Surface-mount technology

Surface-mount components on a USB flash drive's circuit board. The small rectangular chips with numbers are resistors, while the unmarked small rectangular chips are capacitors. The capacitors and resistors pictured are 0603 (1608 metric) package sizes, along with a very slightly larger 0805 (2012 metric) ferrite bead. Not shown here, are even smaller chip capacitors of 01005 (0402 metric) and 0201 (0603 metric) sizes.

Surface-mount capacitor

Surface-mount technology (SMT) is a method for producing electronic circuits in which the components are mounted or placed directly onto the surface of printed circuit boards (PCBs). An electronic device so made is called a surface-mount device (SMD). In the industry it has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board. Both technologies can be used on the same board, with the through-hole technology used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors.

An SMT component is usually smaller than its through-hole counterpart because it has either smaller leads or no leads at all. It may have short pins or leads of various styles, flat contacts, a matrix of solder balls (BGAs), or terminations on the body of the component.

History

Surface mounting was originally called "planar mounting".^[1]

Surface-mount technology was developed in the 1960s and became widely used in the late 1980s. Much of the pioneering work in this technology was by IBM. The design approach first demonstrated by IBM in 1960 in a small-scale computer was later applied in the Launch Vehicle Digital Computer used in the Instrument Unit that guided all Saturn IB and Saturn V vehicles.^[2] Components were mechanically redesigned to have small metal tabs or end caps that could be directly soldered to the surface of the PCB. Components became much smaller and component placement on both sides of a board became far more common with surface mounting than through-hole mounting, allowing much higher circuit densities. Often only the solder joints hold the parts to the board, in rare cases parts on the bottom or "second" side of the board may be secured with a dot of adhesive to keep components from dropping off inside reflow ovens if the part has a large size or weight. Adhesive is sometimes used to hold SMT components on the bottom side of a board if a wave soldering process is used to solder both SMT and through-hole components simultaneously. Alternatively, SMT and through-hole components can be soldered together without adhesive if the SMT parts are first reflow-soldered, then a selective solder mask is used to prevent the solder holding the parts in place from reflowing and the parts floating away during wave soldering. Surface mounting lends itself well to a high degree of automation, reducing labor cost and greatly increasing production rates. SMDs can be one-quarter to one-tenth the size and weight, and one-half to one-quarter the cost of equivalent through-hole parts.

Terms

Because "surface-mount" refers to a methodology of manufacturing, there are different terms used when referring to the different aspect of the method, which distinguishes for example the components, technique, and machines used in manufacturing. These terms are listed in the following table:

Assembly techniques

Assembly line with SMT placement machines

Where components are to be placed, the printed circuit board normally has flat, usually tin-lead, silver, or gold plated copper pads without holes, called solder pads. Solder paste, a sticky mixture of flux and tiny solder particles, is first applied to all the solder pads with a stainless steel or nickel stencil using a screen printing process. It can also be applied by a jet-printing mechanism, similar to an inkjet printer. After pasting, the boards then proceed to the pick-and-place machines, where they are placed on a conveyor belt. The components to be placed on the boards are usually delivered to the production line in either paper/plastic tapes wound on reels or plastic tubes. Some large integrated circuits are delivered in static-free trays. Numerical control pick-and-place machines remove the parts from the tapes, tubes or trays and place them on the PCB.

The boards are then conveyed into the reflow soldering oven. They first enter a pre-heat zone, where the temperature of the board and all the components is gradually, uniformly raised. The boards then enter a zone where the temperature is high enough to melt the solder particles in the solder paste, bonding the component leads to the pads on the circuit board. The surface tension of the molten solder helps keep the components in place, and if the solder pad geometries are correctly designed, surface tension automatically aligns the components on their pads. There are a number of techniques for reflowing solder. One is to use infrared lamps; this is called infrared reflow. Another is to use a hot gas convection. Another technology which is becoming popular again is special fluorocarbon liquids with high boiling points which use a method called vapor phase reflow. Due to environmental concerns, this method was falling out of favor until lead-free legislation was introduced which requires tighter controls on soldering. Currently, at the end of 2008, convection soldering is the most popular reflow technology using either standard air or nitrogen gas. Each method has its advantages and disadvantages. With infrared reflow, the board designer must lay the board out so that short components don't fall into the shadows of tall components. Component location is less restricted if the designer knows that vapor phase reflow or convection soldering will be used in production. Following reflow soldering, certain irregular or heat-sensitive components may be installed and soldered by hand, or in large-scale automation, by focused infrared beam (FIB) or localized convection equipment.

