In this article, you’ll discover the steps required in the PCB assembly process. Note that as an example, the process described will assume that surface-mount technology will be used.
Before adding components to the board, solder paste has to be applied to the areas where solder is needed. Often, these are the component pads. This step makes use of a solder screen, too. The paste is made of flux mixed with tiny grains of solder.
With a solder screen which has been placed in the correct and direct position on the board, a runner moves across it and squeezes the paste through the screen’s holes and finally onto the board. The screen has holes on corresponding locations on the solder pads, making sure that the solder is placed only on the pads. The solder is meticulously controlled to make sure that the amount deposited is just right.
Pick and Place
In this step of the PCB assembly, the board (that has the solder paste) moves on to the process known as “pick and place”. Here, reels of components are placed in a machine which picks the components and places them on their proper locations on the PCB. Once in place, the components are kept in place with the use of solder paste, which creates the right amount of tension. This is often sufficient to maintain their position, given that the board isn’t jolted.
A number of PCB assembly procedures use machines which add tiny dots of glue to keep the components on the board in place. This is only really done if the PCB is going to undergo wave soldering, though. One downside of this type of PCB assembly is that the glue can make any future repairs a lot more difficult (although there are some types of glue designed to degrade throughout the process).
The design of the printed circuit board contains the correct data on the positioning and other component information needed for programing the machine – simplifying this step of assembly.
Now that the components are on the board, the next step of the PCB assembly is passing the board through the soldering machine. Reflow soldering is more commonly employed these days compared to wave soldering*.
Some PCBs may be passed through a wave-soldering machine instead, although in this case (for PCBs with surface mount components), this process is rarely utilized. Instead, the solder paste isn’t added on the board – this is because solder is already included in the wave soldering machine.
As the boards pass through the soldering step, they are often inspected, too. Manual inspection used to be done – but never for surface-mount PCBs, which have hundreds of components at a time. Instead, a more viable, efficient solution comes in the form of automated optical inspection. Here, machines are able of inspecting PCBs, detecting poorly constructed joints, components in the wrong locations, and in rare cases, boards that have incorrect components.
Testing and Feedback
It is required to test all electronic products. PCBs are no exception. There are different testing methods available, depending on the manufacturer and methods used.
Also, to make sure that the manufacture of PCBs is running smoothly and according to quality standards, it is required to monitor the output as well. To do this, any detected failures during testing have to be further investigated. The ideal phase to do this is during the optical inspection step mentioned above. Since it takes place immediately after soldering, the defects can be identified quickly and the proper rectifications can be made before too many PCBs end up with the same defect.