Simplified PCB Assembly Process

Printed Circuit Board (PCB) assembly is the process of placing electronic components to the circuit board. The two methods of assembling are surface mount technology (SMT) and thru-hole assembly. Each PCB assembly method uses almost similar procedures, except on the use of PCB, and mounting the electronic components on the board. SMT uses surface pad of the board to mount components while thru-hole uses PCB with pre-drilled holes to solder a component to the PCB.

The process of assembling a functional PCB
Surface Mount design/BOM

In SMT, surface mount design is the first in the process. It also includes a bill of materials (BOM). On the BOM, the electronic components, circuit type, packaging directions, components density, and other pertinent information are defined.

Stencil Printing

Pcb assemblyWhen everything is ready for assembling, stencil printing is next. It is important because it tells the machine how much solder paste is needed. This is the most critical stage since the printed stencil establishes electrical connections between the components. Coming up with a correct stencil guarantees that the right amount of solder paste will be placed and optimum results for the finish product will be achieved.


After adding solder paste, component placement should follow immediately. If the process involves the electro-mechanical, components wrapped in plastic tubes are loaded into the pick-and-place machine. The machine places the components on the board. The solder paste acts as an adhesive while the PCB goes for a “reflow”.

Reflow or Soldering

Reflow is the process of permanently connecting the electronic components to the board by melting the solder paste. In a through-hole assembly, soldering includes cutting the excess lead pin of the electronic component. For SMT process, the success of this step depends on paste stenciling (stencil printing).

Manual Assembly

Even if you use the electro-mechanical method in SMT, manual assembly may still be required depending on the stated BOM. The step can either be done after reflow, when some components are too small for the machine to place on the board, or after inspection and testing when the board does not pass quality control.

Inspection and Testing

Due to industry standards and customer satisfaction, PCB assembly companies are placing great importance on quality control. During this stage, manual inspection is the best method to see if all the electronic components are in place and that the solder joints are in excellent condition. Thus, the person in charge of quality control must have extensive knowledge on how an excellent solder joint should look like.

Before the circuit board is sent for washing and packaging, testing the electrical connections should be done. If the board does not function accordingly, the faulty PCB is brought to the rework area. Other SMT PCB assembly companies outsource reworks, usually to a thru-hole assembly company.


Washing is an essential part of the assembly as this stage removes excess solder paste or any unwanted material accumulated during the process. However, there are cases that this stage is totally removed in the entire procedure because some solder pastes are no-wash types.


The last process is packaging. This part is still in line with the BOM specifications. Correct packaging is essential as this will keep the PCB in perfect condition. As much as possible, shock and vibration during transportation should be kept under control as these affect the finished product. Keeping the shock and vibration to a minimum eliminates the need to spend a lot on reworks and decreases return rates.

The above mentioned series of steps are commonly followed by assembly companies. Well, there are still procedures unique to a PCB assembly company. Such procedures are added to ensure that the finished PCB is of excellent quality.