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Introduction of FPC manufacturing and assembly process

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Introduction of FPC manufacturing and assembly process

FPC is also known as flexible circuit board, FPC PCBA assembly welding process and hard circuit board assembly is very different, because the hardness of THE FPC board is not enough, more soft, if you do not use a special carrier, can not complete the fixation and transmission, also can not complete printing, SMT, furnace and other basic SMT process.  

one  Pretreatment of FPC  

FPC board is soft, generally not vacuum packaging when leaving the factory, easy to absorb moisture in the air during transportation and storage process, need to be pre-baked before SMT cast line, the moisture slowly forced discharge.  Otherwise, under the high temperature impact of reflow welding, the water absorbed by FPC will be quickly vaporized into water vapor and protrusion of FPC, which is easy to cause bad delamination and blistering of FPC.  

Introduction of FPC manufacturing and assembly process  

 

Pre-baking conditions are generally 80-100℃ for 4-8 hours, under special circumstances, the temperature can be raised to more than 125℃, but the baking time should be shortened accordingly.  Always make a sample test before baking to determine if the FPC can withstand the set baking temperature, and consult the FPC manufacturer for suitable baking conditions.  When baking, FPC stack should not be too much, 10-20PNL is suitable, some FPC manufacturers will put a piece of paper between each PNL for isolation, it is necessary to confirm whether the paper used for isolation can withstand the set baking temperature, if not, it is necessary to remove the isolation paper before baking.  After baking, FPC should have no obvious discoloration, deformation, warping and other defects, and should be inspected by IPQC before casting line.  

two  Special loading plate production  

According to the CAD file of the circuit board, the hole positioning data of FPC is read to manufacture the high-precision FPC positioning template and the special carrier, so that the diameter of the positioning pin on the positioning template and the positioning hole on the carrier and the aperture of the positioning hole on the FPC match.  Many FPC because to protect part of the line or design reasons are not the same thickness, some places thick and some places to thin, and some strengthen the metal plate, so the joint of the board and FPC needs to be processed and polished according to the actual situation, the role is to ensure that the FPC is flat when printing and mounting.  The material of the carrier plate requires light weight, high strength, less heat absorption, fast heat dissipation, and small warping deformation after several thermal shocks.  Commonly used plate materials include synthetic stone, aluminum plate, silica gel plate, special high temperature resistant magnetized steel plate and so on.  

3.  The production process.  

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

1. Fixation of FPC:  

Before performing SMT, the FPC needs to be precisely fixed to the board first.  Special attention should be paid to the minimum storage time between the time the FPC is fixed on the board and the time it is printed, mounted and welded.  There are two kinds of board with and without the positioning pin.  Loading board without positioning pin, and should take the positioning pin, the positioning of the template to load board on the set of the dowel pin in the template, make the dowel locating hole on the load board, FPC piece set on revealing the dowel pin, reoccupy adhesive plaster, and then let the load board isolation, FPC positioning template for printing, SMT and soldering.  The carrier plate with positioning pin has been fixed with a number of spring positioning pins about 1.5mm long, you can directly cover the FPC one piece on the spring positioning pin of the carrier plate, and then fixed with adhesive tape.  In the printing process, the spring positioning pin can be completely pressed into the loading plate without affecting the printing effect.  

Method 1 (single-side adhesive tape fixation) : The four sides of FPC are fixed on the loading plate with thin single-side adhesive tape with high temperature resistance, so as to prevent the FPC from deviation and buckling. The viscosity of the adhesive tape should be moderate, and it must be easy to peel off after reflow welding, and there is no residual adhesive agent on the FPC.  If the use of automatic tape machine, can quickly cut the length of the same tape, can significantly improve efficiency, cost saving, avoid waste.  

Method two (double-sided adhesive tape fixation) : first use high-temperature double-sided adhesive tape to stick on the carrier plate, the effect is the same as that of the silica gel plate, and then paste the FPC to the carrier plate. Special attention should be paid to the viscosity of the tape not too high, otherwise, when stripping after reflow welding, it is easy to cause THE FPC tear.  After repeatedly going through the furnace, the viscosity of the double-sided tape will gradually become low, and the viscosity is so low that FPC cannot be reliably fixed that it must be replaced immediately.  This station is the key station to prevent FPC smudging, so it needs to wear finger sleeves.  Before the board is reused, it should be cleaned properly. It can be cleaned with non-woven cloth dipped in cleaning agent, or anti-static dust roller can be used to remove dust, tin beads and other foreign bodies on the surface.  Do not use too much force when removing FPC. FPC is fragile and easy to produce creases and fractures.  

2. Solder paste printing of FPC:  

FPC has no special requirements for the composition of solder paste. The size and metal content of solder ball particles are subject to fine spacing IC on FPC, but FPC has higher requirements for the printing performance of solder paste. Solder paste should have excellent thixotropy, and solder paste should be able to easily print and release and firmly adhere to the SURFACE of FPC.  There will be no bad demoulding blocking steel mesh leakage or collapse after printing.  

Because FPC is loaded on the loading board, FPC has high temperature resistant adhesive tape used for positioning, so that its plane is not consistent, so the printing surface of FPC can not be as flat as PCB and the thickness of hardness is the same, so it is not suitable to use metal scraper, but should use the hardness of 80-90 degrees polyurethane scraper.  Solder paste printing machine is best with optical positioning system, otherwise it will have a great impact on the printing quality. Although FPC is fixed on the board, there will always be some tiny gap between FPC and the board, which is the biggest difference with PCB hard board, so the setting of equipment parameters will also have a great impact on the printing effect.  

