PCB Assembly Process – All you need to know

PCB Assembly line illustration

PCB assembly refers to the act of installing components on the printed circuit board. It involves the use of procedures such as DIP and SMT. The level of SMT automation is improving every day due to the development of advanced equipment. A lot of changes take place in the assembly process, so it’s always good to stay updated with the changing process.

PCB assembly requirements

  • PCB
  • Soldering flux
  • Soldering materials such as preforms, bar, wire, and paste
  • Electrical components (resistors, integrated circuits, capacitors, etc.)
  • Soldering equipment such as soldering station, soldering machine, SMT and testing tools

PCB assembly process

Some of the stages that you will come across in the PCB assembly process includes soldering paste to the keyboard, picking up and placing components, soldering, and lastly, inspection and testing. If you want to produce high-quality devices, all these processes are vital.

Solder Paste

Before the components are added on the board, soldering paste is added to the area of the board where the solder will be applied. These areas are typically the component pads. Solder screen is the tool that usually used for this process

Solder paste refers to a mixture of flux and a paste of small grains of solder. This mixture can be placed on the board on a similar process to the printing process. Placing the solder paste on the correct position is the first step of the assembly process.

The solder paste is placed directly using the solder screen. Then a runner squeezes a minimal amount of solder to the PCB’s holes as well as on the board. Because the solder screen is generated using the PCB files, its holes which are directly proportional to the solder pads. This is the main reason why solder is always deposited on the solder pads directly. However, the depositing solder paste is always controlled.

Pick and place

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In this step of the assembly process, where the board has already been added solder paste, it is then passed through the pick and place machines. They load the PCB with some reels of selected components. It is in this step where the components are placed on the correct position of the PCB.

The tension of the solder paste is the one that holds all the parts onto the board.  As long as the board will not be jolted, this tension is good enough to keep them in place. Some assembly processes pick and place steps that may involve machines placing some small glue dots that can secure the board. It mostly used, especially when soldering is about to take place.

The only setback of this process is that it makes repair hard because of the glue presence. But some glues can degrade during the soldering process so as to pave the way for repair. A printed circuit board design provides the component information and the correct position.  This design is beneficial, especially to the pick and place programming. 


After the components have been added to the board, they are then passed through the soldering machines. Soldering is a critical stage of the assembly process because it helps to hold the components firm.  Although some boards will require to be passed through many soldering machines, this process is rarely used for surface mount components today. Solder paste is not added if wave soldering is used. However, reflow soldering is the one that is mostly used today rather than the wave soldering.


The next step that is taken when soldering is completed is the inspection. You cannot inspect the boards that have passed through the SMT process manually. The reason being that surface mount boards can hold very many boards.  So an automatic optical inspection is the one that is widely used.

Machines used to inspect SMT boards are so efficient that they can detect misplaced components. Besides, these machines can also detect weak joints, and even when the wrong part has been used.


After all the boards have been inspected, then they pass through the testing phase. This is the last step taken before these boards leave the factory. There are many ways in which testing of the PCBA’s can be carried out.


If you want to ensure that the manufacturing process is doing a great job w, the outputs need to be closely monitored. This step involves a process investigating any failures that are associated with the PCBA. The optical inspection stage is the one that is mostly involved in giving feedback. Mainly because it’s the closest stage to the soldering stage of this assembly process. So the manufactures will be able to make the required corrections before many boards are produced.

Wave soldering procedure

An electromagnetic a bump with the jet flow, or a mechanical bump is used to transform the melted solder to the required wave. So all the PCB boards that have components installed on them have to pass through the solder wave. 

The main steps for wave soldering include component molding, mounting soldering, and cooling. All the molded components are inserted on the PCB, depending on the requirements. On top of that, after the PCB has been populated by the necessary parts, it is then passed to the wave soldering system.

After being pushed by the transmission device into the wave soldering system, the solder flux is sprayed. It is then heated in the pre-heating zone. Lastly, the final step carried out is the wave soldering and cooling.

Soldering Basics

There are three categories of soldering that can be used in the PCB assembly process.

Fusion welding- This one is mostly used in the production of small scale PCBAs.

Pressure welding – Usually used for a large scale, and it involves the use of high temperatures.

Brazing- It is further classified into soft and hard.

Final thoughts

PCB assembly process involves populating an empty board with components. On top of that, the assembly process also makes sure that the correct parts are added in the right position. During the inspection stage, feedback is sent back to the soldering stage so as to correct the mistakes before it is too late. Also, before the PCBA board leaves the factory, they are passed through the testing phase. This step is crucial because it ensures that the boards are working as intended.

After reading this article, you should already know the pcb assembly process. If you want to browse more information about pcb assembly , please visit this link: https://www.pcbonline.com/PCB-Assembly/PCB-Assembly-Overview