Circuit board production process: SMT
Hardware engineers, in order to deal with factories frequently, must fully understand the basic processes and principles of SMT factories.
PCBA=PCB Assembled. The assembled PCB. Strictly speaking, PCBA=PCB+components+SMT production+firmware+testing.
There are so many components on the circuit board, it is definitely not enough to solder by hand, and it has to be patched by machine. No matter whether you have been to the factory or not, you must have seen such a picture on TV: a mechanical hand moves on the circuit board, and stabs it down a few times, and there are components on the circuit board. This is a link in PCB becoming PCBA, SMT.
The principle of machine welding circuit is the same as that of manual welding circuit, tinning, placing components, and heating. It’s just that the machine is much faster than the human hand, and can place several components in one second.
The first thing to do is to tin the board. As mentioned earlier, there is a paste mask file in the Gerber file, which is used to open the steel mesh. The steel mesh is a thin steel sheet, very flat, with a thickness of about 0.1mm. According to the graphics on the paste mask file, there are corresponding hollow holes. Cover the stencil on the circuit board and align it. At this time, you can see that the pads that need to be soldered will be exposed. The stencil is the stencil of solder paste. Brush a layer of solder paste on the stencil, and the solder paste in the place with holes will be printed on the pads of the circuit board, and there will be no solder if there are no openings. The thickness of the solder paste is the same as the thickness of the steel mesh, which is also 0.1mm thick.
The equipment for tinning is called “printing machine”. Insert the stencil into the machine, and then put the circuit board in. The equipment will automatically hold up the circuit board, position it well, and firmly press it under the stencil. There is a brush above the stencil, pushing a large amount of solder paste, going back and forth from the upper layer of the stencil, and a layer of solder paste will be piled up at the position of the opening of the stencil and the groove formed by the circuit board. Take the circuit board off again, and the tinning on the circuit board is complete. Next, you need to enter another machine and start placing components.
Holes are opened on the steel mesh, corresponding to the component pads of the circuit board
Solder paste is printed on the circuit board through the stencil
SMT, Surface Mount Technology, Surface Mount. As the name implies, components are mounted on the surface of the circuit board. The reason why it is called a sticker is because the solder paste is sticky to a certain extent, and it can stick to the components when it is not melted. SMT is also called patch. It means attaching the chip to the circuit board.
Because the patch is the most important link in the entire PCBA processing process, the PCBA processing factory is also called the patch factory.
The principle of the patch is extremely simple. When soldering manually, the components are placed on the circuit board with tweezers, and the placement machine is placed on the circuit board with the manipulator.
However, the actual situation of the patch is very complicated, and the equipment is also very sophisticated. Think about it too, if there is no technical content, why is there always a SMD lens on the TV, instead of a printed or baked lens? We can first look at the following questions:
Where are the components placed?
Components with relatively small size, including chips, are all tape-storage. Through paper or plastic tape, the components are embedded into the tape one by one in the same order, and then rolled into rolls. There are many standard-sized holes on the material belt. These holes can be stuck on the gears of the material conveyor, and the gears carry the material forward bit by bit.
Material conveyor, called Feida. The name is purely transliterated, Feeder. The original meaning is a feeder, a breeder. It vividly expresses the function of this thing: feeding material to the placement machine.
The feeders are neatly arranged at both ends of the placement machine. The manipulator of the placement machine, according to the program settings, picks up components from the feeders and places them on the circuit board.
For large-sized components or bulk materials that are not woven into tapes, they can also be placed on a pallet, and the robot can also pick up materials from the pallet.
How did the manipulator pick up such a small component?
In fact, the mechanical arm of the placement machine does not rely on fingers to pick up components, but vacuum suction. There are multiple nozzles on each arm, and each nozzle can pick up a component. With more suction nozzles, the robotic arm can pick up many components in one move, and place them many times, so the production efficiency will be higher.
