CN101384136A - Surface mounting process for flexible circuit board and magnetic jig and steel mesh used by surface mounting process - Google Patents

Abstract

The invention relates to a manufacturing technology of a printed circuit board, in particular to a flexible printed circuit board (FPC) Surface Mount Technology (SMT) and a magnetic jig and a steel mesh used in the FPC surface mount technology. Magnetic jig including pressing and detaining apron and magnetic tray, magnetic tray for having magnetic base plate, press and detain the sheet metal of apron for can being attracted by the base plate, be provided with the slotted hole that supplies flexible circuit board printing tin cream and paster to use on pressing and detaining the apron, its characterized in that, press and detain the steel sheet of apron for can being attracted by magnet, last etching have with press and detain the ladder layer that the apron shape is the same, the degree of depth on ladder layer is the same with the thickness of steel sheet. Before printing, the magnetic tray is fixed on the positioning base, then the flexible circuit board and the pressing buckle cover plate are sequentially placed, after positioning is accurate, the flexible circuit board and the magnetic jig are taken out of the positioning base together, and solder paste printing, surface mounting and reflow soldering processes are carried out.

Description

Flexible circuit board surface mount technology and its magnetic fixture and steel mesh

【Technical field】

The invention relates to the manufacturing technology of printed circuit boards, in particular to a surface mounting process of flexible circuit boards and magnetic jigs and steel nets used in the process.

【Background technique】

Flexible printed circuit (FPC) has the characteristics of high wiring density, light weight, and thin thickness. It is mainly used in many products such as mobile phones, notebook computers, PDAs, and digital cameras. The surface mount technology (SMT) process of flexible circuit board mainly includes three basic links of solder paste printing, placement and reflow soldering. Solder paste printing refers to the printing of solder paste on the pads of the circuit board through the template of the predetermined pattern; patch is to accurately install the surface mount components on the fixed position of the flexible circuit board; reflow soldering refers to melting the solder paste to make the electronic Mechanical and electrical connections are made between component pins and circuit board pads.

In these links, there are very precise requirements for solder paste printing graphics and placement. Once the solder paste printing graphics and placement are not accurate enough, it will lead to poor connection between the electronic component pins and the flexible circuit board pads. , causing the product to be scrapped or requiring rework. The surface mount process is completed on the flexible circuit board, so the flexible circuit board must be accurately positioned to ensure the quality of the surface mount. However, the FPC itself is soft and its mechanical strength is not high, so its position is higher than that of ordinary The PCB is difficult to fix. In order to prevent the deformation of the flexible circuit board during the surface mount process and ensure the accuracy of the solder paste printing and the placement of the patch, the flexible circuit board to be surface mounted is usually placed on a carrier board. Then surface mount process. At present, there are two methods of fixing the flexible circuit board on the carrier board that are widely used in the industry. The first method is to stick high-temperature resistant double-sided adhesive tape around the carrier board to fix the four sides of the flexible circuit board. The first method does not fix the middle part of the flexible circuit board, so that the flexible circuit board in the middle part is floating, so that the middle part of the flexible circuit board has empty soldering and poor bridging after surface mounting electronic components; the second method, A silicone layer is set on the surface of the carrier board, so that the center part and four sides of the flexible circuit board are fixed on the carrier board. After soldering, it is difficult to separate the FPC from the carrier board, and it is easy to break when removing and attaching, and the residual glue on the FPC is easy to stick to dust and be polluted. When the silica gel layer is repeatedly used, the viscosity of the silica gel layer will become low. When the viscosity of the silica gel is too low, the fixed FPC will not be firm enough, which will easily cause the FPC to be dislocated.

Therefore, it is necessary to provide a surface mount mounting fixture which can fully fix the flexible circuit board and prevent the deformation of the flexible circuit board during the surface mount process.

The State Intellectual Property Office of China discloses a jig for assembling flexible circuit boards (FPC) (public number: CN200944704, publication date: 2007.09.05), which is characterized in that: the jig includes: a cover plate and a carrier plate, The cover plate is an elastically deformable thin metal plate of ferromagnetic material. Positioning pins and slot holes that need to be exposed when the flexible circuit substrate is assembled are provided on the cover plate. Positioning holes corresponding to the positioning pins and supply holes are provided on the carrier plate. The slot holes that need to be exposed when the flexible circuit substrate is assembled are also provided with a set of permanent magnet blocks on the carrier plate. The positioning pins are matched with the positioning holes, and the permanent magnet blocks and the cover plate are integrated by magnetic attraction.

