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FPC PCBA assembly soldering process is very different from the assembly of rigid circuit boards, because the hardness of the FPC board is not enough, more flexible, if you do not use a dedicated carrier board, it will not be able to complete the fixing and transmission, and will not be able to complete the printing, mounting, over the furnace and other basic SMT processes.
1.Pre-treatment of FPC
FPC board is relatively soft, the factory is generally not vacuum packaging, in the transportation and storage process is prone to absorb moisture in the air, the need for SMT casting line before the pre-baking process, the moisture will be slowly and forcibly discharged. Otherwise, under the impact of the high temperature of reflow soldering, the water absorbed by the FPC quickly vaporizes into water vapor protruding from the FPC, which is easy to cause FPC delamination, blistering and other defects.
Pre-baking conditions are generally temperature 80-100 ℃ time 4-8 hours,under special circumstances, the temperature can be adjusted to 125 ℃ or more, but need to correspondingly shorten the baking time. Before baking, must first make a small sample test to determine whether the FPC can withstand the set baking temperature. When baking, the FPC should not be stacked too much, 10-20PNL is more suitable. After baking, the FPC should have no obvious defects such as discoloration, deformation, warpage, etc., and should be qualified by IPQC before being put into the line.
2.Specialized carrier board production
According to the CAD file of the circuit board, read the hole positioning data of the FPC to manufacture high-precision FPC positioning templates and special carrier boards, so that the diameter of the positioning pins on the positioning templates and the positioning holes on the carrier boards, the positioning holes on the FPC to match the diameter of the holes. Many FPC because of the need to protect part of the circuit or design reasons is not the same thickness, some places thick and some places to be thinner, and some have to strengthen the metal plate, so the combination of the carrier plate and FPC need to be processed in accordance with the actual situation of grinding and grooving, the role is to ensure that the FPC in the printing and pasting is flat. Carrier plate material requirements for thin, light, high-strength, less heat absorption,fast heat dissipation, and after a number of thermal shock warpage deformation is small. Commonly used carrier materials include synthetic stone, aluminum plate, silicone plate, and special high-temperature resistant magnetized steel plate.
FPC PCBA assembly
3. Production process
Here we take the common carrier board as an example to describe the SMT points of FPC, the use of silicone boards or magnetic fixtures, FPC fixation to be much more convenient, do not need to use tape, and printing, mounting, soldering and other processes are the same process points.
3.1 FPC fixing
Before SMT, the first need to accurately fix the FPC on the carrier board. Particular attention should be paid to the FPC fixed on the carrier board and then, to print, paste and soldering between the storage time is as short as possible. Carrier boards are available with or without locating pins. Carrier plate without positioning pins, need to be used in conjunction with the positioning template with positioning pins, first set the carrier plate on the positioning pins of the template, so that the positioning pins through the positioning holes on the carrier plate exposed, the FPC piece by piece on the exposed positioning pins, and then fixed with tape, and then let the carrier plate and the FPC positioning template to separate, for printing, mounting and soldering. The carrier board with locating pins has already fixed a number of spring locating pins of about 1.5mm in length, so that the FPC can be put on the spring locating pins of the carrier board directly one by one, and then fixed with adhesive tapes. In the printing process, the spring locating pins can be completely pressed into the carrier board by the stencil, which will not affect the printing effect.
Method 1 (single-sided tape fixing) with a thin, high-temperature single-sided tape to fix the four sides of the FPC on the carrier board, so that the FPC does not have offset and warpage, the viscosity of the tape should be moderate, the reflow soldering must be easy to peel off, and there is no residue of adhesive on the FPC. If you use automatic tape machine, can quickly cut the length of the tape, can significantly improve efficiency, cost savings, to avoid waste.
Method 2 (double-sided tape fixing) first use high temperature resistant double-sided tape on the carrier board, the effect is the same as the silicone board, and then stick the FPC to the carrier board, pay special attention to the viscosity of the tape can not be too high, otherwise it is easy to cause tearing of the FPC after reflow soldering when peeled off. After repeated passes through the furnace, the viscosity of the double-sided tape will gradually become lower, viscosity is so low that it can not be reliably fixed FPC must be replaced immediately. This station is the most important station to prevent FPC from being dirty, and it is necessary to wear finger cots to work. Before reuse of the carrier board, it should be cleaned properly, either with a nonwoven cloth dipped in cleaning agent or with an anti-static dust roller to remove surface dust, tin beads and other foreign matter. Do not use too much force to remove or place FPC, as FPC is fragile and prone to crease and breakage.
