November 2, 2023

Basic Things You Should Know About PCB Assembly Process

Basic Things You Should Know About PCB Assembly Process

The assembly process of PCB or Circuit Boards includes various steps, guidelines, and rules that need to be followed to get a precise and fully functional product at the end. However, to successfully achieve this goal, PCB assemblers use controlled cooling and heating phenomena and screen templates to regulate the placement of electrical components

Assemblers must choose the right technology to assemble the circuit board based on component type. All pieces and parts must align correctly in the designated place. Furthermore, any slight displacement of the component may lead to dysfunctionality. To understand the initial steps, refer to What Is PCB Fabrication?.

How PCB Assembly Works?

PCB (Printed Circuit Board) assembly is the process of bringing crucial PCB parts together. It led to the development of commercial electronic devices and home appliances. The PCB assembly’s first step is fabrication. It involves the etching of the copper traces and the pads on the surface of the substrate. After this, the technicians begin with the actual PCB assembly. If you're designing your own PCB, you might find How To Design A PCB Layout helpful.

Solder paste is important for this process, and it uses a stencil. A solder is made from different solder granules and solder fluxes. It adds to the strength of the solder mixture and adheres to the components on the PCB. It is also crucial to carefully place the PCB parts on the board and position them in the right spots while soldering. It takes place through pick-and-place machines that are highly precise and fast in this job. 

Once the machines have positioned all the PCB parts, it's time to take the PCB through a reflow process. Normally, a reflow oven is used for this purpose. 

The purpose is to fully heat up the PCB along with its components. The solder paste will melt from its heat and then undergo solidification. It will make strong bonds between the PCB components and the surface of PCBs, which will add to the quality of PCB performance on the while. The next step is to inspect the PCBs and check if there are any defects or not. There are different methods of PCB inspection available in this regard. AOI is a common technique. Learn more about the machines used in this process by reading Guide to Automated Optical Inspection (AOI).

Manual inspection is also used by different PCB manufacturers, which helps to identify possible errors within the PCBs before dispatching them. After inspection, these PCBs undergo further enhancement procedures like potting and conformal coating. These additional steps are important to make the PCBs immune to adverse environmental damage and possible negative impacts. Learn more about how to identify components on printed circuit boards.

In this way, the manufacturers make the PCBs durable and more reliable in the long run. The last step is to test the PCBs to see if these can properly function in unfavorable circumstances or not. Functional testing, scan testing, and in-circuit testing are some common examples. 

Types of PCB Assembly and Terms

Electronic manufacturers are prone to face many challenges in developing their prototypes. As technology keeps advancing, the need to develop a high-quality prototype is also increasing. 

Therefore, there are several technical approaches that make the process of PCB assembly much easier and efficient. To understand the phenomenon of PCB assembly, you must dive into the following technologies that play a vital role in the process of PCB assembly.

Close-up of SMT assembly process on PCB
  • SMT Assembly

This type of assembly represents a process of assembler incorporating the electrical components on the board surface directly using solder paste. For an in-depth look at Surface-Mount Technology, check out SMT Assembly.

  • Mechanical Assembly

This process helps in assembling electrical components of PCB on production or assembly lines. Furthermore, it also represents all the assembled products and components that were made this way. In simple words, it indicates the phenomenon of putting parts together to conduct a certain function.

  • BGA Assembly

BGA assembly refers to a surface-mount package. This process utilizes Integrated circuits and pins. Moreover, BGA allows the use of HDI PCBs at an affordable price, which makes it the best option for assembling electrical components on board surfaces with utmost care and precision. Discover more about BGA and its role in multilayer PCBs in our Comprehensive Guide on Multilayer PCB.

  • Cable Assembly

Cable assembly involves a process of putting together so many cables into one group. Cable assemblies have the ability to transmit power. This exquisite feature of cable assembly enables it to deliver signals and power across the board. Furthermore, using an assembled group of cables also allows easy maintenance and replacements as compared to a single cable. To understand the components involved in cable assemblies, visit Electronic Circuit Board Components.

  • Axial Horizontal Assembly

Axial leads pop out from each end of box-shaped or cylindrical components. The group of axial cables has the same shape as wire jumpers. These axials are mostly used in short-space PCBs. Furthermore, the axial horizontal assembly has two distinct types of settings, which involve right-angle SMT and PTH. In these settings, leads come from the other end (opposite) of electronic components.

