Introduction

Surface mount technology (SMT) is an aspect of electronic assembly where electronic components, also called surface mount devices (SMD), are directly mounted onto the surface of a printed circuit board (PCB). Because of its cost and quality efficiency, SMT has become highly sought after in the industry.

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What is Surface Mount Technology (SMT)?

Surface mount technology (SMT) is an assembly and production method that applies electronic components directly onto the surface of a printed circuit board (PCB). This process allows for automated production to complete more of the required assembly to create a working board. Any electrical component mounted in this manner is referred to as a surface-mount device (SMD). Unlike conventional assembly, SMT does not require components to be inserted through holes, rather components are soldered onto the board directly through reflow soldering.

Initially called Planar Mounting, SMT was first developed and applied by IBM to build small-scale computers in the s, thereby becoming a replacement for its predecessor, Through-Hole Technology. However, it did not take off until when surface-mounted components managed to reach 10% market popularity. By , surface-mounted devices or SMDs could be found in the majority of all high-tech printed circuit assemblies (PCAs).[1]

SMT components were designed to have small tabs where solder could be applied to attach SMDs to the surface of the PCB. In the era of Through-Hole Technology, components were mounted through lead holes that were drilled into PCBs. The holes were sized to fit each component to hold each piece tightly, then the grip was soldered. With SMT, the hole drilling step is bypassed as SMDs are quickly sorted and attached to the top of the PCB with little to no hole leads; thereby significantly cutting the process of device assembly.

The SMT assembly process can be tedious and time-consuming if done manually, due to the precision it requires to create high-quality surface mount assembly (SMA). So for better efficiency, most SMT manufacturing is done through automated assembly machines, especially when production is large scale.[2]

SMT components are significantly smaller than through-hole components, which makes for the production of sleek and attractive electronic devices suitable for modern times. Therefore these days, SMT is used in almost every electronic device ranging from toys to kitchen appliances, to laptops and smartphones.[3]

SMT Manufacturing Process

The SMT manufacturing process is broadly grouped into 3 stages, namely: solder paste printing, components placement, and reflow soldering. However, due to the demands of the SMT production process, these stages are further analyzed in the following outline:

1. SMC and PCB Preparation

This is the preliminary stage where the SMCs are selected and PCB designed. The board usually contains flat, commonly silver, tin-lead, or gold plated copper pads devoid of holes, called solder pads. Solder pads support the pins of the components like transistors and chips.

Another vital tool is the stencil, which is used to provide a fixed position for the next phase of the process (solder paste printing), according to the predetermined positions of solder pads on the PCB. These materials, along with others that are to be used in the manufacturing process must be properly examined for flaws. 

2. Solder Paste Printing

This is a critical phase in the SMT process. During this phase, a printer applies solder paste using the prepared stencil and squeegee (a tool for cleaning in printing) at an angle ranging from 45° to 60°. Solder paste is a putty-like mixture of powdered metal solder and sticky flux. The flux serves as a temporary glue to hold the surface mount components in place as well as cleansing the soldering surfaces of impurities and oxidation.

The solder paste, on the other hand, is used to connect the SMC and solder pads on the PCB. It is pertinent that each pad is coated in the correct quantity of paste. Otherwise, there will be no connection established when the solder is melted in the reflow oven. In the electronics manufacturing industry, a reflow oven is an electronic heating device used in surface mount technology (SMT) to place electronic components on printed circuit boards (PCBs).

3. Components Placement

Next, pick-and-place machines are used to mount components on the PCB. Each component is removed from its packaging using a vacuum or a gripper nozzle, and the placement machine places it in its designed location. The PCB is carried on a conveyor belt while the electronic components are placed on it by the quick and accurate machines, some of which can place 80,000 individual components per hour.

Accuracy is required in this process because any erroneous placement soldered into a position can be costly and time-consuming to rework.

4. Reflow Soldering

After SMCs are placed, the PCB is then conveyed into the reflow soldering oven, where it passes through the following zones to undergo the soldering process:

Preheat zone: this is the first zone in the oven, where the temperature of the board and all the attached components is raised simultaneously and gradually. Temperature is cranked up at the rate of 1.0&#;-2.0&#; per second until it enters 140&#;-160&#;.

Soak zone: here, the board will be kept at a temperature between 140&#; and 160&#; for 60-90 seconds.

Reflow zone: the boards then enter a zone where the temperature is ramped up at 1.0&#;-2.0&#; per second to the maximum of 210&#;-230&#; to melt the tin in the solder paste, welding the component leads to the pads on the PCB. While this is going on, the components are kept in place by the surface tension of the molten solder.

Cooling zone: this is the final section that ensures solder freezes upon exiting the heating zone to avoid joint defects.

If the printed circuit board is double-sided then these processes may be repeated using either solder paste or glue to hold the SMCs in place.

