Precise positioning and fixing of SMD on FPC is the key

PCB (Printed Circuit Board) is a printed circuit board, or simply hard board. FPC (Flexible Printed Circuit) is a flexible circuit board, also known as flexible circuit board or flexible circuit board, referred to as soft board. The miniaturization of electronic products is an inevitable trend. A considerable portion of consumer products are surface mounted. Because of the assembly space, SMDs are mounted on FPCs to complete the assembly of the entire machine. FPCs are used in calculators, mobile phones, and digital cameras. Digital products such as digital video cameras have been widely used. SMD surface mounting on FPC has become one of the trends in SMT technology.

Figure 1-1 Sample of FPC

Figure 1-2 FPC sample with metal reinforcement

The process requirements of FPC surface SMT have many differences with the conventional hard PCB PCB SMT solutions. In order to complete the FPC SMT process, the most important thing is positioning. Because the hardness of the FPC board is not enough, it is relatively soft. If the dedicated carrier board is not used, it cannot be fixed and transported, and thus the basic SMT process such as printing, patching, and furnaces cannot be completed. The following is a detailed description of the process points of the pre-processing, fixing, printing, patching, reflowing, testing, inspection, and sub-boarding of FPC in FPC SMT production.

Figure 2 Engineering plastic (synthetic stone) carrier board

Figure 3 aluminum carrier board

Figure 4 Silicone plate and positioning template

Figure 5 disc automatic tape dispenser

Figure 6 anti-static dust roller

Figure 7 high temperature single-sided tape

Figure 8 high temperature double-sided tape

2. FPC solder paste printing:

FPC has no special requirements on the composition of the solder paste. The size and metal content of the solder ball particles are not based on the fine-pitch IC on the FPC, but the FPC has higher requirements on the printing performance of the solder paste. The solder paste should have excellent Thixotropic, solder paste should be able to easily print release and can be firmly attached to the FPC surface, there will be no poor mold release blocking the stencil hole or print after the collapse and other defects.

Because the carrier board is loaded with FPC, FPC has positioning high-temperature tape, so that its plane is inconsistent, so the printed surface of the FPC is not as flat as the PCB and the hardness and hardness of the same, so the metal scraper should not be used, but the hardness should be 80 -90 degree polyurethane type scraper. Paste printing machine is best equipped with optical positioning system, otherwise it will have a greater impact on the print quality, although the FPC is fixed on the carrier board, but there will always be some small gap between the FPC and the carrier board, which is with the PCB hard The biggest difference is the board, so the setting of the equipment parameters will also have a big impact on the printing effect.

The printing station is also a key station for preventing the contamination of the FPC. It is necessary to wear a finger jacket, keep the station clean, clean the steel net, and prevent the solder paste from contaminating the gold finger and gold-plated keys of the FPC.

3. FPC patch:

According to the characteristics of the product, the number of components and the efficiency of the patch, it can be mounted with a medium and high-speed placement machine. Since there is an optical MARK mark for positioning on each FPC, the SMD mounting on the FPC is not significantly different from mounting on the PCB. It should be noted that although the FPC is fixed on the carrier board, its surface may not be as flat as the PCB board, and there must be some gaps between the FPC and the carrier board. Therefore, the nozzle drop height, blowing pressure, etc. To be precisely set, the nozzle movement speed needs to be reduced. At the same time, the majority of FPCs are FPCs, and the yield of FPCs is relatively low. Therefore, it is normal for the entire PNL to contain some bad PCS. This requires the SMT machine to have the BAD MARK recognition function. If PNL is a good board, production efficiency will be greatly reduced.

4. FPC reflow soldering:

A forced hot air convection infrared reflow soldering furnace should be used, so that the temperature on the FPC can be more evenly changed and the occurrence of poor welding can be reduced. If single-sided tape is used, because only the four sides of the FPC can be fixed, the middle portion of the FPC can be deformed under hot air, and the pad can be easily inclined. The molten tin (liquid tin at high temperature) can flow and generate air welding, continuous welding, Tin beads, the higher the rate of defects in the process.

1) Temperature curve test method:

Because of the different endothermic properties of the carrier board, the different types of components on the FPC, the different speeds at which the temperature rises during heating in the reflow process, and the amount of heat absorbed is also different. Therefore, the temperature profile of the reflow oven is carefully set to the quality of the soldering. Great influence. A more secure method is to place two FPC-loaded carrier boards before and after the test board according to the carrier board spacing during actual production. At the same time, components are mounted on the FPC of the test carrier board, and the test temperature is measured with a high-temperature solder wire. The probe is welded to the test point and the probe wire is fixed to the carrier plate with a high-temperature tape. Note that high-temperature tape cannot cover the test points. The test points should be chosen near the solder joints on the sides of the carrier board and the QFP pins. Such test results can better reflect the real situation.

2) Temperature curve settings:

In the furnace temperature debugging, because the temperature uniformity of the FPC is not good, it is better to adopt the temperature curve of temperature increase/retention/reflow, so that the parameters in each temperature zone are easy to control, and the impact of FPC and components on thermal shock is small. some. According to experience, it is best to adjust the furnace temperature to the lower limit of the solder paste technical requirements. The reflow furnace wind speed generally adopts the minimum wind speed that the furnace can use. The stability of the reflow oven chain is better and there should be no jitter.

5. FPC inspection, testing and boarding:

Since the carrier plate absorbs heat in the furnace, especially the aluminum carrier plate, the temperature at the time of tapping is high, so it is better to add a forced cooling fan at the tap outlet to help reduce the temperature quickly. At the same time, the operator must wear heat-resistant gloves to avoid being burned by the high-temperature carrier board. When the FPC that has completed the welding is taken from the carrier plate, the force must be uniform and no brute force can be applied to prevent the FPC from being torn or creasing.

The removed FPC was visually inspected under a magnifying glass with a magnification of 5 times or more, and the problems such as surface adhesive residue, discoloration, tinned tin solder, tin beads, IC pin air soldering, and continuous soldering were inspected. Because the surface of the FPC cannot be very smooth, the erroneous rate of the AOI is very high. Therefore, the FPC generally does not cooperate with the AOI inspection. However, the FPC can complete the ICT and FCT tests by using a dedicated test fixture.

Due to the fact that FPC is mostly based on splicing plates, it may be necessary to do the slicing before testing ICT and FCT. Although blades, scissors, and other tools can be used to complete the slicing operation, the operating efficiency and the quality of work are low, and the scrap rate is high. If it is a large-volume production of shaped FPCs, it is recommended to make special FPC stamping dividing board molds for punching and dividing, which can greatly improve the working efficiency. At the same time, the punched out FPC edges are neat and beautiful, and the internal stress generated when punching the cutting board is very low. Can effectively avoid solder cracks.

Figure 9 FPC stamping board module

Figure 10 FPC FCT Test Fixture

four. summary

SMD placement on the FPC, the precise positioning and fixing of the FPC is the key, the key to fixing is to make a suitable carrier board. Followed by FPC pre-baking, printing, patch and reflow. Obviously, the difficulty of FPC SMT process is much higher than PCB hard board, so it is necessary to set precise process parameters. At the same time, strict production process management is equally important. It is necessary to ensure that the operator strictly enforces every rule on the SOP. Engineers and IPQCs should strengthen inspections, identify abnormal conditions on the production line, analyze the causes, and take necessary measures so that the defect rate of the FPCSMT production line can be controlled within dozens of PPMs.