FPC is also called flexible circuit board. The PCBA assembly and welding process of FPC is very different from that of rigid circuit board. Because the hardness of FPC board is not enough, it is relatively soft. If you do not use a dedicated carrier board, it cannot be fixed and transmitted. Basic SMT processes such as printing, placement, and furnace cannot be completed.
One. FPC pretreatment
The FPC board is soft, and it is generally not vacuum-packed when it leaves the factory. It is easy to absorb moisture in the air during transportation and storage. It needs to be pre-baked before SMT is put into the line to force the moisture out slowly. Otherwise, under the high temperature impact of reflow soldering, the moisture absorbed by the FPC will quickly vaporize and become water vapor to protrude from the FPC, which will easily cause defects such as FPC delamination and blistering.
The pre-baking conditions are generally at a temperature of 80-100°C and a time of 4-8 hours. Under special circumstances, the temperature can be adjusted to above 125°C, but the baking time needs to be shortened accordingly. Before baking, be sure to make a sample test to determine whether the FPC can withstand the set baking temperature, or consult the FPC manufacturer for suitable baking conditions. When baking, FPC should not be stacked too much. 10-20PNL is more suitable. Some FPC manufacturers will put a piece of paper between each PNL for isolation. It is necessary to confirm whether this isolation paper can withstand the set baking Temperature, if it is not necessary to remove the release paper, then bake. The baked FPC should have no obvious discoloration, deformation, warping and other defects, and it can be put into line only after being qualified by IPQC.
two. Production of dedicated carrier board
According to the CAD file of the circuit board, read the hole positioning data of the FPC to manufacture the high-precision FPC positioning template and the special carrier board, so that the diameter of the positioning pin on the positioning template is the same as the positioning hole on the carrier board and the hole diameter of the FPC. match. Many FPCs are not of the same thickness because they want to protect part of the circuit or for design reasons. Some places are thicker, some places are thinner, and some have reinforced metal plates. Therefore, the joint of the carrier board and the FPC needs to be The actual situation is to process, polish and dig grooves, which is to ensure that the FPC is flat during printing and placement. The material of the carrier board requires light and thin, high strength, low heat absorption, fast heat dissipation, and small warpage deformation after multiple thermal shocks. Commonly used carrier materials include synthetic stone, aluminum plate, silica gel plate, and special high-temperature magnetized steel plate.
three. production process.
Here we take a common carrier board as an example to detail the SMT points of FPC. When using silicone plates or magnetic jigs, the fixing of FPC is much more convenient, without the need to use tape. The process points of printing, patching, welding and other processes are the same.
1. Fixing of FPC:
Before SMT, the FPC needs to be accurately fixed on the carrier board. It is particularly important to note that the shorter the storage time between printing, mounting and soldering after the FPC is fixed on the carrier board, the better. There are two kinds of carrier boards with and without positioning pins. The carrier board without positioning pins needs to be used in conjunction with the positioning template with positioning pins. First put the carrier board on the positioning pins of the template, so that the positioning pins are exposed through the positioning holes on the carrier board, and put the FPC piece by piece. Fix the exposed positioning pins with tape, and then separate the carrier board from the FPC positioning template for printing, patching and welding. The carrier board with positioning pins has been fixed with several spring positioning pins about 1.5mm in length. The FPC can be directly put on the spring positioning pins of the carrier board one by one, and then fixed with tape. In the printing process, the spring positioning pin can be completely pressed into the carrier plate by the steel mesh without affecting the printing effect.
Method one (fixed with single-sided tape): Use thin, high-temperature single-sided tape to fix the four sides of the FPC on the carrier board to prevent the FPC from shifting and warping. The viscosity of the tape should be moderate, and it must be easy to peel off after reflow soldering. There is no glue residue on it. If you use an automatic tape machine, you can quickly cut tapes of the same length, which can significantly improve efficiency, save costs, and avoid waste.
Method two (fixed with double-sided tape): first use high-temperature resistant double-sided tape to stick on the carrier board, the effect is the same as the silicone plate, and then paste the FPC to the carrier board, pay special attention to the tape viscosity not too high, otherwise it will peel off after reflow soldering It is easy to cause the FPC to tear. After repeated ovens, the viscosity of the double-sided tape will gradually decrease. If the viscosity is too low to reliably fix the FPC, it must be replaced immediately. This station is the key station to prevent the FPC from getting dirty and requires finger cots to work. Before the carrier is reused, it needs to be properly cleaned. It can be wiped with a non-woven cloth dipped in detergent, or an anti-static sticky roller can be used to remove surface dust, tin beads and other foreign objects. Don't use too much force when picking up the FPC. The FPC is fragile and prone to creases and breaks.