6. Cleaning and Inspection

After soldering, the board is cleaned and checked for flaws. If any is found, the defects are repaired and then the product is stored. Common ways used for SMT inspection include the use of magnifying lenses, AOI (Automated Optical Inspection), flying probe tester, X-ray inspection, etc. Instead of the naked eye, machines are used for quick and accurate results.

                                                   Soldering of PCB with electronic components

SMT: Pros and Cons

SMT has proven beneficial for PCB assembly (PCBA), PCB manufacturing, and electronics production in many ways including these:

• Allows for smaller components

• SMT process encourages increased automation

• Maximum flexibility in building PCBs

• Improved reliability and performance

• Reduced manual intervention for component placement

• Smaller, lighter boards

• Ease of PCB assembly, using both sides of the board without the hole limitations that exist in the conventional method

• Can co-exist with through-hole components, even on the same board

• Increased density i.e  more SMD components in the same space, or the same number of components in a much smaller frame

• Low Cost of materials

• Simplifies the production process and reduces the production cost. [4]

Conversely, disadvantages of SMT to electronic manufacturing include:

• Small volume

• Easily broken due to fragility

• High requirements for soldering technology

• Components can be easily dropped or damaged when installed.

• It is not easy to use visual inspection, which is difficult to test.

• Miniaturization and numerous solder joint types complicate the process and inspection.

• Large investment in equipment such as the SMT machine 

• Technical complexity requires high training and learning costs.

• Rapid development requires continuous follow-up. [5]

SMT vs SMD

SMT and SMD are frequently misunderstood and used simultaneously. Indeed, any technology and its actual components can be deeply entwined, creating confusion. Such is the case of SMT and SMD. This is why knowing the difference between an SMT assembly and individual SMD components is important.

In a simpler vein, SMT is the process in the technology, while SMD is the device involved in the technology. SMT is the technology that uses the method of directly placing and soldering electronic components on a PCB. These components are also sometimes called surface mount devices or SMDs. They are designed to be mounted on a printed circuit board (PCB).

SMDs make for devices produced faster, with more flexibility and less cost, without sacrificing functionality. They promise more functionality because smaller components allow for more circuits on small board space. This miniaturization is the major feature of SMD.[6]

Both SMT and SMD work together to provide users with faster, more energy-efficient, and more dependable PCBs.

Closing Thoughts

Smaller size, quicker production, and reduced weight are the major allures of SMT, leading to much easier electronic circuitry design and production, especially crucial in complex circuits. This higher level of automation has saved time and resources throughout the electronics manufacturing industry. As such, even though there is always a chance of developing new technology, SMT has undoubtedly secured its relevance.

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References

1. SCRIBD. Surface-Mount Technology: History. .[cited Jun 8]. Available from: https://www.scribd.com/document//SMD

2. Das S. SMT Machine and SMT Machine Manufacturers. [cited Jun 8]. Available from: http://www.electronicsandyou.com/smt-machine-and-smt-machine-manufacturers.html

3. Geospace Technology. SMT ASSEMBLY TECHNOLOGY: ALL YOU NEED TO KNOW. .[cited Jun 9]. Available from: https://geospacemfg.com/blog/smt-assembly-technology/

4. Blog Industry News. Top Benefits of Surface-mount Technology (SMT) PCB Assembly. . [cited Jun 9]. Available from: https://www.pcbnet.com/blog/benefits-surface-mount-technology-smt-pcb-assembly/

5. PCBWay. Advantages and Disadvantages of Surface Mounting Technology. .[cited Jun 9]. Available from: https://www.pcbway.com/blog/PCB_Assembly/Advantages_and_Disadvantages_of_Surface_Mounting_Technology.html

6. History Computer (HC). Surface-Mount Technology. . [cited Jun 9]. Available from: https://history-computer.com/smt-surface-mount-technology/

Surface Mount Technology & SMT Devices

Surface mount technology, SMT and its associated surface mount devices, SMDs considerably speed up PCB assembly as the components simply mount on the board.

Surface Mount Technology, SMT Includes:
What is SMT     SMD packages     Quad flat pack, QFP     Ball grid array, BGA     Plastic leaded chip carrier, PLCC    

Look inside any piece of commercially made electronic equipment these days and it is filled with minute devices. Rather than using traditional components with wire leads like those that may be used for home construction and kits, these components are mounted onto the surface of the boards and many are minute in size.

This technology is known as Surface Mount Technology, SMT and it has the associated SMT components or surface mount devices or SMDs.

Typical PCB using surface mount technology

Virtually all today's equipment that is manufactured commercially uses surface mount technology, SMT, because it offers significant advantages during PCB manufacture, and in view of the size the use of SMT components enables far more electronics to be packed into a much smaller space.

In addition to the size, surface mount technology allows automated PCB assembly and soldering to be used, and this brings significant improvements in reliability as well as enormous savings in cost.

