The main advantages of SMT over the older through-hole technique are:
Smaller components. As of 2017 smallest component is metric 0201 measuring 0.25mm × 0.125mm
Much higher component density (components per unit area) and many more connections per component.
Components can be placed on both sides of the circuit board.
Higher density of connections because holes do not block routing space on inner layers, nor on back-side layers if components are mounted on only one side of the PCB.
Small errors in component placement are corrected automatically as the surface tension of molten solder pulls components into alignment with solder pads. (On the other hand, through-hole components cannot be slightly misaligned, because once the leads are through the holes, the components are fully aligned and cannot move laterally out of alignment.)
Better mechanical performance under shock and vibration conditions (partly due to lower mass, and partly due to less cantilevering)
Lower resistance and inductance at the connection; consequently, fewer unwanted RF signal effects and better and more predictable high-frequency performance.
Better EMC performance (lower radiated emissions) due to the smaller radiation loop area (because of the smaller package) and the lesser lead inductance.
Fewer holes need to be drilled. (Drilling PCBs is time-consuming and expensive.)
Lower initial cost and time of setting up for mass production, using automated equipment.
Simpler and faster automated assembly. Some placement machines are capable of placing more than 136,000 components per hour.
Many SMT parts cost less than equivalent through-hole parts.
A surface mount package is favored where a low profile package is required or the space available to mount the package is limited. As electronic devices become more complex and available space is reduced, the desirability of a surface mount package increases. Concurrently, as the device complexity increases, the heat generated by operation increases. If the heat is not removed, the temperature of the device rises shortening the operational life. It is therefore highly desirable to develop surface mount packages having high thermal conductivity.