Printing station is also the key station to prevent FPC dirty, need to wear finger sleeve operation, at the same time to keep the station clean, wipe the steel mesh frequently, prevent solder paste pollution FPC gold finger and gold button.  

3. FPC patch:  

According to the characteristics of the product, the number of components and the efficiency of the SMT, medium and high speed SMT machine can be used for mounting.  Since there is an optical MARK for positioning on each FPC, SMD mounting on FPC is not much different from that on PCB.  It should be noted that although FPC is fixed on the board, its surface can not be as flat as PCB hard board. There will certainly be local gaps between FPC and the board. Therefore, the drop height and blowing pressure of the nozzle need to be set accurately, and the moving speed of the nozzle needs to be reduced.  At the same time, FPC is in the majority of combined plates, and the yield of FPC is relatively low, so it is very normal for the whole PNL to contain some defective PCS, which requires the SMT machine to have the BAD MARK recognition function, otherwise, the production efficiency will be greatly reduced in the case of producing such non-whole PNL with good plates.  

4. Reflow welding of FPC:  

Mandatory hot air convection infrared reflow furnace should be used so that the temperature on the FPC can change more evenly and reduce the occurrence of bad welding.  If it is the use of single-sided tape, because only the four sides of the FPC can be fixed, the middle part due to deformation in the hot air state, the pad is easy to form tilt, molten tin (liquid tin under high temperature) will flow and produce empty welding, welding, tin bead, so that the process defect rate is higher.  

1) Temperature curve test method:  

Due to the different absorbability of the carrier plate and the different types of components on the FPC, the temperature rises at different rates and absorbs different heat during reflow welding. Therefore, carefully setting the temperature curve of reflow welding furnace has a great influence on the welding quality.  The more reliable method is, according to the actual production of the load plate interval, in the test board before and after each two loaded with FPC load plate, at the same time in the test board of THE FPC affixed with components, with high temperature solder wire test temperature probe welding on the test point, at the same time with high temperature resistant tape probe wire fixed on the load plate.  Note that the heat resistant tape cannot cover the test point.  The test points should be selected near the solder joints and QFP pins of each side of the load plate, so that the test results can reflect the real situation.  

2) Setting of temperature curve:  

In the furnace temperature debugging, because the homogenization of FPC is not good, so it is best to use the temperature curve of heating/insulation/reflux, so that the parameters of each temperature zone is easy to control some, in addition, FPC and components are less affected by thermal shock.  According to experience, it is best to adjust the furnace temperature to the lower limit of solder paste technical requirements. The wind speed of backwelding furnace generally adopts the lowest wind speed that the furnace can use. The chain stability of backwelding furnace is better, and there can be no jitter.  

5. FPC inspection, testing and sub-board:  

Because the load plate absorbs heat in the furnace, especially the aluminum load plate, the temperature is higher when it comes out of the oven, so it is best to increase the forced cooling fan at the oven mouth to help cool down quickly.  At the same time, operators need to wear heat insulation gloves to avoid being scalded by the high temperature load plate.  Remove the welded FPC from the carrier plate with even force. Do not use brute force to prevent the FPC from being torn or creased.  

The removed FPC is placed under a magnifying glass of more than 5 times for visual inspection, focusing on the surface residual glue, discoloration, gold fingers with tin, tin beads, IC pin empty welding, continuous welding and other problems.  FPC is generally not suitable for AOI inspection because the surface of FPC may not be very smooth, resulting in a high misjudgment rate of AOI. However, FPC can complete ICT and FCT tests with the help of special test jig.  

Since FPC is dominated by connecting plates, it may be necessary to make splitter plates before conducting ICT and FCT tests. Although splitter plates can also be completed by using tools such as blades and scissors, the operation efficiency and quality are low and the scrap rate is high.  If it is the mass production of special-shaped FPC, it is recommended to make special FPC punching parting die for stamping and segmentation, which can greatly improve the working efficiency. At the same time, the edges of the FPC are neat and beautiful, and the internal stress generated by stamping and cutting plate is very low, which can effectively avoid solder joint tin crack.  

In the assembly and welding process of PCBA flexible electronics, the precise positioning and fixation of FPC is the key point, and the key of fixation is to make the appropriate carrier plate.  FPC is followed by pre-baking, printing, SMT and reflow welding.  FPC SMT process obviously is higher than the PCB board and a lot of difficulty, so it is necessary to accurately set the process parameters, and at the same time, strict production process control are also important, must ensure that operators strictly implement SOP on each article, with line engineers and IPQC inspection should be strengthened, timely discover the unusual situation of production line, analyze the reason and take the necessary measures to  In order to FPC SMT production line defect rate control in dozens of PPM.  

In the PCBA production process, it is necessary to rely on a lot of machines and equipment to complete the sub-assembly of a board, often the quality level of a factory’s machinery and equipment directly determines the manufacturing capacity.  

The basic equipment needed for PCBA production is solder paste printing machine, laminators, reflow welding, AOI detector, component shearing machine, wave soldering, tin furnace, washing machine, ICT test fixture, FCT test fixture, aging test rack, etc., different scale OF PCBA processing plants, equipped with equipment will be different.

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