If the circuit board is double-sided then this printing, placement, reflow process may be repeated using either solder paste or glue to hold the components in place. If a wave soldering process is used, then the parts must be glued to the board prior to processing to prevent them from floating off when the solder paste holding them in place is melted.

After soldering, the boards may be washed to remove flux residues and any stray solder balls that could short out closely spaced component leads. Rosin flux is removed with fluorocarbon solvents, high flash point hydrocarbon solvents, or low flash solvents e.g. limonene (derived from orange peels) which require extra rinsing or drying cycles. Water-soluble fluxes are removed with deionized water and detergent, followed by an air blast to quickly remove residual water. However, most electronic assemblies are made using a "No-Clean" process where the flux residues are designed to be left on the circuit board [benign]. This saves the cost of cleaning, speeds up the manufacturing process, and reduces waste.

Certain manufacturing standards, such as those written by the IPC - Association Connecting Electronics Industries require cleaning regardless of the solder flux type used to ensure a thoroughly clean board. Even no-clean flux leaves a residue which, under IPC standards, must be removed. Proper cleaning removes all traces of solder flux, as well as dirt and other contaminants that may be invisible to the naked eye. However, while shops conforming to IPC standard are expected to adhere to the Association's rules on board condition, not all manufacturing facilities apply IPC standard, nor are they required to do so. Additionally, in some applications, such as low-end electronics, such stringent manufacturing methods are excessive both in expense and time required.

Finally, the boards are visually inspected for missing or misaligned components and solder bridging. If needed, they are sent to a rework station where a human operator repairs any errors. They are then usually sent to the testing stations (in-circuit testing and/or functional testing) to verify that they operate correctly.

Advantages

The main advantages of SMT over the older through-hole technique are:

• Smaller components. As of 2012 smallest was 0.4 × 0.2 mm (0.016 × 0.008 in: 01005). Expected to sample in 2013 are 0.25 × 0.125 mm (0.010 × 0.005 in, size not yet standardized)
• Much higher component density (components per unit area) and many more connections per component.
• Higher density of connections because holes do not block routing space on inner or back-side layers.
• Lower initial cost and time of setting up for production.
• Fewer holes need to be drilled.
• Simpler and faster automated assembly. Some placement machines are capable of placing more than 136,000 components per hour.
• Small errors in component placement are corrected automatically as the surface tension of molten solder pulls components into alignment with solder pads.
• Components can be placed on both sides of the circuit board.
• Lower resistance and inductance at the connection; consequently, fewer unwanted RF signal effects and better and more predictable high-frequency performance.
• Better mechanical performance under shake and vibration conditions.
• Many SMT parts cost less than equivalent through-hole parts.
• Better EMC performance (lower radiated emissions) due to the smaller radiation loop area (because of the smaller package) and the smaller lead inductance.^[3]

Disadvantages

• Manual prototype assembly or component-level repair is more difficult and requires skilled operators and more expensive tools, due to the small sizes and lead spacings of many SMDs.^[4]
• SMDs cannot be used directly with plug-in breadboards (a quick snap-and-play prototyping tool), requiring either a custom PCB for every prototype or the mounting of the SMD upon a pin-leaded carrier. For prototyping around a specific SMD component, a less-expensive breakout board may be used. Additionally, stripboard style protoboards can be used, some of which include pads for standard sized SMD components. For prototyping, "dead bug" breadboarding can be used.^[5]
• SMDs' solder connections may be damaged by potting compounds going through thermal cycling.
• Solder joint dimensions in SMT quickly become much smaller as advances are made toward ultra-fine pitch technology. The reliability of solder joints becomes more of a concern, as less and less solder is allowed for each joint. Voiding is a fault commonly associated with solder joints, especially when reflowing a solder paste in the SMT application. The presence of voids can deteriorate the joint strength and eventually lead to joint failure.^[6][7]
• SMT is unsuitable for large, high-power, or high-voltage parts, for example in power circuitry. It is common to combine SMT and through-hole construction, with transformers, heat-sinked power semiconductors, physically large capacitors, fuses, connectors, and so on mounted on one side of the PCB through holes.
• SMT is unsuitable as the sole attachment method for components that are subject to frequent mechanical stress, such as connectors that are used to interface with external devices that are frequently attached and detached.

