Introduction to FPC assembly process

FPC is also called flexible circuit board. The PCBA assembly and welding process of FPC is very different from that of hard circuit board. Because the FPC board is not hard enough and soft enough, it cannot be fixed and transmitted without the use of a special carrier plate, and it cannot complete the basic SMT processes such as printing, mounting, and heating.

1、 Pretreatment of FPC

The FPC board is relatively soft, and it is generally not vacuum packed when leaving 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 production to slowly and forcibly discharge the moisture. Otherwise, under the impact of high temperature of reflow welding, the moisture absorbed by FPC will quickly vaporize into steam and extrude out of FPC, which is easy to cause FPC delamination, blistering and other defects.

Multilayer FPC

The pre baking conditions are generally 80-100 ℃, 4-8 hours. Under special circumstances, the temperature can be raised to above 125 ℃, but the baking time needs to be shortened accordingly. Before baking, make sure to conduct a sample test to determine whether FPC can withstand the set baking temperature, or consult the FPC manufacturer for appropriate baking conditions. During baking, the FPC stack should not be too much. 10-20PNL is appropriate. Some FPC manufacturers will place a piece of paper between each PNL for isolation. It is necessary to confirm whether this piece of paper for isolation can withstand the set baking temperature. If it is not necessary to remove the isolation paper, bake again. The baked FPC shall be free from obvious discoloration, deformation, warping and other defects, and can be put into production only after passing the spot check by IPQC.

2、 Manufacture of special carrier plate

According to the CAD file of the circuit board, read the hole positioning data of FPC to manufacture high-precision FPC positioning template and special carrier plate, so that the diameter of the positioning pin on the positioning template matches the diameter of the positioning hole on the carrier plate and the positioning hole on the FPC. Many FPCs are not of the same thickness because of the need to protect some lines or design reasons. Some parts are thick, some parts are thin, and some have reinforced metal plates. Therefore, the joint between the carrier plate and FPC needs to be processed, polished and grooved according to the actual situation to ensure that the FPC is flat during printing and mounting. The material of carrier plate shall be thin, high strength, less heat absorption, fast heat dissipation, and small warpage deformation after multiple thermal shocks. Commonly used carrier plate materials include synthetic stone, aluminum plate, silica gel plate, special high temperature resistant magnetized steel plate, etc.

3、 Production process

Here we take the ordinary carrier plate as an example to detail the SMT key points of FPC. When using silica gel plate or magnetic fixture, the fixation of FPC is much more convenient, without the use of tape, while the process key points of printing, mounting, welding and other processes are the same.

1. Fixing of FPC:

Before SMT, the FPC needs to be accurately fixed on the carrier plate. In particular, it should be noted that the storage time from FPC fixed on the carrier plate to printing, mounting and welding should be as short as possible. There are two kinds of carrier plates with and without locating pins. The carrier plate without locating pin shall be used together with the locating template with locating pin. First, the carrier plate shall be sleeved on the locating pin of the template to expose the locating pin through the locating hole on the overload plate. Then, the FPC shall be sleeved on the exposed locating pin piece by piece and fixed with tape. Then, the carrier plate shall be separated from the FPC locating template for printing, pasting and welding. Several 1.5mm long spring locating pins have been fixed on the carrier plate with locating pins. FPC can be directly sleeved on the spring locating pins of the carrier plate piece by piece, and then fixed with adhesive tape. In the printing process, the spring locating pin can be completely pressed into the carrier plate by the steel mesh without affecting the printing effect.

Method 1 (fixing with single-sided tape): Fix the four sides of FPC on the carrier plate with thin heat-resistant single-sided tape to prevent FPC from shifting and warping. The tape viscosity should be moderate. After reflow welding, it must be easy to peel off, and there is no residual adhesive on the FPC. If an automatic belt conveyor is used, it can quickly cut tapes with the same length, which can significantly improve efficiency, save costs and avoid waste.

Method 2 (fixed with double-sided tape): First, stick the high temperature resistant double-sided tape on the carrier plate, with the same effect as the silica gel plate, and then stick the FPC to the carrier plate. Pay special attention to the tape viscosity, otherwise the FPC will easily tear when peeling off after reflow welding. After repeated furnace passes, 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 FPC from being dirty, and it needs to wear finger cots for operation. Before the carrier plate is reused, it needs to be properly cleaned. You can use non-woven cloth dipped in cleaning agent to scrub, or use an anti-static dust roller to remove the surface dust, tin beads and other foreign matters. Do not use too much force when taking and placing FPC. FPC is fragile and prone to crease and fracture.

2. Solder paste printing of FPC:

FPC has no special requirements on the composition of solder paste. The size and metal content of solder ball particles are subject to whether there is a fine pitch IC on the FPC. However, FPC has high requirements on the printing performance of solder paste. The solder paste should have excellent thixotropy. The solder paste should be able to easily print and release the mold and firmly adhere to the surface of the FPC, without defects such as poor mold release blocking the steel mesh leak or collapse after printing.