2. FPC solder paste printing:
FPC does not have very special requirements for the composition of solder paste. The size and metal content of the solder ball particles are subject to whether there are fine-pitch ICs on the FPC. However, FPC has higher requirements for the printing performance of solder paste, and the solder paste should have excellent Thixotropy, the solder paste should be easy to print and release and firmly adhere to the surface of the FPC, and there will be no defects such as poor release, blocking the stencil leakage or collapse after printing.
Because the FPC is loaded on the carrier board, there is a high temperature resistant tape for positioning on the FPC, which makes the plane inconsistent, so the printed surface of the FPC cannot be as flat as the PCB and the thickness and hardness are consistent. Therefore, it is not suitable to use a metal scraper, but a hardness of 80 -90 degree polyurethane type scraper. The solder paste printer is best equipped with an optical positioning system, otherwise it will have a greater impact on the printing quality. Although the FPC is fixed on the carrier board, there will always be some tiny gaps between the FPC and the carrier board. The biggest difference between the board, so the setting of equipment parameters will also have a greater impact on the printing effect.
The printing station is also a key station to prevent the FPC from being dirty. It is necessary to wear finger cots to work, while keeping the station clean, frequently rubbing the steel mesh to prevent the solder paste from contaminating the FPC's gold fingers and gold-plated buttons.
3. FPC patch:
According to the characteristics of the product, the number of components and the placement efficiency, medium and high-speed placement machines can be used for placement. Since there is an optical MARK mark for positioning on each FPC, there is not much difference between SMD mounting on FPC and mounting on PCB. It should be noted that although the FPC is fixed on the carrier board, its surface cannot be as flat as a PCB hard board. There must be a partial gap between the FPC and the carrier board. Therefore, the suction nozzle drop height, blowing pressure, etc. It needs to be set accurately, and the moving speed of the nozzle needs to be reduced. At the same time, FPCs are mostly connected boards, and the yield of FPCs is relatively low. Therefore, it is normal for the whole PNL to contain some bad PCS. This requires the placement machine to have the BAD MARK recognition function. Otherwise, the production of such non-integrated When PNL is a good board, the production efficiency will be greatly reduced.
4. Reflow soldering of FPC:
The forced hot air convection infrared reflow oven should be used, so that the temperature on the FPC can change more uniformly, reducing the occurrence of poor soldering. If you use a single-sided tape, because you can only fix the four sides of the FPC, the middle part is deformed under hot air, the pad is easily inclined, and the molten tin (liquid tin at high temperature) will flow, resulting in empty soldering, continuous soldering, Tin beads make the process defect rate higher.
1) Temperature curve test method:
Due to the different heat absorption properties of the carrier board and the different types of components on the FPC, the temperature rises at different speeds after being heated during the reflow soldering process, and the heat absorbed is also different. Therefore, carefully set the temperature curve of the reflow oven to improve the quality of soldering. Great influence. A more reliable method is to place two FPC-equipped carrier boards before and after the test board according to the carrier board interval during actual production, and at the same time mount components on the FPC of the test carrier board, and use high-temperature solder wire to test the temperature. The probe is welded on the test point, and at the same time, the probe wire is fixed on the carrier board with a high temperature resistant tape. Note that the high temperature resistant tape cannot cover the test point. The test points should be selected near the solder joints and QFP pins on each side of the carrier board, so that the test results can better reflect the real situation.
2) Setting of temperature curve:
In the furnace temperature debugging, because the FPC's temperature uniformity is not good, it is best to use the temperature curve method of heating/heat preservation/reflow, so that the parameters of each temperature zone are easier to control, and the FPC and components are less affected by thermal shock some. According to experience, it is best to adjust the furnace temperature to the lower limit of the solder paste technical requirements. The wind speed of the reflow furnace generally adopts the lowest wind speed that the furnace can use, and the chain of the reflow furnace should be stable without jitter.
5. FPC inspection, testing and sub-board:
Since the carrier plate absorbs heat in the furnace, especially the aluminum carrier plate, the temperature is higher when it is out of the furnace, so it is best to add a forced cooling fan at the furnace outlet to help cool down quickly. At the same time, operators need to wear insulated gloves to avoid being burned by the high temperature carrier. When taking the soldered FPC from the carrier board, the force should be even, and brute force should not be used to prevent the FPC from being torn or creases.
