PCB Assembly and Production Process

- an overview of the PCB assembly process involved in building a surface mount technology (SMT) board using pick and place techniques.

Within a printed circuit board electronics assembly / production or manufacturing process there are a number of individual stages. However it is necessary for them all to work together to form an integrated overall process. Each stage of assembly and production must be compatible with the next, and there must be feedback from the output to the input to ensure that the highest quality is maintained. In this way any problems are detected quickly and the process can be adjusted accordingly.

PCB assembly process overview

The various stages in the PCB assembly process including adding solder paste to the board, pick and place of the components, soldering, inspection and test. All these processes are required, and need to be monitored to ensure that product of the highest quality is produced. The PCB assembly process described below assumes that surface mount components are being used as virtually all PCB assembly these days uses surface mount technology.

  • Solder paste:   Prior to the addition of the components to a board, solder paste needs to be added to those areas of the board where solder is required. Typically these areas are the component pads. This is achieved using a solder screen.

    The solder paste is a paste of small grains of solder mixed with flux. This can be deposited into place in a process that is very similar to some printing processes.

    Using the solder screen, placed directly onto the board and registered in the correct position , a runner is moved across the screen squeezing a small mount of solder paste through the holes in the screen and onto the board. As the solder screen has been generated from the printed circuit board files, it has holes on the positions of the solder pads, and in this way solder is deposited only on the solder pads.

    The amount of solder that is deposited must be controlled to ensure the resulting joints have the right amount of solder.
  • Pick and place:   During this part of the assembly process, the board with the added solder paste is then passed into the pick and place process. Here a machine loaded with reels of components picks the components from the reels or other dispensers and places them onto the correct position on the board.

    The components placed onto the board are held in place by the tension of the solder paste. This is sufficient to keep them in place provided that the board is not jolted.

    In some assembly processes, the pick and place machines add small dots of glue to secure the components to the board. However this is normally done only if the board is to be wave soldered. The disadvantage of the process is that any repair is made far more difficult by the presence of the glue, although some glues are designed to degrade during the soldering process.

    The position and component information required to programme the pick and place machine is derived from the printed circuit board design information. This enables the pick and place programming to be considerably simplified.
  • Soldering:   Once the components have been added to the board, the next stage of the assembly, production process is to pass it through the soldering machine. Although some boards may be passed through a wave soldering machine, this process is not widely used for surface mount components these days. If wave soldering is used, then solder paste is not added to the board as the solder is provided by the wave soldering machine. Rather than using wave soldering, reflow soldering techniques are used more widely.
  • Inspection:   After the boards have been passed through the soldering process they are often inspected. Manual inspection is not an option for surface mount boards employing a hundred or more components. Instead automatic optical inspection is a far more viable solution. Machines are available that are able to inspect boards and detect poor joints, misplaced components, and under some instances the wrong component.
  • Test:   It is necessary to test electronic products before they leave the factory. There are several ways in which they may be tested. Further views of test strategies and methods may be found on the "Test and Measurement" section of this website.
  • Feedback:   To ensure that the manufacturing process is running satisfactorily, it is necessary to monitor the outputs. This is achieved by investigating any failures that are detected. The ideal place is at the optical inspection stage as this generally occurs immediately after the soldering stage. This means that process defects can be detected quickly and rectified before too many boards are built with the same problem.

Summary

The PCB assembly process for the manufacture of loaded printed circuit boards has been considerably simplified in this overview. The PCB assembly and production processes are generally optimized to ensure very low levels of defects, and in this way produce the highest quality product. In view of the number of components and solder joints in today's products, and the very high demands placed on quality, the operation of this process is critical to the success of the products that are manufactured.

By Ian Poole

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