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One-stop solution from flexible PCB Manufacturing to flex circuit assembly
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FPC is short for flexible printed circuit . The printed circuit board assembly (PCBA) and soldering process of FPCs (Flex 918kiss plus rebat or flex circuit Assembly ) are very different from that of the rigid PCB board. Because of the lack of hardness of an Flex PCB, it is relatively flexible. If you do not use a dedicated carrier board, it cannot be fixed and transmitted; also, basic surface-mount technology (SMT) processes such as printing, placement, and reflow oven procedures cannot be completed
The FPC board is relatively soft, and it is generally not vacuum packed when it leaves the factory. It can thus easily absorb moisture in the air during transportation and storage, so it needs to be pre-baked before it undergoes the SMT process 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 protruding from the FPC, which will likely cause defects such as FPC delamination and blistering.
The pre-baking conditions are generally at a temperature of 80-100°C and are for a time of 4-8 hours. Under special circumstances, the temperature can be adjusted to above 125°C, but the baking time must 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, the FPC should not be stacked too much—10-20PNL is 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, then bake if it is not necessary to remove the release paper. The baked FPC should have no obvious discoloration, deformation, warping, or other defects, and it can be put into line only after being qualified by IPQC.
According to the CAD file for the circuit board, read the drilling hole layer of the FPC to manufacture the high-precision FPC positioning template and the special carrier fixture. Ensure that the diameter of the pin on the positioning template is the same as the positioning hole on the carrier board and the hole diameter of the FPC. Many FPCs are not the same thickness; for example, some might be different to protect part of the circuit, and some for design reasons. Some sections are thicker, some sections are thinner, and some have stiffer materials like FR-4 or PI Aluminum.
Therefore, the joint of the carrier board and the FPC needs to follow actual situation in order process, as well as to polish and dig grooves, which ensures that the FPC is flat during printing and placement. The material of the carrier board requires lightness and thinness, high strength, low heat absorption, fast heat dissipation, and little 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.
Here we use a common carrier board as an example to explain the SMT points of FPCs. When using silicone plates or magnetic fixtures, the fixing of FPC is much more convenient, without the need to use tape. The process points of printing, patching, soldering and other processes are the same.
Before surface mounting, the FPC needs to be accurately fixed to 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: those 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 such that the positioning pins are exposed through the 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 soldering. The carrier board has been fixed with several spring positioning pins about 1.5mm in length. The FPC can be directly put on the spring positioning pins one by one, and then fixed with tape. During the printing process, the spring positioning pin can be completely pressed into the carrier plate by the steel mesh without negative results.
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 to the carrier fixture (the effect is the same as the silicone plate), and then paste the FPC to the carrier fixture. Pay attention to the viscosity of the tape; it should not be too high, otherwise it will peel off after reflow. It is easy to cause the FPC to tear. After passing through several ovens, the viscosity of the double-sided tape will gradually decrease. it must be replaced immediately.
This working station is to prevent the FPC from becoming dirty, and it is necessary to wear finger cots for work. Before the carrier board 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.
FPCs do 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 integrated circuits (ICs) on the FPC. However, FPCs have higher requirements for the printing performance of solder paste, and the solder paste should have excellent thixotropy; solder paste should be easy to print and release and firmly adhere to the surface of the FPC, and there should 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. This 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 key to preventing the FPC from being dirty. It is necessary to wear finger cots for 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.
According to the characteristics of the product, the number of components, and the placement efficiency required, medium and high-speed placement machines can be used for placement. Since there is an optical Mark for positioning on each FPC, there is not much difference between surface mount device (SMD) mounting on FPCs and rigid PCB mounting. It should be noted that although the FPC is fixed on the carrier board, its surface cannot be as flat as a rigid PCB.
There will definitely be a partial gap between the FPC and the carrier board, so the suction nozzle drop height, blowing pressure, etc. need 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.
The forced hot air convection infrared reflow oven should be used, so that the temperature on the FPC can be changed more uniformly, reducing the occurrence of poor soldering. If you use 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 or continuous soldering. Tin beads make the process defect rate higher.
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. This has a significant influence on the results.
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 this tape cannot cover the test point. The test points should be selected near the solder joints and quad flat package (QFP) pins on each side of the carrier board, so that the test results can better reflect the real situation.
In the oven 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. This way, the parameters of each temperature zone are easier to control, and the FPC and components are less affected by thermal shock. Based on experience, it is best to adjust the furnace temperature to the lower limit of the solder paste’s technical requirements. The wind speed of the reflow furnace is generally the lowest wind speed that the furnace can use. The chain of the reflow furnace should be stable and free of jitter.
Since the carrier plate absorbs heat in the oven (especially the aluminum carrier plate), the temperature is relatively high when the oven is discharged, so it is best to add a forced cooling fan at the oven outlet to help it 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 from being produced.
The removed FPC is visually inspected under a magnifying glass of more than 5 times, looking for residual glue on the surface, discoloration, golden finger tin, tin beads, IC pin empty welding, continuous welding, and other problems. Because the surface of FPCs cannot be very smooth, which makes AOI’s misjudgment rate very high, FPC is generally not suitable for automated optical inspection (AOI); but with the help of special test fixture, FPCs can complete ICT and function tests (FCTs).