Compared with the first two methods of fixing flexible circuit boards by gluing, this fixture has the characteristics of simple structure, fast and convenient use, good reliability, strong versatility, and can be reused many times, which improves production efficiency and saves production. Cost, while improving the excellent rate of the product. Yet in this utility model, described cover plate must be the metal thin plate of ferromagnetic material, and this thin plate also has certain mechanical strength, because common iron sheet is because its toughness is not enough, if thickness is too thin (less than 0.1mm), It is easy to break, which makes it very difficult to remove and attach, and the labor cost is greatly increased. At the same time, its service life is also very short. If the thickness is too thick (greater than 0.1mm), if the cover and carrier are used in the solder paste printing stage Fixing the flexible circuit board will affect its printing quality, resulting in uneven distribution of solder paste and affecting the subsequent process. Therefore, the current process that has been adopted in the industry is: set a layer of adhesive paper on the carrier board, bond the FPC to the carrier board, and then perform the solder paste printing process, and then place the cover plate on the FPC after the solder paste is printed. , SMT and reflow soldering process. Therefore, this technology is actually an improvement on the above-mentioned method 2, and the FPC is pressed by the cover plate to solve the deformation problem of the FPC during the placement and reflow soldering process. The use of adhesive paper in this process has the same disadvantages as setting a layer of silica gel on the carrier in method 2. Therefore, in order to fundamentally solve the above-mentioned series of problems, a new magnetic material, metal sheet, must be used. cover plate.

Since the cover plate is used to cover the FPC, a layer of stencil needs to be set on the cover plate when solder paste is printed, and the cover plate has a certain thickness. If the screen is directly covered on the steel plate, it will make the FPC Too much solder paste will affect the subsequent placement and reflow soldering process, thereby affecting the quality of the entire placement process. Therefore, the stencil must be removed to a certain thickness to ensure the quality of the entire process.

【Content of invention】

The main purpose of the present invention is to provide a new type of magnetic jig used in the surface mount process (SMT) of flexible circuit boards (FPC), and a new flexible circuit produced by using the magnetic jig. board (FPC) surface mount process (SMT).

A magnetic fixture used in the surface mount process (SMT) of a flexible circuit board (FPC). The cover plate is a thin metal plate that can be attracted by the substrate, and slots for printing solder paste and patches on the flexible circuit board are arranged on the press-button cover plate. Attractive steel sheet.

The steel sheet is a stainless steel sheet with a thickness of 0.05-0.1 mm and a hardness of 480-700 Vickers hardness. The steel sheet is made of austenitic stainless steel or martensitic stainless steel through cold working. The chemical composition of the selected steel is as follows:

C:Si:Mn:P:S:Ni:Cr:Fe＝(0.08-0.15):(0.75-1.00):(1.00-2.00):(0.04-0.045):(0.025-0.030):(6.00-10.00 ):(16.0-19.0):(68.0-76.0).

The magnetic tray is a substrate made of aluminum alloy, synthetic stone, glass fiber board and other materials inlaid with high temperature resistant permanent magnets.

The magnetic jig also includes a positioning base, on which at least three positioning pins are arranged, the magnetic tray is provided with a base positioning hole and a tray positioning hole corresponding to the positioning pins of the base, and the press buckle The cover plate is provided with a cover plate positioning hole corresponding to the tray positioning hole, and the flexible circuit board is provided with an FPC positioning hole, and the tray positioning hole, the cover plate positioning hole and the FPC positioning hole cooperate; the described Marking positioning holes corresponding to the optical markings for positioning the flexible circuit board are also arranged on the pressing cover plate.

The magnetic tray is also provided with a positioning groove with the same shape as the flexible circuit board, a groove or a cooling hole corresponding to the position of the components on the circuit board, and the groove is used for double-sided bonding of the flexible circuit board. Components can be filled during assembly.

The steel mesh is a non-magnetic steel sheet with a thickness of 0.08mm or more, on which is etched a stepped layer with the same shape as the press button cover plate, the depth of the stepped layer is the same as the thickness of the steel sheet, and the stepped layer is It is made by partial thinning by etching process.