3.2 FPC solder paste printing
FPC on the composition of the solder paste is not very special requirements, the size of the tin ball particles and metal content, etc. in order to FPC with or without fine pitch IC as standard, but FPC on the solder paste printing performance requirements are higher, the solder paste should have excellent thixotropy, solder paste should be able to easily print off the mold and firmly attached to the surface of the FPC, will not appear off the mold bad blocking stencil leakage holes or after printing to produce collapse. There will not be any defects such as mold release blocking stencil leakage holes or collapsing after printing.
Because the carrier board is loaded with FPC, FPC on the positioning of high temperature tape, so that the plane is not consistent, so the printing surface of the FPC can not be as flat as the PCB and the thickness of the same hardness, so it is not appropriate to use a metal squeegee, but should be used in the hardness of 80-90 degrees of polyurethane squeegee. The best solder paste printing machine with optical positioning system, otherwise there will be a greater impact on the printing quality, although the FPC is fixed on the carrier board, but the FPC and the carrier board will always produce some small gaps between them, which is the biggest difference with the PCB hardboards, and囙setting parameter setting will also have a greater impact on the printing effect.
Printing station is also to prevent FPC pollution of the key workstation, need to wear finger cots work, and at the same time to maintain the cleanliness of the workstation, wipe the stencil diligently, to prevent solder paste contamination of FPC gold fingers and gold-plated keys.
3.3 FPC Lamination
According to the characteristics of the product, the number of components and the efficiency of the paste, the use of high-speed and medium-sized paste machine for the paste can be. Since each FPC has an optical MARK mark for positioning, SMD mounting on FPC is not much different from mounting on PCB. It should be noted that, although the FPC is fixed on the carrier board, but its surface can not be as flat as the PCB hardboard, FPC and the carrier board there will be a local gap between, so the nozzle drop height, blowing pressure, etc. need to be set accurately, and the nozzle moving speed needs to be reduced.
3.4 FPC reflow soldering
Should use forced hot air convection infrared reflow soldering furnace, so that the temperature on the FPC can be more uniform changes to reduce the generation of welding defects. If the use of single-sided tape, because it can only fix the four sides of the FPC, the middle part of the hot air state due to deformation, the soldering pad is easy to form a tilt, molten tin (liquid tin at high temperatures) will flow and produce empty soldering, continuous soldering, solder beads, so that the process of the higher rate of defects.
3.4.1 Temperature Curve Test Methods
Due to the different heat absorption of the carrier board, the different types of components on the FPC, they are heated in the reflow process, the speed of temperature rise is different, the amount of heat absorbed is also different, so the temperature curve of the reflow oven is set carefully,which has a great impact on the quality of the soldering. A more secure method is to be based on the actual production of the carrier board interval, in front of and behind the test board each put two pieces of carrier boards equipped with FPC,and at the same time in the test of the carrier board of the FPC affixed to the components, high-temperature soldering wire to test temperature probe soldered to the test point, and at the same time with high-temperature-resistant adhesive tape to fix the probe lead on the carrier board. Note that the temperature-resistant tape should not cover the test point. The test point should be selected near the solder joints and QFP pins on each side of the carrier board, so that the test result can better reflect the real situation.
3.4.2 Temperature Curve Setting
In the furnace temperature debugging, because the FPC temperature uniformity is not good, so it is best to use the warming / warming / reflux of the temperature curve pipeline, so that the parameters of the temperature zone is easier to control, in addition to the FPC and the components by the thermal shock of the impact of the smaller. According to experience, it is best to adjust the furnace temperature to the lower limit of the value of the solder paste technology requirements, the reflow furnace wind speed is generally the lowest wind speed that can be used in the furnace, the reflow furnace chain stability should be good, there can be no jitter.
3.5 FPC Inspection, Testing and Depaneling
Since the carrier board in the furnace heat absorption, especially aluminum carrier board, out of the furnace temperature is higher, so it is best to add a new strong cooling fan in the furnace outlet to help quickly cool down the temperature. At the same time, workers need to wear heat-insulating gloves to avoid being burned by the high-temperature carrier plate. When removing the finished FPC from the carrier board, the force should be even, not brute force, so as to avoid the FPC being torn or creased.
Place the removed FPC under a magnifying glass of 5 times or more for visual inspection, focusing on surface residue, discoloration, gold finger tin, tin beads, IC pin soldering, 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 with the help of specialized test fixtures, FPC can complete the ICT, FCT test.
