The Advantages and Disadvantages of Surface Mount Technology (SMT)
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Surface mount technology is a part of the electronic assembly that deals with the mounting of electronic components to the surface of a PCB. Electronic components mounted this way are called surface-mounted devices (SMD).
SMT was developed to minimize manufacturing costs while making efficient use of board space. The advent of surface mount technology has empowered manufacturers to create intricate circuit boards in smaller sizes. Throughout this article, we'll explore the numerous pros and cons associated with surface mount technology.
Surface mount technology emerged in the s and gained widespread usage in the s, becoming prevalent in most high-end PCB assemblies by the s. It involved redesigning conventional electronic components to incorporate metal tabs or end caps that could be directly attached to the board's surface, eliminating the need for wire leads to pass through drilled holes. This shift to SMT resulted in significantly smaller components and allowed for component placement on both sides of the board. The introduction of surface mounting facilitated increased automation, reducing labor costs, and expanding production rates, thereby advancing the development of circuit boards.
Surface mount technology (SMT) allows electrical components to be affixed to the board surface without requiring drilling. Most electronic applications favor surface mount components due to their compact size and the ability to install them on either side of a printed circuit. They are particularly suitable for applications with high routing densities, featuring smaller leads or no leads at all compared to through-hole components.
1. Applying solder paste to the fabricated circuit board using stencils. Solder paste comprises flux and tin particles.
2. Attaching the surface mount components.
3. Employing a reflow method for soldering.
In through-hole technology, component leads are inserted into drilled holes on the board. Subsequently, these leads are soldered to pads on the opposite side using wave soldering or reflow soldering tools. This method provides strong mechanical bonds, ensuring high reliability. However, the process of drilling PCBs during production tends to elevate manufacturing costs. Additionally, through-hole technology constrains the routing area for signal traces, particularly below the top layer of multi-layer PCBs.
1. SMT eliminates the constraints on board space imposed by through-hole mounting during the manufacturing process.
2. Through-hole components typically incur higher manufacturing costs compared to SMT components.
3. Utilizing SMT demands advanced design and production skills compared to through-hole technology.
4. SMT components can accommodate a higher pin count in comparison to through-hole components.
5. Unlike through-hole technology, SMT allows for assembly automation, making it suitable for high-volume production at lower costs compared to through-hole production.
6. SMT components offer higher component density due to their compactness compared to through-hole mounting.
7. Although surface mount technology reduces production costs, the machinery's capital investment required is higher than that for through-hole technology.
8. Through-hole mounting is more suitable for manufacturing large and bulky components that experience occasional mechanical stresses or for high-voltage and high-power parts.
9. SMT facilitates achieving higher circuit speeds due to its reduced size and fewer holes.
1. Component stability under external stress exposure
2. Thermal management and heat dissipation ease
3. Part availability and alternatives
4. Cost-effectiveness in assembly
5. High performance and package lifespan
6. Rework feasibility in case of board failure
1. SMT supports microelectronics by enabling closer placement of components on the board, resulting in more lightweight and compact designs.
2. The production setup process for SMT is faster than through-hole technology as components are mounted using solder paste instead of drilled holes, saving time and reducing labor-intensive work.
3. Components can be placed on both sides of the circuit board, allowing for higher component density and more connections per component.
4. Due to the compact package size, higher-density traces can be accommodated on the same layer.
5. Molten solder's surface tension aligns components with solder pads, automatically correcting minor placement issues.
6. Unlike through holes, SMT components do not expand during operation, enabling reduced inter-packaging space.
7. SMT boards achieve electromagnetic compatibility easily due to their compact package and lower lead inductance.
8. Lower resistance and inductance at connections in SMT mitigate undesired effects of RF signals, providing better high-frequency performance.
9. Compactness allows more parts to fit on the board, resulting in shorter signal paths and enhanced signal integrity.
10. SMT components dissipate less heat compared to through-hole components.
11. SMT reduces board and material handling costs.
12. Enables a controlled manufacturing process, particularly beneficial for high-volume PCB production.
1. Reliability concerns arise when relying solely on surface mounting for components exposed to mechanical stress, as component connectors are required for interfacing with external devices that are periodically removed and reattached.
2. Solder connections for surface-mounted devices (SMDs) may sustain damage due to thermal cycles during operations.
3. Skilled operators and expensive tools are necessary for component-level repair and manual prototype assembly due to smaller sizes and lead spaces.
4. Most SMT component packages cannot be installed in sockets, hindering easy installation and replacement of failed components.
5. The reduced solder used in SMT raises reliability concerns for solder joints, with void formation potentially leading to solder joint failures.
6. SMDs are typically smaller than through-hole components, providing limited surface area for marking part IDs and component values, complicating identification during prototyping and PCB repair.
7. Solder in SMT can melt under intense heat, limiting its use in electrical load circuits with high heat dissipation.
8. Implementation of SMT in PCBs requires higher installation costs due to the expensive equipment involved, such as hot air rework stations, pick and place machines, solder paste screen printers, and reflow ovens.
