SMT components are commonly called pin-free components or chip components. SMT passive components, such as chip resistor, capacitor and inductor, are also known as SMC (Surface Mounted Components). While active devices, such as small outline transistor (SOT) and quad flat pack (QFP), are called SMD (Surface Mounted Devices).
Both SMC and SMD have the same functions as traditional through-hole components. At first, they were manufactured to reduce the volume. However, once they were published, they showed great vitality. Their advantages such as smaller volume, higher frequency characteristics, vibration resistance and compact installation were beyond the traditional through-hole components, which greatly stimulated the development of electronic products to be multi-functional, high performance,miniaturized and low-cost. At the same time, these micro-electronic products have promoted SMC and SMD to continue to develop towards miniaturization. Chip resistor and capacitor has been reduced from 3.2mm*1.6mm in the early stage to 0.4mm*0.2mm. The center distance of IC pin has been reduced from 1.27mm to 0.3mm. With the development of bare chip technology, the multi-pin devices BGA and CSP have been widely used in production. In addition, some electromechanical components, such as switch, relay, filter and delay line, have also been chip-type.
On the electrodes of SMT components, some welding ends have no lead at all, and some have very short lead; the spacing between adjacent electrodes of SMT integrated circuits is much smaller than the standard lead spacing of traditional THT integrated circuits (2.54mm), and the spacing between pin centers has been 0.3mm at present. Under the same integration degree, the volume of SMT device is much smaller than that of THT device; under the same volume, the integration degree of SMT device is increased many times.
SMT components are directly mounted on the surface of PCB, and the electrodes are welded on the same pad as the components. In this way, the diameter of through-hole on PCB is only determined by the technology level of metallization hole when making sound circuit board. There is no pad around the through-hole, which greatly improves the wiring density and assembly density of PCB.