FPC SMT production process

There are many differences between the process requirements of SMT on the surface of FPC and the SMT solution of traditional rigid PCB. If you want to do a good job in the SMT process of FPC, the most important thing is positioning. Because the FPC board is not enough rigid, and relatively soft. If not use a special carrier board,  cannot finish the fixing and transmission, and cannot complete the basic SMT processes such as printing, placement, and furnace. The following is a detailed description of the process points of FPC pretreatment, fixing, printing, patch, reflow soldering, testing, inspection, board splitting and other processes in FPC SMT production.

Preprocessing of FPC

The FPC board is relatively flexible, and generally not vacuum-packed when leaves the factory.So easily absorbs moisture from the air during transportation and storage. Flex circuit needs to be pre-baked before SMT lines to discharge the moisture slowly and forcibly. If not under the high-temperature impact of reflow soldering, the Flexible circuit board moisture will quickly vaporize and become water vapor protruding from the FPC, which will easily cause defects such as Flex PCB delamination and blistering.

The pre-baking conditions are generally at a temperature of 80-100°C for 4-8 hours. In special cases, the temperature can be increased to above 125°C, but need to be shortened the baking time accordingly. Before baking, do a small sample test first to determine whether the Flexible circuit can withstand the set baking temperature? Or inquiry the our FPC production department and engineer teams for suitable baking conditions. In baking, the FPC boards stackup cannot be too much, 10-20PNL is more appropriate, some Flexible circuit board manufacturers will put a piece of paper between each PNL for isolation, to confirm whether the paper used for isolation can withstand the set baking temperature. If not, remove the isolation paper and then bake. After baking, the Flex circuits will have no obvious discoloration, deformation, warping, etc., and the IPQC must inspected this samples before flexible assembly can be put into production.

Dedicated carrier boards Production

Production of dedicated carrier boards

According to the Flexible circuit board CAD file, get the FPC board hole positioning data to manufacture a high-precision FPC positioning template and a special carrier board, so match the diameter of the positioning pin on the positioning template with the positioning hole on the carrier plate and the positioning hole on the Flex circuit.

Many FPCs are not at same thickness because of the need to protect some circuits or design reasons. Some places are thicker and some places are thinner, and some have reinforced metal plates, so the joint between the carrier board and the FPC needs to be pressed. In the actual situation, the groove is processed, polished, and the function is to ensure that the FPC is flat during printing and patching.

The carrier board material requires thinness, high strength, less heat absorption, fast heat dissipation, and small warping deformation after multiple thermal shocks. Commonly used carrier materials include synthetic stone, aluminum plate, silica gel plate, special high temperature resistant magnetized steel plate, etc.

Flexible circuit SMT Production process

Fixing of FPC

Before SMT, the FPC needs to be precisely fixed on the carrier board first. Special attention should be paid to the shorter the storage time between the FPC fixed on the carrier board and the printing, mounting and welding, the better. There are two types of carrier boards with and without positioning pins. The carrier board without positioning pins needs to be used together 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 set the FPCs one by one on the exposed positioning pins are fixed with adhesive tape, and then the carrier board is separated from the FPC positioning template for printing, placement and welding. Several spring positioning pins with a length of about 1.5mm have been fixed on the carrier board with positioning pins. FPCs can be directly placed 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 by the steel mesh without affecting the printing effect.

First method (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 tape viscosity should be moderate, and it must be easy to peel off after reflow soldering. No glue residue. 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.

Second Method (fixed with double-sided tape): first stick the high-temperature-resistant double-sided tape on the carrier board, the effect is the same as that of the silicone board, and then paste the FPC on the carrier board. Pay special attention to the viscosity of the tape not being too high, otherwise it will be peeled off after reflow soldering when, it is easy to cause FPC tearing. After repeated furnace passes, the viscosity of the double-sided tape will gradually decrease, and it must be replaced immediately when the viscosity is too low to reliably fix the FPC. This station is the key station to prevent FPC from getting dirty, and it is necessary to wear finger cots for operation. Before the carrier board is reused, it needs to be properly cleaned. It can be scrubbed with a non-woven cloth dipped in a cleaning agent, or an anti-static sticky dust roller can be used to remove foreign matter such as surface dust and tin beads. Do not use too much force when picking and placing the FPC, as the FPC is fragile and prone to creases and breaks.