Rework

Removal of surface-mount device using soldering tweezers

Defective surface-mount components can be repaired by using soldering irons (for some connections), or using a non-contact rework system. In most cases a rework system is the better choice because SMD work with a soldering iron requires considerable skill and is not always feasible. There are essentially two non-contact soldering/desoldering methods: infrared soldering and soldering with hot gas.

Infrared

With infrared soldering, the energy for heating up the solder joint is transmitted by long- or short-wave infrared electromagnetic radiation.

Advantages:

• Easy setup
• No compressed air required
• No requirement for different nozzles for many component shapes and sizes, reducing cost and the need to change nozzles
• Fast reaction of infrared source (depends on system used)

Disadvantages:

• Central areas will be heated more than peripheral areas
• Temperature control is less precise, and there may be peaks
• Nearby components must be shielded from heat to prevent damage, which requires additional time for every board
• Surface temperature depends on the component's albedo: dark surfaces will be heated more than lighter surfaces
• The temperature additionally depends on the surface shape. Convective loss of energy will reduce the temperature of the component
• No reflow atmosphere possible

Hot gas

During hot gas soldering, the energy for heating up the solder joint is transmitted by a hot gas. This can be air or inert gas (nitrogen).

Advantages:

• Simulating reflow oven atmosphere
• Some systems allow switching between hot air and nitrogen
• Standard and component-specific nozzles allow high reliability and faster processing
• Allow reproducible soldering profiles
• Efficient heating, large amounts of heat can be transferred
• Even heating of the affected board area
• Temperature of the component will never exceed the adjusted gas temperature
• Rapid cooling after reflow, resulting in small-grained solder joints (depends on system used)

Disadvantages:

• Thermal capacity of the heat generator results in slow reaction whereby thermal profiles can be distorted (depends on system used)

Reworking usually corrects some type of error, either human- or machine-generated, and includes the following steps:

• Melt solder and remove component (s)
• Remove residual solder
• Print solder paste on PCB, directly or by dispensing
• Place new component and reflow.

Sometimes hundreds or thousands of the same part need to be repaired. Such errors, if due to assembly, are often caught during the process. However, a whole new level of rework arises when component failure is discovered too late, and perhaps unnoticed until the end user of the device being manufactured experiences it. Rework can also be used if products of sufficient value to justify it require revision or re-engineering, perhaps to change a single firmware-based component. Reworking in large volume requires an operation designed for that purpose.

Packages

Surface-mount components are usually smaller than their counterparts with leads, and are designed to be handled by machines rather than by humans. The electronics industry has standardized package shapes and sizes (the leading standardisation body is JEDEC). These include:

The codes given in the chart below usually tell the length and width of the components in tenths of millimeters or hundredths of inches. For example, a metric 2520 component is 2.5 mm by 2.0 mm which corresponds roughly to 0.10 inches by 0.08 inches (hence, imperial size is 1008). Exceptions occur for imperial in the two smallest rectangular passive sizes. The metric codes still represent the dimensions in mm, even though the imperial size codes are no longer aligned. Problematically, some manufacturers are developing metric 0201 components with dimensions of 0.25 mm × 0.125 mm (0.0098 in × 0.0049 in),^[8] but the imperial 01005 name is already being used for the 0.4 mm × 0.2 mm (0.0157 in × 0.0079 in) package.

Example of component sizes, metric and imperial codes and comparison included

SMD capacitors (on the left) with two through-hole capacitors (on the right)

Two-terminal packages

Rectangular passive components

Mostly resistors and capacitors.

Tantalum capacitors^[14][15]

Aluminium capacitors^[16][17][18]

Small outline diode (SOD)

Metal electrode leadless face^[25] (MELF)

Mostly resistors and diodes; barrel shaped components, dimensions do not match those of rectangular references for identical codes.

