With the development of electronic component package technologies towards miniature, light weight and high performance, it has been a development trend of electronic components to increase component function density and decrease spacing between input terminals and output terminals, which is best displayed by automatic assembly technology featured by SMT (surface mount technology). In order to implement surface mounting of components, the first step is to manufacture corresponding pads on PCB so that structured PCB can be obtained. Then, stencil printing technology is applied to cover solder paste on the surface of PCB pads. Finally, heating is carried out to transform solder paste into liquid that is formed into liquid bridge between pins of components and PCB pad. Under the influence of solder mask on PCB, melting solder paste is limited in the corresponding solder pad area in order to prohibit bridging between solder joints so that automatic assembly of chip on PCB is implemented. According to different package types, circular and rectangular solder pads are majorly selected, that is, BGA (ball grid array) and QFN (quad flat no-lead) package. If you want to know more about BGA, just FOUR steps are sufficient.