degrees before use). The improper storage environment will shorten the service life of the covering layer. At the end of its service life, the fluidity of the adhesive is very small. If the lamination temperature and pressure are not high, a covering film that fills the gaps in the pattern and has high bonding strength cannot be obtained. Before pasting the covering layer, the circuit surface should be cleaned to remove surface pollution and oxidation. After removing the release film from the covering film with window holes, it should be pasted on the etched substrate. Use butanone to stick the edge of the pattern to prevent position deviation during lamination. After tearing off the release film, the covering layer has no supporting strength and has many window holes. Be careful when positioning it with the substrate. Do not force the film to be stretched or make wrinkles on the film, otherwise it will cause misalignment or uneven appearance.

[0034] S8: Hot air leveling and gold plating: Due to the high water absorption of polyimide, it should be pre-baked in a 120C oven for 2 to 4 hours before hot air leveling to prevent the metallized holes from breaking or the inner layer from stratifying and bubbling when subjected to thermal shock. The printed circuit board should be hot-air leveled immediately after baking to prevent the board from absorbing moisture again. It is recommended to use a rigid frame to reinforce the flexible circuit board during hot air leveling, and adjust the process parameters at the same time: shorten the lead-tin immersion time (from 5 seconds to 2 to 3 seconds) and reduce the front and rear air knife pressure (from 4MPa to 25Mpa). In order to avoid the flexible circuit board from being wrinkled or torn after heat treatment and improve the qualified rate of finished products, chemical nickel / gold plating process is also used;

[0035] S9: Shape processing: Most of the processing of holes and shapes of flexible printed circuit boards is done by punching, but it is not the only method. Depending on the situation, various methods or combinations of methods can be used for processing.

[0036] Among them, in step S1, the lead and the pad should be chamfered to prevent the occurrence of micro cracks. If the pad strength is required to be very high or the component on the pad is subjected to stress during operation, the pad should be designed to be reinforced. In order to avoid stress concentration, the lead should avoid right-angle corners and should use arc-shaped corners. In step S5, when designing for SMT, the length direction of the component should avoid the bending direction of the flexible board; large QFP or BGA should be attached to the back of the flexible board with a reinforcing plate or glued under the IC. The material of the reinforcing plate is FR4, with a thickness greater than 0.2 mm and an area greater than 0.5 mm of the outer edge of the component. Two positioning holes should be made near the edge of the flexible board, and two identification pads for patches are also required. The diameter is 1 mm and the distance from other pads is at least 3.5mm, gold-plated or tin-plated on the surface, good flatness. If the large flexible board is composed of many small boards, a ring-shaped (inner diameter 3.2mm) bad circuit board identification pad should be made on each small board. If the small circuit board is damaged, the identification pad can be painted black with a black marker to avoid future operations. The minimum distance between the component and the edge of the circuit board is to avoid 2.5mm, and the minimum spacing between components is 0.5mm. There should be no vias under the components, because the flux will flow through the vias and cause pollution. In step S4, the hole metallization process needs to pay special attention to adjusting the degreasing station: because the adhesive of the flexible circuit board has poor alkali resistance, the alkaline degreasing process (temperature of about 60C) usually used in printed circuit board production is not suitable. Flexible circuit boards are suitable for this purpose, so it is recommended to use an acidic degreasing adjustment process. In addition, a fast and highly active chemical copper plating solution is used to reduce the contact time between acrylic resin and polyimide and the alkaline copper plating solution to prevent the swelling of the flexible material caused by a long reaction time. At the same time, avoid excessive speed to cause holes and poor mechanical properties of the coating. In step S5, special attention should be paid to the dry film. The copper-clad board is relatively soft, so a thin plate laminator should be used to ensure that the dry film is closely attached to the substrate. The exposure parameters are the same as those of conventional printed boards, but the vacuum absorption may be poor due to the thinness and unevenness of the board. For this reason, a scraper can be used to drive away the air to ensure that the flexible circuit board has good vacuum absorption. The dry film after photopolymerization is less flexible than the unpolymerized dry film. During operation, the board should be bent to avoid causing the dry film to sag. It is recommended to use a rigid board for traction during development to avoid bending or jamming of the flexible circuit board due to the spray pressure of the developer and the influence of hot and cold wind. In step S5, special attention should be paid to the dry film pasting. The copper clad board is relatively soft, and a thin plate laminator should be used to ensure that the dry film is closely attached to the substrate. The exposure parameters are the same as those of conventional printed boards. However, due to the thinness and unevenness of the board, the vacuum absorption may be poor. For this reason, a scraper can be used to drive away the air to ensure that the flexible circuit board has good vacuum absorption. The dry film after photopolymerization is less flexible than the unpolymerized dry film. During operation, the board should be prevented from bending to cause the dry film to fall off from the substrate. It is recommended to use a rigid board for traction during development to avoid bending or jamming of the flexible circuit board due to the spray pressure of the developer and the influence of hot and cold wind.

