The main advantages of SMT over the older through-hole technique are:
Smaller components. As of 2012 smallest was 0.4 × 0.2 mm (0.016 × 0.008 in: 01005). Expected to sample in 2013 are 0.25 × 0.125 mm (0.010 × 0.005 in, size not yet standardized)
Much higher component density (components per unit area) and many more connections per component.
Higher density of connections because holes do not block routing space on inner or back-side layers.
Components can be placed on both sides of the circuit board.
Small errors in component placement are corrected automatically as the surface tension of molten solder pulls components into alignment with solder pads.
Better mechanical performance under shake and vibration conditions.
Lower resistance and inductance at the connection; consequently, fewer unwanted RF signal effects and better and more predictable high-frequency performance.
Fewer holes need to be drilled.
Lower initial cost and time of setting up for production.
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.
Better EMC performance (lower radiated emissions) due to the smaller radiation loop area (because of the smaller package) and the smaller lead inductance.