PCBs or printed circuit boards have paved the way for enjoying highly advanced technologies and devices. These compact circuit boards allow for high frequency, high speed electronic processing necessary in devices such as computers and wireless communications. Over the years, newer materials have been developed to use in PCB assembly. The process remains basically unchanged through the years since the PCB was first developed in the 1950s. A few touch ups were added to the process as newer materials were incorporated to improve PCB design and function.
PCB Assembly Process
Modern PCBs are manufactured using the SMT or surface mount technology. This process follows the following process:
• Applying solder paste
This is a paste material composed of a mix of small solder grains and flux. This paste is applied on the circuit board in a process very much like a printing process.
The solder paste needs to be applied on the board surface before the components are added. It is applied on the surface of the board where the solder is to be applied later on in the process. These areas are typically the component pads. To apply, a solder screen is used.
The solder screen is prepared by matching it to the design of the desired printed circuit board. It is then placed directly on the board surface. A runner is then moved across the solder screen. As the runner is moved, a small amount of solder paste is squeezed onto the board through the screen. The solder paste squeezed out only passes through the screens that matches the designated position of the solder pads (according to the printed circuit board design).
Hence, the solder paste is only applied on the solder pads and not anywhere else on the board surface. It is very important that the amount of solder placed on the solder pads is carefully controlled. This is to ensure that the joints have just the right amount of solder in them.
• Process of pick and place
After applying the solder paste, the board is subjected to the pick and place process. The board is placed on a machine that contains reels of PCB components. As the board passes through, the machine picks the right components off the reels and attaches them in the designated positions on the board. The components are held into place from the tension produced by the solder paste. The tension is enough to keep the components in place, as long as the board is not shaken.
Sometimes, the machines used for the pick and place process add glue in small dots to further secure the components into the board. This can only be done if the board is to be subjected to wave soldering. Adding glue can be a disadvantage if repairs may become necessary later in the process.
The pick and place machine derives the information on where to place the components from the programmed design of the printed circuit board.
The board is then passed through a soldering machine. This part of the PCB assembly process will solder the components in place. Some machine use a wave soldering system, but this is fairly uncommon with the SMT.
After the soldering process, the board is inspected. An automatic optical inspection is used rather than annual inspection of the soldered boards. Manual inspection is not a good idea if there are hundreds of manufactured boards. Inspection machines are designed to check the boards for poor joints, wrong components and misplaced components.
Quick turn PCB assembly requires testing for function before they are packaged and shipped. Testing includes proper electronic flow, properly placed components and if components overheat and detach while in use.
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