The term printed circuit board assembly (or PCB assembly) refers to the process of attaching electronic components to a printed circuit board in order to create a fully functional completed printed circuit assembly. There are two types of construction used in PCB assembly, through-hole construction and surface-mount construction. Both methods have their advantages and disadvantages and are both integral to the process.
Through-hole construction involves the leads of components being pushed through holes in the PCB and soldered to pads on the other side. This can either be done by hand or with the use of mounting machines. Through-hole replaced point-to-point construction and was used for almost every component on all circuit boards from 1950s until surface-mount construction became popular in the 1980s.
Surface-mount construction involves components being attached directly to the surface of a PCB. Surface-mount components can be much smaller than there through-hole equivalents as they have much smaller leads or even none at all. They sometimes use pins, flat contacts or solder balls as an alternative to leads.
Surface-mount components have several advantages over the older style through-hole components. As mentioned above they can be much smaller but they can also have many more connections. They also allow for much faster and simpler assembly and components can be placed on both sides of the board. Additionally, they are also cheaper than through-hole components as a general rule.
It is definitely fair to say that surface-mount components are preferred in modern China PCB fabrication and assembly methods but both types are still used in most cases. There are still some components that are only available in one type or the other and through-hole construction provides some extra strength to the connection that may sometimes be required. Due to the complex nature of attaching surface-mount components, amateurs hobbyists are much more likely to use through-hole components.
Components are attached to PCBs using a number of different soldering methods. Large scale production is usually done with machine placement and most likely reflow ovens. Very tiny components may be attached by highly skilled professionals with the help of a microscope and precision tools.
Once assembly is complete the finished circuit is usually tested by visual inspection to check the quality of the construction, analog signature analysis while the power is off, in circuit testing to check voltage and frequency while the power is on and finally a functional test to make sure the circuit operates in the way it was intended to.