FPC board is also called a flexible printed circuit board. The PCBA assembly and welding process of the FPC board is very different from that of the rigid circuit board. Because the hardness of the FPC board is not enough, it is soft. If you do not use a dedicated carrier board, it will not be able to complete the fixing and transmission. Basic SMT processes such as printing, placement, and furnace cannot be completed.
Here is the flexible printed circuit FPC manufacturing process.
FPC pretreatment
The FPC board is relatively soft, and it is generally not vacuum-packed when it leaves 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 the line to force the moisture out slowly.
Otherwise, under the high-temperature impact of reflow soldering, the moisture absorbed by the FPC will quickly vaporize and become water vapor to protrude from the FPC, which will easily cause defects such as FPC delamination and blistering.
The pre-baking conditions are generally at a temperature of 80-100°C and a time of 4-8 hours. Under special circumstances, the temperature can be adjusted to above 125°C, but the baking time needs to be shortened accordingly.
Before baking, a small sample test must be done to determine whether the FPC can withstand the set baking temperature. You can also consult the FPC manufacturer for suitable baking conditions.
When baking, FPC should not be stacked too much. 10-20PNL is more suitable. Some FPC manufacturers will put 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 release paper, then bake it.
The baked FPC should have no obvious discoloration, deformation, warping, and other defects, and it can be put into the line after being qualified by IPQC sampling.
FPC production of the dedicated carrier board
According to the CAD file of the circuit board, read the hole positioning data of the FPC to manufacture the high-precision FPC positioning template and the special carrier board, so that the diameter of the positioning pin on the positioning template and the positioning hole on the carrier board and the aperture of the positioning hole on the FPC is the same. match.
Many FPCs are not of the same thickness because they want to protect part of the circuit or for design reasons. Some places are thick, some are thinner, and some have reinforced metal plates. Therefore, the junction of the carrier board and the FPC needs to be The actual situation is to process, polish, and dig grooves, and the function is to ensure that the FPC is flat during printing and placement.
The material of the carrier board requires light and thin, high strength, low heat absorption, fast heat dissipation, and small warpage 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.
FPC production process
Here we take a common carrier board as an example to detail the SMT points of FPC. When using silicone plates or magnetic jigs, the fixing of FPC is much more convenient, without the use of tape, and the process points of printing, patching, welding, and other processes are the same.
Fixing of FPC
Before SMT, the FPC needs to be accurately fixed on the carrier board. In particular, it should be noted that the storage time between printing, mounting, and soldering after the FPC is fixed on the carrier board is as short as possible.
There are two kinds of carrier boards with positioning pins and without positioning pins. The carrier board without positioning pins needs to be used in conjunction 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 put the FPC piece by piece. The exposed positioning pins are then fixed with tape, and then the carrier board is separated from the FPC positioning template for printing, patching, and welding.
The carrier board with positioning pins has been fixed with several spring positioning pins about 1.5mm in length. The FPC can be directly put 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 plate by the steel mesh without affecting the printing effect.
Method one (fixed with single-sided tape):
Fix the four sides of the FPC on the carrier board with thin high-temperature-resistant single-sided tape to prevent the FPC from shifting and warping.
The viscosity of the tape should be moderate, and it must be easy to peel off after reflow soldering, and there should be no adhesive residue on the FPC. 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.
Method two (fixed with double-sided tape):
First, use high-temperature resistant double-sided tape to stick to the carrier board, the effect is the same as the silica gel board, and then paste the FPC to the carrier board. Pay special attention to the viscosity of the tape not too high, otherwise, it will easily cause the FPC to tear when it is peeled off after reflow soldering.
After repeated ovens, 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 the FPC from getting dirty, and it is necessary to wear finger cots for work. Before the carrier is reused, it needs to be properly cleaned. It can be wiped with a non-woven cloth dipped in detergent, or an anti-static sticky roller can be used to remove surface dust, tin beads, and other foreign objects.
Don’t use too much force when picking up and placing FPC. FPC is fragile and prone to creases and breaks.
FPC solder paste printing
FPC does not have very special requirements for the composition of solder paste. The size and metal content of the solder ball particles is subject to whether there are fine-pitch ICs on the FPC.
