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.smt pcb assembly process
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.
