(SMT) Surface-Mount Technology: Meaning, Definition, and ...

Author: Bonny

Dec. 06, 2023

Machinery

Key Points

  • SMT stands for Surface-Mount Technology and is method for attaching electrical components onto a motherboard.
  • SMT has another big advantage over the former ‘through-hole technology’ method.
  • It is a lot of terms to associate with the technology.

What is Surface-Mount Technology?: Complete Explanation

Surface-Mount Technology, or SMT, is a method for attaching electrical components directly onto the surface of a printed circuit board, or PCB. This process allows for automated production to complete more of the required assembly to create a working board. It lowers the cost of production and increases the maximum output by eliminating bottlenecks on the assembly line.

SMT has another big advantage over the former ‘through-hole technology’ method. In the older method, electronic components were mounted to a circuit board through specifically cut holes in the board for the component.

This required larger components, precise handling, and an extra solder to attach each component firmly. With SMT, electrical components, called surface-mount devices (SMD), are quickly sorted and attached to the top of the PCB with either minuscule leads or no leads at all.

Your motherboard is responsible for ensuring all parts of your computer are able to function properly.

©prawest/Shutterstock.com

SMT components are significantly smaller than through-hole components which are achieved by machine handling. While there may still be short pins, flat contacts, solder balls, or terminations on the circuit body, the components are still so much smaller than what is required by through-hole technology that the end result is much more compact as well. This often translates into sleek, attractive electric devices.

Along with SMT, there is a slew of other related terms that you may want to know. Here is a quick breakdown:

  • SMD – Surface-mount devices: active, passive, and electrochemical components
  • SMT – Surface-mount technology: assembling and mounting technology
  • SMA – Surface-mount assembly: module assembled with SMT
  • SMC – Surface-mount components: components used for SMT
  • SMP – Surface-mount package: SMD case forms
  • SME – Surface-mount equipment: SMT assembling machines

It is a lot of terms to associate with the technology, but it can be understood in an easier way. SMT is the method used by SMEs to place SMDs made of SMPs around SMCs onto PCBs. The finished assembly is an SMA.

Surface-Mount Technology: Pros and Cons!

Pros!Cons!SMT allows for smaller components
SMT is not great for devices that need to be frequently attached and detached from other componentsMachine precision can be used to organize smaller components into a much denser populated board
SMDs solder connections are smaller and can be damaged by potting compounds during thermal cyclingComponents can be placed on both sides of the PCB
Manual assembly and repair are difficult and require expensive tools and a skilled operatorSmall errors in placement are automatically corrected by surface tension when the solder is heated
Many SMT components are incompatible with socketsImproved mechanical performance when shocked or vibrating
SMDs are not plug-and-play friendly and will likely require specialized PCBs for prototypingLower resistance and inductance at the connection
Due to the size of SMDs being smaller and PCBs being more densely populated, part ID codes must be made even smaller and often coded or shorthandBetter EMC performance
Fewer to no holes needed
Lower cost and less time to set up for mass production and automation
Less expensive components

Surface-Mount Technology: An Exact Definition

SMT, or Surface-Mount Technology, is an assembly and production method for mounting components to a printed circuit board (PCB) directly to the surface. It was designed to replace the previous method known as ‘through-hole technology’.

The method was developed in the 1960s. It took until 1986 before surface-mounted components were able to reach 10% of the market. By 1990, SMDs were inside the majority of all high-tech printed circuit assemblies.

Most of the work pioneering surface-mount technology was done by IBM. IBM presented its first demonstration of SMT in 1960 with a small-scale computer that was later used in the Launch Vehicle Digital Computer for the Instrument Unit that guided all Saturn IB and Saturn V vehicles.

Surface-mount technology components were designed to have small tabs or end caps where solder could be applied to mount SMDs to the surface of the PCB. Previously, components were mounted through lead holes that had to be drilled into PCBs.

The holes were drilled to component size to hold each piece tightly. The grip was then soldered. SMT takes the hole drilling step out of the process. By designing SMDs that required less or no hole leads, the process of device assembly was dramatically cut. There was a much greater advantage.

SMT allowed for much smaller components to be designed that could still be held tightly to the PCB. With smaller components, comes greater component density. The advancement caused by using the SMT method has been noted under “Moore’s Law” which states that the density of components on a motherboard would double every year from 1965 to 1975. He then amended the statement to say that the density would double every two years after that.

