pcb assembly components, commonly known as leadless components, were born in the 1960s. Usually, surface-mount passive components, such as chip resistors, capacitors, inductors, etc., are called pcb assembly components (SMC), while active components, such as small outline transistors, quad flat packaging, etc., are called pcb assembly devices ( SMD). Both SMC and SMD are functionally identical to the Through Hole Component/Device (THCD).
pcb assembly components have the following characteristics:
(1) There are no pins or only very short pins on the soldered end of the surface-mount components, the lead pitch is reduced, and the degree of integration is improved. The standard lead pitch of conventional through-hole ICs is 2.54mm. At present, the minimum pitch of surface-mount components has reached 0.3mm. In the case of the same degree of integration, the pcb assembly components are much smaller than the via-hole components in volume, or the integration of pcb assembly components is many times higher than that of conventional circuit chips of the same size.
(2) The pcb assembly components are directly mounted on the surface of the PCB, and the leads are soldered to the welding pads on the same side on the components. Thus, the through holes on the PCB are typically only used as electrical connections to the multilayer circuit board, the diameter of which is determined by the level of metallization of the printed circuit board. There are no welding pads around the vias, which greatly increases the wiring density and assembly density of the PCB.
(2) The development of chip-forming components of pcb assembly components is unbalanced, and the development of RC components, transistors, and integrated circuits is relatively fast. The development of special-shaped components, sockets, and oscillators is slow, and some high-power, high-voltage, and high-plugging forces connector, can't be sliced.
(4) Components that have been sliced have not yet been fully standardized, and products from different countries and even different manufacturers have large differences. Therefore, when designing and selecting components, it is necessary to figure out the model, manufacturer and performance of the components firstly in order to avoid defects caused by poor interchangeability.
(5) The surface-assembled components are very close to the surface of the PCB, and the gap with the substrate is small, which causes difficulty in cleaning.
(6) Due to the small size and high assembly density of SMC/SMD, it brings some problems such as poor heat dissipation performance and difficulty in PCB design and manufacturing.
(7) The above is the information provided by FASTPCBA, I hope it’s helpful to you!