Solder paste printing for Flexible Circuit

FPC has no special requirements on the composition of solder paste. The size of solder ball particles and metal content are based on whether there are fine-pitch ICs on FPC. However, FPC has higher requirements on the printing performance of solder paste, and solder paste should have excellent thixotropy, the solder paste should be easy to print and release and can firmly adhere to the surface of the FPC, and there will be no defects such as blocking the leak of the stencil or causing 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 printing surface of the FPC cannot be as flat as the PCB and has the same thickness and hardness. Therefore, it is not suitable to use a metal scraper, but should use a hardness of 80. -90 degree polyurethane type scraper. The solder paste printing machine should be equipped with 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, which is different from the hard PCB. The biggest difference between boards, so the setting of equipment parameters will also have a great impact on the printing effect.

The printing station is also an important station to prevent FPC from being dirty. It is necessary to wear finger cots for operation. At the same time, it is necessary to keep the station clean and wipe the stencil frequently to prevent solder paste from polluting the gold fingers and gold-plated buttons of the FPC.

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 each FPC has an optical MARK for positioning, there is little 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 local gap between the FPC and the carrier board. Therefore, the drop height of the suction nozzle, the blowing pressure, etc. Precise setting is required, and the nozzle moving speed needs to be reduced. At the same time, most FPCs are connected boards, and the yield rate of FPCs is relatively low, so it is normal for the whole PNL to contain some bad PCS. When the PNL boards are all good boards, the production efficiency will be greatly reduced.

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 you can only fix the four sides of the FPC, the middle part is deformed in the hot air state, the pad is easy to tilt, and the molten tin (liquid tin at high temperature) will flow to produce empty soldering, continuous soldering, Tin beads make the process defect rate higher.

(1) Temperature curve test method:

Due to the different heat absorption of the carrier board and the different types of components on the FPC, their temperature rises at different speeds after being heated during the reflow soldering process, and the heat absorbed is also different. Therefore, carefully setting the temperature curve of the reflow soldering oven has a great impact on the soldering quality. Big impact. A more secure method is to place two carrier boards with FPCs on the front and back of the test board according to the carrier board spacing 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 the probe wire is fixed on the carrier board with high temperature resistant tape. Note that the high temperature resistant tape cannot cover the test points. 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 temperature uniformity of FPC is not good, it is best to use the temperature curve method of heating/holding/reflow, so that the parameters of each temperature zone are easy to control, and the influence of thermal shock on FPC and components should be small. 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 adopt. The chain stability of the reflow furnace should be good without shaking.

Flexible circuit inspection, testing and Splitting board

Since the carrier board absorbs heat in the furnace, especially the aluminum carrier board, the temperature is high when it comes 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, the operator needs to wear heat-insulating gloves to avoid being scalded by the high-temperature carrier board. When taking the welded FPC from the carrier board, use even force 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, and the focus is on inspection of residual glue on the surface, discoloration, gold finger staining with tin, tin beads, IC pin empty soldering, continuous soldering and other problems. 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, but by using special test fixtures, FPC can complete ICT and FCT tests.

Since FPC is mostly connected boards, it may be necessary to divide boards before ICT and FCT tests. Although tools such as blades and scissors can also be used to complete board division operations, the work efficiency and quality are low, and the scrap rate is high. If it is a mass production of special-shaped FPC, it is recommended to make a special FPC stamping splitting mold for stamping and splitting, which can greatly improve the working efficiency. It can effectively avoid tin cracking of solder joints.

RTR continuous tape

Roll-to-roll or reel-to-reel is RTR, continuous tape method.

Our factory has built RTR production lines.

Our pass rate of FPC produced on the RTR production line is over 99.8%. Due to the expansion of the TAB, COF and RTR technology applications, the TAB and COF FPCs production, this continuous production FPC equipment and process began to exerte advantages. Our RTR production of FPC technical has been relatively mature now.

Schematic diagram of REEL TO REEL production line:

From Flex PCB prototype to flexible PCB manufacturing, and FPC Assembly, BETON PCBs factory has covered for all your flexible PCB needs. Our knowledge and experience with Flexible PCB (FPC) assembly this specific type Flex PCB gives us a competitive edge in the PCB assembly industry. We also have no MOQ and always promise the best price and customer service and provide the high quality FPCA products to our customers. Any FPC Assembly demands, please contact cathy@beton-tech.com.