[0037] Embodiment 1

[0038] A high-density circuit layout process for a flexible circuit board comprises the following steps:

[0039] S1: FPC cutting: Except for some materials, the materials used for flexible circuit boards are basically in roll form. Since not all processes must be processed using the tape process, some processes must be cut into sheets for processing, such as the drilling of metallized holes in double-sided flexible circuit boards. Currently, drilling can only be done in sheet form. Therefore, the first process of double-sided flexible circuit boards is cutting. Flexible copper-clad laminates have extremely poor tolerance to external forces and are easily damaged. If they are damaged during cutting, the qualified rate of subsequent processes will be seriously affected;

[0040] S2: Drilling: The drilling equipment for flexible circuit boards is the same as that for rigid printed circuit boards, but the drilling conditions are slightly different. Since the adhesive used in FPC boards is soft and easy to stick to the knife, it is recommended to use FPC special tools for drilling, or use new drill bits (the number of holes drilled is less than 1000), and optimize the drilling parameters at the same time to reduce drilling grease or burrs. The drilling of flexible circuit boards, cover layers and reinforcement layers is basically the same. Since the copper clad laminate and the cover layer are both thin, multiple overlapping holes can be drilled when drilling (according to the thickness of the board, the copper clad laminate can be stacked about 7-10 pieces and the cover layer can be stacked 10-15 pieces). The upper pad can use aluminum pads or epoxy yellow pads, and the lower pad can use epoxy yellow pads. The processing conditions for drilling, milling the outer shape of the cover film and reinforcement board are basically the same, but because the adhesive used in the flexible printed circuit board material is soft, it is very easy to adhere to the drill bit, and the drill bit status needs to be frequently inspected, and the drill bit speed should be appropriately increased;

[0041] S3: Surface treatment of copper foil: The surface of the flexible circuit board needs to be cleaned before hole metallization to improve the bonding strength between the hole metallization coating and the substrate. At the same time, effective treatment of the copper surface before the hole metallization and the pattern imaging is also very necessary to enhance the adhesion strength between copper and the anti-corrosion layer. The surface of the FPC board is not cleaned before the hole metallization, and the hole plating may be delaminated and blistered: the copper surface is not cleaned and roughened enough before the pattern imaging, and the bonding with the anti-corrosion agent is not strong. It is almost impossible to make fine circuit patterns, and it is easy to seep plating in the subsequent electroplating process. This causes uneven line edges or even short circuits, affecting the pattern accuracy;

[0042] S4: Hole metallization: Before hole metallization of flexible multilayer boards, grease removal is required. There are two commonly used degreasing processes: one is alkaline potassium permanganate solution treatment, and the other is plasma treatment. Because acrylic adhesives have poor alkali resistance, alkaline potassium permanganate process is only suitable for epoxy adhesives. Therefore, plasma treatment process is often used for flexible circuit boards. It can not only effectively remove grease, but also etch the insulating medium, so that the inner copper layer forms a reliable three-dimensional structure, enhance the bonding force of the hole wall plating, and use a hole inspection mirror to observe the state of the hole wall. After plasma treatment, the complete copper ring is clearly protruding, ensuring that the hole wall is clean and free of drilling dirt. When hole metallization is performed, the contact area is increased and the plating bonding is better;

[0043] S5: Graphics production: The graphic transfer of flexible circuit boards is not much different from that of ordinary printed boards. The commonly used anti-corrosion layers are liquid resists and dry films. When applying the liquid anti-corrosion layer, avoid electrostatic dust and floating objects adsorbed into the anti-corrosion layer. In addition, the viscosity of the anti-corrosion agent and the time and temperature of the baking board should be selected to ensure that the adhesion between the anti-corrosion agent and the copper plate reaches the maximum and remains dry, so as to prevent the base plate from sticking during the image transfer, resulting in incomplete graphics or uneven edges.

[0044] S6: Graphic plating and etching: The biggest feature of flexible circuit boards is flexibility. Therefore, when plating, avoid excessively thick or rough coating. The thicker the line, the worse the flexibility. Rough coating affects the appearance of the graphics. When making flexible circuit boards, it is necessary to analyze and adjust the main salt and additive content of the electroplating solution, select the appropriate current density and electroplating time. The coating thickness is generally 15 to 20um. In order to ensure reliability, the thickness of the locking layer of military flexible circuit boards is generally greater than 25um. When clamping, the characteristics of the flexible circuit board should be considered, and the frame should be used to strengthen the support of the flexible circuit board. If the board area is too large, the circuit board will be damaged during the electroplating. During the copper plating process, the air stirring should be weakened or turned off, and only the circulation filtration and cathode movement should be used to prevent the board from wrinkling. The film removal process of the flexible circuit board should be strictly controlled. The alkali resistance of polyimide is poor, and the alkali resistance of acrylic glue is even worse. Therefore, when using a strong alkaline solution to remove the film, the temperature, concentration and film removal time of the film removal solution must be strictly controlled. After the film is removed, the residual liquid on the board surface and the hole should be washed with clean water immediately. The production of flexible circuit boards has higher requirements for the etching machine. Before etching, the etching machine must be cleaned first, and the crystals on the transmission roller must be cleaned to avoid residual crystals scratching the pattern: the cleaning nozzle ensures the uniformity of the spray corrosion;

