Surface mount technology (SMT)
- an overview of surface mount technology and surface mount devices SMDs
Virtually all of today's mass produced electronics circuitry
is manufactured using surface mount technology (SMT). However it was not until
the 1980 that surface mount technology became widely used. However once SMT
started to be used, the change from conventional leaded components to surface
mount devices, SMDs took place quickly in view of the enormous gains that could
be made using SMT.
Why SMT?
Mass produced electronic circuit boards need to be manufactured in a highly
mechanised manner. The traditional leaded electronic components do not lend
themselves to this approach. Although some mechanisation was possible, component
leads need to be pre-formed, and when they were inserted into boards
automatically problems were often encountered as wires did not fit properly
slowing production rates considerably.
It was reasoned that the wires that had traditionally been
used for connections were not actually needed for printed circuit board
construction. Rather than having leads placed through holes, the components
could be soldered onto pads on the board instead. This also saved the need to
drilling as many holes in boards.
As the components were mounted on the surface of the board,
rather than having connections that went through holes in the board, the new
technology was called surface mount technology or SMT. The idea for SMT was
adopted very quickly because it enabled greater levels of mechanisation to be
used, and it considerably saved on manufacturing costs.
To accommodate surface mount technology, SMT, a completely
new set of components was needed. New SMT outlines were required, and often the
same components, e.g. ICs were sold in both traditional leaded packages and SMT
packages. Despite this, the gains of using SMT proved to be so large that it was
adopted very quickly.
What are SMT components?
SMDs, i.e. SMT components, by their nature are very different to the traditional
leaded components. They can be split into a number of categories:
- Passive SMDs
- Transistors
- Integrated circuits
Passive SMDs: There is quite a variety of
different packages used for passive SMDs. Howeer the majority of passive SMDs
are either resistors or capacitors for which the package sizes are reasonably
standardised. Other components including coils, crystals and others tend to have
more individual requirements and hence their own packages.
Resistor and capacitor packages have a variety of packages.
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 powers 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 elvel 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 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. As the name suggests it is a smaller version of the BGA.
In Use
SMT is used almost exclusively for the manufacture of electronics circuit
boards. Now it is found that SMDs are far more widely used than traditional
leaded components. While there is still a need for many traditional components,
SMDs now form the main line stream for components
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