However, FPC has higher requirements for the printing performance of solder paste, and the solder paste should have excellent Thixotropy, the solder paste should be easy to print and release and firmly adhere to the surface of the FPC, and there will be no defects such as poor release, blocking the leakage of the stencil or 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 printed surface of the FPC cannot be as flat as the PCB and the thickness and hardness are consistent. Therefore, it is not suitable to use a metal scraper, but a hardness of 80-90 degrees polyurethane type scraper.
The solder paste printer is best equipped with an optical positioning system, otherwise, it will have a greater impact on the printing quality. Although the FPC is fixed on the carrier board, there will always be some tiny gaps between the FPC and the carrier board, which is hardly related to the PCB.
The biggest difference between the board and the setting of equipment parameters will also have a greater impact on the printing effect.
The printing station is also a key station to preventing the FPC from getting dirty. It is necessary to wear finger cots for work. At the same time, keep the station clean and wipe the steel mesh frequently to prevent the solder paste from contaminating the FPC’s gold fingers and gold-plated buttons.
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 there is an optical MARK mark for positioning on each FPC, there is little difference between SMD mounting on the FPC and mounting on the PCB.
It should be noted that although the FPC is fixed on the carrier board, its surface cannot be as flat as a PCB hardboard. There will definitely be a partial gap between the FPC and the carrier board. Therefore, the suction nozzle drop height, blowing pressure, etc. It needs to be accurately set, and the moving speed of the suction nozzle needs to be reduced.
At the same time, FPC is a mostly connected board, and the yield of FPC is relatively low. Therefore, it is normal for the whole PNL to contain some bad PCS. This requires the placement machine to have the BAD MARK recognition function, otherwise, in the production of this type of non-integral When PNL is a good board, the production efficiency will be greatly reduced.
FPC reflow soldering
The forced hot air convection infrared reflow oven should be used so that the temperature on the FPC can be changed more uniformly, and the occurrence of poor soldering can be reduced.
If you use single-sided tape because you can only fix the four sides of the FPC, the middle part is deformed under hot air, the pad is easily inclined, and the molten tin (liquid tin at high temperature) will flow, resulting in empty soldering, continuous soldering, Tin beads make the process defect rate higher.
Temperature curve test method
Due to the different heat absorption properties of the carrier board and the different types of components on the FPC, the temperature rises at different speeds after being heated during the reflow soldering process, and the heat absorbed is also different. Therefore, carefully set the temperature curve of the reflow oven to improve the quality of soldering with great influence.
A safer method is to place two FPC-equipped carrier boards before and after the test board according to the carrier board interval during actual production.
At the same time, mount the 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 a high-temperature adhesive tape.
Note that the high-temperature-resistant tape cannot cover the test point. 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.
Setting of the temperature curve
In the furnace temperature debugging, because the FPC’s temperature uniformity is not good, it is best to use the temperature curve method of heating/heat preservation/reflow, so that the parameters of each temperature zone are easier to control, and the FPC and components are affected by thermal shock.
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 is generally the lowest wind speed that the furnace can use. The chain of the reflow furnace should be stable and free of jitter.
FPC inspection, testing, and sub-board
Since the carrier plate absorbs heat in the furnace, especially the aluminum carrier plate, the temperature is higher when it is 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, operators need to wear heat-insulating gloves to avoid being burned by the high-temperature carrier. When taking the soldered FPC from the carrier board, the force should be even, and brute force should not be used to prevent the FPC from being torn or creased.
The removed FPC is visually inspected under a magnifying glass more than 5 times, focusing on the inspection of residual glue on the surface, discoloration, gold finger staining, tin bead, IC pin empty welding, continuous welding, and other problems.
Since the surface of FPC cannot be very smooth, which makes the AOI misjudgment rate high, FPC is generally not suitable for AOI inspection, but by using special test fixtures, FPC can complete ICT and FCT tests.
Since FPCs are mostly connected boards, it may be necessary to split the boards before testing ICT and FCT. Although the splitting can be done with tools such as blades and scissors, the efficiency and quality of the operations are low, and the scrap rate is high.