[0045] S7: Selection of covering layer: One of the biggest differences between flexible circuit boards and rigid boards is the covering layer. The insulating protective layer of the flexible circuit can be made of covering film and printed liquid flexible ink. The screen-printed liquid covering layer ink has low material cost and low mass production cost, so it is widely used in automobiles and some civilian products. However, due to the disadvantages of hardness and poor bending resistance of flexible ink, liquid ink is rarely used for boards that need to be bent multiple times and high-density fine circuit products. Instead, a covering film of the same material as the base film is used. There is a release film (or paper) on the adhesive film of the covering film. The semi-cured epoxy resin adhesive will gradually solidify at room temperature, so it should be stored in a low-temperature refrigerated storage (stored at about 5 degrees before use). The improper storage environment will shorten the service life of the covering layer. At the end of its service life, the fluidity of the adhesive is very small. If the lamination temperature and pressure are not high, a covering film that fills the gaps in the pattern and has high bonding strength cannot be obtained. Before pasting the covering layer, the circuit surface should be cleaned to remove surface pollution and oxidation. After removing the release film from the covering film with window holes, it should be pasted on the etched substrate. Use butanone to stick the edge of the pattern to prevent position deviation during lamination. After tearing off the release film, the covering layer has no supporting strength and has many window holes. Be careful when positioning it with the substrate. Do not force the film to be stretched or make wrinkles on the film, otherwise it will cause misalignment or uneven appearance.

[0046] S8: Hot air leveling and gold plating: Due to the high water absorption of polyimide, it should be pre-baked in a 120C oven for 2 to 4 hours before hot air leveling to prevent the metallized holes from breaking or the inner layer from stratifying and bubbling when subjected to thermal shock. The printed circuit board should be hot-air leveled immediately after baking to prevent the board from absorbing moisture again. It is recommended to use a rigid frame to reinforce the flexible circuit board during hot air leveling, and adjust the process parameters at the same time: shorten the lead-tin immersion time (from 5 seconds to 2 to 3 seconds) and reduce the front and rear air knife pressure (from 4MPa to 25Mpa). In order to avoid the flexible circuit board from being wrinkled or torn after heat treatment and improve the qualified rate of finished products, chemical nickel / gold plating process is also used;

[0047] S9: Shape processing: Most of the processing of holes and shapes of flexible printed circuit boards is done by punching, but it is not the only method. Depending on the situation, various methods or combinations of methods can be used for processing.

[0048] The flexible circuit board produced by the above method can effectively prevent the problem of circuit breakage due to stress concentration, and can effectively prevent cracks from occurring between the circuit and the pad.

[0049] Embodiment 2

[0050] A high-density circuit layout process for a flexible circuit board comprises the following steps:

[0051] S1: FPC cutting: Except for some materials, the materials used for flexible circuit boards are basically in roll form. Since not all processes must be processed using the tape process, some processes must be cut into sheets for processing, such as the drilling of metallized holes in double-sided flexible circuit boards. Currently, drilling can only be done in sheet form. Therefore, the first process of double-sided flexible circuit boards is cutting. Flexible copper-clad laminates have extremely poor tolerance to external forces and are easily damaged. If they are damaged during cutting, the qualified rate of subsequent processes will be seriously affected;

[0052] S2: Drilling: The drilling equipment for flexible circuit boards is the same as that for rigid printed circuit boards, but the drilling conditions are slightly different. Since the adhesive used in FPC boards is soft and easy to stick to the knife, it is recommended to use FPC special tools for drilling, or use new drill bits (the number of holes drilled is less than 1000), and optimize the drilling parameters at the same time to reduce drilling grease or burrs. The drilling of flexible circuit boards, cover layers and reinforcement layers is basically the same. Since the copper clad laminate and the cover layer are both thin, multiple overlapping holes can be drilled when drilling (according to the thickness of the board, the copper clad laminate can be stacked about 7-10 pieces and the cover layer can be stacked 10-15 pieces). The upper pad can use aluminum pads or epoxy yellow pads, and the lower pad can use epoxy yellow pads. The processing conditions for drilling, milling the outer shape of the cover film and reinforcement board are basically the same, but because the adhesive used in the flexible printed circuit board material is soft, it is very easy to adhere to the drill bit, and the drill bit status needs to be frequently inspected, and the drill bit speed should be appropriately increased;

[0053] S3: Surface treatment of copper foil: The surface of the flexible circuit board needs to be cleaned before hole metallization to improve the bonding strength between the hole metallization coating and the substrate. At the same time, effective treatment of the copper surface before the hole metallization and the pattern imaging is also very necessary to enhance the adhesion strength between copper and the anti-corrosion layer. The surface of the FPC board is not cleaned before the hole metallization, and the hole plating may be delaminated and blistered: the copper surface is not cleaned and roughened enough before the pattern imaging, and the bonding with the anti-corrosion agent is not strong. It is almost impossible to make fine circuit patterns, and it is easy to seep plating in the subsequent electroplating process. This causes uneven line edges or even short circuits, affecting the pattern accuracy;

