Electronic Manufacturing Basics: SMT Meaning Explained

smt meaning

If you are reading this, you probably just googled SMT meaning or something along those lines. Here is the short answer: SMT stands for surface-mount technology. The long answer, however, is a little more complicated.

SMT Basics

Surface-mount technology is used in the manufacture of electronic assemblies. The process involves soldering electronic components known as surface-mount devices (SMDs) directly onto the substrate of a printed circuit board (PCB). That substrate, which is usually made of epoxy glass, is the basis where SMD components are attached and connected to one another to form a functioning electronic circuit, also known as a printed circuit board assembly (PCBA).

Surface-mount technology is ubiquitous: virtually all mass-produced electronics today are developed using SMT.

SMT Component Types

Some of the most popular SMT components include surface-mount capacitors and surface-mount resistors. Both capacitors and resistors come in miniature rectangular packages.

SMT Packages

SMT components are available in a variety of “packages.” The types of packages differ depending on the technology being used, the level of interconnectivity required, and a number of other factors.

Most SMT packages are standardized to allow for the easy and efficient production of PCBs using automated equipment. Standardized packages are denoted using numbers or acronyms, such as 0201, 0403, 0603, 0805, 1206, BGA, SOIC, SOT, or QFP.

Other SMT components come in non-standardized packages. These are typically used in situations that require custom technological solutions.

Handling of PCB Components

Standardized surface-mount packages are specially developed for the purposes of automated PCB assembly. That means they must lend themselves to handling by pick-and-place machines. Also known as P&Ps or component placement systems, these machines are automated devices that are used to place SMDs onto printed circuit boards.

SMT packages must also support easy handling throughout the shipping and stock stages of the supply chain.

Ensuring that SMT components can be handled efficiently at all stages of the manufacturing process reduces the production costs and time and guarantees the highest possible product quality.

What Does the Handling Process Entail?

Typically, the smallest SMT components are placed loosely in a hopper before being fed down a tube and picked out as needed.

Larger components such as capacitors, resistors, transistors, and diodes are usually attached to a tape within a reel. A second tape is stuck loosely onto the back of the reel. As the components are fed to the machine, the retention tape is pulled back, exposing the next component in line to be used.

Other components, such as dual in-line packages, are often held in a vertical tube. As the package at the end of the line is removed as required, gravity forces the one right next to it to slide down.

Larger integrated circuits such as quad flat packs (QFPs) and plastic leaded chip carriers (PLLCs) may be held in what is colloquially known as a “waffle pack.” The waffle pack is placed on the pick-and-place machine, and its components are removed one by one as required.

SMT Package Standards

The sizes and dimensions of most SMT components conform to global industry standards. That ensures a degree of predictability and uniformity across the industry and helps simplify the PCB design and manufacturing process. The use of standard-sized packages allows pick-and-place machines to use standard feed for various SMT components, which saves time, costs, and effort.

The Joint Electron Device Engineering Council (JEDEC) specifications are among the main industry standards. The JEDEC Solid State Technology Association is an independent semiconductor engineering standardization body and trade organization. The JEDEC Association has more than 300 members, including some of the most prominent electronics companies. Among other things, it is responsible for managing and developing a wide array of standards for SMT component packages and other devices in the semiconductor industry.

Passive Rectangular Components

Passive surface-mount devices are comprised of SMT capacitors and resistors.

Small SMT capacitors are currently used in nearly all forms of mass-produced electronics. SMT capacitors typically come in small rectangular cuboids, the dimensions for which conform to industry standards. Capacitors may use a wide array of technologies, such as tantalum, multilayer ceramic, electrolytic, as well as certain other, less popular varieties.

When using surface-mount resistors, it is vital to ensure that the indicated power dissipation levels are not exceeded. The maximum figures are much lower than for most types of leaded resistors.

Passive SMDs come in several standard sizes that get smaller and smaller over time as technological advances have made it possible to manufacture and use ever-tinier components. The sizes and names of passive SMDs are derived from their measurements in inches.

Standard Passive SMDs Package Sizes

Standard Passive SMDs Package Sizes
Package Type Dimensions (inches) Dimensions (mm)
2920 0.29 x 0.20

 

7.4 x 5.1
2725 0.27 x 0.25 6.9 x 6.3
2512 0.25 x 0.125 6.3 x 3.2
2010 0.20 x 0.10 5.0 x 2.5
1825 0.18 x 0.25

 

4.5 x 6.4
1812 0.18 x 0.125 4.6 x 3.0
1806 0.18 x 0.06 4.5 x 1.6
1210 0.125 x 0.10

 

3.2 x 2.5
1206 0.12 x 0.06 3.0 x 1.5
1008 0.10 x 0.08 2.5 x 2.0
0805 0.08 x 0.05 2.0 x 1.3
0603 0.06 x 0.03 1.5 x 0.8
0402 0.04 x 0.02 1.0 x 0.5
0201 0.02 x 0.01 0.6 x 0.3
01005 0.016 x 0.008 0.4 x 0.2

Of these, sizes 0603 and 0402 are the most widespread. However, with the clear trend toward miniaturization, sizes 0201 and smaller are increasingly being used across the industry.

Sizes 1812 and 1206 are nowadays only used for specialized SMT components and ones with more substantial power level requirements.

Although surface-mount component packages are mainly used for SMD resistors and capacitors, they are sometimes used in other component types such as SMD inductors. SMD inductors are available in sizes 0603 and 0805.

SMD Packages for Tantalum Capacitors

Due to the special construction and technical characteristics of tantalum SMD capacitors, they require a different type of SMD packages. These conform to specifications developed by the Electronic Industries Alliances (EIA).

