Surface mount technology (SMT) is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards (PCBs). Electronic devices so made are called surface-mount devices or SMDs. In the industry it has largely replaced the previous construction method of fitting components with wire leads into holes in the circuit board (also called through-hole technology).
An SMT component is usually smaller than its leaded counterpart (because it has no leads or smaller leads). It can have short pins, flat contacts, a matrix of balls (BGAs), terminations on the body of the component (passives), or short leads in a gull-wing formation (QFPs).
Assembly techniques
The side on the PCB where the component is to be fitted has flat, usually tinned, copper pads without holes. One technique for assembly coats the pads with a thin layer of solder paste, which also acts as a temporary adhesive to hold the component in place during soldering. Soldering, in this technique (reflow soldering), consists of heating the circuit board and components in an oven; this drives off the flux from the solder paste Surface-mount technology and melts the remaining solder. The surface tension in the liquid solder prevents the component from sliding off while the solder is liquid. The circuit board is then cooled to solidify the solder.
Main advantages
The main advantages of SMT over the older through-hole technique are:
- smaller components
- no need to drill holes through abrasive boards
- simpler automated assembly
- small errors in component placement are corrected automatically (the surface tension of the molten solder pulls the component into alignment with the solder pads)
- components can be fitted to both sides of the circuit board
- lower lead resistance and inductance (leading to better performance for high frequency parts)
Package sizes
Surface-mount components are usually much smaller than their leaded counterparts, and are designed to be handled by machines rather than by humans. The electronics industry has defined a collection of standard package shapes and sizes (the leading standardisation body is JEDEC). These include:
- Rectangular passive components (mostly resistors and capacitors):
- 0201 - 0.02" × 0.01" (0.6 mm × 0.3 mm), two terminals
- 0402 - 0.04" × 0.02" (1.0 mm × 0.5 mm), two terminals
- 0603 - 0.06" × 0.03" (1.5 mm × 0.8 mm), two terminals
- 0805 - 0.08" × 0.05" (2.0 mm × 1.3 mm), two terminals
- 1206 - 0.12" × 0.06" (3.0 mm × 1.5 mm), two terminals
- 1812 - 0.18" × 0.12" (4.6 mm × 3.0 mm), two terminals
- 0201 - 0.02" × 0.01" (0.6 mm × 0.3 mm), two terminals
- Tantalum capacitors:
- Size A (EIA 3216-18): 3.2 mm × 1.6 mm × 1.6 mm
- Size B (EIA 3528-21): 3.5 mm × 2.8 mm × 1.9 mm
- Size C (EIA 6032-28): 6.0 mm × 3.2 mm × 2.2 mm
- Size D (EIA 7343-31): 7.3 mm × 4.3 mm × 2.4 mm
- Size E (EIA 7343-43): 7.3 mm × 4.3 mm × 4.1 mm
- Size A (EIA 3216-18): 3.2 mm × 1.6 mm × 1.6 mm
- Small-Outline Integrated Circuit (SOIC) - small-outline integrated circuit, dual-in-line, 8 or more pins, gull-wing lead form, pin spacing 1.27 mm
- PLCC - plastic leaded chip carrier, square, J-lead, pin spacing 1.27 mm
- TSOP - thin small-outline package, thinner than SOIC with smaller pin spacing of 0.5 mm
- SSOP - shrink small-outline package, pin spacing of 0.635 mm
- TSSOP - thin shrink small-outline package
- QSOP - quarter-size small-outline package, with pin spacing of 0.635 mm
- VSOP - even smaller than QSOP; 0.4, 0.5 mm or 0.65 mm pin spacing
- SOT - small-outline transistor, with three terminals
- SOT-23 - 3 mm × 1.75 mm × 1.3 mm body - three terminals for a transistor, or up to eight terminals for an integrated circuit
- SOT-223 - 6.7 mm × 3.7 mm × 1.8 mm body - four terminals, one of which is a large heat-transfer pad
- SOT-23 - 3 mm × 1.75 mm × 1.3 mm body - three terminals for a transistor, or up to eight terminals for an integrated circuit
- LQFP - Low-profile Quad Flat Package, with pins spaced at 0.5mm
- PQFP - plastic quad flat-pack, a square with pins on all four sides, 44 or more pins
- CQFP - ceramic quad flat-pack, similar to PQFP
- TQFP - thin quad flat pack, a thinner version of PQFP
- QFN - quad flat pack, no-leads, smaller footprint than leaded equivalent
- PQFN - power quad flat-pack, no-leads, with exposed die-pad[s] for heatsinking
- BGA - ball grid array, with a square or rectangular array of solder balls on one surface, ball spacing typically 1.27 mm
- CGA - column grid array, circuit package in which the input and output points are high temperature solder cylinders or columns arranged in a grid pattern.
- CCGA - ceramic column grid array, circuit package in which the input and output points are high temperature solder cylinders or columns arranged in a grid pattern. The body of the component is ceramic.
- μBGA - micro-BGA, with ball spacing less than 1 mm
- COB - chip-on-board; a bare silicon chip, that is usually an integrated circuit, is supplied without a package (usually a lead frame overmolded with epoxy) and is attached, often with epoxy, directly to a circuit board. The chip is then wire bonded and protected from mechanical damage and contamination by an epoxy "glob-top".
- COF - chip-on-flex; a variation of COB, where a chip is mounted directly to a flex circuit.
- COG - chip-on-glass; a variation of COB, where a chip is mounted directly to a piece of glass - typically an LCD display.
- MLP - Leadframe package with a 0.5mm contact pitch. Pictured to the right.
- MQFP - Metric Quad Flat Pack, a QFP package with metric pin distribution.
There are often subtle variations in package details from manufacturer to manufacturer, and even though standard designations are used, designers need to confirm dimensions when laying out printed circuit boards.
Links
- SMT-Soldering by hand (German language with pictures)
- Manual SMT board assembly and soldering (English)
- DIY SMT oven
- using point-to-point construction with SMT components, instead of a custom PCB: Progressive Wiring Techniques
Electronics Topics
The field of electronics is the study and use of systems that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. The design and construction of electronic circuits to solve practical problems is part of the fields of electronic engineering, and the hardware design side of computer engineering. The study of new semiconductor devices and their technology is sometimes considered as a branch of physics.
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