Today, SMT is used in nearly every electronic device from children’s toys to coffee makers to smartphones and laptops. While there is always the possibility of another method arising and replacing SMT, it appears that surface-mount technology will continue to be in use for a very long time.

SMTs are used to mount components to PCBs.

How Does SMT Work?

SMT is a method that makes use of specialized tools to manage tiny components onto a printed circuit board. While this process can be done by hand, it is incredibly time-consuming and tedious. The precision required to create a quality SMA, surface-mount assembly, makes it a skilled position. Most SMT production and assembly are done through automation.

The process starts with a pile of components and any type of printed circuit board. PCBs are typically covered in flat tin-lead, silver, or gold plated copper pads named solder pads. Specialized automatons then cover the solder pads with solder paste using a stainless steel or nickel stencil.

With the solder paste in position, the PCB is sent down the assembly line to pick-and-place machines that take components from a conveyor and place them where they should be on the PCB.

Once the board is covered in components and ready to bake, they are sent through a reflow soldering oven. The temperature of the PCB and its components are gradually raised in a uniform manner to prevent thermal shock.

Once the board is hot enough, the conveyor moves the assembly into a zone that can reach temperatures hot enough to melt the solder paste. The surface tension of the molten solder on the solder pads keeps the components in the correct place as long as the pads’ shapes are correctly designed.

After the PCB is completely soldered, they are usually taken to be washed. The reflow process can sometimes create solder paste residue or have stray solder balls. Any extra solder or conductive material can cause board shorts.

Once the boards are cleaned, they are sent to be visually inspected for missing components or abnormalities. In some more advanced fabrication plants, an automated optical inspection (AOI) system is in place to cut back on human labor and decrease inspection times.

How do You Create Surface-Mount Technology?

To perform the process at home, you will need to get a hold of a few materials:

  • Printed circuit board
  • Solder Paste
  • SMD and required components (this list depends entirely on what is being designed)
  • Solder Gun
  • Cleaning Solvent

Step 1

Place solder paste on the solder pads across the board. If the PCB does not have a printed pattern, it will need to be screened with a conductive pathway according to your device’s schematics.

Step 2

Organize the components across the board where they belong. This pattern relies completely on the intended use and design of the product.

Step 3

Use the solder gun to heat the solder paste on the solder pad of each component. It may be useful to use tweezers to hold each component still.

Step 4

Clean the board for solder residue or runs.

Step 5

Inspect the design for missing components.

The process may sound simple, but the design knowledge required to design circuit boards alone is enormous. If you can manage to successfully design a circuit board, the rest is just a little bit of hard labor. With any luck, you’ll have a useable board. I would personally recommend that you order any custom needs from PCB manufacturers. As the size of components on SMT devices is minuscule, the identification markings can be one of the most difficult facts to read.

Where Did SMT Originate From?

The concept of SMT electronics was presented by IBM in the 1960s to showcase types of small form electronics for functional use. Around the same time, the European Philips company had developed a small surface-mounted button device for watches. It took some time for it to take over, but by 1986 it had reached 10% of the market. The method was originally called Planar Mounting and IBM designed the majority of the components required for the process.

The evolution from Planar Mounting to SMT occurred in three stages. The first stage was between 1970 and 1975. The adoption of IBM’s prototype in the 60s for space vehicle computer systems helped to grow interested in its development.

1970 marked the kick-off for reaching the goal of applying miniaturized electronics to printed circuit boards. One of the greatest early advancements was in the replacement of lead with leadless ceramic chip carriers (LCCC). By the end of 1975, SMT was widely used in civilian quartz electronic watches and electronic calculators.

From 1976 to 1985, SMT went through its second major stage of evolution. The size of components had been significantly reduced which allowed for the creation of multi-functional electronics. This lent SMT heavily for use in photography and headset radios.

At the same time, automation at the factory level of SMT production was advancing to a new point. As the production machinery matured, the foundation for even more rapid advancements in the development of SMT had been laid.

What are the Applications of SMT?

SMT can and is used to create device assemblies used in nearly every electronic device in the world. From digital cameras and smart TVs to laptops, desktops, and smartphones, SMT has made way for densely populated circuit boards which directly translates into better and more functionality.

Any time a product needs a circuit board for electronics function, SMT is used to make it. The density at which components can be placed is without true competition.