The stencil is provided with printing holes for printing solder paste on the flexible circuit board. The printing holes correspond to the parts of the flexible circuit board that need to print solder paste. The printing holes are made by laser cutting .

Use the flexible printed circuit board (FPC) surface mount process (SMT) of above-mentioned magnetic fixture and stencil, it is characterized in that, its processing step is as follows:

(1) Fix the magnetic tray on the positioning base before printing, and then place the flexible circuit board and the press cover in turn, so that the positioning hole of the base of the magnetic tray is buckled with the positioning pin, and the corresponding positioning hole of the cover plate, FPC The positioning hole coincides with the positioning hole of the pallet nail, so that the marking positioning hole coincides with the optical marking point on the flexible circuit board. After the positioning is accurate, take the flexible circuit board and the magnetic jig together from the positioning base and put them into the printing track, etc. After the flexible circuit board enters a certain printing track, the printing machine equipped with a stencil starts to print solder paste.

(2) After the printing is completed, the flexible circuit board enters the placement machine with the magnetic fixture, and the placement machine performs component placement on the flexible circuit board.

(3) After the placement is completed, enter the reflow oven to weld and fix the high-temperature components and the solder joints on the flexible circuit board.

(4) After the reflow soldering is completed, take out the cooled magnetic jig from the track, take off the snap cover, take out the flexible circuit board, and the magnetic jig enters the next cycle.

The fixing direction of the flexible printed circuit board is the vertical direction of the sheet element, and the horizontal direction of SOT and SOP; the printing machine is provided with an optical positioning system, and the optical positioning system can identify the optical positioning point on the flexible printed circuit board; The scraper of the printing machine is a steel scraper, and the angle between the scraper and the flexible circuit board is 60-75 degrees; the printing direction of the printing machine is the left-right or front-back direction, the printing speed is 10mm/s-25mm/s, and the printing pressure is 0.1kg/cm ² -0.3kg/cm ² ; the demoulding speed of the printing machine is 0.1mm/s-0.2mm/s.

The placement machine is provided with a suction nozzle, the suction of the suction nozzle becomes 0 after the placement is completed, and after the suction becomes 0, the suction nozzle is removed from the flexible circuit board.

The reflow soldering process in the step (3) includes a preheating stage, a constant temperature stage, a soldering stage and a cooling stage. In the preheating stage, the temperature rises from room temperature to 110 degrees, and the temperature rise rate is 1°C/S-2°C/ S; in the constant temperature stage, the temperature rises from 110°C to 150°C, and the holding time is 60s-120s; in the welding stage, the temperature rises at a rate of 1-2°C/S; the maximum temperature is less than 230°C, and the temperature is maintained at 200°C-220°C for 20s- For 40s, keep the temperature at 220°C-230°C for 3s-5s; in the cooling stage, the temperature will naturally drop after reaching the highest temperature, and the falling speed is 2°C/S-5°C/S.

Beneficial technical effect of the present invention is:

(1) The present invention uses a magnetic tray and a snap cover as a magnetic fixture to firmly fix the flexible circuit board, so there is no need to use silica gel or double-sided tape during the entire surface mount process, so the flexible circuit board will not be damaged. Silica gel remains on the circuit board, and there will be no dust on the tray, which greatly improves the service life of the tray and eliminates the cost of cleaning.

(2) The stainless steel press-button cover plate has strong toughness, which makes it not easy to break when it is attached, which saves a lot of labor costs and improves the service life of the press-button cover plate.

(3) The thickness of the stainless steel press button cover plate is very thin, and at the same time, after the steel mesh is partially thinned by the etching process, it will not cause excessive solder paste, which ensures the quality of the entire process.

(4) The flexible circuit board is clamped in the middle by a magnetic fixture, which has a good heat insulation effect, so that the flexible circuit board will not be deformed during the reflow soldering process, thus ensuring the soldering of components and flexible circuit boards There will be no virtual welding and empty welding in the connection between the points.

(5) Since the silicone layer or double-sided tape is not used as the fixing tool for the flexible circuit board in the whole process, the fixture can be shared during the printing, patch and reflow soldering stages, which not only simplifies the process, but also saves tools Cost and labor cost, and after the process is simplified, the probability of affecting product quality due to human factors is greatly reduced, and the quality of the product is greatly improved.

(6) After the SMT process is completed, the flexible circuit board can be easily removed only by separating the snap cover from the magnetic tray. There is adhesion between them and the circuit board is broken.