What actually is surface mount technology?

During the s and s the level of automation started to rise for PCB assembly for boards used in a variety of equipment. The use of traditional components with leads did not prove easy for PCB assembly.

Resistors and capacitors needed to have their leads pre-formed so that they would fit through holes, and even integrated circuits needed to have their leads set to exactly the right pitch so that they could be placed through the holes easily.

This approach always proved difficult as leads often missed the holes as tolerances required to ensure they fitted exactly through the holes were very tight. As a result operator intervention was frequently required to resolve the issues of components not fitting properly and stopping the machines. This slowed down the PCB assembly process and considerably increased costs.

For PCB assembly there is actually no need for the component leads to pass through the board. Instead it is quite adequate for components to be soldered directly to the board. As a result, surface mount technology, SMT was born, and the use of SMT components rose very rapidly as their advantages were seen and realised.

Concept of surface mount technology: a typical passive component

Today surface mount technology is the main technology used for PCB assembly within electronics manufacturing. SMT components are able to be made very small, and may types are used in their billions, particularly SMT capacitors and SMT resistors.

SMT devices

Surface mount components are different to their leaded counterparts. Rather than being designed to wire between two points, SMT components are designed to be set down on a board and soldered to it.

Their leads to not go through holes in the board as might be expected for a traditional leaded component. There are different styles of package for different types of component. Broadly the package styles can be fitted into three categories: passive components, transistors and diodes, and integrated circuits and these three categories of SMT components are viewed below.

A selection of surface mount technology components

• Passive SMDs:   There is quite a variety of different packages used for passive SMDs. However the majority of passive SMDs are either SMT resistors or SMT capacitors for which the package sizes are reasonably well standardised. Other components including coils, crystals and others tend to have more individual requirements and hence their own packages.

  Resistors and capacitors have a variety of package sizes. These have designations that include: , , , , , and . The figures refer to the dimensions in hundreds of an inch. In other words the measures 12 x 6 hundredths of an inch.

  The larger sizes such as and were some of the first that were used. They are not in widespread use now as much smaller components are generally required. However they may find use in applications where larger power levels are needed or where other considerations require the larger size.

  The connections to the printed circuit board are made through metallised areas at either end of the package.

• Transistors and diodes:   SMT transistors and SMT diodes are often contained in a small plastic package. The connections are made via leads which emanate from the package and are bent so that they touch the board. Three leads are always used for these packages. In this way it is easy to identify which way round the device must go.

• Integrated circuits:   There is a variety of packages which are used for integrated circuits. The package used depends upon the level of interconnectivity required. Many chips like the simple logic chips may only require 14 or 16 pins, whereas other like the VLSI processors and associated chips can require up to 200 or more. In view of the wide variation of requirements there is a number of different packages available.

  For the smaller chips, packages such as the SOIC (Small Outline Integrated Circuit) may be used. These are effectively the SMT version of the familiar DIL (Dual In Line) packages used for the familiar 74 series logic chips. Additionally there are smaller versions including TSOP (Thin Small Outline Package) and SSOP (Shrink Small Outline Package).

  The VLSI chips require a different approach. Typically a package known as a quad flat pack is used. This has a square or rectangular footprint and has pins emanating on all four sides. Pins again are bent out of the package in what is termed a gull-wing formation so that they meet the board. The spacing of the pins is dependent upon the number of pins required. For some chips it may be as close as 20 thousandths of an inch. Great care is required when packaging these chips and handling them as the pins are very easily bent.

  Other packages are also available. One known as a BGA (Ball Grid Array) is used in many applications. Instead of having the connections on the side of the package, they are underneath. The connection pads have balls of solder that melt during the soldering process, thereby making a good connection with the board and mechanically attaching it. As the whole of the underside of the package can be used, the pitch of the connections is wider and it is found to be much more reliable.

  A smaller version of the BGA, known as the microBGA is also being used for some ICs. As the name suggests it is a smaller version of the BGA.

Read more about . . . . packages for surface mount components.

With the level of adoption for surface mount technology, there is a huge variety of components available. The selection of components available in surface mount packages far exceeds the number available in traditional leaded forms. This is purely because of the demand.

However the popular basic components like transistors and many logic and analogue ICs such as operational amplifiers normally have versions available as a traditional leaded component and also as a surface mount component. A BC109 transistor, for example can be obtained in both formats, as can many operational amplifiers and basic logic chips.

Surface mount technology in design

The main reason for the move to surface mount technology was the great improvement in terms of speed, reliability and cost for the PCB assembly process. Whilst this is the major impact for the adoption of the technology, it also impacts the design and development of new electronic circuits and equipment. Fortunately this transfer brings more advantages to development and circuit performance, than disadvantages.