Here are a few products that you can find SMT processed circuit boards:

  • Laptops
  • Desktops
  • Smart TVs
  • Smartphones
  • Tablets
  • Electronic Toys
  • Robotics
  • VR Headsets
  • Smart Appliances
  • Watches
  • Cameras

Examples of SMT in the Real World

As a method for production, SMT is in use by many different companies. You can find the end products in nearly any electronic device. Here is a quick list of some SMT manufacturers:

  • Giltronics Associates Inc.
  • Janco Electronics, Inc.
  • Bennett Machine & Stamping Co.
  • PGF Technology Group
  • RBB Systems, Inc.
  • DIGICOM Electronics, Inc.
  • MME group, Inc.
  • NAS Electronics
  • Advanced Product Design & Mfg.
  • MH MFG., a SEACOMP Company
  • CSI Electronics
  • NuWaves Engineering
  • Kingston Technologies Inc.
  • American Products, Inc.
  • RPC Electronics, Inc.
  • Providence Enterprise USA, Inc.
  • LeeMAH Electronics, Inc.
  • Electro-Prep, Inc.
  • Accelerated Design and Manufacturing, Inc.
  • Cir-Q-Tek
  • A & M Electronics Inc.
  • Pico Electronics, Inc.
  • Creative Hi-Tech Limited
  • Smart Sourcing, Inc.
  • Galaxy Electronics, Inc.

Next Up…

Method for producing electronic circuits

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. A MOSFET transistor, placed upon a British postage stamp for size comparison.

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.

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

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Surface-mount technology was developed in the 1960s. By 1986 surface mounted components accounted for 10% of the market at most, but was rapidly gaining popularity.[4] By the late 1990s, 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 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.[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.

Common abbreviations

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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)

Assembly techniques

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PCB assembly line: pick-and-place machine followed by an SMT soldering oven

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 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 2008, 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]

Advantages

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SMD resistors in original packaging - this packaging allows for use in a mounting machine

The main advantages of SMT over the older through-hole technique are:[14][15]

  • Smaller components.
  • Much higher component density (components per unit area) and many more connections per component.
  • Components can be placed on both sides of the circuit board.
  • Higher density of connections because holes do not block routing space on inner layers, nor on back-side layers if components are mounted on only one side of the PCB.
  • Small errors in component placement are corrected automatically as the surface tension of molten solder pulls components into alignment with solder pads. (On the other hand, through-hole components cannot be slightly misaligned, because once the leads are through the holes, the components are fully aligned and cannot move laterally out of alignment.)
  • Better mechanical performance under shock and vibration conditions (partly due to lower mass, and partly due to less cantilevering)
  • Lower resistance and inductance at the connection; consequently, fewer unwanted RF signal effects and better and more predictable high-frequency performance.
  • Better EMC performance (lower radiated emissions) due to the smaller radiation loop area (because of the smaller package) and the lesser lead inductance.[16]
  • Fewer holes need to be drilled. (Drilling PCBs is time-consuming and expensive.)
  • Lower initial cost and time of setting up for mass production, using automated equipment.
  • Simpler and faster automated assembly. Some placement machines are capable of placing more than 136,000 components per hour.
  • Many SMT parts cost less than equivalent through-hole parts.

Disadvantages

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  • SMT may be 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.[

    citation needed

    ]
  • SMDs' solder connections may be damaged by potting compounds going through thermal cycling.
  • 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.[17] 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.
  • Many types of SMT component packages cannot be installed in sockets, which provide for easy installation or exchange of components to modify a circuit and easy replacement of failed components. (Virtually all through-hole components can be socketed.)
  • 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.[18]
  • 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.[19][20]
  • SMDs, usually being smaller than equivalent through-hole components, have less surface area for marking, requiring marked part ID codes or component values to be more cryptic and smaller, often requiring magnification to be read, whereas a larger through-hole component could be read and identified by the unaided eye. This is a disadvantage for prototyping, repair, rework, reverse engineering, and possibly for production set-up.

Rework

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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.

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 (may be not required for some components)
  • Print solder paste on PCB, directly or by dispensing or dipping
  • 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.