Completed turnkey PCB assembly with components
  • Turnkey Assembly

Turnkey represents a ready-to-use product that allows a person to use right after purchasing it. Manufacturers and assemblers need to pay utmost attention to every single element of the PCB assembly and manufacturing. This also includes the placement of electrical components and PCB manufacturing. Learn about turnkey solutions in Turnkey PCB Assembly.

  • Assembly Of Radical Vertical

Assemblers use this process when there is limited space on board, but the components still remain to complete the board. This process allows them to incorporate electrical components in a vertical form over the PCB surface. The components are placed in a vertical form. However, in radical vertical assembly, wires come from only one side of the component's end.

Close-up of box build assembly process

Box-build contains a box that consists of an enclosed, small-spaced cabin filled with tons of wires entirely incorporated into an electromechanical system. Assemblers use this box to assess already produced PCB. 

  • Lead-Free Soldering

This type of soldering involves metals like copper, bismuth, antimony, indium, tin, zinc, silver, tin, and some others. Lead-free soldering, as the name suggests, does not use lead in the process of soldering. Moreover, soldering is done with soldering iron. When the tip of the soldering iron gets heated up, it may release dust and fumes into the air, which may prove harmful to the health of the assembler as well as people in the area.

  • Printing 

The printing of boards refers to a process of transforming the digital schematic and design of a PCB into a glossy sheet of paper. To get the best outcomes, the print must be on the shiny side of the paper. Printing plays a vital role in the PCB assembly. However, any small error in printing may lead to malfunction or damage to the PCB in the end.

  • N2 System

N2, or nitrogen gas, is a chemical molecule. It does not have any odor or color and plays quite an essential role in the production of Printed Circuit Boards. The N2 system represents the use of gas in the manufacturing PCB, which includes reflow and some selective soldering. N2 has a unique approach to protect convection reflow and oxidation problems.

  • Varnishing

The varnishing applies on the PCB surface. The main goal of varnishing is to create a protective layer on the board surface, which protects it from damage and corrosion caused by external factors. In simple words, Varnishing assists PCBs in standing against harsh environmental factors.

Technicians testing PCBs in a laboratory
  • Testing

PCB testing plays a crucial role in the production of PCBs. Testing allows manufacturers to identify errors and correct them beforehand. This process enables manufacturers to save additional money that may come up with rework. Moreover, testing also allows the detection of errors that cause PCBs to malfunction.

  • Packaging

PCBs need a packaging that protects them from moisture and physical breakage. Inadequate packaging may lead to damage to the board and affect its functionality. However, adequate packaging is considered vital for the integrity of PCB. Therefore, always make sure to choose a manufacturer that delivers PCBs with proper packaging.

PCB Assembly Process Steps

The process of PCB assembly is not considered complete, regardless of several manual and automatic steps. The PCBA always begins with the basic unit of a PCB.

Step 1: Application of Solder Paste To The PCB


First of all, use a mechanical unit to put a thin sheet of stainless steel model on the surface of the plate. Make sure to spread the solder paste uniformly on the right place of a PCB. 

  • Solder Paste Ingredients

Solder paste is a gray material composed of tiny balls of metals, which is called solder. These mini metal balls contain 0.5% copper, 3% silver, and 96.5% tin. Then, flux is added to the solder to create solder paste. Flux refers to a chemical substance that aids solder in melting and creating a bond. Solder paste must be applied in an adequate amount in the right place on the board surface. For more details on solder paste types and applications, see Solder Paste in Electronics Manufacturing.

In an industrial PCB assembly line, mechanical holders grip the soldering paste and PCB in the place. Then, the applicator applies paste on the right surface of the area. After that, the machine put the slurry uniformly over the model. After the removal of the model, the soldering paste fixes on its desired place. 

Step 2: Pick And Place Machine


After the completion of soldering comes the step of picking and placing electrical components on the board. A pick-and-place robot puts the SMD or Surface-mount assembly over the finished PCB. SMDs help in placing mostly non-connector electrical components on the PCBs.

Before, this step was done manually using forceps. The assembler picked and placed each electrical component manually. However, today, manufacturers just need a robot to accomplish this task automatically. Moreover, the placement of components requires utmost precision. Therefore, this change is needed by both the manufacturer and the PCB.