[0054] S4: Hole metallization: Before hole metallization of flexible multilayer boards, grease removal is required. There are two commonly used degreasing processes: one is alkaline potassium permanganate solution treatment, and the other is plasma treatment. Because acrylic adhesives have poor alkali resistance, alkaline potassium permanganate process is only suitable for epoxy adhesives. Therefore, plasma treatment process is often used for flexible circuit boards. It can not only effectively remove grease, but also etch the insulating medium, so that the inner copper layer forms a reliable three-dimensional structure, enhance the bonding force of the hole wall plating, and use a hole inspection mirror to observe the state of the hole wall. After plasma treatment, the complete copper ring is clearly protruding, ensuring that the hole wall is clean and free of drilling dirt. When hole metallization is performed, the contact area is increased and the plating bonding is better;

[0055] S5: Graphics production: The graphic transfer of flexible circuit boards is not much different from that of ordinary printed boards. The commonly used anti-corrosion layers are liquid resists and dry films. When applying the liquid anti-corrosion layer, avoid electrostatic dust and floating objects adsorbed into the anti-corrosion layer. In addition, the viscosity of the anti-corrosion agent and the time and temperature of the baking board should be selected to ensure that the adhesion between the anti-corrosion agent and the copper plate reaches the maximum and remains dry, so as to prevent the base plate from sticking during the image transfer, resulting in incomplete graphics or uneven edges.

[0056] S6: Graphic plating and etching: The biggest feature of flexible circuit boards is flexibility. Therefore, when plating, avoid excessively thick or rough coating. The thicker the line, the worse the flexibility. Rough coating affects the appearance of the graphics. When making flexible circuit boards, it is necessary to analyze and adjust the main salt and additive content of the electroplating solution, select the appropriate current density and electroplating time. The coating thickness is generally 15 to 20um. In order to ensure reliability, the thickness of the locking layer of military flexible circuit boards is generally greater than 25um. When clamping, the characteristics of the flexible circuit board should be considered, and the frame should be used to strengthen the support of the flexible circuit board. If the board area is too large, the circuit board will be damaged during the electroplating. During the copper plating process, the air stirring should be weakened or turned off, and only the circulation filtration and cathode movement should be used to prevent the board from wrinkling. The film removal process of the flexible circuit board should be strictly controlled. The alkali resistance of polyimide is poor, and the alkali resistance of acrylic glue is even worse. Therefore, when using a strong alkaline solution to remove the film, the temperature, concentration and film removal time of the film removal solution must be strictly controlled. After the film is removed, the residual liquid on the board surface and the hole should be washed with clean water immediately. The production of flexible circuit boards has higher requirements for the etching machine. Before etching, the etching machine must be cleaned first, and the crystals on the transmission roller must be cleaned to avoid residual crystals scratching the pattern: the cleaning nozzle ensures the uniformity of the spray corrosion;

[0057] S7: Selection of covering layer: One of the biggest differences between flexible circuit boards and rigid boards is the covering layer. The insulating protective layer of the flexible circuit can be made of covering film and printed liquid flexible ink. The screen-printed liquid covering layer ink has low material cost and low mass production cost, so it is widely used in automobiles and some civilian products. However, due to the disadvantages of hardness and poor bending resistance of flexible ink, liquid ink is rarely used for boards that need to be bent multiple times and high-density fine circuit products. Instead, a covering film of the same material as the base film is used. There is a release film (or paper) on the adhesive film of the covering film. The semi-cured epoxy resin adhesive will gradually solidify at room temperature, so it should be stored in a low-temperature refrigerated storage (stored at about 5 degrees before use). The improper storage environment will shorten the service life of the covering layer. At the end of its service life, the fluidity of the adhesive is very small. If the lamination temperature and pressure are not high, a covering film that fills the gaps in the pattern and has high bonding strength cannot be obtained. Before pasting the covering layer, the circuit surface should be cleaned to remove surface pollution and oxidation. After removing the release film from the covering film with window holes, it should be pasted on the etched substrate. Use butanone to stick the edge of the pattern to prevent position deviation during lamination. After tearing off the release film, the covering layer has no supporting strength and has many window holes. Be careful when positioning it with the substrate. Do not force the film to be stretched or make wrinkles on the film, otherwise it will cause misalignment or uneven appearance.

