PCB Assembly refers to the attaching of electronic components to a printed circuit board. PCBs are used to mechanically connect and support electronic components using conductive pathways, traces or tracks which are etched from copper sheets laminated onto a non-conductive substrate.
Once the PCB fabrication is completed then electronic components can be attached to produce a printed circuit assembly. The leads of the components are inserted through holes in the PCB, and in surface mount construction, the electronic components are placed on lands or pads on the outer surfaces of the printed circuit board. The component leads in both types of this construction are then mechanically fixed to the PCB with soft metal molten solder.
There are all kinds of various techniques for attaching components to printed circuit boards, and most very high volume production is generally produced by machine placement and bulk wave soldering or reflow ovens.
Sometimes very skilled technicians are employed to solder very small parts by hand under a microscope. This is achieved by using tweezers and a very fine soldering tip. Some parts such as ball grid array packages are impossible to solder by hand.
Very often, surface mount and through the hole construction must be combined on a single printed circuit board, as some of the required electronic components are only available in surface mount packages, while others are only available in through hole packages.
One reason to use both of the above methods is that surface mount techniques take up less space and will go largely unstressed, while through the hole mounting can provide needed strength for any components which are likely to endure any physical stress. When the printed circuit board or PCB fabrication has been built or populated with the desired components, it can be tested in a number of different ways and these may include:
o Power on; functional test, checking if the PCB is doing what it is designed for
o Power on; in circuit test, physical measurements
o Power off; visual inspection & automated optical inspection
o Power off; analogue signature analysis, power off testing
Some printed circuit boards may have a conformal coating applied by dipping or spraying once the components have been soldered. This coating provides a protective layer which helps prevent corrosion and leaking current due to condensation. Assembled PCBs are sensitive to static and are often placed in antistatic plastic bags for handling and transport purposes.