Surface Mount Technology Introduction: Process, Types & Principle

When it comes to PCB assembly, surface mount technology (SMT) is the most used method today. Here, components are mounted or placed directly onto the surface of a PCB. This differs from through-hole construction, where components are inserted into holes on the PCB. So what exactly does the surface mount technology process entail, and why is it so popular?

What is Surface Mount Technology?

Surface mount technology means the method of mounting or placing electronic components on the surface of a printed circuit board (PCB). The main alternative to SMT is through-hole technology, in which components are inserted into holes drilled into the PCB. So, how did it all start and when?

Surface Mount Technology History

While surface mount technology has only become widely used in the last few decades, its origins date back to the early 1960s. However, it wasn’t until the 1980 and 90ss that SMT began to be widely used in commercial applications.

In the present day, SMT technology is the most common method used for PCB assembly. Its market share also continues to grow. In a report by Transparency Market Research, the global pcb smt equipment market was valued at $637 million in 2021 and projected to reach $1.04 billion by 2031.

Surface Mount Technology Process

The surface mount technology process can be broken down into 4 main steps: solder paste application, component placement, reflow soldering, and final inspection.

Solder Paste Application

  • Solder paste is a sticky substance composed of metal particles. It holds components in place on a circuit board during the soldering process.
  • To apply SMT solder paste, a stencil is placed over the PCB and solder paste is dispensed through the openings in the stencil.
  • This process is typically done using an SMT solder paste printer, although it can also be done by hand.

Component Placement

  • Once the solder paste has been applied, the next step is to place the SMT components on the board. This can be done manually or using a pick-and-place machine.
  • If done by hand, a tweezers is typically used to place the components. With a pick-and-place machine, the process is faster and more accurate.
  • The surface mount technology machine places the components on the board according to a predetermined pattern, using a computer-controlled arm.

Reflow Soldering

  • After all of the components have been placed on the board, the next step is to solder them in place. This is done using a process called reflow soldering.
  • In the SMT reflow process, the board is heated to a high temperature (up to 250 degrees Celsius) and then cooled. This melts the solder paste, which bonds the components to the board.

Final Inspection

  • After the soldering process is complete, the board is typically inspected for any defects. Defective boards are repaired or scrapped, while good boards are sent on to the next stage of production.
  • The surface mount technology equipment in this step may include automated optical inspection (AOI) machines and X-ray machines.
A type of surface mount PCBA
A type of surface mount PCBA
Resource: https://www.youtube.com/watch?v=119TGNQ5ynA

Types of Surface Mount Technology

Surface mount technology, when it comes to PCBA or PCB assembly, can be classified into 3 different types: type I, type II, and type III. These types of surface mount technology PCBAs, together with their characteristics, and explained below.

Type I Surface Mount Technology

Type I surface mount technology is the most basic type of SMT PCBA. In this type of assembly, all of the components are SMT types, placed on one side of the PCB (the components side) or both sides. The advantage of this is that it’s a relatively simple process and doesn’t require sophisticated equipment.

Type II Surface Mount Technology

Type II surface mount technology is a more advanced type of SMT. In this type of assembly, both thru hole and SMT PCB assembly components are often used on the primary side, and only SMT on the other side. Although this makes the assembly process more complex, it allows for a higher component density and better electrical performance.

Type III Surface Mount Technology

In this type of SMT PCBA, the components are placed on both sides of the PCB and the soldering, just like in the type II assembly method. However, only thru hole components are mounted on the primary side of the PCB. This design offers the benefits of type II assembling, but in a different way.

SMT PCB manufacturing process
Resource: https://www.youtube.com/watch?v=ngxJGKLqUMI

Surface Mount Technology Principles and Best Practices

Surface mount technology offers many advantages: smaller footprint, higher packing density, faster assembly, and improved reliability. But to get the most out of SMT, you need to understand the basic principles and best practices. Here are 6 of the most important.

1. Proper Spacing: Surface mount technology uses smaller spacing, typically 0.040 inches for smaller components and up to 0.060 inches for larger ones. The right spacing must be ensured to facilitate PCBA repair, inspection, and rework.

2. Component Alignment: Similar SMT components should be aligned in the same way on the board. This serves to ease the assembly process, right from parts placement to soldering and testing or inspection.

3. SMT Via Holes: The vias used to interconnect different layers in a PCB are another consideration in surface mount technology. It’s good practice to reduce these or cover them when using SMT. This is to avoid situations where they trap flux during the process of wave soldering.

4. Thermal Management: Thermal management is important in any electronics design, but it’s even more critical in the SMT PCB assembly process. That is because the smaller components used in SMT have a higher power density. So, proper thermal management must be ensured to avoid problems like overheating and thermal stress.

5. Solder Paste: The surface mount technology process uses a paste of solder flux to attach the components to the board. Options include water-soluble and no-clean flux. The former is easier to remove and provides better wetting, while the latter leaves little or no residue. It’s important to choose the right type of solder paste, depending on the application.

6. SMT Component Size: Surface mount components sizes is another major consideration. SMT devices are much smaller than their through-hole counterparts. This means that they’re more susceptible to damage from handling and thermal stress. To avoid problems, make sure to use the proper component sizes for your application.

These are just a few of the most important principles and best practices to keep in mind when working with surface mount technology. By following these guidelines, you can ensure a successful SMT process and avoid potential problems down the line.

Conclusion

Surface mount technology is an effective and efficient method that can be used to create high-quality, reliable electronic PCB assemblies. By following the best PCBA practices, you can ensure that your SMT assembly process goes smoothly and that your final product meets your expectations, and if having your SMT PCBA made by a professional, that it is manufacturable.

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