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What is SMT technology? Why should OEMs care about it?
Original equipment manufacturers (OEMs) are organizations that manufacture or sell products to other companies. These companies then use these components as part of a broader process to manufacture products for their customers under their own brands. Much of the work of manufacturing products and selling them to other companies revolves around the SMT process.
Original Equipment Manufacturers (OEMs) are organizations that manufacture or sell products to other companies. These companies then use these components as part of a broader process to manufacture products for their customers under their own brand. Much of the work of manufacturing products and selling them to other companies revolves around the SMT process.
What is SMT?
Surface Mount Technology (SMT) is the process of mounting components onto the surface of a printed circuit board. For the vast majority of electronic products, the components are specifically designed to be mounted directly onto the circuit board rather than hard-wired.
Surface mount technology has been widely used since the 1980s. This is because the SMT process is automated, meaning it can save manufacturers and customers both time and money. Previous methods involved manually wiring components together through holes in the circuit boards, meaning this required manual labor and, of course, carried the risk of human error. Compared to the through-hole wiring method, the SMT process is also more adaptable to future technological advancements.
Other advantages of surface mount technology include higher component density and smaller component sizes, as well as better pressure resistance. Using surface mount technology also means that components can be mounted on both sides of the circuit board.
What is the typical SMT process flow?
The surface mount technology process mainly involves three steps, including solder paste printing, component placement, and reflow soldering.
Solder Paste Printing
The solder paste printing process is done by machine, ensuring accuracy and speed.
In this part of the assembly, a printer uses a pre-made PCB stencil and squeegee to apply solder paste. This paste is typically a mixture of flux and tin, used to connect the SMCs and pads on the PCB.
In this process, the correct amount of solder paste must cover each pad. Otherwise, the solder will not be able to make a connection when it melts in the reflow oven (more on this later).
Controlling the quality of the solder paste printing process is crucial. This is because if any printing defects are not detected at this stage, they will lead to other defects later on. Therefore, the design of the stencil is crucial, and the assembly team must exercise care to ensure the process is repeatable and stable. Thankfully, to help the process run smoothly, most solder paste printers have the option of including automatic inspection.
However, sometimes external machines are also used to assess the printing quality. These solder printing inspection machines use 3D technology to allow for more thorough inspection. This is because they check the solder paste volume on each pad, not just the printing area.
Component Placement
Once the PCB passes inspection, it moves to the component placement stage of the SMT assembly process.
In this stage, each component to be mounted on the PCB is picked from its packaging using vacuum nozzles or grippers. The machine then places it in its programmed location. The machines that perform this process are not only highly accurate but also extremely fast. Some advanced machines can place 80,000 individual components per hour.
After all the individual components have been placed on the PCB, they must be inspected to ensure they are correctly placed. This is a very important step in the process because if any placement errors are not detected and the parts are soldered into place, it will lead to significant rework, which is both expensive and time-consuming.
Reflow Soldering
Once the placed components pass inspection, the process moves to the reflow soldering stage. In this part of the SMT process, the PCBs are placed into a reflow soldering machine (some people call it a reflow oven).
Here, all the electrical solder connections are formed between the components and the PCB. Using heat, the previously applied solder paste is transformed into solder. Again, accuracy is crucial at this stage because if the PCB is heated to too high a temperature, the parts or components may be damaged and the PCB will not function as intended. If the temperature is too low, connections may not be made.
To ensure optimal results, all PCBs inside the soldering machine are placed on a conveyor belt. They are then gradually heated in a series of zones before passing through a cooling zone.
To avoid solder joint defects, the PCBs must remain in each zone for the correct amount of time. The PCBs must also be completely cooled before being handled or moved. Otherwise, they may warp.
After the PCBs have passed through the reflow soldering machine, a final inspection is carried out. This inspection is usually performed by a 3D Automated Optical Inspection machine (AOI). This is to ensure that the solder joint quality is as expected and that no errors have occurred during the SMT process. Machines are used for this process because they are much faster than humans and provide more accurate analysis.
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