【Description of drawings】

Fig. 1 is the assembly schematic diagram of flexible circuit board and magnetic jig of the present invention;

Figure 2 is a perspective view of the positioning base of the present invention;

Fig. 3 is the front view of magnetic pallet of the present invention;

Fig. 4 is the front view of the snap cover of the present invention;

Figure 5 is a front view of the flexible circuit board;

Fig. 6 is a front view of the stencil of the present invention.

Among them: 1: Magnetic tray; 11: Base positioning hole; 12: Tray positioning hole; 13: Positioning groove; 14: Groove; 15: Cooling hole; 16: Permanent magnet; 2: Flexible circuit board; 21: Optical mark Point; 22: FPC positioning hole; 3: press button cover plate; 31: cover plate positioning hole; 32: marking positioning hole; 33: slot hole; 4: positioning base; 41: positioning pin; 5: steel mesh; 51: Ladder layer; 52: printing holes.

【Detailed ways】

The invention relates to a magnetic jig and a steel net used in the surface mounting process of a flexible circuit board and a surface mounting process of a flexible circuit board using the magnetic jig and steel net. The magnetic jig includes a magnetic tray and a buckle The cover plate and the snap cover are thin and highly tough stainless steel sheets, and the flexible circuit board is sandwiched between the magnetic tray and the press cover to complete the entire process of solder paste printing, patch and reflow soldering.

Embodiment one:

As shown in Figures 1-5, the magnetic fixture involved in the present invention includes a magnetic tray 1, a snap-on cover 3 and a positioning base 4, and the positioning base 4 is provided with two positioning pins 41 (three positioning pins can also be provided). or more than three locating pins); the magnetic tray 1 is made of blackened aluminum alloy and then inlaid with a permanent magnet 16 that can withstand 300°C high temperature. high temperature resistant permanent magnet), the magnetic tray 1 is provided with a base positioning hole 11 corresponding to the base positioning pin 41 and a tray positioning hole 12, and the magnetic tray 1 is also provided with a flexible circuit board 2 Positioning grooves 13 with the same shape, grooves 14 and cooling holes 15 corresponding to the positions of the components on the circuit board, the grooves 14 can be filled with components when mounting the flexible circuit board 2 on both sides; The snap cover 3 described above is a stainless steel sheet that can be attracted by a magnet. The thickness of the stainless steel sheet is 0.05 mm, and the hardness is 480 Vickers hardness. The steel sheet is made of austenitic stainless steel or martensitic stainless steel through cold working. The chemical composition of the steel is as follows:

C: Si: Mn: P: S: Ni: Cr: Fe = 0.08: 0.7: 1.00: 0.04: 0.025: 6.00: 16.0: 76.0, and the rest are other impurities. The press cover 3 is provided with a cover positioning hole 31 corresponding to the tray positioning hole 12, and the press cover 3 is also provided with a marking positioning hole 32 corresponding to the optical marking point 21 for positioning the flexible circuit board 2 and Slot holes 33 for printing solder paste and patches on the flexible circuit board 2. The flexible circuit board 2 is provided with optical marking points 21 for printing and positioning and FPC positioning points corresponding to the tray positioning holes 12 of the magnetic tray 1. Hole 22.

Before printing, the magnetic tray 1 is fixed on the positioning base 4, and then the flexible circuit board 2 and the snap cover 3 are placed in sequence, so that the positioning hole 11 of the base of the magnetic tray 1 is buckled with the positioning pin 41, so that the corresponding Cover plate positioning hole 31, FPC positioning hole 22 coincides with tray nail positioning hole 12, let mark positioning hole 32 coincide with optical marking point 21 on the flexible circuit board, after the positioning is accurate, place the flexible circuit board 2 and the magnetic fixture together Take it out from the positioning base 4 and put it into the printed track. The fixing direction of the flexible circuit board 2 is the vertical direction of the chip components, and the horizontal direction of the SOT and SOP. After the flexible printed circuit board 2 enters a certain printing track, the printing machine equipped with the stencil 5 starts to print the solder paste.