Since FPCs are mostly connected boards, it may be necessary to split the boards before the ICT and FCT processes. Although the splitting can also be done with tools such as blades and scissors, the efficiency and quality of the operations are low, and the scrap rate is high. With respect to mass production of special-shaped FPCs, it is recommended to make a special FPC stamping and splitting die, which can greatly improve efficiency. At the same time, the edges of the punched FPC are neat and beautiful, and the internal stress generated during stamping and cutting is very low. Thus, we can effectively avoid solder joint cracking.
In the assembly and soldering process of flexible PCBA, the precise positioning and fixing of FPCs 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 FPCs is much more difficult than that of rigid PCBs, so it is necessary to accurately set process parameters.
Strict production process management is also important. It is necessary to ensure that operators strictly implement every requirement on the SOP. Engineers and in-process quality control (IPQC) should strengthen inspections, find abnormal conditions in the production line in time, analyze the causes, 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 machinery and equipment in a factory directly determines the manufacturing capacity.
The basic equipment required for PCBA production includes: a solder paste printer, a pick and place 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.
Modern solder paste printers generally consist of plate loading, solder paste addition, imprinting, and circuit board transfer. Its working principle is this: first fix the flex 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.
Also known as “mounting machine” and “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. It is a device placed on the flex PCB pad, divided into two kinds: manual and automatic.
There is a heating circuit inside the reflow soldering, which heats air or nitrogen to a high enough temperature and blows it to the flex circuit board where the component has been attached. This is done 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 minimize.
Also known by its full name, automatic optical inspection, this is a piece of equipment that detects common defects encountered in welding production based on optical principles. AOI is a new type of testing technology that is developing rapidly, and many manufacturers have introduced AOI testing equipment.
During automatic inspection, the machine automatically scans the flex PCBA through the camera, collects images, compares the tested solder joints with the qualified parameters in the database, and after image processing, checks out the defects on the PCB, and displays/marks the defects through the display or automatic signs. It can then come out for repair by maintenance personnel.
This is used to cut and fashion the pin components.
Wave soldering ensures that the soldering surface of the plug-in board is in direct contact with the high-temperature liquid tin to achieve the purpose of soldering. The high-temperature liquid tin maintains a slope, and a special device makes it form a wave-like phenomenon, hence the name. The main material is solder bars.
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.
This is used to clean the flex PCBA board and can remove the residue on the board after soldering.
ICTs mainly test the flex PCBA circuit open circuit, short circuit, and welding condition of all parts by contacting the test points of the PCB layout with *test probes.
FCT, or functional test, refers to the simulated operating environment (stimulus and load) provided to the test target board (Unit Under Test [UUT]) 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.
The burn-in test rack can test PCBA boards in batches, and test the problematic flexible PCBA boards by simulating the user’s operation for a long time.
PCBA outsourcing processing refers to the PCBA manufacturers sending orders to other capable PCBA processing manufacturers. So, what are the general requirements?
Insert or mount components in strict accordance with the bill of materials, Flex PCB silk screen, and outsourcing processing requirements. When the material does not match the bill, PCB silk screen, or contradicts the process requirements—or if the requirements are ambiguous and cannot be operated—one should timely contact our company to confirm the correctness of materials and process requirements.
- All components are treated as electrostatic-sensitive devices.
- All personnel who come into contact with components and products should wear anti-static clothes, anti-static bracelets, and anti-static shoes.
- Upon entry of the raw materials tothe factory and in the warehouse, the electrostatic sensitive devices are all in anti-static packaging.
- During the operation, we use an anti-static work surface, and use anti-static containers for components and semi-finished products.
- The welding equipment is grounded reliably, and the electric soldering iron adopts anti-static type. All must be tested before use.
- The semi-finished PCB is stored and transported in an anti-static box, and the isolation material uses anti-static pearl cotton.
- The whole machine without shell usesan anti-static packaging bag.
- Polarity components are inserted according to polarity.
- For components with silkscreen on the side (such as high-voltage ceramic capacitors), when inserted vertically, the silk screen faces right; when inserted horizontally, the silk screen faces down. When the components (not including chip resistors) silk-printed on the top are inserted horizontally, the font direction is the same as that of the PCB screen printing direction; when inserted vertically, the font top faces right.
- 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.
- The pin height of the plug-in component on the soldering surface is 1.5～0mm. 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 unacceptable.
- 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.
- Solder joint shape: Conical shape covering the entire pad.
- Solder joint surface: smooth, bright, free of black spots, flux and other impurities, and no defects such as spikes, pits, pores, exposed copper, etc.
- Solder joint strength: fully wetted with pads and pins, no false soldering or false soldering.
- 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 between the lead and the solder. There are no spikes or barbs at the cross section.
- Needle base welding: The needle base must be mounted on the bottom board, and the position and the direction are 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.
To prevent Flex PCBA damage, the following packaging should be used during transportation:
- Container: Anti-static turnover box.
- Isolation material: Anti-static pearl cotton.
- Placement spacing: There is a distance greater than 10mm between Flex PCB board and board,and between PCB board and box.
- 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 cable.
The Flex PCBA board surface should be clean, 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. When washing the board, the following devices should be protected: wires, connecting terminals, relays, switches, polyester capacitors and other easily corrosive devices. The relay is strictly prohibited from ultrasonic cleaning.
All components are not allowed to extend beyond the edge of the flex PCB after installation. When the flex 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, so the repair soldering staff is required to make appropriate corrections.