Components of different sizes have different suction nozzles. It can be seen from high school physics that under the same pressure, the larger the area, the greater the force. Therefore, the suction nozzles for relatively heavy materials such as chips and connectors should be larger. , the suction nozzle that absorbs resistance and capacitance should be smaller, and the components that absorb 0201 need to use a smaller nozzle.
Things with a relatively large weight will have a relatively large inertia when moving, so the placement machine is divided into several areas. The robotic arm in the large component area moves slowly, and the small component area moves much faster.
From the principle of the placement machine sucking components, it is not difficult to understand that for pointed components such as pins and thimbles, the materials are shipped with a plastic cover, because there is no plane, it cannot be sucked up. . For components with small openings on the surface such as the USB port, a small piece of high-temperature adhesive tape will be pasted when leaving the factory, and the purpose is to prevent the nozzle from leaking.
How does the manipulator know where to put the components?
In the production materials, there will be a coordinate file indicating the coordinates of each component on the circuit board. Before going online for placement, the production line engineer will look at the production materials and enter the placement information of each component into the operating software of the placement machine. In this way, the placement machine knows how many components to take from which feeder and where to place them on the circuit board.
This process is called programming in the factory. The SMT factory has a special program engineer who is responsible for entering this information. It takes half a day to program a circuit board with hundreds of components.
How to align the board and components?
The circuit board is sent to the placement machine through the conveyor belt. The components are in the material belt, and they are not rigid and will shake. The placement machine must be able to judge the precise position of the circuit board and place the components precisely in place.
The placement machine uses the camera on the robotic arm to identify circuit boards and components. After each component is picked up, a photo will be taken. Through the image recognition of this photo, it can be seen whether the suction is crooked. If it is crooked, the system will automatically paste it according to the crooked data on the image. The position of the film is compensated to a certain extent, the deviation is moved, and the rotation is skewed. Similarly, there will be several mark points and a circular pad on the circuit board. The camera can identify the current position of the circuit board according to the position of the pad, and then find the components according to the coordinates of the components relative to the circuit board. the location.
Reflow oven with lid open
Once all the components are placed, the board will be pushed out of the pick and place machine for visual inspection by humans, or by an AOI machine, to see if any components are attached crookedly or wrongly. If there is no problem, it will be overheated.
Over the oven, through a stove. The solder paste has to be heated to melt, and to fix the components. Passing through the furnace is to gradually heat the entire circuit board through a reflow furnace until the solder melts, and then gradually cool down. The whole process usually takes about 8 minutes. At present, the furnaces for reflow soldering are mainly heated by hot air. Divided into multiple temperature zones and gradually heated, the time above the melting point of the solder is not long, at most tens of seconds.
When it comes to the name reflow soldering, almost everyone doesn’t know what “reflow” is. It doesn’t mean that the wind inside will flow back and forth, let alone that the solder flows from here to there. Reflow soldering comes from “Reflow Soldering”, the real meaning of this “reflow” is “turning solid solder paste back into a liquid that can flow”.
During the process of passing through the furnace, the solder paste will melt, and the melted solder paste will exhibit the characteristics of a liquid: it is adsorbed to the place where it can be adsorbed, and tension is generated. It is often the case that the patch is rectified when it is pasted, and it is crooked after passing through the oven. This is often caused by the difference in tension between the pads. For example, some pads are large and some chips are small. This need is solved by controlling the amount of tin and the shape of the solder paste, or by dispensing and fixing before passing through the furnace.
There is also a pin-based soldering method, wave soldering, which is still used on old-fashioned circuit boards and large-sized simple circuit boards, and is dedicated to soldering pin circuits. The wave crest of wave soldering means that after the solder is melted, the solder is sprayed out through the nozzle of the equipment, forming a wave shape like a small fountain. Insert the pins of the components into the wave crest, and then the solder can be stained, and the adsorption force of the solder is strong. Solder the pads and pins together.
In the field of intelligent hardware, there are almost no products with full-board plug-in components. Most of them are full-board SMT. Occasionally, some large-sized connectors may need to be wave soldered.