If it is the mass production of special-shaped FPC, it is recommended to make a special FPC stamping and splitting die, which can greatly improve the work efficiency. At the same time, the edges of the punched FPC are neat and beautiful, and the internal stress generated during stamping and cutting is very low. Can effectively avoid solder joint cracking.
In the assembly and welding process of PCBA flexible electronics, the precise positioning and fixing of FPC are the key points. The key to fixing is to make a suitable carrier board. Followed by FPC pre-baking, printing, placement, and reflow soldering. Obviously, the difficulty of the FPC SMT process is much higher than that of the PCB rigid board, so it is necessary to accurately set process parameters.
At the same time, strict production process management is also important. It is necessary to ensure that operators strictly implement every regulation on SOP. Follow-up engineers and IPQC should strengthen inspections to find abnormalities in the production line in time, analyze the reasons and take necessary measures. In order to control the defect rate of the FPC SMT production line within dozens of PPM.
In the PCBA production process, a lot of machinery and equipment are needed to assemble aboard. Often the quality level of a factory’s machinery and equipment directly determines the manufacturing capacity.
The basic equipment required for PCBA production includes a solder paste printer, placement machine, reflow soldering, AOI detector, component clipper, wave soldering, tin furnace, washing machine, ICT test fixture, FCT test fixture, For aging test racks, PCBA processing plants of different sizes will have different equipment.
PCBA production equipment
Solder paste printing machine
Modern solder paste printers generally consist of plate loading, solder paste addition, imprinting, and circuit board transfer.
Its working principle is: first fix the circuit board to be printed on the printing positioning table, and then the solder paste or red glue is printed on the corresponding pad through the stencil by the left and right scrapers of the printer. The transfer station is input to the placement machine for automatic placement.
Mounter
The placement machine is also called the placement machine, the surface mount system (Surface Mount System), in the production line, it is configured after the solder paste printer, and the surface mount components are accurately placed on the PCB pads by moving the placement head A kind of equipment. Divided into two kinds of manual and automatic.
Reflow soldering
There is a heating circuit inside the reflow soldering, which heats air or nitrogen to a high enough temperature and blows it to the circuit board where the component has been attached so that the solder on both sides of the component is melted and bonded to the motherboard.
The advantage of this process is that the temperature is easy to control, oxidation can be avoided during the welding process, and the manufacturing cost is easier to control.
AOI detector
The full name of AOI is automatic optical inspection, which is equipment that detects common defects encountered in welding production based on optical principles. AOI is a new type of test technology that is emerging, but it is developing rapidly, and many manufacturers have introduced AOI test equipment.
During the automatic inspection, the machine automatically scans the PCB through the camera, collects images, compares the tested solder joints with the qualified parameters in the database, after image processing, checks out the defects on the PCB, and displays/marks the defects through the display or automatic signs Come out and be repaired by maintenance personnel.
Component trimming machine
It is used to cut and deform the pin components.
Wave soldering
Wave soldering is to make the soldering surface of the plug-in board directly contact the high-temperature liquid tin to achieve the purpose of soldering. The high-temperature liquid tin maintains a slope, and a special device makes the liquid tin form a wave-like phenomenon, so it is called wave soldering. The main material is solder bars.
Tin furnace
In general, the tin furnace refers to a welding tool used in electronic welding. For discrete component circuit boards, the welding consistency is good, the operation is convenient, fast, and the work efficiency is high. It is a good helper for your production and processing.
Plate washer
It is used to clean the PCBA board and can remove the residue of the board after soldering.
ICT test fixture
ICT Test is mainly to test the PCBA circuit open circuit, short circuit, and welding condition of all parts by contacting the test points of the PCB layout with test probes.
FCT test fixture
FCT (Functional Test) It refers to the simulated operating environment (stimulus and load) provided to the test target board (UUT: Unit Under Test) to make it work in various design states, so as to obtain the parameters of each state to verify the UUT The function of the test method is good or bad.
Simply put, it is to load a suitable stimulus to the UUT and measure whether the response of the output end meets the requirements.
Aging test frame
The burn-in test rack can test PCBA boards in batches, and test out the PCBA boards with problems by waiting for a long time to simulate the user’s operation.