[0058] S8: Hot air leveling and gold plating: Due to the high water absorption of polyimide, it should be pre-baked in a 120C oven for 2 to 4 hours before hot air leveling to prevent the metallized holes from breaking or the inner layer from stratifying and bubbling when subjected to thermal shock. The printed circuit board should be hot-air leveled immediately after baking to prevent the board from absorbing moisture again. It is recommended to use a rigid frame to reinforce the flexible circuit board during hot air leveling, and adjust the process parameters at the same time: shorten the lead-tin immersion time (from 5 seconds to 2 to 3 seconds) and reduce the front and rear air knife pressure (from 4MPa to 25Mpa). In order to avoid the flexible circuit board from being wrinkled or torn after heat treatment and improve the qualified rate of finished products, chemical nickel / gold plating process is also used;

[0059] S9: Shape processing: Most of the processing of holes and shapes of flexible printed circuit boards is done by punching, but it is not the only method. Depending on the situation, various methods or combinations of methods can be used for processing.

[0060] The flexible circuit board produced by the above method can ensure that the flexible circuit board has good vacuum absorption. The dry film after photopolymerization is less flexible than the unpolymerized dry film. During operation, the bending of the board should be avoided to cause the dry film and the substrate to fall off. It is recommended to use a rigid board for traction during development, which can avoid the spray pressure of the developer and the influence of hot and cold air causing the flexible circuit board to bend or jam.

[0061] Embodiment 3

[0062] A high-density circuit layout process for a flexible circuit board comprises the following steps:

[0063] S1: FPC cutting: Except for some materials, the materials used for flexible circuit boards are basically in roll form. Since not all processes must be processed using the tape process, some processes must be cut into sheets for processing, such as the drilling of metallized holes in double-sided flexible circuit boards. Currently, drilling can only be done in sheet form. Therefore, the first process of double-sided flexible circuit boards is cutting. Flexible copper-clad laminates have extremely poor tolerance to external forces and are easily damaged. If they are damaged during cutting, the qualified rate of subsequent processes will be seriously affected;

[0064] S2: Drilling: The drilling equipment for flexible circuit boards is the same as that for rigid printed circuit boards, but the drilling conditions are slightly different. Since the adhesive used in FPC boards is soft and easy to stick to the knife, it is recommended to use FPC special tools for drilling, or use new drill bits (the number of holes drilled is less than 1000), and optimize the drilling parameters at the same time to reduce drilling grease or burrs. The drilling of flexible circuit boards, cover layers and reinforcement layers is basically the same. Since the copper clad laminate and the cover layer are both thin, multiple overlapping holes can be drilled when drilling (according to the thickness of the board, the copper clad laminate can be stacked about 7-10 pieces and the cover layer can be stacked 10-15 pieces). The upper pad can use aluminum pads or epoxy yellow pads, and the lower pad can use epoxy yellow pads. The processing conditions for drilling, milling the outer shape of the cover film and reinforcement board are basically the same, but because the adhesive used in the flexible printed circuit board material is soft, it is very easy to adhere to the drill bit, and the drill bit status needs to be frequently inspected, and the drill bit speed should be appropriately increased;

[0065] S3: Surface treatment of copper foil: The surface of the flexible circuit board needs to be cleaned before hole metallization to improve the bonding strength between the hole metallization coating and the substrate. At the same time, effective treatment of the copper surface before the hole metallization and the pattern imaging is also very necessary to enhance the adhesion strength between copper and the anti-corrosion layer. The surface of the FPC board is not cleaned before the hole metallization, and the hole plating may be delaminated and blistered: the copper surface is not cleaned and roughened enough before the pattern imaging, and the bonding with the anti-corrosion agent is not strong. It is almost impossible to make fine circuit patterns, and it is easy to seep plating in the subsequent electroplating process. This causes uneven line edges or even short circuits, affecting the pattern accuracy;

[0066] S4: Hole metallization: Before hole metallization of flexible multilayer boards, degreasing is required. There are two commonly used degreasing processes: one is alkaline potassium permanganate solution treatment, and the other is plasma treatment. Because acrylic adhesives have poor alkali resistance, alkaline potassium permanganate process is only suitable for epoxy adhesives. Therefore, plasma treatment process is often used for flexible circuit boards. It can not only effectively remove grease, but also etch the insulating medium, so that the inner copper layer forms a reliable three-dimensional structure, enhance the bonding force of the hole wall plating, and use a hole inspection mirror to observe the state of the hole wall. After plasma treatment, the complete copper ring is clearly protruding, ensuring that the hole wall is clean and free of drilling dirt. When hole metallization is performed, the contact area is increased and the plating bonding is better;

[0067] S5: Graphics production: The graphic transfer of flexible circuit boards is not much different from that of ordinary printed boards. The commonly used anti-corrosion layers are liquid resists and dry films. When applying the liquid anti-corrosion layer, static dust and floating objects should be avoided from being adsorbed into the anti-corrosion layer. In addition, the viscosity of the anti-corrosion agent and the time and temperature of the baking board should be selected to ensure that the adhesion between the anti-corrosion agent and the copper plate reaches the maximum and remains dry, so as to prevent the base plate from sticking during the image transfer, resulting in incomplete graphics or uneven edges.

