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    

Look inside any piece of commercially made electronic equipment these days and it is filled with minute devices. Rather than using the more traditional components that are normally 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.

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.

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 components

Surface mount (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.

Passive SMT components

The packages for surface mount resistors and many capacitors are well standardised. They rectangular in shape, having metallised contacts at both ends. The common packages are 1812, 1206, 0805, 0603, 0402, and 0201. The figures describe the package size measurements in Imperial measurements. In other words an 0805 package measures 0.08 by 0.05 inches. Some of the larger packages are now becoming less common. With surface mount techniques now well developed, the smaller packages are more widely used. The larger ones are now generally reserved for more specialised components. For example, resistors requiring to dissipate more than average levels of heat, or larger capacitors may require different packages.

Tantalum capacitors require packages that are somewhat larger than many of the resistors or ceramic capacitors that can comfortably fit into smaller packages. There is a separate set of packages used for them. Also other components such as coils or inductors, crystals and other specialised components have individual requirements and so they tend to have packages that are suited to the individual components. However they are all smaller than those that are normally used with traditional wire ended component technologies.

Transistors and diodes

Diodes and transistors generally use a package called a Small Outline Transistor (SOT). One of the most popular is the SOT23. This package has three leads, one on one side and two on the opposite. The come from the side of the package and are bent in what is termed a gull wing shape. This enables them to make contact with the board.

Even though diodes have only two connections, they still use the same package. By two leads on one side and one on the other, this enables the connections to be differentiated more easily.

The SOT23 package can only be used for low power applications. Where higher powers are needed another package designated the SOT223 may be used. Naturally there is a wide variety of other SOT packages to accommodate different requirements.

Integrated circuits

There is a very wide variety of packages used for integrated circuits. The widely varying requirements for interconnectivity of the different ICs mean that a number of different solutions are required.

For the smaller SMT ICs a variety of packages are used:

  • SOIC Small Outline Integrated Circuit. This is the standard package for logic ICs and it is the equivalent (although smaller) of the DIL package for leaded components.
  • SSOP Shrink Small Outline Package. This is smaller than the SOIC and has a pin spacing of 0.635 mm.
  • TSSOP Thin Shrink Small Outline package. As the name suggests this is thinner than the SSOP.

Many integrated circuit SMDs require much greater levels of interconnectivity than are offered by the smaller packages. Microprocessors and many other chips require over a hundred connections, and these cannot be accommodated by the SOIC, SSOP and TSSOP packages. Some of the main types are summarised below:

  • QFP Quad Flat Pack. This is a square package with leads on all four sides. The leads have a gull wing shape and have a narrow spacing. The spacing varies according to the actual chip. These are very delicate and require handling with great care.
  • BGA Ball Grid Array. The problem with the QFP is that it is difficult to handle, and even with automated pick and place machines, these chips can be damaged very easily. The BGA adopts a different approach. It makes use of the space under the chip and small pads are placed here. They have a small ball of solder - hence the name - and when they pass through the soldering machine these solder balls melt and solder the BGA to the printed circuit board.

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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