FPC is also called flexible circuit board. The PCBA assembly and welding process of FPC is very different from the assembly of rigid circuit board. Because FPC board is not hard enough and soft, fixation and transmission cannot be completed without using a special carrier board. It is impossible to complete basic SMT processes such as printing, patching, and furnace passing.
1. Preprocessing of FPC
FPC boards are relatively soft and are generally not vacuum packed when leaving the factory. They easily absorb moisture in the air during transportation and storage. They need to be pre-baked before SMT is put into the line to slowly and forcibly discharge the moisture. Otherwise, under the high-temperature impact of reflow soldering, the moisture absorbed by the FPC will quickly vaporize and turn into water vapor protruding out of the FPC, easily causing defects such as delamination and blistering of the FPC.
The pre-baking condition is generally a temperature of 80-100°C and a time of 4-8 hours. In special circumstances, the temperature can be raised to above 125°C, but the baking time needs to be shortened accordingly. Before baking, be sure to conduct a sample test to determine whether the FPC can withstand the set baking temperature. When baking, the FPC stack should not be too much, 10-20PNL is more suitable. The baked FPC should have no obvious discoloration, deformation, warping and other defects, and must be inspected by IPQC before it can be put into production.
2. Production of FPC special carrier board
According to the CAD file of the circuit board, read the hole positioning data of the FPC to create a high-precision FPC positioning template and a special carrier board, so that the diameter of the positioning pins on the positioning template is consistent with the positioning holes on the carrier board and the aperture of the positioning holes on the FPC. match. Many FPCs are not of the same thickness due to the need to protect some circuits or design reasons. Some places are thicker and some are thinner. Some have reinforced metal plates, so the joint between the carrier board and the FPC needs to be In actual situations, the purpose of processing, grinding and grooving is to ensure that the FPC is flat during printing and mounting. The material of the carrier board is required to be thin, light, high-strength, absorb less heat, dissipate heat quickly, and have little warping deformation after multiple thermal shocks. Commonly used carrier plate materials include synthetic stone, aluminum plate, silicone plate, special high-temperature resistant magnetized steel plate, etc.
FPC
3. Production process
Here we take an ordinary carrier board as an example to detail the SMT key points of FPC. When using silicone boards or magnetic fixtures, the fixation of FPC is much more convenient and does not require the use of tape. However, the process points of printing, patching, welding and other processes are it's the same.
3.1. Fixing of FPC
Before proceeding with SMT, the FPC first needs to be accurately fixed on the carrier board. Special attention should be paid to the shorter the storage time between the FPC being fixed on the carrier board and printing, mounting and welding, the better. The carrier board is available with or without positioning pins. The carrier board without positioning pins needs to be used 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. Then put the FPC one by one on the template. The exposed positioning pins are then fixed with tape, and then the carrier board is separated from the FPC positioning template for printing, patching and welding. The carrier plate with positioning pins has been fixed with several spring positioning pins about 1.5mm long. You can directly put the FPC piece by piece on the spring positioning pins of the carrier plate and then fix it with tape. During 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 1 (Single-sided tape fixation) Use thin high-temperature resistant 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, it must be easy to peel off after reflow soldering, and it must be on the FPC No adhesive residue. If you use an automatic tape machine, you can quickly cut tapes of consistent length, which can significantly improve efficiency, save costs, and avoid waste.
Method 2 (fixed with double-sided tape) First use high-temperature resistant double-sided tape to stick to the carrier board. The effect is the same as that of the silicone board. Then stick the FPC to the carrier board. Pay special attention to the viscosity of the tape not being too high, otherwise it will peel off after reflow soldering. When used, it is easy to cause FPC tearing. After repeated passes through the oven, the viscosity of the double-sided tape will gradually become lower. When the viscosity is too low to reliably fix the FPC, it must be replaced immediately. This station is a key station to prevent the FPC from getting dirty, and you need to wear finger gloves when working. Before the carrier board is reused, it needs to be properly cleaned. You can use a non-woven cloth dipped in detergent to scrub it, or you can use an anti-static dust-sticking roller to remove surface dust, tin beads and other foreign matter. Be careful not to use too much force when picking up and placing FPC. FPC is fragile and prone to creases and breaks.