For the development engineer, the use of surface mount technology offers many advantages, although there are some points to watch:

• Low spurious capacitance and inductance:   In view of the small size of the components, the levels of spurious inductance and capacitance are much smaller - SMT resistors function in a way that is closer to the perfect resistor than that of a leaded resistor. Similarly an SMT capacitor will exhibit much lower parasitic inductance. As a result faster speeds and higher frequencies are possible with standard SMT components than would be possible with leaded equivalents.
• Lower power ratings:   The power rating of surface mount components is of great importance. The surface mount resistor is the particular example. A standard leaded resistor can dissipate at least 0.25 watts. For surface mount resistors, bing much smaller, the dissipation is also less. Be aware of this and check the manufacturers data.
• Smaller / more dense circuits:   As the drive for more functionality within ever smaller volumes is a common trend across the electronics industry, surface mount technology helps in a major way in allowing for miniaturisation. The components can be made much smaller and additionally they can be mounted on the printed circuit board far closer together than would be possible with traditional leaded components. Combined with the greater level of functionality now obtainable within integrated circuits, this means that the task of the development engineer is made possible.

Although there are a few additional precautions to observe when using surface mount technology in a new design, most elements of the design remain very much the same, although the designs tend to be much more complicated and provide much more functionality. In this way the introduction and use of surface mount technology has facilitated the development of electronics, allowing much greater levels of complexity and providing more capability.

PCB assembly using surface mount technology

SMT is used almost exclusively for the PCB assembly and manufacture these days. It is possible to pack far more electronics into a smaller space using SMT. The surface mount components are smaller and often offer a better level of performance and they can be used with automated pick and place machine that in many cases all bit eliminate the need for manual intervention in the assembly process.

Wired components were always difficult to place automatically because the wires needed to be pre-formed to fit the relevant hole spacing, and even then they were prone to problems with placement.

Today in the PCB assembly process, most of the components on a board are placed automatically. Occasionally some may need manual intervention, but this is being reduced all the time. Traditionally some connectors and possibly a few other components required assisted placement, but the level of manual placement is falling all the time.

Today, printed circuit boards are normally developed to reduce this to an absolute minimum, even to the extent of altering the design to use components that can be placed automatically. In addition to this, component manufacturers have developed some specialised surface mount versions of components that enable virtually complete automated assembly for most boards.

One of the issues with some components has been their resilience to heat. The soldering processes require that the whole component is raised to a high temperature, and this has caused issues with some technologies. Integrated circuits, surface mount resistors and many types of surface mount capacitor are fine.

However, it was for this reason that surface mount electrolytic capacitors were not used initially. Instead surface mount tantalums were used, but now, versions of surface mount electrolytic capacitors have been developed that are able to tolerate the temperatures experienced during soldering.

There are other components that have required special development to enable them to become available in surface mount component formats.

Board expansion & flexure

One of the issues that can occur with surface mount boards arise as a result of temperature changes and also board flexing. With boards using leaded components this is not a major issue because the wires on the components take up the movement and relieve any strain that may be caused.

The same may not be true for surface mount components. The components are soldered down tot he printed circuit board and quite rigidly held in place. Components like surface mount transistors and surface mount integrated circuits where there are leads from the device body onto the surface of the board have some means for accommodating movement, but surface mount resistors and capacitors do not.

The components most sensitive to strain on the board are surface mount capacitors - the ceramic MLCC variety. They tend to crack when put under tensile stress. This obviously is a major issue for reliability.

There are several precautions that can be taken in design and PCB assembly to ensure that warping and temperature expansion etc problems are minimise:

• Ensure PCB power and earth planes are distributed evenly:   When the printed circuit boards are passed though the soldering process during PCB assembly, the boards will be heated significantly and this can lead to warping - the levels can be significant in some large boards. To help alleviate this issue, earth planes and power planes should cover the complete board as far as possible. If they are only present over part of the printed circuit board this can cause warping.
• Shape of components:   Surface mount components with short, wide bodies are preferable to long and thin ones. If the component is short and wide, the effects of expansion and flexing will be less pronounced.
• Mount components at right angle to direction of maximum flex:   Boards tend to warp along the longest length of the board. Mount components in a plane that will be exposed to the minimum flex or bend.

SMT applications

Although it is possible to use some SMT components for home construction, great care is required when soldering them. Additionally even the ICs having a wide pin spacing may be difficult to solder.

Those with fifty or more pins cannot be soldered without special equipment. They are intended only for large scale manufacturing. Even when working on boards that have already been built great care is needed. However these SMT components offer great cost savings to manufacturers and this is why they have been adopted. Fortunately for the home constructor, traditional leaded components that can be soldered manually are still widely available and offer a much better solution for home construction.

Nevertheless SMT components can be used for some home projects where they are applicable - where the SMT component leads and connections are not too small to manage with more traditional soldering irons and other tools.

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