DO-214

Three-terminal packages

Small-outline transistor (SOT)

• SOT-223: 6.7 mm × 3.7 mm × 1.8 mm body: four terminals, one of which is a large heat-transfer pad ^[27]
• SOT-89 (TO-243)^[28] (SC-62):^[29] 4.5 mm × 2.5 mm × 1.5 mm body: four terminals, center pin is connected to a large heat-transfer pad ^[30]
• SOT-23 (TO-236-3) (SC-59): 2.9 mm × 1.3/1.75 mm × 1.3 mm body: three terminals for a transistor ^[31]
• SOT-323 (SC-70): 2 mm × 1.25 mm × 0.95 mm body: three terminals ^[32]
• SOT-416 (SC-75): 1.6 mm × 0.8 mm × 0.8 mm body: three terminals ^[33]
• SOT-663: 1.6 mm × 1.6 mm × 0.55 mm body: three terminals ^[34]
• SOT-723: 1.2 mm × 0.8 mm × 0.5 mm body: three terminals: flat lead^[35]
• SOT-883 (SC-101): 1 mm × 0.6 mm × 0.5 mm body: three terminals: leadless ^[36]

Other

• DPAK (TO-252, SOT-428): Discrete Packaging. Developed by Motorola to house higher powered devices. Comes in three-^[37] or five-terminal^[38] versions
• D2PAK (TO-263, SOT-404): bigger than the DPAK; basically a surface mount equivalent of the TO220 through-hole package. Comes in 3, 5, 6, 7, 8 or 9-terminal versions ^[39]
• D3PAK (TO-268): even larger than D2PAK ^[40]

Five- and six-terminal packages

Small-outline transistor (SOT)

• SOT-23-5 (SOT-25, SC-74A): 2.9 mm × 1.3/1.75 mm × 1.3 mm body: five terminals ^[41]
• SOT-23-6 (SOT-26, SC-74): 2.9 mm × 1.3/1.75 mm × 1.3 mm body: six terminals ^[42]
• SOT-23-8 (SOT-28): 2.9 mm × 1.3/1.75 mm × 1.3 mm body: eight terminals ^[43]
• SOT-353 (SC-88A): 2 mm × 1.25 mm × 0.95 mm body: five terminals ^[44]
• SOT-363 (SC-88, SC-70-6): 2 mm × 1.25 mm × 0.95 mm body: six terminals ^[45]
• SOT-563: 1.6 mm × 1.2 mm × 0.6 mm body: six terminals ^[46]
• SOT-665: 1.6 mm × 1.6 mm × 0.55 mm body: six terminals ^[47]
• SOT-666: 1.6 mm × 1.6 mm × 0.55 mm body: six terminals ^[48]
• SOT-886: 1.5 mm × 1.05 mm × 0.5 mm body: six terminals: leadless
• SOT-886: 1 mm × 1.45 mm × 0.5 mm body: six terminals: leadless ^[49]
• SOT-891: 1.05 mm × 1.05 mm × 0.5 mm body: five terminals: leadless
• SOT-953: 1 mm × 1 mm × 0.5 mm body: five terminals
• SOT-963: 1 mm × 1 mm × 0.5 mm body: six terminals
• SOT-1115: 0.9 mm × 1 mm × 0.35 mm body: six terminals: leadless ^[50]
• SOT-1202: 1 mm × 1 mm × 0.35 mm body: six terminals: leadless ^[51]

Various SMD chips, desoldered

MLP package 28-pin chip, upside down to show contacts

32-pin MQFP chip with manually soldered wires attached for prototyping. The same effect can be achieved using commercially available breakout boards

Packages with more than six terminals

Dual-in-line

• Flatpack was one of the earliest surface-mounted packages.
• Small-outline integrated circuit (SOIC): dual-in-line, 8 or more pins, gull-wing lead form, pin spacing 1.27 mm
• Small-outline package, J-leaded (SOJ), the same as SOIC except J-leaded ^[52]
• Thin small-outline package (TSOP), thinner than SOIC with smaller pin spacing of 0.5 mm
• Shrink small-outline package (SSOP), pin spacing of 0.65 mm, sometimes 0.635 mm or in some cases 0.8 mm
• Thin shrink small-outline package (TSSOP).
• Quarter-size small-outline package (QSOP), with pin spacing of 0.635 mm
• Very small outline package (VSOP), even smaller than QSOP; 0.4, 0.5 mm or 0.65 mm pin spacing
• Dual flat no-lead (DFN), smaller footprint than leaded equivalent