[0068] S6: Graphic plating and etching: The biggest feature of flexible circuit boards is flexibility. Therefore, when plating, avoid excessively thick or rough coating. The thicker the line, the worse the flexibility. Rough coating affects the appearance of the graphics. When making flexible circuit boards, it is necessary to analyze and adjust the main salt and additive content of the electroplating solution, select the appropriate current density and electroplating time. The coating thickness is generally 15 to 20um. In order to ensure reliability, the thickness of the locking layer of military flexible circuit boards is generally greater than 25um. When clamping, the characteristics of the flexible circuit board should be considered, and the frame should be used to strengthen the support of the flexible circuit board. If the board area is too large, the circuit board will be damaged during the electroplating. During the copper plating process, the air stirring should be weakened or turned off, and only the circulation filtration and cathode movement should be used to prevent the board from wrinkling. The film removal process of the flexible circuit board should be strictly controlled. The alkali resistance of polyimide is poor, and the alkali resistance of acrylic glue is even worse. Therefore, when using a strong alkaline solution to remove the film, the temperature, concentration and film removal time of the film removal solution must be strictly controlled. After the film is removed, the residual liquid on the board surface and the hole should be washed with clean water immediately. The production of flexible circuit boards has higher requirements for the etching machine. Before etching, the etching machine must be cleaned first, and the crystals on the transmission roller must be cleaned to avoid residual crystals scratching the pattern: the cleaning nozzle ensures the uniformity of the spray corrosion;

[0069] S7: Selection of covering layer: One of the biggest differences between flexible circuit boards and rigid boards is the covering layer. The insulating protective layer of the flexible circuit can be made of covering film and printed liquid flexible ink. The screen-printed liquid covering layer ink has low material cost and low mass production cost, so it is widely used in automobiles and some civilian products. However, due to the disadvantages of hardness and poor bending resistance of flexible ink, liquid ink is rarely used for boards that need to be bent multiple times and high-density fine circuit products. Instead, a covering film of the same material as the base film is used. There is a release film (or paper) on the adhesive film of the covering film. The semi-cured epoxy resin adhesive will gradually solidify at room temperature, so it should be stored in a low-temperature refrigerated storage (stored at about 5 degrees before use). The improper storage environment will shorten the service life of the covering layer. At the end of its service life, the fluidity of the adhesive is very small. If the lamination temperature and pressure are not high, a covering film that fills the gaps in the pattern and has high bonding strength cannot be obtained. Before pasting the covering layer, the circuit surface should be cleaned to remove surface pollution and oxidation. After removing the release film from the covering film with window holes, it should be pasted on the etched substrate. Use butanone to stick the edge of the pattern to prevent position deviation during lamination. After tearing off the release film, the covering layer has no supporting strength and has many window holes. Be careful when positioning it with the substrate. Do not force the film to be stretched or make wrinkles on the film, otherwise it will cause misalignment or uneven appearance.

[0070] S8: Hot air leveling and gold plating: Due to the high water absorption of polyimide, it should be pre-baked in a 120C oven for 2 to 4 hours before hot air leveling to prevent the metallized holes from breaking or the inner layer from stratifying and bubbling when subjected to thermal shock. The printed circuit board should be hot-air leveled immediately after baking to prevent the board from absorbing moisture again. It is recommended to use a rigid frame to reinforce the flexible circuit board during hot air leveling, and adjust the process parameters at the same time: shorten the lead-tin immersion time (from 5 seconds to 2 to 3 seconds) and reduce the front and rear air knife pressure (from 4MPa to 25Mpa). In order to avoid the flexible circuit board from being wrinkled or torn after heat treatment and improve the qualified rate of finished products, chemical nickel / gold plating process is also used;

[0071] S9: Shape processing: Most of the processing of holes and shapes of flexible printed circuit boards is done by punching, but it is not the only method. Depending on the situation, various methods or combinations of methods can be used for processing.

[0072] The flexible circuit board produced by the above method has the advantages of reliable performance and good appearance.

[0073] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus.

