Surface-mount technology

Surface mount technology (SMT) is a method for constructing electronic circuits in which the components (surface-mounted components/SMCs) are mounted 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.

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.

Contents

History

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 the 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.[1] 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, although parts on the bottom or "second" side of the board are temporarily secured with a dot of adhesive as well. Surface mounted devices (SMDs) are usually made physically small and lightweight for this reason. 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

SMp DIALECT Expanded Form
SMD Surface Mount Devices (active, passive and electromechanical components)
SMT Surface Mount Technology (assembling and montage technology)
SMA Surface Mount Assembly (module assembled with SMT)
SMC Surface Mount Components (components for SMT)
SMP Surface Mount Packages (SMD case forms)
SME Surface Mount Equipment (SMT assembling machines)
SO Small Outline (4 to 28 pins)
VSO Very Small Outline (40 pins)
SOP Small Outline Package (case)
SOD Small Outline Diode
SOT Small Outline Transistor
SOIC Small Outline Integrated Circuit
CC Chip Carrier
LCC Leadless Chip Carrier
PLCC Plastic Leaded Chip Carrier
LCCC Leadless Ceramic Chip Carrier
MELF Metal Electrode Face Bonding
MINI MELF Mini Metal Electrode Face Bonding
MICRO MELF Micro Metal Electrode Face Bonding

Assembly techniques

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 glue is used then the parts must be soldered later using a wave soldering process.

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.

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:

Disadvantages

Rework

Defective surface mount components can be repaired in two ways: by using soldering irons (depends on the kind and number of connections) or using a professional rework system. In most cases a rework system is the first choice because the human influence on the rework result is very low. Generally, two essential soldering methods can be distinguished: infrared soldering and soldering with hot gas.

Benefits and disadvantages of different soldering methods

During infrared soldering, the energy for heating up the solder joint will be transmitted by long or short wave electromagnetic radiation.

Benefits:

Disadvantages:

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

Benefits:

Disadvantages:

A rework process usually undoes some type of error, either human or machine-generated, and includes the following steps:

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 experiences them. Rework may also be used if high-value products require revisions, and re-engineering, perhaps to change a single firmware based component, may revive a once obsolete product. These tasks require a rework operation specifically designed to repair/replace components in volume.

Package sizes

Main article: chip carrier

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:

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
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 coloured backgrounds and lettering can be used.

The black or coloured 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 rarely be seen mounted with the uncoated underside face up, whereby the resistance value code is not visible.

Typical examples of resistance codes
102 = 10 00 = 1000 Ohms = 1K Ohm
0R2 = 0.2 Ohms
684 = 68 0000 = 680 000 Ohms = 680K Ohms

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 100pF.
Medium Grey colour indicates a capacitance anywhere from 10pF to 10nF.
Light brown colour indicates a capacitance in a range from 1nF to 100nF.
Medium brown colour indicates a capacitance in a range from 10nF to 1μF.
Dark brown colour indicates a capacitance from 100nF to 10μF.
Dark grey colour indicates a capacitance in the μF range, generally 0.5 to 50μF, or the device may be an inductor and the dark grey is the colour 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 the larger the physical size, the larger the capacitance or voltage rating will be when all other ratings are held constant. For example, a 100nF 50v capacitor may come in the same package as a 10nF 500v device. If both appear on the same board, the two can be told apart by their usage (context). The device rated 500V will typically be designed into a high voltage or high energy circuit, whereas the 50v rated device would be found in the small signal part of the circuit.

SMD (non electrolytic) capacitors, which are usually monolithic ceramic capacitors, exhibit the same body colour 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 = 100nF = 100 000pF
226 = 22μF = 22 000 000pF

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 colours with complete end caps, not metal strips.

Inductors
Due to the small dimensions of SMDs, inductors are limited to quite small values, generally less than 1mH. Smaller inductances with moderately high current ratings are usually of the ferrite bead type. They are simply a metal conductor passing through a ferrite bead and almost the same as their through-hole versions but possess the 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.

Inductors with larger inductance values will often possess turns of wire or flat strap around the body or embedded in clear epoxy. This wire or strap can often be easily seen and 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.

Like capacitors, the specific values are not usually marked and removal from the circuit and subsequent measurement is the only really reliable way of determining their value.

Discrete Semiconductors
Discrete semiconductors, such as transistors, diodes and F.E.T.s are often marked with a cryptic 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, http://www.marsport.org.uk/smd/mainframe.htm and a similar .pdf list is also avalable, elektronik.googlecode.com/files/SMD_Catalog222.pdf, although these lists do not contain all the device codes one might encounter.

Integrated Circuits
Generally integrated circuit packages are large enough to print the whole number or a significant part of the number which can be fully identified as long as you can identify the manufacturer of the device.

Examples
4066 = Philips HEF4066, or if Motorola, MC14066. (Quad Analog Switch.)
5502M = Fujitsu Electric FA5502. (Boost Architecture Power Factor Correction controller.)

See also

Electronics portal

References

  1. ^ "High-resolution photos of components/PCBs". Spaceaholic.com. http://www.spaceaholic.com/lunar_module_saturn_v.htm. Retrieved 2011-07-07. 
  2. ^ "Voiding Mechanisms in SMT". Indium.com. http://www.indium.com/techlibrary/whitepapers/voiding-mechanisms-in-smt. Retrieved 2011-07-07. 
  3. ^ DerMarderosian, Aaron (1983). The Effects of Entrapped Bubbles in Solder for The Attachment of Leadless Ceramic Chip Carriers. Reliability Physics Symposium. pp. 235–241. ISSN 0735-0791. 
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External links