Because FPC is loaded on the carrier plate, and there is high temperature resistant tape for positioning on the FPC, which makes its plane inconsistent, the printed surface of FPC cannot be as flat as that of PCB with the same thickness and hardness. Therefore, metal scraper should not be used, but polyurethane scraper with hardness of 80-90 degrees should be used. The solder paste printer 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 plate, there will always be some small gaps between the FPC and the carrier plate, which is the biggest difference from the PCB hard plate. Therefore, 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 needs to wear finger cots for operation. At the same time, it needs to keep the station clean and wipe the steel mesh frequently to prevent solder paste from polluting the gold fingers and gold plated keys of FPC.

3. FPC patch:

According to the characteristics of the product, the number of components and the mounting efficiency, the medium and high speed mounting machines can be used for mounting. Since each FPC has an optical MARK mark for positioning, SMD mounting on FPC is not different from mounting on PCB. It should be noted that although the FPC is fixed on the carrier plate, its surface cannot be as flat as the PCB hard board, and there must be a local gap between the FPC and the carrier plate. Therefore, the nozzle lowering height, blowing pressure, etc. need to be set accurately, and the nozzle moving speed needs to be reduced. At the same time, most FPCs are connected boards, and the yield of FPCs is relatively low. Therefore, it is normal to include some defective PCSs in the whole PNL, which requires the placement machine to have the BAD MARK identification function. Otherwise, the production efficiency will be greatly reduced when the production of such non whole PNLs is good boards.

4. Reflow soldering of FPC:

The forced hot air convection infrared reflow soldering furnace shall be used, so that the temperature on the FPC can change more evenly and reduce the occurrence of poor welding. If single-sided tape is used, because only four sides of the FPC can be fixed, and the middle part is deformed in hot air, the bonding pad is prone to tilt, and the molten tin (liquid tin at high temperature) will flow, resulting in empty solder, continuous solder, and solder beads, which makes the defective rate of the process higher.

1) Temperature curve test method:

Due to the different heat absorption of carrier plates and the different types of components on FPC, their temperature rise speed is different after being heated during reflow soldering, and the heat absorbed is also different. Therefore, carefully setting the temperature curve of reflow soldering furnace will greatly affect the welding quality. A relatively safe method is to place two carrier plates with FPC in front of and behind the test plate according to the carrier plate spacing during actual production. At the same time, stick the components on the FPC of the test carrier plate, weld the test temperature probe to the test point with high-temperature solder wire, and fix the probe lead to the carrier plate with high-temperature resistant tape. Note that the high temperature resistant tape cannot cover the test point. The test points should be selected at the solder joints and QFP pins near each side of the carrier plate, so that the test results can better reflect the real situation.

2) Setting of temperature curve:

During the furnace temperature commissioning, because the temperature homogeneity of FPC is not good, it is better to adopt the temperature curve mode of temperature rise/insulation/reflux, so that the parameters of each temperature zone can be easily controlled. In addition, FPC and components are less affected by thermal shock. According to experience, it is better to adjust the furnace temperature to the lower limit of the solder paste technical requirements. The wind speed of the reflow furnace is generally the lowest wind speed that can be used by the furnace. The chain of the reflow furnace has good stability and can not shake.

5. FPC inspection, test and plate division:

As the carrier plate absorbs heat in the furnace, especially the aluminum carrier plate, the temperature is high when it is discharged, so it is better to add a forced cooling fan at the outlet to help rapid cooling. At the same time, operators need to wear insulating gloves to avoid being scalded by high temperature carrier plates. When taking the FPC that has been welded from the carrier plate, the force should be uniform and no brute force should be used to avoid the FPC being torn or creased.

The removed FPC shall be visually inspected under a magnification of more than 5 times, focusing on problems such as glue residue on the surface, discoloration, gold finger tin, solder beads, empty soldering and continuous soldering of IC pins. Because the surface of FPC cannot be very flat, which leads to a high error rate of AOI, FPC is generally not suitable for AOI inspection, but with the help of special test fixture, FPC can complete ICT and FCT tests.

Because most FPCs are connected boards, it may be necessary to perform board splitting before ICT and FCT tests. Although blade, scissors and other tools can also be used to complete board splitting, the work efficiency and quality are low, and the scrap rate is high. If it is a large-scale production of special-shaped FPC, it is recommended to make a special FPC stamping parting die for stamping segmentation, which can greatly improve the operating efficiency. At the same time, the edges of the FPC blanking are neat and beautiful, and the internal stress generated during stamping and cutting is very low, which can effectively avoid solder joint tin cracking.

In the assembly and welding process of PCBA flexible electronics, the precise positioning and fixation of FPC is the key point, and the key to good or bad fixation is to make a suitable carrier plate. The second is the pre baking, printing, mounting and reflow of FPC. Obviously, the SMT process of FPC is much more difficult than that of PCB hard board, so it is necessary to set the process parameters accurately. At the same time, strict production process management is also important. It is necessary to ensure that operators strictly implement every provision of SOP. The line following engineer and IPQC should strengthen patrol inspection, timely find the abnormal conditions of the production line, analyze the causes and take necessary measures to control the defective rate of FPC SMT production line within dozens of PPMs.

In the process of PCBA production, it is necessary to rely on a lot of machinery and equipment to complete the assembly of 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 solder paste printing machine, chip mounter, reflow soldering, AOI detector, component leg shearing machine, wave soldering, tin furnace, plate washer, ICT test fixture, FCT test fixture, aging test rack, etc. The equipment provided by PCBA processing plants of different sizes will be different.