What is SMT in PCB Assembly and How is It Done?

Technological advancements have significantly changed the lives of men today. From the development of the earliest electronic devices, up to the latest inventions, we have been benefiting from these gadgets greatly. But have you ever wondered how these device works?

Every electronic device is composed of circuitries, made up of electronic components assembled or soldered on a printed circuit board (PCB). The components and the circuit board are joined together in a process called PCB assembly. In circuit board assembly, there are various mounting technologies that are used today. The latest and most preferred of these technologies is called Surface Mount technology (SMT), or planar mounting.

SMT was developed in the 1960s and gained popularity in the 1980s. Because of its advancement, it eventually bested out its competitors in the industry. In fact, 90% of the majority prefers this mounting technology.

SMT has also become the solution to the various problems of its predecessors, providing different advantages and ease in PCB assembly today. Some examples of these advantages include:

  • Surface mount components (SMCI) were made smaller compared to the other components, taking lesser space on the board. This allows more components to be soldered on the circuit board.
  • Pin count of components greatly increased
  • Production cost per unit was reduced.
  • SMC can be mounted on both sides of the PCB.

PCB Assembly

 

In SMT, the components are mounted directly on the PCB. It involves several processes, involving the following:

 

1. Apply the soldering paste:

Before mounting the components on the circuit board, the solder paste should be applied. Solder paste is powder metal solder mixed with flux. It is used to connect electronic components on the surface of the circuit board.

Solder paste is applied on component pads using a solder stencil or screen. To apply using a screen, place the tool to the areas where solder is required. Squeeze the solder paste onto the screen holes and onto the circuit board as you move the runner across the screen.

To apply using a stencil, tape down the large and small L-shaped board fixtures to prevent them from moving. Then place the PCB in it. Put the stencil over the SMT pads and make sure it is aligned. Apply the solder paste from one point and run it across the holes.

Make sure to control the amount of solder pasted you deposit. This is to guarantee that the resulting joints have the correct quantity of solder.

2. Run through the pick and place machine:

Circuit Board AssemblyOnce the solder paste is applied, it will then pass into the pick and place machine. The said machine contains the electronic components, which will be dispensed on their designated spots on the circuit board accurately.

With the PCB design information, the machine is programmed to position the components correctly on the surface of the board. The solder paste applied previously also makes the components be held in place.

3. Solder the components onto the circuit board:

In SMT, the electronic components are soldered onto the circuit board with the reflow technique. In this soldering technique, the components and the PCB go through a conveyor oven. The heat of the oven will then melt the solder paste, joining the components and the board together.

4. Clean the circuit board:

After soldering, flux residues should be removed from the circuit board. There are various ways to do this, including the use of cleaning agents, use of heat, and agitation.

5. Inspect for quality assurance:

Once cleaned, the assembled PCB will then be sent for inspection. In SMT PCB assembly, manual checking is not preferable for boards with hundreds of components. Instead, automatic optical inspection (AOI) is favored.

In AOI, machines are used to detect mislaid components, poor joints, and even incorrect components. On the other hand, other inspection techniques such as sample checks and automatic x-ray inspection can also be performed.

6. Test the assembled circuit boards:

To ensure that the circuit boards are working, a series of tests must be performed. For instance, in-circuit test (ICT) is performed to check each component for possible errors and mistakes, such as open and short circuits.

In ICT, a specialist automated test equipment (ATE) is required to perform the manufacturing defects analysis (MDA).

7. Reworking:

Reworking is usually done if a component is found malfunctioning. In this process, defective parts are replaced and undergo reheating to solder them onto the board’s surface.

Each process in SMT PCB assembly is complex and requires careful attention to achieve a fully working device. And thanks to this process, we are able to enjoy the gadgets we much love today.