[0074] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A high-density circuit layout process for a flexible circuit board, characterized in that: The following steps are involved: S1: FPC cutting: Except for some materials, the materials used for flexible circuit boards are basically in roll form. Since not all processes must be processed using the tape process, some processes must be cut into sheets for processing, such as the drilling of metallized holes in double-sided flexible circuit boards. Currently, drilling can only be done in sheet form. Therefore, the first process of double-sided flexible circuit boards is cutting. Flexible copper-clad laminates have extremely poor tolerance to external forces and are easily damaged. If they are damaged during cutting, the qualified rate of subsequent processes will be seriously affected; S2: Drilling: The drilling equipment for flexible circuit boards is the same as that for rigid printed circuit boards, but the drilling conditions are slightly different. Since the adhesive used in FPC boards is soft and easy to stick to the knife, it is recommended to use FPC special tools for drilling, or use new drill bits (the number of holes drilled is less than 1000), and optimize the drilling parameters at the same time to reduce drilling grease or burrs. The drilling of flexible circuit boards, cover layers and reinforcement layers is basically the same. Since the copper clad laminate and the cover layer are both thin, multiple overlapping holes can be drilled when drilling (according to the thickness of the board, the copper clad laminate can be stacked about 7-10 pieces and the cover layer can be stacked 10-15 pieces). The upper pad can use aluminum pads or epoxy yellow pads, and the lower pad can use epoxy yellow pads. The processing conditions for drilling, milling the outer shape of the cover film and reinforcement board are basically the same, but because the adhesive used in the flexible printed circuit board material is soft, it is very easy to adhere to the drill bit, and the drill bit status needs to be frequently inspected, and the drill bit speed should be appropriately increased; S3: Surface treatment of copper foil: The surface of the flexible circuit board needs to be cleaned before hole metallization to improve the bonding strength between the hole metallization coating and the substrate. At the same time, effective treatment of the copper surface before the hole metallization and the pattern imaging is also very necessary to enhance the adhesion strength between copper and the anti-corrosion layer. The surface of the FPC board is not cleaned before the hole metallization, and the hole plating may be delaminated and blistered: the copper surface is not cleaned and roughened enough before the pattern imaging, and the bonding with the anti-corrosion agent is not strong. It is almost impossible to make fine circuit patterns, and it is easy to seep plating in the subsequent electroplating process. This causes uneven line edges or even short circuits, affecting the pattern accuracy; S4: Hole metallization: Before hole metallization of flexible multilayer boards, degreasing is required. There are two commonly used degreasing processes: one is alkaline potassium permanganate solution treatment, and the other is plasma treatment. Because acrylic adhesives have poor alkali resistance, alkaline potassium permanganate process is only suitable for epoxy adhesives. Therefore, plasma treatment process is often used for flexible circuit boards. It can not only effectively remove grease, but also etch the insulating medium, so that the inner copper layer forms a reliable three-dimensional structure, enhance the bonding force of the hole wall plating, and use a hole inspection mirror to observe the state of the hole wall. After plasma treatment, the complete copper ring is clearly protruding, ensuring that the hole wall is clean and free of drilling dirt. When hole metallization is performed, the contact area is increased and the plating bonding is better; S5: Graphics production: The graphic transfer of flexible circuit boards is not much different from that of ordinary printed boards. The commonly used anti-corrosion layers are liquid resists and dry films. When applying the liquid anti-corrosion layer, static dust and floating objects should be avoided from being adsorbed into the anti-corrosion layer. In addition, the viscosity of the anti-corrosion agent and the time and temperature of the baking board should be selected to ensure that the adhesion between the anti-corrosion agent and the copper plate reaches the maximum and remains dry, so as to prevent the base plate from sticking during the image transfer, resulting in incomplete graphics or uneven edges. S6: Graphic plating and etching: The biggest feature of flexible circuit boards is flexibility. Therefore, when plating, avoid excessively thick or rough coating. The thicker the line, the worse the flexibility. Rough coating affects the appearance of the graphics. When making flexible circuit boards, it is necessary to analyze and adjust the main salt and additive content of the electroplating solution, select the appropriate current density and electroplating time. The coating thickness is generally 15 to 20um. In order to ensure reliability, the thickness of the locking layer of military flexible circuit boards is generally greater than 25um. When clamping, the characteristics of the flexible circuit board should be considered, and the frame should be used to strengthen the support of the flexible circuit board. If the board area is too large, the circuit board will be damaged during the electroplating. During the copper plating process, the air stirring should be weakened or turned off, and only the circulation filtration and cathode movement should be used to prevent the board from wrinkling. The film removal process of the flexible circuit board should be strictly controlled. The alkali resistance of polyimide is poor, and the alkali resistance of acrylic glue is even worse. Therefore, when using a strong alkaline solution to remove the film, the temperature, concentration and film removal time of the film removal solution must be strictly controlled. After the film is removed, the residual liquid on the board surface and the hole should be washed with clean water immediately. The production of flexible circuit boards has higher requirements for the etching machine. Before etching, the etching machine must be cleaned first, and the crystals on the transmission roller must be cleaned to avoid residual crystals scratching the pattern: the cleaning nozzle ensures the uniformity of the spray corrosion; S7: Selection of covering layer: One of the biggest differences between flexible circuit boards and rigid boards is the covering layer. The insulating protective layer of the flexible circuit can be made of covering film and printed liquid flexible ink. The screen-printed liquid covering layer ink has low material cost and low mass production cost, so it is widely used in automobiles and some civilian products. However, due to the disadvantages of hardness and poor bending resistance of flexible ink, liquid ink is rarely used for boards that need to be bent multiple times and high-density fine circuit products. Instead, a covering film of the same material as the base film is used. There is a release film (or paper) on the adhesive film of the covering film. The semi-cured epoxy resin adhesive will gradually solidify at room temperature, so it should be stored in a low-temperature refrigerated storage (stored at about 5 degrees before use). The improper storage environment will shorten the service life of the covering layer. At the end of its service life, the fluidity of the adhesive is very small. If the lamination temperature and pressure are not high, a covering film that fills the gaps in the pattern and has high bonding strength cannot be obtained. Before pasting the covering layer, the circuit surface should be cleaned to remove surface pollution and oxidation. After removing the release film from the covering film with window holes, it should be pasted on the etched substrate. Use butanone to stick the edge of the pattern to prevent position deviation during lamination. After tearing off the release film, the covering layer has no supporting strength and has many window holes. Be careful when positioning it with the substrate. Do not force the film to be stretched or make wrinkles on the film, otherwise it will cause misalignment or uneven appearance. S8: Hot air leveling and gold plating: Due to the high water absorption of polyimide, it should be pre-baked in a 120C oven for 2 to 4 hours before hot air leveling to prevent the metallized holes from breaking or the inner layer from stratifying and bubbling when subjected to thermal shock. The printed circuit board should be hot-air leveled immediately after baking to prevent the board from absorbing moisture again. It is recommended to use a rigid frame to reinforce the flexible circuit board during hot air leveling, and adjust the process parameters at the same time: shorten the lead-tin immersion time (from 5 seconds to 2 to 3 seconds) and reduce the front and rear air knife pressure (from 4MPa to 25Mpa). In order to avoid the flexible circuit board from being wrinkled or torn after heat treatment and improve the qualified rate of finished products, chemical nickel / gold plating process is also used; S9: Shape processing: Most of the processing of holes and shapes of flexible printed circuit boards is done by punching, but it is not the only method. Depending on the situation, various methods or combinations of methods can be used for processing.

