The Advantages and Disadvantages of Surface Mount Technology (SMT)

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 1960s and gained widespread usage in the 1980s, becoming prevalent in most high-end PCB assemblies by the 1990s. 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.

Key characteristics of SMT and through-hole technology

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.

The SMT assembly process involves:

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.

Key distinctions between through-hole technology and surface mount technology:

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.

Considerations before opting for SMT or through-hole technology:

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

Advantages of surface mount technology (SMT):

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.

Disadvantages of surface mount technology (SMT):

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.

When to use surface mount technology?

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.

Guidelines for SMT component placement

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.