Surface Mount Technology & SMT Devices

- an overview of surface mount technology, SMT, why it is used, and surface mount devices, SMDs or SMT components

Surface Mount Technology, SMT Includes:
What is SMT     SMD packages     Quad flat pack, QFP     Ball grid array, BGA     Plastic leaded chip carrier, PLCC    

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, manufactured commercially uses surface mount technology, SMT, because it offers significant advantages during 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 production and soldering to be used, and this brings significant improvements in reliability.

Typical PCB PCB using surface mount technology - components include a large surface mount FPGA
Typical PCB using surface mount technology

What actually is SMT?

During the 1970s and 1980s the level of automation in the construction of electronic equipment started to increase. The use of traditional components with leads did not prove easy. 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.

For printed circuit board technology there is 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.

Concept of surface mount technology
Concept of surface mount technology: a typical passive component

Today surface mount technology is the main technology used for electronics manufacturing. SMT components are able to be made very small, and may types are used in their billions, particularly capacitors and resistors.

SMT devices

Surface mount technology, SMT, components, or surface mount devices, SMDs, as they are often called are different to their leaded counterparts. Rather than being designed to wire between two points SMT components are designed to be set down on a board and soldered to it. Their leads to not go through holes in the board as might be expected for a traditional leaded component. There are different styles of package for different types of component. Broadly the package styles can be fitted into three categories: passive components, transistors and diodes, and integrated circuits and these three categories of SMT components are viewed below.

Selection of a variety of surface mount technology devices: resistors and capacitors through to integrated circuits
A selection of surface mount technology components
  • Passive SMDs:   There is quite a variety of different packages used for passive SMDs. However the majority of passive SMDs are either resistors or capacitors for which the package sizes are reasonably well standardised. Other components including coils, crystals and others tend to have more individual requirements and hence their own packages.

    Resistors and capacitors have a variety of package sizes. These have designations that include: 1812, 1206, 0805, 0603, 0402, and 0201. The figures refer to the dimensions in hundreds of an inch. In other words the 1206 measures 12 hundreds by 6 hundreds of an inch. The larger sizes such as 1812 and 1206 were some of the first that were used. They are not in widespread use now as much smaller components are generally required. However they may find use in applications where larger power levels are needed or where other considerations require the larger size.

    The connections to the printed circuit board are made through metallised areas at either end of the package.
  • Transistors and diodes:   These components are often contained in a small plastic package. The connections are made via leads which emanate from the package and are bent so that they touch the board. Three leads are always used for these packages. In this way it is easy to identify which way round the device must go.
  • Integrated circuits:   There is a variety of packages which are used for integrated circuits. The package used depends upon the level of interconnectivity required. Many chips like the simple logic chips may only require 14 or 16 pins, whereas other like the VLSI processors and associated chips can 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. These are effectively the SMT version of the familiar DIL (Dual In Line) packages used for the familiar 74 series logic chips. Additionally there are smaller versions including TSOP (Thin Small Outline Package) and SSOP (Shrink Small Outline Package).

    The VLSI chips require a different approach. Typically a package known as a quad flat pack is used. This has a square or rectangular footprint and has pins emanating on all four sides. Pins again are bent out of the package in what is termed a gull-wing formation so that they meet the board. The spacing of the pins is dependent upon the number of pins required. For some chips it may be as close as 20 thousandths of an inch. Great care is required when packaging these chips 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. Instead of having the connections on the side of the package, they are underneath. The connection pads have balls of solder that melt during the soldering process, thereby making a good connection with the board and mechanically attaching it. As the whole of the underside of the package can be used, the pitch of the connections is wider and it is found to be much more reliable.

    A smaller version of the BGA, known as the microBGA is also being used for some ICs. As the name suggests it is a smaller version of the BGA.

SMT in use

SMT is used almost exclusively for the manufacture of electronic circuit boards these days. They are smaller, 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.

Although many connectors and some other components still require assisted placement, 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.

SMT applications

Although it is possible to use some SMT components for home construction, great care is required when soldering them. Additionally even the ICs having a wide pin spacing may be difficult to solder. Those with fifty or more pins cannot be soldered without special equipment. They are intended only for large scale manufacturing. Even when working on boards that have already been built great care is needed. However these SMT components offer great cost savings to manufacturers and this is why they have been adopted. Fortunately for the home constructor, traditional leaded components that can be soldered manually are still widely available and offer a much better solution for home construction. Nevertheless SMT components can be used for some home projects where they are applicable - where the SMT component leads and connections are not too small to manage with more traditional soldering irons and other tools.

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