The removed FPC is visually inspected under a magnifying glass of more than 5 times, focusing on the inspection of residual glue on the surface, discoloration, golden finger tin, tin bead, IC pin empty welding, continuous welding and other problems. Because the surface of FPC cannot be very smooth, which makes the AOI misjudgment rate very high, FPC is generally not suitable for AOI inspection, but by using special test fixture, FPC can complete ICT and FCT tests.
Since FPCs are mostly connected boards, it may be necessary to split the boards before testing ICT and FCT. Although the splitting can be done with tools such as blades and scissors, the efficiency and quality of the operations are low, and the scrap rate is high. If it is mass production of special-shaped FPC, it is recommended to make a special FPC stamping and splitting die, which can greatly improve the work efficiency. At the same time, the edges of the punched FPC are neat and beautiful, and the internal stress generated when stamping and cutting the board is very low. Can effectively avoid solder joint cracking.
In the assembly and welding process of PCBA flexible electronics, the precise positioning and fixing of FPC are the key points. The key to fixing is to make a suitable carrier board. Followed by FPC pre-baking, printing, placement and reflow soldering. Obviously, the SMT process of FPC is much more difficult than PCB hard board, so it is necessary to accurately set process parameters. At the same time, strict production process management is also important. It is necessary to ensure that operators strictly implement every requirement on SOP and follow the line Engineers and IPQC should strengthen inspections, discover the abnormal conditions of the production line in time, analyze the reasons and take necessary measures to control the defect rate of the FPC SMT production line within dozens of PPM.
In the PCBA production process, a lot of machinery and equipment are needed to assemble a board. Often the quality level of a factory's machinery and equipment directly determines the manufacturing capacity.
The basic equipment required for PCBA production includes solder paste printer, placement machine, reflow soldering, AOI detector, component trimming machine, wave soldering, tin furnace, washing machine, ICT test fixture, FCT test fixture, For aging test racks, PCBA processing plants of different sizes have different equipment.
4. PCBA production equipment
1. Solder paste printing machine
Modern solder paste printers generally consist of plate loading, solder paste addition, imprinting, and circuit board transfer. Its working principle is: first fix the circuit board to be printed on the printing positioning table, and then the solder paste or red glue is printed on the corresponding pad through the stencil by the left and right scrapers of the printing machine. The transfer station is input to the placement machine for automatic placement.
2. Mounter
Mounting machine: also known as "mounting machine", "Surface Mount System" (Surface Mount System), in the production line, it is configured after the solder paste printer, by moving the placement head to accurately mount the surface components A device placed on the PCB pad. Divided into two kinds of manual and automatic.
3. Reflow soldering
There is a heating circuit inside the reflow soldering, which heats air or nitrogen to a high enough temperature and blows it to the circuit board where the component has been attached, so that the solder on both sides of the component is melted and bonded to the motherboard. The advantage of this process is that the temperature is easy to control, oxidation can be avoided during welding, and the manufacturing cost is easier to control.
4. AOI detector
The full name of AOI (Automatic Optic Inspection) is automatic optical inspection, which is based on optical principles to detect common defects encountered in welding production. AOI is a new type of testing technology that is emerging, but it is developing rapidly, and many manufacturers have introduced AOI testing equipment. During automatic inspection, the machine automatically scans the PCB through the camera, collects images, compares the tested solder joints with the qualified parameters in the database, after image processing, checks out the defects on the PCB, and displays/marks the defects through the display or automatic signs Come out for repair by maintenance personnel.
5. Component trimming machine
It is used to cut and deform the pin components.
6. Wave soldering
Wave soldering is to make the soldering surface of the plug-in board directly contact the high-temperature liquid tin to achieve the purpose of soldering. The high-temperature liquid tin maintains a slope, and a special device makes the liquid tin form a wave-like phenomenon, so it is called "wave soldering". The main material is solder bars.
7. Tin furnace
In general, tin furnace refers to a welding tool used in electronic welding. For discrete component circuit boards, the welding consistency is good, the operation is convenient, fast, and the work efficiency is high. It is a good helper for your production and processing.
8. Plate washer
It is used to clean the PCBA board and can remove the residue on the board after soldering.
9. ICT test fixture
ICT Test is mainly *test probe contacting the test points of the PCB layout to detect the PCBA circuit open circuit, short circuit, and welding condition of all parts
10. FCT test fixture
FCT (Functional Test) It refers to the test target board (UUT: Unit Under Test) that provides a simulated operating environment (stimulus and load) to make it work in various design states, so as to obtain the parameters of each state to verify the UUT The function is good or bad test method. Simply put, it is to load a suitable stimulus to the UUT and measure whether the response of the output end meets the requirements.