Quad-in-line

• Plastic leaded chip carrier (PLCC): square, J-lead, pin spacing 1.27 mm
• Quad flat package (QFP): various sizes, with pins on all four sides
• Low-profile quad flat-package (LQFP): 1.4 mm high, varying sized and pins on all four sides
• Plastic quad flat-pack (PQFP), a square with pins on all four sides, 44 or more pins
• Ceramic quad flat-pack (CQFP): similar to PQFP
• Metric quad flat-pack (MQFP): a QFP package with metric pin distribution
• Thin quad flat-pack (TQFP), a thinner version of PQFP
• Quad flat no-lead (QFN): smaller footprint than leaded equivalent
• Leadless chip carrier (LCC): contacts are recessed vertically to "wick-in" solder. Common in aviation electronics because of robustness to mechanical vibration.
• Micro leadframe package (MLP, MLF): with a 0.5 mm contact pitch, no leads (same as QFN)
• Power quad flat no-lead (PQFN): with exposed die-pads for heatsinking

Grid arrays

• Pin grid array (PGA)
• Ball grid array (BGA): A square or rectangular array of solder balls on one surface, ball spacing typically 1.27 mm (0.050 in)
• Land grid array (LGA): An array of bare lands only. Similar to in appearance to QFN, but mating is by spring pins within a socket rather than solder.
• Fine-pitch ball grid array (FBGA)]: A square or rectangular array of solder balls on one surface
• Low-profile fine-pitch ball grid array (LFBGA): A square or rectangular array of solder balls on one surface, ball spacing typically 0.8 mm
• Thin fine-pitch ball grid array (TFBGA): A square or rectangular array of solder balls on one surface, ball spacing typically 0.5 mm
• Column grid array (CGA): A circuit package in which the input and output points are high-temperature solder cylinders or columns arranged in a grid pattern.
• Ceramic column grid array (CCGA): A 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.
• Micro ball grid array (μBGA): Ball spacing less than 1 mm
• Lead less package (LLP): A package with metric pin distribution (0.5 mm pitch).

Non-packaged devices

Although surface-mount, these devices require specific process for assembly.

• Chip-on-board (COB), 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".
• Chip-on-flex (COF), a variation of COB, where a chip is mounted directly to a flex circuit.
• Chip-on-glass (COG); a variation of COB, where a chip, typically a liquid crystal display (LCD) controller, is mounted directly on glass:.

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.

Identification

Resistors

For 5% precision SMD resistors usually are marked with their resistance values using three digits: two significant digits and a multiplier digit. These are quite often white lettering on a black background, but other colored backgrounds and lettering can be used.

The black or colored coating is usually only on one face of the device, the sides and other face simply being the uncoated, usually white ceramic substrate. The coated surface, with the resistive element beneath is normally positioned face up when the device is soldered to the board, although they can be seen in rare cases mounted with the uncoated underside face up, whereby the resistance value code is not visible.

For 1% precision SMD resistors, the code is used, as three digits would otherwise not convey enough information. This code consists of two digits and a letter: the digits denote the value's position in the E96 sequence, while the letter indicates the multiplier.^[53]

Typical examples of resistance codes

• 102 = 10 00 = 1,000 Ω = 1 kΩ
• 0R2 = 0.2 Ω
• 684 = 68 0000 = 680,000 Ω = 680 kΩ
• 68X = 499 × 0.1 = 49.9 Ω

There is an online tool to translate codes to resistance values. Resistors are made in several types; a common types uses a ceramic substrate. Resistance values are available in several tolerances defined in EIA Decade Values Table :

• E3, 50% tolerance (no longer used)
• E6, 20% tolerance (now seldom used)
• E12, 10% tolerance
• E24, 5% tolerance
• E48, 2% tolerance
• E96, 1% tolerance
• E192, 0.5, 0.25, 0.1% and tighter tolerances

Capacitors

Non electrolytic capacitors are usually unmarked and the only reliable method of determining their value is removal from the circuit and subsequent measurement with a capacitance meter or impedance bridge. The materials used to fabricate the capacitors, such as Nickel Tantalate, possess different colours and these can give an approximate idea of the capacitance of the component.