2. A high-density circuit layout process for a flexible circuit board according to claim 1, characterized in that: In step S1, the lead and the pad should be chamfered to prevent the occurrence of microcracks. If the pad strength is required to be very high or the component on the pad is subjected to stress during operation, the pad should be designed to be reinforced. In order to avoid stress concentration, the lead should avoid right-angle corners and should use arc-shaped corners.

3. A high-density circuit layout process for a flexible circuit board according to claim 1, characterized in that: In step S5, when designing for SMT, the length direction of the component should avoid the bending direction of the flexible board; a large QFP or BGA should be attached with a reinforcing plate on the back of the flexible board or glued under the IC, the reinforcing plate is made of FR4, the thickness is greater than 0.2 mm, and the area is 0.5 mm larger than the outer edge of the component. Two positioning holes are made near the edge of the flexible board, and two patch identification pads are also needed. The diameter is 1 mm, the distance from other pads is at least 3.5 mm, the surface is gold-plated or tin-plated, and the flatness is good. If the large flexible board is composed of many small boards, a ring-shaped (inner diameter 3.2 mm) bad circuit board identification pad is made on each small board. If this small circuit board is damaged, this identification pad can be painted black with a black marker to avoid subsequent operations. The minimum distance between the component and the edge of the circuit board is to avoid 2.5 mm, and the minimum spacing between components is 0.5 mm. There should be no vias under the component because the flux will flow through the vias and cause pollution.

4. A high-density circuit layout process for a flexible circuit board according to claim 1, characterized in that: In step S4, the hole metallization process requires special attention to the adjustment of the degreasing station: because the adhesive of the flexible circuit board has poor alkali resistance, the alkaline degreasing process (temperature of about 60°C) usually used in the production of printed circuit boards is not suitable for flexible circuit boards. Therefore, it is recommended to use an acidic degreasing adjustment process. In addition, a fast and highly active chemical copper plating solution is used to reduce the contact time between the acrylic resin and the polyimide and the alkaline copper plating solution to prevent the swelling of the flexible material caused by a long reaction time, and to avoid excessive speed causing hole voids and poor mechanical properties of the coating.

5. The high-density circuit layout process of a flexible circuit board according to claim 1, characterized in that: In step S5, special attention should be paid to the dry film laminating. The copper clad laminate is relatively soft, so a thin plate laminating machine should be used to ensure that the dry film is closely attached to the substrate. The exposure parameters are the same as those of conventional printed circuit boards. However, since the board is thin and uneven, the vacuum absorption may be poor. For this reason, a scraper can be used to drive away the air to ensure that the flexible circuit board has good vacuum absorption. The dry film after photopolymerization is less flexible than the unpolymerized dry film. During operation, the board should be bent to avoid falling off the dry film and the substrate. It is recommended to use a rigid plate for traction during development, which can avoid the spray pressure of the developer and the influence of hot and cold winds that cause the flexible circuit board to bend or jam.

6. A high-density circuit layout process for a flexible circuit board according to claim 1, characterized in that: In step S6, in order to master the spraying time, first of all, a curve chart of spraying time and etching speed should be prepared, and then the spraying time should be selected according to the required etching amount. Generally, the copper at the edge of the board will be etched away first during corrosion, and local uncleanness can be solved by point spraying. During operation, it is necessary to avoid over-corrosion or under-corrosion of the lines, otherwise the neatness of the edges of the wires will be poor. The substrate after corrosion is softer, so it is recommended to use hard board traction to prevent carding and folding of the board.

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