What does SMD mean in electronics

Surface-mounted device

Board with pure SMD assembly

Surface-mounted device (short SMD) is a technical term from electronics and describes so-called "surface-mounted" components. These components (e.g. resistors or capacitors) have no wire connections, but are soldered directly to a circuit board using solderable connection surfaces.

construction

While the connection wires of conventional components, as they were still common until the end of the 1980s, are passed through assembly holes and have to be soldered on the back of the circuit board (or via inner layers) (through-hole plating), this is not necessary with SMD components. This enables very dense assemblies and, above all, two-sided assembly of the circuit board and the space requirement of the components is less. As a result, the devices can be made smaller and at the same time much cheaper.

For hobby electronics there is a disadvantage, however, since SMD components are very difficult to solder without machines. Therefore, in the hobby area, SMD components are avoided as far as possible.

Printed circuit boards with SMD components are usually assembled on solder paste and soldered using the reflow process. SMD components on the underside of a printed circuit board with mixed components are first glued to the underside, and then soldered together with the conventionally assembled components in a wave or surge pool.

The main advantages of using SMDs

  • Smaller component dimensions. (results in higher component density)
  • There is no need to drill holes in the circuit board. (Reduction of "drill jobs")
  • Simple machine equipment.
  • Automatic optical inspection (AOI) of all critical optically testable factors is possible for most designs.
  • Small positional errors during component assembly are automatically corrected during soldering (reflow soldering only) (the components are pulled into the correct position by the surface tension of the molten tin).
  • Components can also be fitted directly "one below the other" on both sides of the circuit board.
  • low connection resistance and impedance (important for high-frequency assemblies).

Disadvantages of SMDs

  • Components can be detached from mechanical stress more easily than THTs (especially critical for plugs, switches, etc.)
  • Production of SMD-equipped circuit boards requires trained personnel and special machinery
  • In the case of components with connections on the underside of the component (such as BGAs), the soldering points cannot be visually checked (X-ray inspection necessary)

Designs

There are different types of SMD components Designs. These shapes differ, among other things, in the dimensions and the way in which the connections are attached to the component.

Chip design

Rectangular designs are common for resistors and capacitors (with low capacitance). These differ significantly in terms of size, which is specified e.g. with 1206. 12 stands for the length and 06 for the width. In many cases, the basic unit of measurements is still inches. In the meantime, however, metric dimensioning of components is becoming more and more popular. In addition, the names of the designs can differ from manufacturer to manufacturer, as there is no generally applicable standard for naming components.

inch metric
Design mil (milli-inch) Design mm
1206 120 × 60 3216 3,2 × 1,6
0805 80 × 50 2012 2,0 × 1,2
0603 60 × 30 1608 1,6 × 0,8
0402 40 × 20 1005 1,0 × 0,5
0201 20 × 10
01005 10 × 5

Tantalum capacitors

  • Type A (EIA 3216-18): 3.2 mm × 1.6 mm × 1.6 mm
  • Design B (EIA 3528-21): 3.5 mm × 2.8 mm × 1.9 mm
  • Design C (EIA 6032-28): 6.0 mm × 3.2 mm × 2.2 mm
  • Design D (EIA 7343-31): 7.3 mm × 4.3 mm × 2.4 mm
  • Design E (EIA 7343-43): 7.3 mm × 4.3 mm × 4.1 mm

With these designs (as in most cases) the connections are on the narrow side of the component. The design is similar to the SOD design common for diodes, but the connections for the tantalum are directly on the component body (with SOD, they usually protrude 0.4 mm from the body). The components are either yellow with orange polarity marking or black with white marking.

SOT

"Small-outline-transistor" with 3, 5 or 6 connections. Usually houses a transistor (or a transtorarry), but an IC can also be an LED or a diode array.

  • SOT23 3 mm × 1.75 mm × 1.3 mm component body with three connections
  • SOT25 3 mm × 1.75 mm × 1.3 mm component body with five connections
  • SOT26 3 mm × 1.75 mm × 1.3 mm component body with six connections

Other designs

For ICs, transistors, diodes and capacitors there are: BGA (ball grind array), PLCC, SO, SOD, Melf.

Due to the progressive miniaturization of components, it is now becoming more and more difficult to check circuit boards with SMD components with the naked eye or with the aid of a microscope; AOI systems have now been developed for this purpose.

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This article is based on the article Surface-mounted device from the free encyclopedia Wikipedia, some text passages may have been adopted. A list of the authors is available on Wikipedia.
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