How to assemble PCB through SMT production line

How to assemble PCB through SMT production line

PCBA has been widely used in modern world, ranging from military defense areas, automobiles, marine areas, aerospace areas, to smart phones and remote controls, so it has close contact with our daily life, but how does a PCB has been assembled. Following passage will provide you the concret steps of manufacturing PCBA through SMT production line.

Table of Contents
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    Bare board loading

    The first step to assemble printed circuit board (PCB) is arrange the bare boards in order, and then put them on the magazine, and the machine will automatically send the boards into the SMT production line.

    Solder paste printing

    The first step is to print solder paste when a printed circuit board enterring the SMT production line. In this process, the solder paste will be printed on the solder pads of the PCB parts that need to be soldered. During the reflow oven, it will melt and solder the electronic parts to the circuit board.

    In addition, when testing new products, some people will use film board or adhesive cardboard instead of solder paste, which can increase the efficiency and waste of SMT adjustment machines.

    Solder paste inspector

    Since the quality of solder paste printing is related to the quality of soldering of subsequent parts, some SMT production line will use optical instruments to check the quality of solder paste printing. Knock off, wash off the solder paste on it and reprint, or remove excess solder paste by repairing.

    Pick and place speed machine

     some small electronic parts

    Here, some small electronic parts, such as small resistors, capacitors, and inductors, will be printed on the circuit board first, and these parts will be slightly stuck by the solder paste just printed on the circuit board, so even if the speed of printing is as fast as a machine gun, while the parts on the board will not fly apart, but large parts are not suitable for use in a fast machine, and it will slow down the speed of small parts. 

    Second,  the parts may be shifted from the original position due to the rapid movement of the board.

    Pick and place general machine

    Also known as “slow machine”, here will be some relatively large electronic parts, such as BGA IC, connectors, etc., these parts need more accurate positions, so the alignment is very important, before the parts will be printed using a camera to take a picture to confirm the position of the parts, so the speed is much slower than that. Due to the size of the parts here, not all of them will be packed in tape-on-reel, and some may be packed in trays or tubes.

    Generally, the traditional pick and place machine uses the principle of suction to move electronic parts, so there must be a plane on the top of these electronic parts for the suction nozzle of the pick and place machine to pick up the parts However, some electronic parts cannot have a flat surface for these machines. At this time, it is necessary to use special nozzles for these special-shaped parts, or add a layer of flat tape to the parts, or wear a flat surface cap.

    Hand place component or visual inspection

    After all the parts are soldered on the circuit board, then going through the high temperature reflow furnace. A check point is usually set up to pick out the defects of part deviation or missing parts. Because if a problem found after going through the high temperature furnace, you have to use a soldering iron, which will affect the quality of the product, and there will be additional costs. In addition, for some reasons, some larger electronic parts or traditional DIP/THD parts that cannot be operated by the punching/mounting machine will also be manually placed here.

    In addition, the SMT of some mobile phone boards will also design an AOI to confirm the quality before reflow. Sometimes it is because the parts will have a shielding frame on the top, which will make it impossible to use AOI to check after the reflow furnace.

    Reflow

    Reflow

    The purpose of reflow is to melt the solder paste and form a non-metallic compound (IMC) on the component feet and the circuit board, that is, to solder the electronic components on the circuit board, and the rise and fall of temperature curves often affects the soldering quality of the entire circuit board.
    According to the characteristics of the solder, the general reflow furnace will set the preheating zone, soaking zone, reflow zone, and cooling zone to achieve the best soldering effect.

    The melting point of SAC305 solder paste in the current lead-free process is about 217°C, which means that the temperature of the reflow furnace must be at least higher than this temperature to remelt the solder paste. In addition, the maximum temperature in the reflow furnace should not exceed 250°C, otherwise many parts will be deformed or melted because they cannot withstand such a high temperature.

    Basically, after the circuit board passes through the reflow oven, the assembly of the entire circuit board is considered complete. If there are hand-soldered parts, the rest is to check and test the circuit board for defects or malfunctions.

    AOI

    Not every SMT production line has an optical inspection machine (AOI). The purpose of setting up AOI is is because some circuit boards with too high density cannot be used for subsequent open and short circuit electronic tests (ICT), so AOI is used.  