  • Surface-Mount Technology

The SMT device starts the process of pick-and-place using vacuum clamps. The clamp picks up the PCB and puts it inside the station. There, the robot starts applying SMT over the surface of the PCB. These electrical components are put over the solder paste in the pre-designed position.

Step 3: Solidifying The Solder Paste


For a firm incorporation of electrical components, solder paste must go through a reflowing process to keep the components in their place for a longer time. 

After the placement process, the PCB then transfers to the conveyor strap. This belt passes through a long furnace of reflow, which contains a number of heaters. As the PCB passes through these heaters, they slowly heat it to 250 Degrees, which melts the solder. Detailed techniques can be found in PCB Soldering.

  • Reflow Soldering

After melting the solder, the PCB passes through a big oven. As the PCB crosses several cooling systems, the molten starts cooling down, which solidifies the solder and creates a solder joint. This generates a firm and permanent joint, which is utilized to join the PCB and SMD. 

Many PCBAs need special consideration during the soldering reflow, particularly for two-sided PCB assembly. This type of assembly particularly needs modeling and soldering reflow of both sides separately. However, the side of the board with fewer components is modeled, placed, and then reflowed first; then, it moves on to the second side.

Step 4: Check the PCB Assembly


After the completion of the third step, the soldering of mounted electronic components, then comes the step of inspection of the PCB. The board needs to go through several analyses and tests that depict its functionality. However, quality control PCBAs inspection include:

  • Manual Inspection

A designer performs a live visual analysis to confirm the PCB quality, which is called manual inspection.

  • Automatic Optical Inspection

This type of inspection is considered most suitable for mass PCBAs. AOI stands for Automated Optical Inspection. This testing approach uses high-power camera detectors that view solder joints from distinct angles. Learn about inspection methods like AOI in What is Automated Optical Inspection (AOI) in PCB.

  • X-Ray Inspection

This inspection involves deep analysis by spotting problems in the PCB layers.

Step 5: Insert The Plated Through Hole Component

Inserting plated through-hole component into PCB

The selection of soldering type is solely based on the board type. Assemblers also use other approaches for soldering. Besides SMT components, they use PTS assembly.

A PTH refers to a type of hole that is plated in the PCB all through the circuit. These holes are used to pass signals from one edge to the other edge of the PCB. In such scenarios, the solder paste does not suit because the paste may pass through the spaces and leave no way for adhesion. Moreover, PTH needs a more professional welding method. 

  • Manual Soldering

This involves the manual insertion of leads through holes.

  • Wave Soldering

Wave soldering differs from manual soldering. It is an automated approach that uses molten to drill holes in the board of PCB.

Complete A Final Inspection

After the completion of soldering, the last step remains, which involves the functional analysis and testing of the PCB. Manufacturers run analog signals and power to test the electrical characteristics of the PCB. If a PCB shows fluctuations in transferring signals during the inspection, then it has faults in it. For some reason, if the inspection fails, the manufacturer has to start the whole process.

Testing plays a crucial role in the entire process of PCB assembly because this will tell whether the PCB functions properly or not. 

PCB Assembly Process - Inspection & Testing

We cannot deny the fact that inspecting PCBs proves to be not only effective but safe for the users. Fully inspected PCBs provide more protection and serve longer in the run. 

The PCB providers use different inspecting methods to ensure that PCBs are in a good place. Therefore, it is crucial for the retailer PCB brands to completely analyze the PCBs and see if there are some defects in them or not. 


The common methods in this regard are X-ray inspection and manual inspection for intense quality control of PCBs. Keep one thing in mind: these methods vary as per your budget; therefore, determining their budget beforehand will be crucial. 

  1. X-ray inspection is common, and many providers afford it too, which deeply analyzes the PCBs for their defects. 
  2. Manual inspection is the most affordable method and does not require any advanced equipment as well. 
  3. AOI is another option to go for, which precisely checks the PCBs and pinpoints hidden faults in them, which ultimately enhances their quality for long-term usage. 


At the end of the day, you need to rely on industrial-grade PCBs for your operations. Defected PCBs are not only dangerous but also prove to be less durable for long-term dependence. 

Due to this reason, we invite you to check our PCBs for once yourself and then decide whether collaborating with us will be beneficial for you or not. We know your investment is important, and our PCBs fully adhere to your needs and requirements successfully. 

Article written by Alice Lee