Since FPC is mostly made of joint boards, it may be necessary to do board splitting before doing ICT and FCT tests. Although board splitting can be done by using tools such as blades and scissors, the efficiency and quality of work are low. If the mass production of shaped FPC, can make a special FPC stamping board mold, stamping division, can greatly improve the efficiency of the work, while stamping out of the FPC edges neat and beautiful, stamping and cutting the board produced by the internal stress is very low, can be effectively avoided solder joints tin cracks.
In the PCBA flexible electronic assembly welding process, the precise positioning and fixation of FPC is the key point, the key to fix the good or bad is to make the appropriate carrier board. Next is the FPC pre-baking, printing, laminating and reflow soldering. Obviously, FPC SMT process is much more difficult than PCB, so it is necessary to set the process parameters accurately, and at the same time, strict production process management is equally important to ensure that the staff strictly enforce every provision of the SOPs, follow the line engineers and IPQC should strengthen the inspections, to find abnormalities on the production line in a timely manner, to analyze the causes and take the necessary measures, in order to control the defective rate of FPC SMT production line to a few percent. In order to control the defective rate of FPCSMT production line within dozens of PPM.
FPC before PCBA assembly
4.PCBA production equipment
PCBA production of basic equipment required for solder paste printing machine, mounting machine, reflow soldering, AOI detector, component clipper, wave soldering, soldering furnace, washing machine, ICT test fixtures, FCT test fixtures, aging test racks and so on.
4.1 Solder Paste Printing Machine
Modern solder paste printing machine generally consists of mounting plate, adding solder paste, embossing, transmission circuit board and other components. Its working principle is to fix the PCB to be printed on the printing positioning table, and then the left and right squeegees of the printing machine will leak the solder paste or red glue through the steel mesh to print on the corresponding pads, and then the PCB with even leakage will be inputted into the laminating machine through the transmission table to carry out automatic lamination.
4.2 Mounter
SMD machine is also known as “SMD machine”, “Surface Mount System” (SurfaceMountSystem), in the production line, it is put aside after the solder paste printing machine, it is a kind of equipment to accurately place the surface mount components on the PCB pads by moving the mounting head. Pad on a device, divided into two kinds of manual and fully automatic.
4.3 Reflow soldering
Reflow soldering internal heating circuit, the air or nitrogen heated to a sufficiently high temperature after blowing to the circuit board has been affixed components, so that components on both sides of the solder melted and bonded with the motherboard. The advantage of this process is that the temperature is easy to control, the soldering process can also avoid oxidation, manufacturing costs are also easier to control.
4.4 AOI inspector
AOI (AutomaticOpticInspection) is the full name of the automatic optical inspection, is based on the principle of optics to the common defects encountered in the production of welding inspection equipment. The machine automatically scans the PCB through the monitor, collects the image, compares the tested solder joints with the qualified parameters in the database, detects the defects on the PCB through image processing, and shows/marks the defects through the display or the automatic sign for the maintenance personnel to repair.
4.5 Component Cutter
It is used to cut and deform the pin components.
4.6 Wave Soldering
Wave soldering is to let the soldering surface of the insert board contact with the high temperature liquid tin directly to achieve the purpose of soldering, its high temperature liquid tin keeps a slanting surface, and the liquid tin forms a wave-like phenomenon by the special device, so it is called “Wave soldering”, and its main material is the solder bar.
4.7 Soldering Furnace
Generally, the soldering furnace is a soldering tool used in electronic soldering. For discrete components circuit board welding consistency is good, easy to operate, fast, high efficiency.
4.8 board washing machine
Used for PCBA board cleaning, can remove the residue of the board after welding.
4.9 ICT test fixture
ICTTest is mainly a test probe contact PCBlayout out of the test point to detect the PCBA line open, short circuit, all parts of the welding situation.
4.10 FCT Test Fixture
FCT (Functional Test) which refers to the test target board (UUTUnitUnderTest) to provide an analogous operating environment (excitation and load), so that it works in a variety of design states, so as to obtain the parameters of the various states to verify the functionality of the UUT good or bad test method. Simply put, it is to load the UUT with appropriate excitation and measure whether the output response meets the requirements.
4.11 Aging Test Rack
Aging test rack can be batch testing of PCBA boards, through a long period of time and other operations similar to the user to test out the problematic PCBA boards.
Abbey Circuits specializes in PCB and FPC board production and assembly, component procurement,SMT SMD processing,assembly and testing one-stop service.Positioned as a professional and fast volume one-stop EMS service provider. The company is equipped with high-precision equipment,fully automatic multifunctional SMD machine,reflow soldering,wave soldering, selective wave soldering,X-Ray inspection machine,AOI equipment and so on. Our products cover a wide range of fields such as automotive electronics,medical equipment, industrial control, aerospace,communication equipment, and so on.