Surface-mount components on a flash drive's circuit board
Surface-mount technology (SMT) is a method for constructing electronic circuits in which the components (SMC, or Surface Mounted Components) are mounted directly onto the surface of printed circuit boards (PCBs). Electronic devices so made are called surface-mount devicesorSMDs. 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.
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 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. (See Saturn Launch Vehicle Digital Computer article for a description of this type of electronic packaging as of 1964.)
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.
Assembly techniques
Where components are to be placed, the printed circuit board 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 stencil using a thick film process. If components are to be mounted on the second side, a numerically controlled (NC) machine places small liquid adhesive dots at the locations of all second-side components. The boards then proceed to the pick-and-place machines, where they are placed on a conveyor belt. Small SMDs are usually delivered to the production line on paper or plastic tapes wound on reels. Integrated circuits are typically delivered stacked in static-free plastic tubes or trays. NC pick-and-place machines remove the parts from the reels or tubes and place them on the PCB. Second-side components are placed first, and the adhesive dots are quickly cured with application of low heat or ultraviolet radiation. The boards are flipped over and first-side components are placed by additional NC machines.
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. This helps minimize thermal stresses when the assemblies cool down after soldering. 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. At one time special fluorocarbon liquids with high boiling points were used in a method called vapor phase reflow. Due to environmental concerns, this method is falling out of favor. Today, it is more common to use nitrogen gas or nitrogen gas enriched air in a convection oven. 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) equipment.
After soldering, the boards are washed to remove flux residue and any stray solder balls that could short out closely spaced component leads. Rosin flux is removed with fluorocarbon solvents, high flash pointhydrocarbon solvents, or limonene (derived from orange peels). Water soluble fluxes are removed with deionized water and detergent, followed by an air blast to quickly remove residual water. When aesthetics are unimportant and the flux doesn't cause shorting or corrosion, flux residues are sometimes left on the boards, saving the cost of cleaning and eliminating the waste disposal.
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 corrects any errors. They are then sent to the testing stations to verify that they operate correctly.
Main advantages
The main advantages of SMT over the older through-hole technique are:
Much higher number of components and many more connections per component
Fewer holes need to be drilled 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 placed on both sides of the circuit board
Lower resistance and inductance at the connection (leading to better performance for high frequency parts)
Better mechanical performance under shake and vibration conditions
SMT parts generally cost less than through-hole parts
Fewer unwanted RF signal effects in SMT parts when compared to leaded parts, yielding better predictability of component characteristics.
Main disadvantages
The manufacturing processes for SMT are much more sophisticated than through-hole boards, raising the initial cost and time of setting up for production
Manual prototype assembly or component-level repair is more difficult (more so without a steady hand and the right tools) given the very small sizes and lead spacings of many SMDs.
SMDs can't be used with breadboards (a quick snap-and-play prototyping tool), requiring a custom PCB for every prototype. The PCB costs dozens to hundreds of dollars to fabricate and must be designed with specialized software.
Reworking defective SMD components
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
Infrared soldering:
During infrared soldering, the energy for heating up the solder joint will be transmitted by long or short wave electromagnetic radiation.
Benefits
Easy setup
No compressed air required
No component-specific nozzles (low costs)
Fast reaction of infrared source (depends on used system)
Disadvantages
Central areas will be heated more than peripheral areas
Temperature can hardly be controlled, peaks cannot be ruled out
Covering of the neighboured components is necessary
No component-specific nozzles but covering neighbored components (to prevent damage) needs additional time for every board
Surface temperature depends on the component's reflection characteristics: 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
Conventional hot gas soldering
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
Simulating reflow oven atmosphere
Switching between hot gas and nitrogen (economic use)
Standard and component-specific nozzles allow high reliability and reduced process time
Allow reproducible soldering profiles
Efficient heating, large heat amounts can be transmitted
Even heating of the affected board area
Temperature of the component will never exceed the adjusted gas temperature
Rapid cool down after reflow, resulting in small-grained solder joints (depends on used system)
Disadvantages
Thermal capacity of the heat generator results in slow reaction whereby thermal profiles can be distorted (depends on used system)
A rework process usually undoes some type of error, either human or machine-generated, and includes the following steps:
Melt solder and component removal
Residual solder removal
Printing of solder paste on PCB, direct component printing or dispensing
Placement and reflow of new component.
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.
Eutectic solder can also be used for low-volume manual rework.
MLP package 28-pin chip, upside down to show contacts32-pin MQFP chip with manually-soldered wires attached for prototypingVarious SMD chips, desolderedSMD capacitors (on the left) with two through-hole capacitors (on the right)
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:
MELF — Metal Electrode Leadless Face — (mostly resistors and diodes): Barrel shaped components, dimensions do not match those of rectangular references for identical codes.
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
DPAK (TO-252): discrete packaging. Developed by Motorola to house higher powered devices. Comes in three- or five-terminal versions [3]
D2PAK (TO-263) - 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 [4]
LCC - Leadless Chip Carrier, contacts are recessed vertically to "wick-in" solder. Common in aviation electronics because of robustness to mechanical vibration.
MLP - Leadframe package with a 0.5 mm contact pitch, no leads [8]
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
LGA - Same Manufacture process of BGA, has Advantage being cooler than BGA by quick Heat Deception feature. No Balls only Patch.
FBGA - fine pitch ball grid array, with a square or rectangular array of solder balls on one surface
LFBGA - low profile fine pitch ball grid array, with a square or rectangular array of solder balls on one surface, ball spacing typically 0.8 mm
TFBGA - thin fine pitch ball grid array, with a square or rectangular array of solder balls on one surface, ball spacing typically 0.5 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
LLP - Lead Less Package, a package with metric pin distribution (0.5 mm pitch).
Non-packaged devices (although surface mount, these devices require specific process for assembly):
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.
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.
