Surface mount devices (SMDs) are the electronic circuits mounting onto the surface of PCBs by surface mount technology. They are very different to the traditional leaded components by their nature. They can be divided into some categories:
The majority of passive SMDs are either resistors or capacitors for which the package sizes are reasonably well standardised. The connections to the PCB are made through metallised areas at either end of the package. These resistors and capacitors have a variety of package sizes that include: 1812, 1206, 0805, 0603, 0402, and 0201. The larger sizes such as 1812 and 1206 were some of the first for use. But they are not in widespread use now as much smaller components are generally required. They may find use in applications where larger power levels are needed or where other considerations require the larger size.
•Transistors and diodes:
These components are often contained in a small plastic packages. To identify which way the device must go, the connections are made through three leads that emanate from the packages and bent to touch the board.
There is a variety of packages which are used for integrated circuits. The use of package depends on the level of interconnectivity. Many chips may only require 14 or 16 pins, whereas others require up to 200 or more.
In view of the wide variation of requirements there is a number of different packages available. For the smaller chips, packages such as the SOIC (Small Outline Integrated Circuit) may be used. Additionally there are smaller versions including TSOP (Thin Small Outline Package) and SSOP (Shrink Small Outline Package). For some chips it may be as close as 20 thousandths of an inch. Great care is required when packing and handling them as the pins are very easily bent.
Other packages are also available. One known as a BGA (Ball Grid Array) is used in many applications. The connections are underneath the package.It makes a good connection with the board through the solder balls that melt during the soldering process.