Close this search box.

PCB Assembly Process Explained in 7 Steps

A PCB cannot be used without electronic components. The installation of these components, or what’s called the PCB assembly process, is a crucial stage in the manufacture of any electronic circuit board. To familiarize you with this process, we will go through every step that is involved, from component placement in PCB to soldering and testing.

What is PCB Assembly Process?

PCB assembly is the process of placing the different types of electronic components on the PCB and then soldering them in place. Several steps are used. These, depending on the complexity of the project and other factors, can be done by hand or with the help of automated PCB assembly machines.

Manual PCB Assembly Process

In manual PCB assembly process, a technician places the components on the PCB by hand. The manual method is slow and more prone to errors than automated assembly. So it’s mostly used when small numbers of PCBs need to be assembled, such as prototypes, or small PCBAs with only a few components.

Automated PCB Assembly Process

PCB assembly today is largely done by machines. Automatic PCB assembly machines include pick-and-place machines, solder paste machines, solder reflow ovens, and so on. The automated PCB assembly process is faster and more accurate than manual assembly, making it suitable for mass production.

During this assembly method, a robot picks and places components on the circuit board according to the design files. The entire process is controlled by a computer, using the XY coordinates file that was produced at the design stage.

The PCB assembly robot, using these coordinates, places the component in the correct position and at the right angle on the PCB. Once all components are in place, the soldering process can begin. This will connect the components to the PCB.

Image shows one of the PCB assembly steps

PCB Assembly Steps

There are several steps involved in the PCB assembly process, from component placement to soldering and testing. Here is a brief overview of each of these PCB assembly steps:

Stencil for PCB assembly
Stencil for PCB assembly

1. Solder Paste Application

When using “surface-mount technology” the first step of the PCB assembly process is to apply solder paste to the printed circuit board, either manually or with an automated applicator.

Solder paste is a special type of solder that comes in the form of a thick paste. It’s usually made up of tiny balls of solder suspended in a flux material.

The paste is applied to the PCB using a stencil. A stencil for PCB assembly is a thin piece of metal with holes in it that match the pattern of the pads on the PCB. The paste is squeegeed onto the PCB through the stencil, making sure that each part has an equal amount of paste on it.

Pick and place machine for PCB assembly
Pick and place machine for PCB assembly

2. Component Placement

Next, electronic components are placed on the PCB. This is done using a machine called a pick-and-place machine. The typical pick-and place machine for PCB assembly uses a vacuum to hold the components before placing them on the board.

The PCB assembly robot, as mentioned earlier, is controlled by a computer that has the XY coordinates of the component pads. Its function is to place the components on the PCB in the correct position and at the right angle.

PCB assembly reflow soldering oven
PCB assembly reflow soldering oven

3. Reflow Soldering

Once all the components are in place, the next step in the PCB assembly process is to solder them to the PCB. This process is called reflow soldering when the components are to be mounted on the surface of the PCB.

In reflow soldering, the PCB is placed in a special oven that gradually heats it to a high temperature, up to about 250°C. At this temperature, the solder paste melts and flows, making a connection between the component and the PCB pad.

The soldering process is completed when the PCB cools down and the solder paste solidifies again. After the solder has cooled and solidified, the components are securely attached to the PCB. In situations that involve double sided PCB assembly, the side with fewer components is usually reflowed first.

Note that in the thru-hole PCB assembly process, the soldering process is called wave soldering. In this method, the PCB is placed on a conveyor belt that takes it through a wave of molten solder.

PCB assembly optical inspection
PCB assembly optical inspection

4. Quality Control and Inspection

After the soldering process is complete, the PCBs are subjected to quality control and inspection. This is done to ensure that they are assembled correctly and meet all the specifications. Inspection can be done manually or using automated optical inspection (AOI) or X-ray inspection machines.

PCB Assembly Visual Inspection

PCB assembly visual inspection involves looking at the PCBs with the naked eye to check for defects. This is usually done after the soldering process is complete.

The technician looks for defects such as missing or misplaced components, shorts, and open circuits. If any of these defects are found, the PCB is repaired or scrapped.

This inspection method is only suitable when only a few PCBs need to be checked. Also, when the PCBs only have a small number of components and easy-to-spot defects. For mass production, automated optical inspection machines are used.

PCB Assembly Optical Inspection (AOI)

AOI or is a type of machine that uses cameras and light sources (infrared, LED, fluorescent, or UV) to scan the PCB and look for defects. A typical AOI machine has several cameras that take pictures of the PCB from different angles.

The pictures are then analyzed by the machine’s software, which looks for defects such as missing or misplaced components, and other errors. If any of these defects are found, the PCB assembly will either be repaired or discarded.

AOI machines are suitable for mass production because they can inspect a large number of PCBs in a short amount of time. With ability to thoroughly scan a PCB assembly, optical inspection is also more accurate than manual inspection.

X-Ray Inspection

X-ray PCBA inspection uses X-rays to scan the PCB and look for defects. The machine takes an X-ray image of the PCB and then analyzes it to look for defects.

X-ray PCB assembly inspection is more accurate than AOI because it can see through the solder paste and detect defects that are not visible to the naked eye or with a camera.

Because it can penetrate the solder paste, X-ray inspection is mostly used to check complex PCBs with complex designs, but mostly multilayered types.

PCB assembly testing
PCB assembly testing

5. PCBA Functional Test

After the PCBs are assembled and inspected, they are subjected to a functional test. This is done to ensure that the PCBs are working correctly in their intended application and environment.

The PCB assembly, or PCBA functional test, checks the PCBs for things such as proper power supply, correct signal levels, and communication with other devices. If any of these tests fail, the PCB is cannot proceed to the other steps of the process.

During this test, the electrical characteristics of the PCB assembly (PCBA) are measured and compared to the specification. A rise or fall time of voltage and current will indicate abnormalities in the function of the device.

This test is important because it verifies that the PCBs are working correctly before they are shipped to the customer. It also helps to identify and fix any problems early on, before they become expensive problems later.

Bench top ultrasonic PCB cleaning machine
Bench top ultrasonic PCB cleaning machine

6. PCB Assembly Cleaning

After the functional test is complete, the PCBs are cleaned to remove any residual flux or other contaminants. That’s because, during the printed circuit board assembly process, the PCBs are exposed to a variety of chemicals and contaminants.

To clean PCBA assemblies, solvents such as acetone or ethyl alcohol are used. De-ionized or de-mineralized water is also used to wash off the PCBAs. After the cleaning process is complete, the PCBAs are often dried using compressed nitrogen gas.

An ultrasonic PCB cleaning machine is a popular way to automate the cleaning process. This machine is often used alongside manual cleaning methods. In addition to automating the cleaning process, ultrasonic cleaners also provide a way to clean hard-to-reach areas of the PCB.

PCB assembly packaging
PCB assembly packaging

7. Packaging PCB Assembly

The last step in the PCB assembly process is packaging shipping the PCBs to the customer. The PCBs are carefully packaged to prevent damage during shipping.

The packaging must also meet the requirements of the transportation method being used. For example, packaging that can withstand high vibration and shock is usually required.


The PCB assembly process is complex and critical in the electronics manufacturing industry. Because of this, it is important to partner with a reputable and experienced contract manufacturer. The right pcb assemble manufacturer will have the knowledge, experience, and facilities to meet your specific needs.


Table of Contents