3.2. FPC solder paste printing
FPC does not have special requirements for the composition of solder paste. The size and metal content of solder ball particles are subject to whether there are fine-pitch ICs on FPC. However, FPC has higher requirements for the printing performance of solder paste, and solder paste should have excellent Being thixotropic, the solder paste should be able to be easily printed and demoulded and firmly attached to the FPC surface, without defects such as poor demoulding blocking the stencil leaks or collapse after printing.
Because FPC is loaded on the carrier board, and there is high-temperature resistant tape for positioning on the FPC, which makes the plane inconsistent. Therefore, the printed surface of FPC cannot be as flat and consistent in thickness and hardness as PCB. Therefore, it is not suitable to use a metal scraper, but should use a hardness of 80 -90 degree polyurethane type scraper. It is best for the solder paste printer to have an optical positioning system, otherwise it will have a great impact on the printing quality. Although the FPC is fixed on the carrier board, there will always be some small gaps between the FPC and the carrier board. This is related to the hardness of the PCB. The biggest difference is that the setting of this equipment parameter will also have a great impact on the printing effect.
The printing station is also a key station to prevent FPC from getting dirty. You need to wear finger gloves when working. At the same time, you must keep the station clean and wipe the steel mesh frequently to prevent solder paste from contaminating the gold fingers and gold-plated buttons of the FPC.
3.3. FPC patch
Depending on the characteristics of the product, the number of components and the placement efficiency, medium or high-speed placement machines can be used for placement. Since each FPC has optical MARK marks for positioning, 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 will definitely be a local gap between the FPC and the carrier board. Therefore, the drop height of the nozzle, blowing pressure, etc. It needs to be set accurately and the moving speed of the nozzle needs to be reduced.
3.4. FPC reflow soldering
A forced hot air convection infrared reflow oven should be used so that the temperature on the FPC can change more evenly and reduce the occurrence of poor soldering. If you use single-sided tape, because it can only fix the four sides of the FPC, the middle part will be deformed under hot air, and the pad will easily tilt, and the molten tin (liquid tin at high temperature) will flow, resulting in empty soldering, continuous soldering, and Tin beads make the process defective rate higher.
3.4.1. Temperature curve test method
Due to the different heat absorption properties of the carrier board and the different types of components on the FPC, their temperatures rise at different rates after being heated during the reflow soldering process, and the heat they absorb is also different. Therefore, carefully set the temperature curve of the reflow soldering furnace to ensure Welding quality has a great impact. A more reliable method is to place two carrier boards with FPC in front and behind the test board according to the actual production carrier board spacing. At the same time, components are mounted on the FPC of the test carrier board, and the test temperature is removed with high-temperature solder wire. The probe is soldered on the test point, and the probe wire is fixed on the carrier board with high temperature resistant tape. Note that the test points cannot be covered with high temperature resistant tape. The test points should be selected near the solder joints and QFP pins on all sides of the carrier board, so that the test results can better reflect the real situation.
3.4.2. Setting of temperature curve
In the furnace temperature debugging, because the temperature uniformity of FPC is not good, it is best to use a temperature curve pipeline of heating/insulation/reflow, so that the parameters of each temperature zone are easier to control, and the impact of thermal shock on FPC and components is small. Some. According to experience, it is best to adjust the furnace temperature to the lower limit of the solder paste technology requirements. The wind speed of the reflow furnace is generally the lowest wind speed that the furnace can use. The reflow furnace chain should have good stability and should not be shaken.
3.5. Inspection, testing and subboarding of FPC
Since the carrier plate absorbs heat in the furnace, especially the aluminum carrier plate, the temperature is higher when it comes out of the oven, so it is best to add a forced cooling fan at the outlet to help cool down quickly. At the same time, workers need to wear heat-insulating gloves to avoid being burned by the high-temperature carrier plate. When picking up the welded FPC from the carrier board, the force must be even and do not use brute force to prevent the FPC from being torn or creased.