11. Aging test frame
The burn-in test rack can test PCBA boards in batches, and test PCBA boards with problems through long-term simulation of user operations.
PCBA outsourcing processing refers to PCBA processing manufacturers sending PCBA orders to other powerful PCBA processing manufacturers. So, what are the general requirements for PCBA outsourcing processing?
1. Bill of Materials
Insert or mount components in strict accordance with the bill of materials, PCB silk screen and outsourcing processing requirements. When the material does not match the bill, PCB silk screen, or contradicts the process requirements, or the requirements are ambiguous and cannot be operated, it should be timely Contact our company to confirm the correctness of materials and process requirements.
2. Anti-static requirements
1. All components are treated as electrostatic sensitive devices.
2. All personnel who come into contact with components and products should wear anti-static clothes, anti-static bracelets, and anti-static shoes.
3. During the raw materials entering the factory and in the warehouse, the electrostatic sensitive devices are all anti-static packaging.
4. During the operation, use an anti-static work surface, and use anti-static containers for components and semi-finished products.
5. The welding equipment is grounded reliably, and the electric soldering iron adopts anti-static type. All must be tested before use.
6. The semi-finished PCB board is stored and transported in an anti-static box, and the isolation material uses anti-static pearl cotton.
7. The whole machine without shell uses anti-static packaging bag.
Third, the requirements for the orientation of the component appearance mark
1. Polarity components are inserted according to polarity.
2. For components with silk-screened on the side (such as high-voltage ceramic capacitors), the silk-screen faces right when they are inserted vertically; when they are inserted horizontally, the silk-screen faces down. When the components (not including the chip resistors) silk-printed on the top are inserted horizontally, the font direction is the same as that of the PCB screen printing; when inserted vertically, the font top faces right.
3. When the resistance is inserted horizontally, the error color ring faces right; when the resistance is inserted vertically, the error color ring faces down; when the resistance is inserted vertically, the error color ring faces the board.
Four, welding requirements
1. The pin height of the plug-in component on the soldering surface is 1.5～2.0mm. SMD components should be flat against the board surface, and the solder joints should be smooth, without burrs, and slightly arc-shaped. The solder should exceed 2/3 of the height of the solder end, but should not exceed the height of the solder end. Less tin, ball-shaped solder joints or solder covered patches are all bad;
2. The height of the solder joints: the height of solder climbing pins is not less than 1mm for single-sided boards, and not less than 0.5mm for double-sided boards and requires tin penetration.
3. Solder joint shape: Conical shape and covering the entire pad.
4. Solder joint surface: smooth, bright, free of black spots, flux and other impurities, and no defects such as spikes, pits, pores, exposed copper, etc.
5. Solder joint strength: fully wetted with pads and pins, no false soldering or false soldering.
6. Solder joint cross-section: the component's cutting foot should not be cut to the solder part as much as possible, and there should be no cracking on the contact surface of the pin and the solder. There are no spikes or barbs at the cross section.
7. Needle base welding: The needle base must be mounted on the bottom board, and the position is correct and the direction is correct. After the needle base is welded, the bottom floating height should not exceed 0.5mm, and the skew of the base body should not exceed the silk screen frame. Rows of needle holders should also be kept neat and not allowed to be misaligned or uneven
Five, transportation
To prevent PCBA damage, the following packaging should be used during transportation:
1. Container: anti-static turnover box.
2. Isolation material: anti-static pearl cotton.
3. Placement spacing: there is a distance greater than 10mm between the PCB board and the board, between the PCB board and the box.
4. Placement height: There is a space greater than 50mm from the top surface of the turnover box to ensure that the turnover box is not pressed against the power supply, especially the power supply of the wire.
Six, washing plate requirements
The board surface should be clean and free of tin beads, component pins, and stains. Especially at the solder joints on the plug-in surface, there should not be any dirt left by soldering. The following devices should be protected when washing the board: wires, connecting terminals, relays, switches, polyester capacitors and other easily corrosive devices, and the relay is strictly prohibited from cleaning with ultrasonic.
7. All components are not allowed to go beyond the edge of the PCB board after installation.
8. When PCBA passes the furnace, because the pins of the plug-in components are washed by the tin flow, some plug-in components will be tilted after the furnace is soldered, causing the body of the component to exceed the silk screen frame. Therefore, the repair soldering personnel after the tin furnace is required to properly perform it. Fix.
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