9. Miniaturization and various solder joints can make procedures and inspections more challenging.
10. The compact size increases the risk of solder overflow, leading to short circuits and solder bridges.
While the majority of current products are manufactured using surface mount technology, it's important to note that SMT might not be suitable for all cases. Consider using SMT if:
1. High component density needs to be accommodated.
2. The product requires a compact or small form factor.
3. The final product needs to be sleek and lightweight while maintaining high component density.
4. High-speed or high-frequency functionality is required.
5. Large quantities need to be produced using automated technology.
6. The product should generate minimal or no noise.
1. Position components as close as possible to minimize routing distance.
2. Follow the schematic's signal path when placing components.
3. Avoid placing components in the return path of sensitive signals to prevent signal integrity issues.
4. For high-speed devices, position bypass capacitors nearer to their power pins to minimize parasitic inductance.
5. Group SMDs together for power supply circuits to enable shorter routing and reduce connection inductance.
6. Consider placing SMT components on one side of the board to reduce costs related to stencils and assembly.
7. Adhere to the manufacturer's specifications for minimal spacing between test points and SMT components, which may vary based on component height.
Surface-mount technology (SMT), originally called planar mounting,[1] is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB).[2] An electrical component mounted in this manner is referred to as a surface-mount device (SMD). In industry, this approach has largely replaced the through-hole technology construction method of fitting components, in large part because SMT allows for increased manufacturing automation which reduces cost and improves quality.[3] It also allows for more components to fit on a given area of substrate. Both technologies can be used on the same board, with the through-hole technology often used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors.
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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.
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Surface-mount technology was developed in the s. By surface mounted components accounted for 10% of the market at most, but was rapidly gaining popularity.[4] By the late s, the great majority of high-tech electronic printed circuit assemblies were dominated by surface mount devices. Much of the pioneering work in this technology was done by IBM. The design approach first demonstrated by IBM in 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.[5] 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 and smaller circuit boards and, in turn, machines or subassemblies containing the boards.
Often the surface tension of the solder is enough to 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 is above the limit of 30g per square inch of pad area.[6] 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 on the same side of a board without adhesive if the SMT parts are first reflow-soldered, then a selective solder mask is used to prevent the solder holding those 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.
Conversely, SMT does not lend itself well to manual or low-automation fabrication, which is more economical and faster for one-off prototyping and small-scale production, and this is one reason why many through-hole components are still manufactured. Some SMDs can be soldered with a temperature-controlled manual soldering iron, but unfortunately, those that are very small or have too fine a lead pitch are impossible to manually solder without expensive hot-air solder reflow equipment[dubious discuss]. 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, but on the other hand, the costs of a certain SMT part and of an equivalent through-hole part may be quite similar, though rarely is the SMT part more expensive.
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Different terms describe the components, technique, and machines used in manufacturing. These terms are listed in the following table:[3]
SMp term Expanded form SMD Surface-mount devices (active, passive and electromechanical components) SMT Surface-mount technology (assembling and mounting 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)[
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PCB assembly line: pick-and-place machine followed by an SMT soldering ovenWhere 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 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.[7]
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 to prevent thermal shock. 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. At the end of , convection soldering was 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 do not 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, since they are considered harmless. This saves the cost of cleaning, speeds up the manufacturing process, and reduces waste. However, it is generally suggested to wash the assembly, even when a "No-Clean" process is used, when the application uses very high frequency clock signals (in excess of 1 GHz). Another reason to remove no-clean residues is to improve adhesion of conformal coatings and underfill materials.[8] Regardless of cleaning or not those PCBs, current industry trend suggests to carefully review a PCB assembly process where "No-Clean" is applied, since flux residues trapped under components and RF shields may affect surface insulation resistance (SIR), especially on high component density boards.[9]
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. Proper cleaning removes all traces of solder flux, as well as dirt and other contaminants that may be invisible to the naked eye. No-Clean or other soldering processes may leave "white residues" that, according to IPC, are acceptable "provided that these residues have been qualified and documented as benign".[10] 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.[11][12] 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.
Automated optical inspection (AOI) systems are commonly used in PCB manufacturing. This technology has proven highly efficient for process improvements and quality achievements.[13]
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SMD resistors in original packaging - this packaging allows for use in a mounting machineThe main advantages of SMT over the older through-hole technique are:[14][15]
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Handling of small SMT components can be difficult, requiring tweezers, unlike nearly all through-hole components. Whereas through-hole components will stay in place (under gravitational force) once inserted and can be mechanically secured prior to soldering by bending out two leads on the solder side of the board, SMDs are easily moved out of place by a touch of a soldering iron. Without developed skill, when manually soldering or desoldering a component, it is easy to accidentally reflow the solder of an adjacent SMT component and unintentionally displace it, something that is almost impossible to do with through-hole components.[
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Removal of surface-mount device using soldering tweezersDefective 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.
Reworking usually corrects 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 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.
There are essentially two non-contact soldering/desoldering methods: infrared soldering and soldering with hot gas.[21]
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With infrared soldering, the energy for heating up the solder joint is transmitted by long-, medium- or short-wave infrared electromagnetic radiation.
Advantages:
Disadvantages:
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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:
Disadvantages:
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Hybrid rework systems combine medium-wave infrared radiation with hot air
Advantages:
Disadvantages
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Example of component sizes, metric and imperial codes for two-terminal packages and comparison includedSurface-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).
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citation needed
] Generally physical size is proportional to capacitance and (squared) voltage 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 picofarads or pF, (1012 farad.)μH.
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