PCBA outsourcing processing means that PCBA processing manufacturers send PCBA orders to other powerful PCBA processing manufacturers. So, what are the general requirements for PCBA outsourcing processing?
Bill of materials
Insert or mount components in strict accordance with the bill of materials, PCB silkscreen, and outsourcing processing requirements. When the material does not match the bill, the PCB silkscreen, or conflicts with the process requirements, or the requirements are vague and the operation cannot be performed, it should be timely Contact our company to confirm the correctness of materials and process requirements.
Anti-static requirements
- All components are treated as electrostatic-sensitive devices.
- All personnel who come into contact with components and products wear anti-static clothes, anti-static bracelets, and anti-static shoes.
- During the raw materials entering the factory and in the warehouse stage, the electrostatic-sensitive devices are all anti-static packaging.
- During the operation, use an anti-static work surface and use anti-static containers for the components and semi-finished products.
- The welding equipment is grounded reliably, and the electric soldering iron adopts an anti-static type. All need to be tested before use.
- The semi-finished PCB board is stored and transported in an anti-static box, and the isolation material uses anti-static pearl cotton.
- The whole machine without a shell uses an anti-static packaging bag.
Requirements for the orientation of component appearance marking
- Polarity components are inserted according to polarity.
- For components with silk-screened on the side (such as high-voltage ceramic capacitors), when inserted vertically, the silkscreen faces right; when inserted horizontally, the silkscreen faces down. When the components (not including the chip resistors) silk-printed on the top are inserted horizontally, the font direction is the same as the direction of the PCB screen printing; when inserted vertically, the upper side of the font faces right.
- When the resistance is inserted horizontally, the error color ring faces to the right; when the resistance is inserted vertically, the error color ring faces down; when the resistance is inserted vertically, the error color ring faces the board.
Welding requirements
- The pin height of the plug-in component on the soldering surface is 1.5～2.0mm. SMD components should be flat against the board surface, and the solder joints should be smooth, without burrs, and slightly arc-shaped. The solder should exceed 2/3 of the height of the solder end, but should not exceed the height of the solder end. Less tin, ball-shaped solder joints, or solder-covered patches are all bad;
- The height of the solder joints: the height of the solder climbing pins should not be less than 1mm for the single-sided board, and the double-sided board should not be less than 0.5mm and the tin must be penetrated.
- Solder joint shape: Conical shape and covering the entire pad.
- The surface of the solder joints: smooth, and bright, with no black spots, flux, or other debris, no spikes, pits, pores, copper exposed, and other defects.
- Solder joint strength: fully wetted with pads and pins, no false soldering or false soldering.
- Solder joint cross-section: the component’s cutting foot should not be cut to the soldered part as much as possible, and there should be no cracking on the contact surface between the lead and the solder. There are no spikes or barbs at the cross-section.
- Needle base welding: The needle base must be mounted on the bottom board, and the position is correct and the direction is correct. After the needle base is welded, the bottom floating height should not exceed 0.5mm, and the skew of the base body should not exceed the silkscreen frame. Rows of needle holders should also be kept neat and not allowed to be misaligned or uneven.
Transport
In order to prevent PCBA damage, the following packaging should be used during transportation:
- Container: anti-static turnover box.
- Isolation material: anti-static pearl cotton.
- Placement spacing: there is a distance greater than 10mm between the PCB board and the board and between the PCB board and the box.
- Placement height: There is a space greater than 50mm from the top surface of the turnover box to ensure that the turnover box is not pressed against the power supply, especially the power supply of the wire.
Washing requirements and others
The board surface should be clean and free of tin beads, component pins, and stains. Especially at the solder joints on the plug-in surface, there should not be any dirt left by soldering.
The following devices should be protected when washing the board: wires, connecting terminals, relays, switches, polyester capacitors, and other easily corrosive devices, and the relays are strictly prohibited to be cleaned by ultrasonic.
After all, components are installed, they are not allowed to extend beyond the edge of the PCB board.
When PCBA passes the furnace, because the pins of the plug-in components are washed by the tin flow, some plug-in components will be tilted after the furnace is soldered, causing the body of the component to exceed the silkscreen frame, so the repair soldering staff after the tin furnace is required to make appropriate corrections.
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