What are the characteristics of PCBA surface mount components?

The widespread use of microelectronics has promoted the development of SMC and SMD towards miniaturization. At the same time, some electromechanical components, such as switches, relays, filters, delay lines, thermal and varistor, have also been sliced. Surface mount components in PCBA processing plants have the following salient features

(1) On the electrodes of SMT components, some soldered ends have no leads at all, and some have very small leads. The spacing between adjacent electrodes is smaller than the lead pitch (2.54mm) of conventional dual in-line integrated circuits. Many, the pin center distance of the IC has been reduced from 1.27mm to 0.3mm; in the case of the same integration, the area of the SMT component is much smaller than the traditional components, the chip resistor and capacitor have been from the early 3.2mm × 1.6mm is reduced to 0.6mm × 0.3mm; with the development of bare chip technology, BGA and CSP high pin count devices have been widely used in production.

(2) SMT components are directly mounted on the surface of the printed circuit board, and the electrodes are soldered to the pads on the same side of the smt component. In this way, there are no pads around the through holes on the PCB printed circuit board, which greatly increases the wiring density of the printed circuit board.

(3) Surface mount technology not only affects the area occupied by the wiring on the printed circuit board, but also affects the electrical characteristics of the device and components. Lead-free or short leads reduce component parasitic capacitance and parasitic inductance, which improves high-frequency characteristics and helps increase the frequency of use and circuit speed.

(4) The shape is simple, the structure is firm, and it is close to the surface of the PCB printed circuit board, which improves the reliability and shock resistance; there is no lead bending and trimming during assembly, and the insertion is reduced when manufacturing the printed circuit board. Through-holes of components: Standardized in size and shape, can be automatically mounted by automatic placement machine, high efficiency, high reliability, easy to mass production, and low overall cost.

(5) In the traditional sense, surface-mount components have no pins or short pins. Compared with the components, the solderability test method and requirements are different, and the entire surface component is subjected to higher temperatures. However, the surface-assembled pins or terminals can withstand lower temperatures during soldering than the DP pins.

Of course, surface-mount components also have shortcomings. For example, a sealed chip carrier is expensive and generally used for high reliability products, which requires matching with the thermal expansion coefficient of the substrate. Even then, the solder joint is still prone to failure during thermal cycling; since the components are closely attached to the surface of the substrate The components and the surface of the PCB are very close, and the gap on the substrate is quite small, which causes difficulty in cleaning. To achieve the purpose of cleaning, it is necessary to have a relatively good process control: the components are small in size, and the resistors and capacitors are generally not marked. Once messed up, it is not easy to figure out; there is a difference in thermal expansion coefficient between components and PCB, and such problems must be noted in SMT products.

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