• Light grey body colour indicates a capacitance which is generally less than 100 pF.
• Medium Grey colour indicates a capacitance anywhere from 10 pF to 10 nF.
• Light brown colour indicates a capacitance in a range from 1 nF to 100 nF.
• Medium brown colour indicates a capacitance in a range from 10 nF to 1 μF.
• Dark brown colour indicates a capacitance from 100 nF to 10 μF.
• Dark grey colour indicates a capacitance in the μF range, generally 0.5 to 50 μF, or the device may be an inductor and the dark grey is the color of the ferrite bead. (An inductor will measure a low resistance to a multimeter on the resistance range whereas a capacitor, out of the circuit, will measure a near infinite resistance.)

Generally physical size is proportional to capacitance and voltage^2 for the same dielectric. For example, a 100 nF 50 V capacitor may come in the same package as a 10 nF 150 V device.

SMD (non electrolytic) capacitors, which are usually monolithic ceramic capacitors, exhibit the same body color on all four faces not covered by the end caps.

SMD electrolytic capacitors, usually tantalum capacitors, and film capacitors are marked like resistors, with two significant figures and a multiplier in units of pico Farads or pF, (10⁻¹² Farad.)

Examples

• 104 = 100 nF = 100,000 pF
• 226 = 22 μF = 22,000,000 pF

The electrolytic capacitors are usually encapsulated in black or beige epoxy resin with flat metal connecting strips bent underneath. Some film or tantalum electrolytic types are unmarked and possess red, orange or blue body colors with complete end caps, not metal strips.

Inductors

Due to the small dimensions of SMDs, SMT inductors are limited to values of less than about 10 mH. Smaller inductance with moderately high current ratings are usually of the ferrite bead type. They are simply a metal conductor looped through a ferrite bead and almost the same as their through-hole versions but possess SMD end caps rather than leads. They appear dark grey and are magnetic, unlike capacitors with a similar dark grey appearance. These ferrite bead type are limited to small values in the nH (nano Henry), range and are often used as power supply rail decouplers or in high frequency parts of a circuit. Larger inductors and transformers may of course be through-hole mounted on the same board.

SMT inductors with larger inductance values often have turns of wire or flat strap around the body or embedded in clear epoxy, allowing the wire or strap to be seen. Sometimes a ferrite core is present also. These higher inductance types are often limited to small current ratings, although some of the flat strap types can handle a few amps.

As with capacitors, component values and identifiers are not usually marked on the component itself; if not documented or printed on the PCB, measurement, usually removed from the circuit, is the only way of determining them.

Discrete semiconductors

Discrete semiconductors, such as transistors, diodes and FETs are often marked with a two- or three-symbol code in which the same code marked on different packages or on devices made by different manufacturers can translate to different devices.

Many of these codes, used because the devices are too small to be marked with more traditional numbers used on through-hole equivalent devices, correlate to more familiar traditional part numbers when a correlation list is consulted.

GM4PMK in the United Kingdom has prepared a correlation list, and a similar .pdf list is also available, although these lists are not complete.

Integrated circuits

Generally, integrated circuit packages are large enough to be imprinted with the complete part number which includes the manufacturer's specific prefix, or a significant segment of the part number and the manufacturer's name or logo.

Examples of manufacturers' specific prefixes:

• Philips HEF4066 or Motorola MC14066. (a 4066 Quad Analog Switch.)
• Fujitsu Electric FA5502. (a 5502M Boost Architecture Power factor correction controller.)

See also

• Board-to-board connectors
• Chip carrier
• Electronics
• Electronics manufacturing services
• List of electronics package dimensions
• Plastic leaded chip carrier
• Point-to-point construction
• Printed circuit board
• RoHS
• SMT placement equipment
• Through-hole technology
• Wire wrap

References

Wikimedia Commons has media related to Surface mount technology.