As shown in Figure 6, the steel mesh 5 is a non-magnetic steel sheet with a thickness of 0.08 mm, on which a stepped layer 51 having the same shape as the snap-on cover plate 3 is etched, and the depth of the stepped layer 51 is 0.05 mm. The stepped layer 51 is made by local thinning by etching process; the stencil 5 is provided with a printing hole 52 for printing solder paste on the flexible circuit board 2, and the printing hole 52 is connected with the flexible circuit board 2 Corresponding to the part to be printed with solder paste, the printing hole 52 is made by laser cutting.

The printing machine is provided with an optical positioning system, which can identify the optical positioning point 21 on the flexible circuit board 2; the scraper of the printing machine is a steel scraper, and the angle between the scraper and the flexible circuit board is 60° The printing direction of the printing machine is the left-right direction (or the front-back direction), the printing speed is 10mm/s, and the printing pressure is 0.1kg/cm ² ; the demoulding speed of the printing machine is 0.1mm/s.

After the printing is completed, the flexible circuit board enters the placement machine with the magnetic fixture, and the placement machine performs component placement on the flexible circuit board; after the placement is completed, it enters the reflow oven to weld the high-temperature components and the solder joints on the flexible circuit board Fixed; after the reflow soldering is completed, take out the cooled magnetic fixture from the track, take off the snap cover, take out the flexible circuit board, and the magnetic fixture enters the next cycle.

The placement machine is provided with a suction nozzle, the suction of the suction nozzle becomes 0 after the placement is completed, and after the suction becomes 0, the suction nozzle is removed from the flexible circuit board.

The reflow soldering process includes a preheating stage, a constant temperature stage, a soldering stage and a cooling stage. In the preheating stage, the temperature rises from room temperature to 110 degrees, and the temperature rise rate is 1°C/S; in the constant temperature stage, the temperature rises from 110 degrees to 150 degrees. , the holding time is 60s; in the welding stage, the temperature rise rate is 1°C/S; the maximum temperature is 220°C, the temperature is maintained at 200°C for 20s, and the temperature is maintained at 220°C for 3s; 2°C/S.

Embodiment two:

As shown in Figures 1-5, the magnetic fixture involved in the present invention includes a magnetic tray 1, a snap-on cover 3 and a positioning base 4, and the positioning base 4 is provided with two positioning pins 41 (three positioning pins can also be provided). or more than three locating pins); the magnetic tray 1 is made of blackened aluminum alloy and then inlaid with a substrate made of permanent magnets resistant to 300°C high temperature (also can be made of synthetic stone, high temperature resistant glass fiber and inlaid with durable magnets). high-temperature permanent magnets), the magnetic tray 1 is provided with a base positioning hole 11 corresponding to the base positioning pin 41 and a tray positioning hole 12, and the magnetic tray 1 is also provided with a The same positioning groove 13, the groove 14 and the cooling hole 15 corresponding to the position of the components on the circuit board, the groove 14 can be filled with components when the flexible circuit board 2 is mounted on both sides; The press button cover plate 3 is a stainless steel sheet that can be attracted by a magnet. The thickness of the stainless steel sheet is 0.1mm, and the hardness is 700 Vickers hardness. The steel sheet is made of austenitic stainless steel or martensitic stainless steel through cold processing. The chemical composition of steel is as follows:

C:Si:Mn:P:S:Ni:Cr:Fe=0.15:1.00:2.00:0.045:0.030:10.00:19.0:67.0, and the rest are other impurities. The press cover 3 is provided with a cover positioning hole 31 corresponding to the tray positioning hole 12, and the press cover 3 is also provided with a marking positioning hole 32 corresponding to the optical marking point 21 for positioning the flexible circuit board 2 and Slot holes 33 for printing solder paste and patches on the flexible circuit board 2. The flexible circuit board 2 is provided with optical marking points 21 for printing and positioning and FPC positioning points corresponding to the tray positioning holes 12 of the magnetic tray 1. Hole 22.

Before printing, the magnetic tray 1 is fixed on the positioning base 4, and then the flexible circuit board 2 and the snap cover 3 are placed in sequence, so that the positioning hole 11 of the base of the magnetic tray 1 is buckled with the positioning pin 41, so that the corresponding Cover plate positioning hole 31, FPC positioning hole 22 coincides with tray nail positioning hole 12, let mark positioning hole 32 coincide with optical marking point 21 on the flexible circuit board, after the positioning is accurate, place the flexible circuit board 2 and the magnetic fixture together Take it out from the positioning base 4 and put it into the printed track. The fixing direction of the flexible circuit board 2 is the vertical direction of the chip components, and the horizontal direction of the SOT and SOP. After the flexible printed circuit board 2 enters a certain printing track, the printing machine equipped with the stencil 5 starts to print the solder paste.