Common Specifications of SMD Tantalum Capacitor Packages

Common Specifications of SMD Tantalum Capacitor Packages
SMD Package Type Dimensions (mm) EIA Standard
Size A 3.2 x 1.6 x 1.6 EIA 3216-18
Size B 3.5 x 2.8 x 1.9 EIA 3528-21
Size C 6.0 x 3.2 x 2.2 EIA 6032-28
Size D 7.3 x 4.3 x 2.4 EIA 7343-31
Size E 7.3 x 4.3 x 4.1 EIA 7343-43

Other Types of Passive SMD Components

There are certain types of passive SMD components that cannot conform to the standard SMT component sizes used by the vast majority of SMD capacitors and resistors such as the EIA.

Examples of such components include various types of:

  • Filters
  • Inductors
  • Transformers
  • Ceramic resonators
  • Quartz crystal resonators
  • Temperature-controlled crystal oscillators (TCXOs)

These components often require non-standard-sized packages that are often much larger than the ones used for SMT capacitors and resistors. Given their unique requirements, the industry is unlikely to introduce uniform SMT package sizes for these components. That said, however, non-standard packages still have to be able to be handled by pick-and-place machines and lend themselves to PCB assembly automation.

Diode and Transistor Packages

The same types of packages are often used for both SMT diodes and transistors. While diodes come with two electrodes only, an SMT package that has three diodes allows for the orientation to be correctly selected.

There is a wide variety of SMT diode and transistor packages currently available on the market. However, the two most popular options are SOT-23 and SOT-223. In both cases, SOT stands for “small outline transistor.”

The SOT23 SMD package measures 3 mm x 1.75 mm x 1.3 mm. It is by far the most common outline for small signal surface-mount transistors in the industry. While the SOT23 originally comes with three diode terminals, more pins may be added if needed, making it possible to use the package for small integrated circuits such as operational amplifiers.

In contrast, SOT-223 packages are typically used for higher-power devices such as higher power surface-mount transistors. The SOT-223 is significantly larger than the SOT-23, measuring 6.7 mm x 3.7 mm x 1.8 mm. A typical SOT-223 comes with four terminals, at least one of which is a large heat-transfer pad. This pad makes it possible to easily and safely transfer heat to the printed circuit board.

Integrated Circuit SMD Packages

While there are many types of packages that can be used for surface-mount integrated circuits (ICs), different package types tend to have specific applications.

The most commonly used SMD package types for ICs are:

1. SOIC or small outline integrated circuit. This surface-mount IC package features a dual-in-line configuration, gull-wing leads, and a pin spacing of 1.27 mm.

2. SOP or small outline package. SOPs come in are several sub-types, namely:

  • TSOP or thin small outline package, which is generally thinner than SOIC packages and features a smaller pin spacing of just 0.5 mm.
  • SSOP or shrink small outline package, with a slightly wider pin spacing of 0.635 mm.
  • TSSOP or thin shrink small outline package.
  • QSOP or quarter-size small outline package with a pin spacing of 0.635 mm.
  • VSOP or very small outline package. These are smaller than the QSOP and come with a pin spacing of 0.4, 0.5, or 0.65 mm.

3. QFP or quad flat pack. This package is the generic flat package for surface-mount ICs. It features very thin gull-wing leads that protrude from all four sides. A distinctive feature of QFPs is that they can be extremely thin and easy to bend on surface-mount ICs with a high pin count. Once bent, these are nearly impossible to reform into a different position. As a result, great care must be taken when handling these devices during the PCB assembly process.

QFPs are quite versatile and are available in several variants, as follows:

  • LQFP or low profile quad flat pack, which comes with pins on all four sides. While the spacing between the pins varies depending on the IC type, the height of the pack is a standard 1.4 mm.
  • PQFP or plastic quad flat pack, which is a square rather than a rectangular package. It features an equal number of gull-wing style pins on each side, with a relatively narrow pin spacing and 44 or more pins. PQFPs are typically used for VLSI integrated circuits.
  • CQFP or ceramic quad flat pack, which is a ceramic variant of the plastic PQFP.
  • TQFP or thin quad flat pack. The TQFP is a slightly thinner version of the PQFP.

4. PLCC or plastic leaded chip carrier. These packages are square-shaped and feature J-lead pins with a pin spacing of 1.27 mm.

5. BGA or ball grid array packages have all their contact pads underneath the package. As a result, these pads look like tiny balls before being soldered, hence the name of the package type. The spacing between the balls is typically 1.27 mm.

There are many benefits to having contact pads under the device. On the one hand, that reduces the surface area. On the other hand, it maintains the number of available connections.

This package style also solves some of the problems that come with the use of the extremely thin leads associated with quad flat packs, making the package more robust.

When BGA packages were first introduced, there were some doubts about the reliability of having contact pads soldered under the package. However, as long as the PCB assembly process is carried out properly, there are no issues associated with the use of BGAs.

SMTs: Final Thoughts

Well, there you have it: you now no longer need to look up SMT meaning in your search engine.

The key takeaway is this: while there are a great many SMD package types currently on the market, industry standards help ensure conformity and make it possible to automate PCB assembly. That minimizes the margin of error in the manufacturing process and makes it easier, faster, more efficient, and most cost-effective overall.

And if you are looking to buy PCB or PCBA products in bulk or need a custom manufacturing service, get in touch with us. MKTPCB is China’s leading PCB solution supplier. We provide full-spectrum PCB services, from PCB design to mass production and everything in between.

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