Surface Mount Technology is part of the electronic assembly that deals with the mounting of electronic components to the surface of the PCB. Electronic components mounted this way are called surface mounted devices (SMD). SMT was developed to minimize manufacturing costs while making efficient use of PCB space. The introduction of surface mount technology has enabled PCB design services for highly complex electronic circuits with smaller assemblies. There are various advantages and disadvantages of surface mount technology which we will discuss over the course of this article.
Through the course of this article, we will cover the following topics:
- The advent of surface mount technology
- Major differences between through-hole technology and Surface Mounted Technology (SMT)
- Advantages of surface mount technology
- Disadvantages of surface mount technology
- When to use surface mount technology?
- Surface Mount Device (SMD) packages
- SMD sizes in Inches or Metric system
The advent of surface mount technology
Surface-mount technology was developed in the 1960s and was broadly used in the 1980s. By the 1990s, they were used in most high-end PCB assemblies. Conventional electronic components were redesigned to include metal tabs or end caps that could be attached directly to the board surface. This replaced typical wire leads which needed to pass through drilled holes. SMT led to much smaller components and enabled component placement on both sides of the board more frequently than with through-hole mounting. Surface mounting enables a higher degree of automation minimizing labor costs and expanding production rates resulting in advanced PCB design and development.
Given below are the salient features of Surface Mounted and Through-Hole Technology:
Surface Mount Technology (SMT)
SMT allows electrical components to be mounted on the surface of a PCB without any drilling. These components have smaller leads or no leads at all and are smaller than through-hole components. As surface mount components do not require many drilled holes, they are more compact and are suitable for higher routing density.
Through-hole technology has been used for nearly all PCBs for years. Such mounting involves inserting the electronic component leads into holes drilled on the PCB and soldering the same to pads located on the other side of the PCB. Since through-hole mounting offers strong mechanical bonds, it is highly reliable. However, drilling PCBs during production tends to increase manufacturing costs. Also, through-hole technology limits the routing area for signal traces below the top layer on multi-layer boards.
Major differences between through-hole technology and Surface Mounted Technology (SMT)
There are several differences between surface-mounted and through-hole technology. Here are a few key distinctions between them:
- SMT frees up the limitation on board-space posed by the through-hole mounting manufacturing process
- Through-hole components involve higher manufacturing costs than SMT components
- SMT components do not have leads and are directly mounted on to the PCB. Through-hole components require lead wires that are placed in drilled holes and soldered.
- You require advanced design and production skills for using SMT when compared to through-hole technology.
- SMT components can have higher pin-count as compared to through-hole components
- Unlike through-hole technology, SMT enables assembly automation which is suitable for high production volumes at lower costs when compared to through-hole production.
- SMT components are more compact leading to higher component density as compared to through-hole mounting.
- While SMT leads to lower production costs, capital investment for machinery is higher than needed for through-hole technology
- Through-hole mounting is better suited to the production of large and bulky components that are subjected to periodic mechanical stresses or even high-voltage and high-power parts.
- SMT makes it easier to achieve higher circuit speeds because of its reduced size and because fewer holes are used, stray capacitance and inductance are reduced.
Advantages of surface mount technology
Overall advantages of SMT are several in number as given below:
- SMT enables smaller PCB design by allowing more components to be placed closer together on the board. This leads to designs that are more lightweight and compact.
- The process for production setup is faster when it comes to SMT when compared to through-hole technology. This is because drilled holes are not required for assembly which also enables lower costs.
- SMT enables higher circuit speeds since PCBs created with the SMT process is more compact.
- Components can be placed on both sides of the circuit board along with a higher component density with more connections possible per component.
- The compact package and lower lead inductance in SMT mean Electromagnetic Compatibility, (EMC) will be more easily achievable.
- SMT enables lower resistance and inductance at the connection, mitigating the undesired effects of RF signals providing better high-frequency performance
Advantages of SMT based on design:
- Significant reduction in weight
- Optimal utilization of board space
- Considerable electrical noise reduction.
Advantages of SMT based on manufacturing:
- Reduced board costs.
- Minimized material handling costs.
- Controlled manufacturing process.
Disadvantages of surface mount technology:
Even though SMT has several advantages, the technology to surface mount a device also brings with it certain disadvantages:
- When you subject components to mechanical stress, it is not reliable to use surface mounting as the sole method of attachment to the PCB. These components include connectors used to interface with external devices and that is periodically removed and re-attached.
- Solder connections for SMDs might be damaged through thermal cycles during operations
- You would higher-skilled or expert level operators and expensive tools for component-level repair and manual prototype assembly. This is because of the smaller sizes and lead spaces.
- Most SMT component packages can’t be installed in sockets that enable easy installation and replacement of failed components.
- You use less solder for solder joints in SMT, therefore the reliability of solder joints becomes a concern. Void formation might lead to solder joint failures here.
- SMDs are typically smaller than through-hole components leaving lesser surface area for marking part IDs and component values. This makes identifying components a challenge during prototyping and repair or rework.
When to use surface mount technology?
The majority of products manufactured at this time utilize surface mount technology. But SMT is not suitable in all cases. As a rule, SMT should be considered if:
- You need to accommodate a high density of components.
- The need is for a compact or small product.
- Your final product needs to be sleek and light despite component density.
- The requirement specifies the high-speed/frequency functioning of the device.
- You need to produce large quantities with automated technology.
- Your product should produce very little noise (if any at all).
Surface Mount Device (SMD) packages
SMD packages come in a broad range of shapes and sizes as given below:
Common passive discrete components
These components are mostly resistors and capacitors and are a part of most electronic devices available today.
Given below are SMD package details for capacitors and resistors.
The common type of packages for transistors are as follows:
- SOT-23 (Small Outline Transistor) with dimensions 3 x 1.75 x 1.3mm
- SOT-223 (Small Outline Transistor) with dimensions 6.7 x 3.7 x 1.8Mmm
Integrated Circuit (IC) packages
Integrated Circuit packages come in a wide range as given below:
- Small Outline Integrated Circuit (SOIC)
Small Outline Package (SOP)
TSOP (Thin Small Outline Package) is thinner than SOIC
- Quad Flat Pack (QFP)
Quad flat packs are generic square, flat IC packages.
- Ball Grid Array (BGA)
BGA packages include an arrangement of solder balls on the chip underside in the place of pins. The ball spacing typically is 1.27, 0.8, 0.5, 0.4, and 0.35mm
- Plastic Leaded Chip Carrier
The chip is enclosed in a plastic mold. It can either be square or in a rectangular shape.
SMD sizes in Inches or Metric system
Surface mount component standards are specified by the Joint Electron Device Engineering Council (JEDEC) Solid State Technology Association (JEDEC.org). JEDEC is an independent semiconductor engineering trade organization and standardization body which has its headquarters in Arlington, Virginia, United States.
You can measure SMD size in inches in the Imperial system and millimeters in the Metric system. For the 0201 imperial components, the dimensions are 0.02 x 0.01 inches. For the 0201 metric component 0.2 x 0.1 mm.
While SMT has its advantages over through-hole mounting methods, the component mounting method to be used still needs to be decided on the application of the device. Understanding the advantages and disadvantages of surface mount technology is essential to comprehend its role in the electronics industry. Such increased understanding will always help in optimizing one’s PCB design and assembly capabilities.
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