As shown in Figure 6, the steel mesh 5 is a non-magnetic steel sheet with a thickness of 0.1 mm, etched with a stepped layer 51 of the same shape as the press button cover plate 3, and the depth of the stepped layer 51 is 0.1 mm. The stepped layer 51 is made by local thinning by etching process; the stencil 5 is provided with a printing hole 52 for printing solder paste on the flexible circuit board 2, and the printing hole 52 is connected with the flexible circuit board 2 Corresponding to the part to be printed with solder paste, the printing hole 52 is made by laser cutting.

The printing machine is provided with an optical positioning system, which can identify the optical positioning point 21 on the flexible circuit board 2; the scraper of the printing machine is a steel scraper, and the included angle between the scraper and the flexible circuit board is 75° The printing direction of the printing machine is the left-right direction (or the front-rear direction), the printing speed is 25mm/s, and the printing pressure is 0.3kg/cm ² ; the demoulding speed of the printing machine is 0.2mm/s.

After the printing is completed, the flexible circuit board enters the placement machine with the magnetic fixture, and the placement machine performs component placement on the flexible circuit board; after the placement is completed, it enters the reflow oven to weld the high-temperature components and the solder joints on the flexible circuit board Fixed; after the reflow soldering is completed, take out the cooled magnetic fixture from the track, take off the snap cover, take out the flexible circuit board, and the magnetic fixture enters the next cycle.

The placement machine is provided with a suction nozzle, the suction of the suction nozzle becomes 0 after the placement is completed, and after the suction becomes 0, the suction nozzle is removed from the flexible circuit board.

The reflow soldering process includes a preheating stage, a constant temperature stage, a soldering stage and a cooling stage. In the preheating stage, the temperature rises from room temperature to 110 degrees, and the temperature rise rate is 2°C/S; in the constant temperature stage, the temperature rises from 110 degrees to 150 degrees. , the holding time is 120s; in the welding stage, the temperature rise rate is 2°C/S; the maximum temperature is 230°C, the temperature is maintained at 220°C for 40s, and the temperature is maintained at 230°C for 5s; 5°C/S.

In a word, the present invention mainly provides a kind of press button cover which is very thin and can fix the flexible wire board, so that the magnetic jig does not need to be fixed by silica gel. It is this function of press button cover that makes the whole The placement process of the flexible circuit board is greatly simplified, and the production cost is greatly reduced.

Because steel has advantages that iron does not have, but ordinary steel is not attracted by magnets. It is precisely because of this that in production applications, it is difficult for people to solve the problem that the press button cover can be attracted by magnets. The inventor is doing this In the course of the invention, many steel sheets that can be attracted by magnets have been developed. Although the present invention only provides a stainless steel as the material of the press cover, it cannot be understood as being limited to stainless steel as the material of the press cover of the present invention. , any steel sheet that can be attracted by a magnet falls within the protection scope of the present invention within the range of thickness and hardness described in the present invention.

Claims (10)