There are essentially two non-contact soldering/desoldering methods: infrared soldering and soldering with hot gas.[21]

Infrared

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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:

  • Easy setup
  • No compressed air required for the heating process (some systems use compressed air for cooling)
  • No requirement for different nozzles for many component shapes and sizes, reducing cost and the need to change nozzles
  • Very uniform heating possible, assuming high quality IR heating systems
  • Gentle reflow process with low surface temperatures, assuming correct profile settings
  • Fast reaction of infrared source (depends on system used)
  • Closed loop temperature control directly on the component possible by applied thermocouple or pyrometric measurement. This allows compensation of varying environmental influences and temperature losses. Enables use of the same temperature profile on slightly different assemblies, as the heating process adapts itself automatically. Enables (re)entry into the profile even on hot assemblies
  • Direct setting of target profile temperatures and gradients possible through direct control of component temperature in each individual soldering process.
  • No increased oxidation due to strong blowing of the solder joints with hot air, reduces flux wear or flux blowing away
  • Documentation of the temperature elapsed on the component for each individual rework process possible

Disadvantages:

  • Temperature sensitive nearby components must be shielded from heat to prevent damage, which requires additional time for every board
  • On short wavelength IR only: Surface temperature depends on the component's albedo: dark surfaces will be heated more than lighter surfaces
  • Convective loss of energy at the component possible
  • No reflow atmosphere possible (but also not required)

Hot gas

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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:

  • Some systems allow switching between hot air and nitrogen
  • Standard and component-specific nozzles allow high reliability and faster processing
  • Allow reproducible soldering profiles (depends on system used)
  • Efficient heating, large amounts of heat can be transferred
  • Even heating of the affected board area (depends on system / nozzle quality used)
  • 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)
  • Precise, sometimes very complex, component-specific hot gas nozzles are needed to direct the hot gas to the target component. These can be very expensive.
  • Today, nozzles can often no longer be deposited on the PCB by neighboring components, which means that there is no longer a closed process chamber and adjacent components can be blown on strongly from the side. This can lead to the blowing of adjacent components and even to thermal damage. In this case, adjacent components must be protected from the air flow, e.g. by covering them with polyimide tape.
  • Local turbulence of the hot gas can create hot and cold spots on the heated surfaces, resulting in uneven heating. Perfectly designed, high-quality nozzles are therefore a must!
  • Swirls at component edges, especially at bases and connectors, can heat these edges significantly more than other surfaces. Overheating can occur (burns, melting of plastics)
  • Losses due to environmental influences are not compensated for, since the component temperature is not measured in the production process
  • Creation of a suitable reflow profile requires an adjustment and test phase, in some cases involving several stages
  • A direct temperature control of the component is not possible, because the measurement of the actual component temperature is difficult due to the high gas velocity (measurement failure!)

Hybrid technology

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Hybrid rework systems combine medium-wave infrared radiation with hot air

Advantages:

  • Easy setup
  • The low flow velocity hot air supporting the IR radiation improves heat transfer, but cannot blow away components
  • Heat transfer does not depend entirely on the flow velocity of hot gas at the component/assembly surface (see hot gas)
  • No requirement for different nozzles for many component shapes and sizes, reducing cost and the need to change nozzles
  • Adjustment of the heating surface possible through various attachments if required
  • Heating even very large / long and exotically shaped components possible, depending on the type of top heater
  • Very uniform heating possible, assuming high quality hybrid heating systems
  • Gentle reflow process with low surface temperatures, assuming correct profile settings
  • No compressed air required for the heating process (some systems use compressed air for cooling)
  • Closed loop temperature control directly on the component possible by applied thermocouple or pyrometric measurement. This allows compensation of varying environmental influences and temperature losses. Enables use of the same temperature profile on slightly different assemblies, as the heating process adapts itself automatically. Enables (re)entry into the profile even on hot assemblies
  • Direct setting of target profile temperatures and gradients possible through direct control of component temperature in each individual soldering process.
  • No increased oxidation due to strong blowing of the solder joints with hot air, reduces flux wear or flux blowing away
  • Documentation of the temperature elapsed on the component for each individual rework process possible

Disadvantages

  • Temperature sensitive nearby components must be shielded from heat to prevent damage, which requires additional time for every board. Shield must cover also from gas flow
  • Convective loss of energy at the component possible

Packages

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Example of component sizes, metric and imperial codes for two-terminal packages and comparison included

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).

Identification

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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. 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 Series of values, while the letter indicates the multiplier.[22]
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.[

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, (10−12 farad.)
Inductors
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 nanohenry (nH) 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 for smaller inductors 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. Larger inductors, especially wire-wound types in larger footprints, usually have the value printed on the top. For example, "330", which equates to a value of 33

 

μH.
Discrete semiconductors
Discrete semiconductors, such as diodes and transistors are often marked with a two- or three-symbol code. The same code marked on different packages or on devices from 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 larger packages, 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.

See also

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References

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(SMT) Surface-Mount Technology: Meaning, Definition, and ...

Surface-mount technology

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