Look inside any piece of commercially made electronic equipment these days and it is filled with minute devices. Rather than using traditional components with wire leads like those that may be used for home construction and kits, these components are mounted onto the surface of the boards and many are minute in size.
This technology is known as Surface Mount Technology, SMT and SMT components. Virtually all today's equipment that is manufactured commercially uses surface mount technology, SMT, because it offers significant advantages during PCB manufacture, and in view of the size the use of SMT components enables far more electronics to be packed into a much smaller space.
In addition to the size, surface mount technology allows automated PCB assembly and soldering to be used, and this brings significant improvements in reliability as well as enormous savings in cost.
During the 1970s and 1980s the level of automation started to rise for PCB assembly for boards used in a variety of equipment. The use of traditional components with leads did not prove easy for PCB assembly. Resistors and capacitors needed to have their leads pre-formed so that they would fit through holes, and even integrated circuits needed to have their leads set to exactly the right pitch so that they could be placed through the holes easily.
This approach always proved difficult as leads often missed the holes as tolerances required to ensure they fitted exactly through the holes were very tight. As a result operator intervention was frequently required to resolve the issues of components not fitting properly and stopping the machines. This slowed down the PCB assembly process and considerably increased costs.
For PCB assembly there is actually no need for the component leads to pass through the board. Instead it is quite adequate for components to be soldered directly to the board. As a result, surface mount technology, SMT was born, and the use of SMT components rose very rapidly as their advantages were seen and realised.
Today surface mount technology is the main technology used for PCB assembly within electronics manufacturing. SMT components are able to be made very small, and may types are used in their billions, particularly SMT capacitors and SMT resistors.
SMT is used almost exclusively for the PCB assembly and manufacture these days. It is possible to pack far more electronics into a smaller space using SMT. The surface mount components are smaller and often offer a better level of performance and they can be used with automated pick and place machine that in many cases all bit eliminate the need for manual intervention in the assembly process.
Wired components were always difficult to place automatically because the wires needed to be pre-formed to fit the relevant hole spacing, and even then they were prone to problems with placement.
Today in the PCB assembly process, most of the components on a board are placed automatically. Occasionally some may need manual intervention, but this is being reduced all the time. Traditionally some connectors and possibly a few other components required assisted placement, but the level of manual placement is falling all the time. Today, printed circuit boards are normally developed to reduce this to an absolute minimum, even to the extent of altering the design to use components that can be placed automatically. In addition to this, component manufacturers have developed some specialised surface mount versions of components that enable virtually complete automated assembly for most boards.
One of the issues with some components has been their resilience to heat. The soldering processes require that the whole component is raised to a high temperature, and this has caused issues with some technologies. Integrated circuits, surface mount resistors and many types of surface mount capacitor are fine.
However, it was for this reason that surface mount electrolytic capacitors were not used initially. Instead surface mount tantalums were used, but now, versions of surface mount electrolytic capacitors have been developed that are able to tolerate the temperatures experienced during soldering.
There are other components that have required special development to enable them to become available in surface mount component formats.