1. The magnetic fixture used in the surface mount process (SMT) of the flexible circuit board (FPC), the magnetic fixture includes a press button cover plate and a magnetic tray, and the magnetic tray is a substrate with magnetism, The snap-on cover is a thin metal plate that can be attracted by the substrate. There are slots for printing solder paste and patch on the snap-on cover. It is characterized in that the snap-on cover is A piece of steel attracted by a magnet. 2. The magnetic fixture according to claim 1, wherein the steel sheet is a stainless steel sheet with a thickness of 0.05-0.1mm and a hardness of 480-700 Vickers hardness, and the steel sheet is made of Austenitic Body stainless steel or martensitic stainless steel is made by cold working. The chemical composition of the selected steel is as follows: C:Si:Mn:P:S:Ni:Cr:Fe=(0.08-0.15):(0.75-1.00):(1.00 -2.00):(0.04-0.045):(0.025-0.030):(6.00-10.00):(16.0-19.0):(68.0-76.0). 3. The magnetic jig according to claim 1, wherein the magnetic tray is a substrate made of aluminum alloy, synthetic stone, glass fiber board and other materials embedded with high temperature resistant permanent magnets. 4. The magnetic jig according to claim 1, wherein the magnetic jig also includes a positioning base, at least two positioning pins are arranged on the positioning base, and the magnetic tray is provided with a The base positioning hole and the tray positioning hole corresponding to the positioning pin, the cover plate positioning hole corresponding to the tray positioning hole is provided on the press button cover plate, and the FPC positioning hole is provided on the flexible circuit board. The positioning hole of the tray, the positioning hole of the cover plate and the positioning hole of the FPC are matched; the pressing cover plate is also provided with a marking positioning hole corresponding to the optical marking point used for positioning the flexible circuit board. 5. The magnetic jig according to claim 1, characterized in that, the magnetic tray is also provided with cooling holes, positioning grooves with the same shape as the flexible circuit board or corresponding to the position of the components on the circuit board It can fill the grooves of components when mounting on both sides of flexible circuit boards. 6. The steel mesh used in the surface mount process (SMT) of the flexible circuit board (FPC), is characterized in that the steel mesh is a non-magnetic steel sheet with a thickness of 0.08mm or more than 0.08mm, etched with The stepped layer with the same shape as the press-button cover plate has the same depth as the thickness of the steel sheet, and the stepped layer is made by partially thinning the etching process; the steel mesh is provided with a flexible circuit board A printing hole for printing solder paste. The printing hole corresponds to the part of the flexible circuit board that needs to be printed with solder paste. The printing hole is made by laser cutting. 7. Use the flexible circuit board (FPC) surface mount process (SMT) of the flexible circuit board (FPC) surface mount technology (SMT) that uses as described in claim 1-5 any one magnetic fixture and steel net as claimed in claim 6, it is characterized in that, its processing step is as follows : (1) Fix the magnetic tray on the positioning base before printing, and then place the flexible circuit board and the press cover in turn, so that the positioning hole of the base of the magnetic tray is buckled with the positioning pin, and the corresponding positioning hole of the cover plate, FPC The positioning hole coincides with the positioning hole of the pallet nail, so that the marking positioning hole coincides with the optical marking point on the flexible circuit board. After the positioning is accurate, take the flexible circuit board and the magnetic jig together from the positioning base and put them into the printing track, etc. After the flexible circuit board enters a certain printing track, the printing machine equipped with a stencil starts to print solder paste. (2) After the printing is completed, the flexible circuit board enters the placement machine with the magnetic fixture, and the placement machine performs component placement on the flexible circuit board. (3) After the placement is completed, enter the reflow oven to weld and fix the high-temperature components and the solder joints on the flexible circuit board. (4) After the reflow soldering is completed, take out the cooled magnetic jig from the track, take off the snap cover, take out the flexible circuit board, and the magnetic jig enters the next cycle. 8. The surface mount process of flexible circuit board according to claim 7, characterized in that, the fixing direction of the flexible circuit board is the vertical direction of the chip element, the horizontal direction of SOT and SOP; An optical positioning system is provided, which can identify the optical positioning points on the flexible circuit board; the scraper of the printing machine is a steel scraper, and the angle between the scraper and the flexible circuit board is 60-75 degrees; The printing direction is left-right or front-back direction, the printing speed is 10mm/s-25mm/s, the printing pressure is 0.1kg/cm ² -0.3kg/cm ² ; the demoulding speed of the printing machine is 0.1mm/s-0.2mm/s . 9. The surface mount process for flexible printed circuit boards according to claim 7, characterized in that, said placement machine is provided with a suction nozzle, the suction of the suction nozzle becomes 0 after the completion of the placement, and after the suction becomes 0 , the suction nozzle is removed from the flexible circuit board. 10. The surface mount process for flexible circuit boards according to claim 7, wherein the reflow soldering process in the step (3) includes a preheating stage, a constant temperature stage, a soldering stage and a cooling stage, and the preheating stage , the temperature rises from room temperature to 110 degrees, and the temperature rise rate is 1°C/S-2°C/S; in the constant temperature stage, the temperature rises from 110°C to 150°C, and the holding time is 60s-120s; in the welding stage, the temperature rise rate is 1 -2°C/S; the maximum temperature is less than 230°C, the temperature is maintained at 200°C-220°C for 20s-40s, and the temperature is maintained at 220°C-230°C for 3s-5s; in the cooling stage, the temperature naturally drops after reaching the maximum temperature, and the rate of decline is 2 ℃/S-5℃/S.

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