Printed board circuit assembly that uses surface-mount technology (SMT) involves a number of different stages – application of solder paste to the board, placement of devices, soldering, inspection, and testing. All of these stages are needed and have to be monitored to guarantee the best quality. Currently, almost all PCB assembly processes use surface-mount technology.
Before putting in the components to a PCB, solder paste has to be applied in the areas of the PCB where solder is needed. This method is done using a solder screen. A solder paste is basically a powdered metal solder mixed with flux.
The solder screen is directly placed into the PCB, and then a runner moves through the screen dispensing a small amount of solder paste into the holes of the screen and into the PCB. The solder screen is previously created from the PCB files, so it has holes on the solder pads. This way, the solder paste is only placed on the solder pads.
During this stage of the PCB assembly process, the PCB with solder paste goes through the pick-and-place (P&P) method. A machine that holds the components picks up and correctly places the components into the PCB. The components placed on the PCB are held in position by the solder paste. Given that the PCB is not shaken, this is enough to keep the components steady.
In other assembly methods, the P&P machines affix a small amount of glue to keep the components secured to the PCB. However, this is usually done only if the PCB will go through wave soldering. A drawback of this method is that a needed repair will be much more difficult to perform because of the glue.
When the components have been placed, the PCB has to go through a soldering machine. While some PCBs may go through a wave soldering machine, surface-mount components do not commonly use this method. The solder paste is not applied into the PCB if it will go through a wave soldering machine because this type of machine provides the solder. Instead, the reflow soldering method is more extensively used.
Manual inspection is not usually done in SMT when using over one hundred components in PCB assembly. A much more practical method is an automatic optical inspection. There are machines that can inspect PCB’s and identify misplaced components, poor connections, and in some cases, an incorrect component.
Any electronic product must be tested before it leaves the factory. There are a number of ways a PCB can be tested. Power-off testing and analog signature analysis can be done while the power is off. In-circuit test is done while the power is on in which physical measurements are performed. Another test that can be done while the power is on is called functional test where the PCB is checked if it does what it is supposed to.
It is essential to check the outputs to make sure that the assembly method is running smoothly. This can be done by examining if a failure has been discovered. The ideal stage is during the optical inspection because this commonly happens after the soldering stage. Through this, the faults in the process can be identified immediately and corrected before too many PCB’s are created with the same fault.
The PCB assembly and manufacturing methods are continually improving to make sure there are only a few defects. Because of that, it will produce only the best quality electronic products. Considering the high demand in quality and the high number of components and solder connections used in the products nowadays, the assembly method is crucial in providing electronic products of the best possible quality.
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