SMT was developed to minimize manufacturing costs while making efficient use of board space. The introduction of SMT technology made it possible to manufacture smaller complex circuit boards. There are several advantages and disadvantages of surface mount technology that we will discuss throughout this article.
The birth of surface mount technology
Surface mount technology was developed in the 1960s and was widely used in the 1980s. In the 1990s, it was used in most high-end PCB assemblies.
Conventional electronic components were redesigned to include metal separators or covers that could be attached directly to the board surface. This replaced the typical wires that had to go through drilled holes.
SMT led to much smaller components and allowed for the placement of components on both sides of the board. Surface mounting allows for a greater degree of automation, minimizing labor costs and the expansion of production rates, which results in the advanced development of the plates.
Key features of SMT and through-hole technology
SMT allows electrical components to be mounted on the board surface without any drilling. Most electronic applications prefer to use surface-mount components since they are compact and can be installed on both sides of a printed circuit board. They are suitable for applications with higher routing densities. These components are smaller or have no cables and are smaller than components with through-hole components.
The process involved in the SMT assembly is:
- Apply solder paste to the fabricated circuit board using stencils. Solder paste is composed of flux and tin particles.
- Place the mounting components on the surface.
- Use a reflow soldering method.
In through-hole technology, component cables are inserted into holes drilled in the board. These cables are then soldered to pads on the opposite side using wave soldering tools or reflow soldering tools. Because hole mounting offers strong mechanical connections, it is highly reliable. However, drilling PCBs during production tends to increase manufacturing costs. Additionally, through-hole technology limits the routing area for signal traces below the top layer of multilayer PCBs.
Key differences between through-hole technology and surface mount technology
- SMT frees the board space limitation placed by through-hole technology.
- Through-hole components entail higher manufacturing costs than SMT components.
- Advanced design and production skills are required to utilize SMT technology compared to through-hole technology.
- SMT components may have a larger number of pins compared to hole components.
- Unlike through-hole technology, SMT allows assembly automation, which is suitable for high production volumes at lower costs compared to through-hole production.
- SMT components are more compact, which leads to a higher component density compared to mounting through holes.
- Although surface mounting leads to lower production costs, the capital investment in machines is higher than that required for through-hole technology.
- Hole mounting is more suitable for producing large, bulky components subject to periodic mechanical stress or even high-tension, high-power parts.
- SMT makes it easier to achieve higher circuit speeds due to its small size and smaller number of holes.
Factors to consider before choosing SMT technology or through-hole technology:
- Component stability when exposed to external stresses.
- Easy thermal management/heat dissipation.
- Availability of the part and its alternative.
- Cost-effectiveness of the assembly.
- High performance and package lifespan.
- Facilitate rework in case of board failure.
Advantages of surface mount technology:
- Surface mount technology supports microelectronics by allowing more components to be placed closer to each other on the board. This leads to lighter and more compact designs.
- The SMT production setup process is faster when compared to through-hole technology. This is due to the fact that the components are assembled using solder paste instead of instead of holes. It saves time and intensive work.
- The components can be placed on either side of the circuit board, along with a higher density of components with more possible connections per component.
- Due to the compact size of the package, the traces can be of greater density and can be accommodated on the same layer.
- The surface tension of the melted solder pulls components into alignment with the solder pads, which automatically corrects minor placement problems.
- Compared to through holes, these do not increase in size during operation. Thus, it is possible to reduce the space between packages.
- Electromagnetic compatibility is easily achieved in SMT plates due to their compact packaging and lower lead inductance.
- SMT technology allows for lower resistance and inductance in the connection. It mitigates the undesirable effects of RF signals and provides better performance at high frequencies.
- More parts can easily fit the board due to its compactness, resulting in shorter signal paths. This improves signal integrity. The heat dissipated is also lower than that of the through-hole components.
- SMT reduces plate and material handling costs.
- It allows you to have a controlled manufacturing process. This is especially suitable for the production of high-volume PCBs for better organization and appearance, since the image we pass on to those who visit us makes all the difference.
Disadvantages of surface mount technology
While SMT has several advantages, it also has some drawbacks:
- When subjecting components to mechanical stress, it is not reliable to use surface mounting as the only method of attachment to the printed circuit board. This is due to the fact that it is necessary to use component connectors to interface with external devices that are periodically removed and replaced.
- Solder connections for SMDs can be damaged by thermal cycles during operations.
- Highly qualified or specialized operators and expensive tools are required for component repair and manual prototype assembly. This is due to smaller sizes and breakthrough spaces.
- Most SMT component packages cannot be installed in sockets that allow easy installation and replacement of faulty components.
- Less solder is used for SMT solder joints, so the reliability of solder joints becomes a concern. The formation of voids can lead to failure of the welding joints.
- SMDs are typically smaller than through-hole components, leaving a surface for marking part IDs and component values. This fact makes identifying components a challenge during printed circuit board prototyping and repair.
- The solder may melt when exposed to intense heat. Therefore, SMT cannot be implemented in electrical charge circuits with high heat dissipation.
- Printed circuit boards that use this technology require more installation costs. This is due to the fact that most SMT equipment, such as the hot air rework station, pick-up and placement machine, solder paste screen printer, and reflow oven are expensive.
- The miniaturization and variety of welding joints can make the most difficult procedure and process inspection difficult.
- Due to the compact size, there is a greater likelihood of solder overflow that can result in short circuits and solder bridges.
When to use surface mount technology?
Most of the products currently manufactured use surface mount technology.
But SMT is not suitable in all cases. Consider SMT if:
- It is necessary to accommodate a high density of components.
- The need is for a compact or small product.
- Your final product needs to be elegant and lightweight, despite the density of components.
- The requirement specifies the high speed/frequency operation of the device.
- It is necessary to produce large quantities with automated technology.
- Your product should produce very little noise (if any).
Welding techniques used in SMT
Solder reflow and wave solder are widely used to mount components on the board. Depending on the nature of the components, the designer may choose one of these methods for surface mounting technology.
Wave soldering: Since the solder will flow through the holes to form a bond, wave soldering is primarily used for through-hole components. You can use wave soldering for most surface mount components.
Solder reflow: This process is generally preferred in SMT. Here, the solder on one pin melts and reflows faster than the other. The only disadvantage is the fact that it causes a tombstoning effect, in which the component detaches from the unmelted cushion. This effect is common for surface-mount components such as resistors, capacitors, and inductors.
Guidelines for placing SMT components
Here are some recommendations for placing SMD to maintain good signal integrity and the energy of your board:
- Keep components as close together as possible to minimize routing distance.
- Follow the signal path according to the scheme, when placing the components.
- Never place components in the return path of sensitive signals. This leads to signal integrity issues.
- For high speed devices, place the shunt capacitors closer to their power pins. This will reduce parasitic inductance.
- Place the SMDs together in the power circuits. This will help you provide shorter cuts and reduce inductance in the connections.
- Try to keep the SMT components on one side of the board to reduce the associated costs associated with stencils and assembly.
- Maintain minimum spacing between test points and SMT components, as specified by your manufacturer. This spacing may vary depending on the height of the component.
To facilitate the assembly process, ensure that all component names, polarities, orientations, and component placements are correctly marked on the mounting drawing. The prints present in the drawings must match the actual pieces. Consult your manufacturer for their kit assembly guidelines if you are considering consigned assembly.