    But because AOI has its blind spots for optical interpretation, for example, the solder under the part cannot be judged. At present, it can only check whether the part is standing on its side, missing parts, displacement, polarity direction, solder bridge, empty soldering, etc., but it cannot judge false soldering, BGA solderability, resistance value, capacitance value, inductance value and other parts quality, so there is no way to completely replace ICT.

    Therefore, if only AOI is used to replace ICT, there are still some risks in terms of quality, but ICT is not 100%, it can only be said to complement each other in test coverage.

    Unloading

    After the board is assembled, it will be retracted to the magazine. These magazines have been designed to allow the SMT production line to automatically pick and place the board without affecting its quality.

    Visual inspection

    Whether there is an AOI station or not, the general SMT production line will still set up a visual inspection area, the purpose is to check whether there are any defects after the circuit board assembly is completed. If there is an AOI station, it can reduce the visual inspection personnel quantity, because it is still necessary to check some places that cannot be interpreted by AOI, or check the badness of AOI.

    AOI

    Many factories will provide a key visual inspection template at this station, which is convenient for visual inspection personnel to inspect some key parts and part polarity.

    Touch-up

    If some parts cannot be printed by SMT production line, it is necessary to touch-up, which is usually after the finished product inspection to distinguish whether the defect comes from process after SMT production lineor.

    When soldering iron at a certain high temperature to the part to be soldered until the temperature rises enough to melt the tin wire,  then the parts are soldered to the circuit board after the tin wire cools down.

    There will be some smoke generated when welding parts by hand, which will contain a lot of heavy metals, so smoke exhaust equipment must be set up in the operating area, and try not to let the operator inhale these harmful fumes.

    It needs to be reminded that some parts will be arranged in a later stage of the process due to the needs of the process.

    In-circuit test

    The purpose of ICT setting is mainly to test whether the parts and circuits on the circuit board are open or short circuited. It can also measure the basic characteristics of most parts, such as resistance, capacitance, and inductance, to judge whether these parts have been reflowed at high temperature. After the furnace, whether the function is damaged, wrong parts, missing parts etc.

    Circuit testing machines are further divided into advanced and basic machines

    Circuit testing machines are further divided into advanced and basic machines. The basic testing machines are generally called MDA (Manufacturing Defect Analyzer).

    In addition to all the functions of the basic model, the high-end test machine can also send power to the board under test, start the board under test and execute the test program. The advantage is that it can simulate the function of the circuit board under the actual power-on condition The test can partly replace the following function test machine . However, a test fixture of this kind of high-end test machine can probably buy a private car, which is 15~25 times higher than that of a low-end test fixture, so it is generally used in mass-produced products. more suitable.

    Function test

    Functional testing is to make up for the ICT, because ICT only tests the open and short circuits on the circuit board, and other functions such as BGA and products are not tested. So it is necessary to use a functional testing machine to test all functions on the circuit board.

    Assembly board de-panel

    General circuit boards will be panelized to increase the efficiency of SMT production line. There are usually so-called “multi-in-one” boards, such as 2 in 1, 4 in 1. After all the assembly work is completed, it needs to be cut (de-panel) into a single board, and some circuit boards that only have a single board also need to cut off some redundant board edges (break-away).

    There are several ways to cut the circuit board. You can design the V-cut using the blade cutting machine or directly manually fold the board (not recommended). For more precise circuit boards, use the path splitting cutting machine or router, it will not damage electronic parts and circuit boards, but the cost and working hours are relatively long.

    Conclusion

    Based on what have been talked about above, a brief understanding of PCBA manufacturing must have been formed. It’s a complicated process requires lots of highly advanced SMT production line and skilled workers. Feel free to contact us to get your high-quality PCB/PCBA products and reliable  PCBA Manufacturer.

    FAQ

    The first step in the assembly of circuit boards is to arrange the bare boards neatly, and then put them on the magazine, and the machine will automatically send the boards one by one into the SMT assembly line.

    The first step for a printed circuit board to enter the SMT production line is to print solder paste. Here, the solder paste will be printed on the solder pads of the PCB parts that need to be soldered. During the reflow oven, it will melt and solder the electronic parts to the circuit board.

    Since the quality of solder paste printing is related to the quality of soldering of subsequent parts, some SMT factories will use optical instruments to check the quality of solder paste printing after solder paste printing in order to ensure stable quality. Knock off, wash off the solder paste on it and reprint, or remove excess solder paste by repairing.

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