The removed FPC is visually inspected under a magnifying glass of more than 5 times, focusing on the surface glue residue, discoloration, gold fingers stained with tin, tin beads, IC pin air soldering, continuous soldering and other issues. Since the surface of FPC cannot be very flat, the misjudgment rate of AOI is very high, so FPC is generally not suitable for AOI inspection. However, by using special test fixtures, FPC can complete ICT and FCT tests.
Since most FPCs are connected boards, it may be necessary to separate boards before testing ICT and FCT. Although tools such as blades and scissors can also be used to complete the board separation work, the work efficiency and work quality are low. If it is mass production of special-shaped FPC, a special FPC stamping and splitting mold can be made for stamping and splitting, which can greatly improve work 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. It can effectively prevent solder joints from cracking.
In the assembly and welding process of PCBA flexible electronics, the precise positioning and fixation of FPC is the focus. The key to the quality of fixation is to make a suitable carrier board. Followed by FPC pre-baking, printing, patching and reflow soldering. Obviously, the SMT process of FPC is much more difficult than that of PCB hard board, so it is necessary to accurately set the process parameters. At the same time, strict production process management is equally important. It is necessary to ensure that workers strictly implement every regulation on the SOP and follow the line. Engineers and IPQC should strengthen inspections, promptly discover abnormalities in the production line, analyze the causes and take necessary measures to control the defective rate of the FPCSMT production line within dozens of PPM.
4. PCBA production equipment
The basic equipment required for PCBA production includes solder paste printers, placement machines, reflow soldering, AOI detectors, component clippers, wave soldering, tin ovens, plate washers, ICT test fixtures, FCT test fixtures, Aging test rack, etc.
4.1. Solder paste printing machine
Modern solder paste printing machines generally consist of mechanisms such as plate mounting, solder paste addition, imprinting, and circuit board delivery. Its working principle is to first fix the circuit board to be printed on the printing positioning table, and then use the left and right scrapers of the printing machine to leak the solder paste or red glue through the steel mesh onto the corresponding pads. For PCBs with uniform leakage, pass through the transmission The machine is input to the placement machine for automatic placement.
4.2. SMT machine
The placement machine is also called "mounting machine" or "Surface Mount System" (Surface Mount System). In the production line, it is configured after the solder paste printing machine. It accurately places surface mount components on the PCB by moving the placement head. A device on the disk, divided into two types: manual and fully automatic.
4.3. Reflow soldering
There is a heating circuit inside the reflow soldering, which heats the air or nitrogen to a high enough temperature and then blows it to the circuit board where the components have been attached, allowing the solder on both sides of the components to melt and bond with the motherboard. The advantage of this process is that the temperature is easy to control, oxidation can be avoided during the welding process, and the manufacturing cost is also easier to control.
4.4. AOI detector
The full name of AOI (Automatic Optic Inspection) is automatic optical inspection. It is a device based on optical principles to detect common defects encountered in welding production. The machine automatically scans the PCB through the monitor, collects images, and compares the tested solder joints with the qualified parameters in the database. After image processing, defects on the PCB are detected, and the defects are displayed/marked through the display or automatic signs. Maintenance crew repairs.
4.5. Component shearing machine
Used for trimming and deforming pin components.
4.6. Wave soldering
Wave soldering is to make the soldering surface of the plug-in board directly contact with the high-temperature liquid tin for 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 bar.
4.7. Tin furnace
Generally speaking, a tin furnace refers to a soldering tool used in electronic welding. For discrete component circuit boards, the welding consistency is good, the operation is convenient and fast, and the work efficiency is high.
4.8. Plate washer
Used to clean PCBA boards and remove residues from the boards after welding.
4.9. ICT test fixture
ICTTest mainly uses the test probe to contact the test points coming out of the PCBlayout to detect open circuits, short circuits, and the welding conditions of all parts of the PCBA.
4.10. FCT test fixture
FCT (Functional Test) refers to providing an analog operating environment (excitation and load) to the test target board (UUTUnitUnderTest), allowing it to work in various design states, thereby obtaining the parameters of each state to verify the functionality of the UUT. test methods. Simply put, it is to load the appropriate stimulus to the UUT and measure whether the output response meets the requirements.
4.11. Aging test stand
The aging test rack can test PCBA boards in batches and test out problematic PCBA boards by comparing the operations used by users for a long time.