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A Guide to Surface Mount Soldering

Surface mount technology is the most common method to assemble PCBs today, and surface mount soldering one the most important parts of the process. Below, we take a deeper dive into what surface mount technology soldering means, the type of equipment used, and a lot more.

What is Surface Mount Soldering?

Surface mount soldering is the process of bonding electrical components to a printed circuit board (PCB) using solder. It involves placing components on the PCB, melting solder to create electrical and mechanical connections between them and the board, and then cleaning any residual flux from the assembled PCB.

Soldering and Surface Mount Technology

Surface mount technology (SMT) is a method of electronic assembly that uses small surface-mounted components placed on the PCB. The SMT soldering process eliminates the need for through-hole soldering, which requires more board space and is more labor-intensive.

But just like With THT, SMT components must be soldered to the PCB to make electrical and mechanical connections. This is where surface mount technology soldering comes in.

Surface Mount Soldering Tools

Sometimes, and especially when there are only a few components to solder or a few PCBAs to produce, manual soldering is all that’s needed. These situations include when producing prototypes, making laboratory work PCBAs, or reworking PCBAs. In such cases, the following surface mount soldering tools are used:

  • Surface mount soldering iron
  • Tweezers
  • Flux pen/brush
  • Microscope
  • Hot air station or soldering rework station

Surface Mount Soldering Equipment

Most of the time and for larger production runs, automated pieces of equipment is necessary. The surface mount soldering equipment for mass production of PCBAs includes:

  • Solder paste printing machine
  • Pick and place machine
  • Reflow soldering oven or wave soldering system
Surface mount LED soldering
Surface mount LED soldering
Resource: https://www.youtube.com/watch?v=qFibR88JEKs

How to Do Surface Mount Soldering

The SMT soldering process is largely divided into four steps – preparation of the PCB, soldering paste application, component placement and reflow soldering. These steps are covered in detail below.

Step 1: PCB Preparation

The first step to solder surface mount components is to prepare the PCB for soldering. The PCB board is printed with pads which will have the surface mount components soldered on them. The board should be clean, free of corrosion and properly oriented.

Step 2: Surface Mount Solder Paste

The next step of the SMT soldering process is to apply the solder paste on the PCB pads. A solder paste consists of a metal solder held in a putty-like suspension of flux. This paste is then applied to the PCB pads using a stencil and squeegee or screen printer.

The right amount of solder paste should be used, or the solder may not be able to bond properly, resulting in poor electrical and mechanical connections. This is especially critical for sensitive components such as when doing surface mount IC soldering.

Step 3: Surface Mount Components

After applying the solder paste, SMT components are placed onto their corresponding pads on the PCB. This can be done manually with tweezers or using an automated pick-and-place machine. When manually placing components, it is important to make sure that they are properly aligned and placed in the correct orientation.

The automated pick-and-place machine is preferred in mass production, as it can place components accurately and quickly. It uses a vacuum gripper to pick up components from a feeder, and then places them onto the PCB, sometimes at a rate of up to 80,000 components per hour.

Step 4: Soldering Surface Mount Parts

The last step is in surface mount soldering is reflow soldering. In this process, the PCB and its components are heated up gradually and in various stages. The stages of surface mount device soldering are:

  • Pre-heat – This is where the PCB and components are heated up to a certain temperature, usually between 140-160 °C.
  • Soak – During this stage, the temperature is maintained so that the solder paste melts completely, usually for between 1 to 11/2 minutes.
  • Reflow – Here, the solder reflows onto the component pads and creates electrical connections. To enable that, the temperature is raised to a maximum of between 210 and 230°C.
  • Cool-down – When the solder has reflowed, the temperature is then reduced back to room temperature to allow the connections to cool and harden.

Once these steps are completed, any residual flux or paste should be removed from the board using an appropriate cleaning agent. After that, the assembled PCB is checked for proper connections and function. And that’s it! The surface mount soldering process is complete.

Wave soldering for through hole PCB
Wave soldering for through hole PCB
Resource: https://www.youtube.com/watch?v=dLUWKLG_E-I

Surface Mount Soldering vs. Through Hole

Through hole and surface mount soldering are the two main types of PCB assembly processes. While both methods can be used to attach components to a board, they differ in several ways. THT soldering involves inserting component leads into holes on the board and then melting solder over them.

SMT soldering, on the other hand, takes place with no holes and requires components to be placed directly onto pads pre-printed on the PCB. Here is a quick comparison of surface mount soldering vs. through hole soldering.

  • The through hole method is traditionally used for larger, heavier components.
  • Surface mount soldering is more suitable for smaller and lighter components as there is less stress on connections due to their size.
  • Surface mount soldering is also much faster and easier than through-hole soldering: fewer steps are required, seeing than no holes are drilled. This makes it ideal for high-volume production runs.
  • The components used in surface mount technology must be specifically created to fit this process. Through-hole components, on the other hand, can be used with both methods.

Overall, surface mount soldering offers many advantages over through-hole soldering and is the preferred method for most PCB assembly applications. With the right equipment and knowledge, it can be done quickly and effectively to ensure that your board will perform as intended.

Conclusion

The above guide provides a comprehensive overview of surface mount soldering technology and its associated tools, equipment, and techniques. This is meant to give you a basic understanding of what to expect when working with these types of PCBs and how the process would be carried out.

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