What are the Processes Involved in Printed Circuits Assembly?

Printed circuit boards are the main backbone of most electronic devices. These boards are one of the crucial developing procedures for manufacturing the best electronics. Printed circuits assembly is very important because they can update digital devices without any problem.

 With the increase in the use of PCBs, there is more demand for PCB assembly. Electronic products won’t function without printed circuit board assembly. Therefore, the importance of PCBA isn’t far-fetched.

What is Printed Circuits Assembly?

Printed circuit assembly is the process of attaching components on bare PCB boards. Without these electronic components, a PCB is just a bare board. The presence of electronic components on a board makes it a PCB assembly. Therefore, PCBA is different from PCB manufacturing. PCB assembly process is primarily focused on mounting components on a bare PCB.

The presence of these components gives birth to a functional board. Therefore, this PCBA is widely used in the production of electronic devices. The placement of electronic components can be mechanical or manual. A PCBA refers to the finished board after the placement of all important components.

Installing the electronic components to the bare PCB is what creates an operational electronic device. A circuit board before the attachment of electronic components is a bare PCB. Once components are already installed, the board becomes a printed circuits assembly.

Also, printed circuit board assembly involves installing various components on a bare PCB. There are two methods of assembling these components. These are the automatic and the manual process. PCB manufacturers drill holes to create space to attach these components. Several actions occur during the assembly process.

The purpose of the PCBA will determine the type of assembly to opt for. There are different types of circuit board assembly. Also, it is important to carry out the assembly process with extreme care. This is because this process determines the functionality of the board. Also, it affects the performance of the electronic device it will be used for.

Printed Circuit Board Assembly Process

The PCBA process involves several steps. The type of Printed circuit board assembly will determine these steps. However, there are major steps to achieving printed circuit board assembly.

Solder paste screening

This first step involves applying solder paste to the bare circuit board. Here, a stencil is placed over the bare PCB.  Therefore, this leaves the pads for surface mount components assembly uncovered. A machine holds the stencil and an applicator distributes solder paste across the uncovered spaces. Also, the assembler must ensure that the solder paste is only applied to the necessary areas.  

Components placement

After applying the solder paste, the next step is the placement of components. Also, the pick and place machines help to achieve this step. Here, the machine picks and places the component on the necessary areas. To ensure maximum efficiency and accuracy, component placement should be machine automated.

Reflow soldering

With the right placement of components, the PCB has to go through the reflow soldering phase. Reflow soldering is the commonest method for PCB assembly. This is because it is more flexible as regards PCB layout requirements. During reflow soldering, the components will withstand heat for a longer period. This is not the case when it comes to manual or wave soldering.

A wide range of modern SMT components implement reflow soldering. However, reflow soldering isn’t suitable for THT components for this reason. The assembler will need to reflow each side of the board for double-sided SMT PCBs.


It is important to inspect circuit boards after a reflow cycle. An X-ray inspection is ideal after reflow cycle. This is because they penetrate and reflect the metal connections in the board. Automatic optical inspection is another type of inspection widely used.

Types of PCB Assembly Technique

printed circuit assemblies

There are several methods of assembling PCB. Simple boards are manually assembled. However, complex boards require the use of automated PCB assembling machine. The two widely used PCB assemblies are the surface-mount and through-hole. The major difference between these assemblies is the electronic components used.

Also, the two types of components are through-hole devices and surface-mount devices. Surface mount devices are smaller. Therefore, they reduce the size of the PCB. However, through-hole device are larger. SMT PCB assembly is different from THT in terms of soldering process and components.

Surface mount PCB assembly

For surface mount PCBA, the assembler solders SMDs on the bare board. Also, surface mount PCBA requires extreme care since their pins are very small. It is easy to mount electronic components with SMT. This is because this PCB assembly technique uses an automated machine to pick and place components.

In addition, surface mount PCBA involves soldering components directly on a PCB surface. Surface mount PCBA is popular among PCB assemblers. The method of this assembly is different from through-hole assembly. SMT helps to make good use of PCB space and reduce manufacturing costs.

SMT PCBA involves a reflow soldering process. In this process, the circuit board will have to withstand extreme heat. The method of component placement and type of reflow soldering distinguish SMT from THT. With SMT, it is easy to manufacture highly effective and small-sized PCBs.

Through hole PCB assembly

This assembly is different from the SMT assembly. Here, the assembler places through hole components on one side of the board. After this, the components are carefully soldered on the other side. Rather than using solder paste, THT used solder wire. Solder wire is a more preferred option because solder paste can pass through the holes.

Also, THT involves drilling of holes. Here the assembler drills holes inside the circuit board. These holes are properly drilled using automated drilling machines. The soldering technique for THT assembly is completely different. This assembly uses manual soldering or wave soldering.

The manual soldering is a simple and easy soldering method. Here, a mechanical fixture holds the circuit board from the edges. Also, this soldering method involves using both hands. Wave soldering involves PCBs going through an oven for soldering. Here, the assembler passes the board into the oven via a conveyor belt. Also, molten solder washes the pins and they cool off. However, this soldering is only ideal for single-sided boards.

Mixed PCB assembly

This is another PCB assembly technique. Mixed assembly integrates both SMT and THT techniques.  Also, this assembly combines the strengths and weaknesses of both SMT and THT. Mixed technology assembly encompasses several approaches. Some of these approaches are a single-side mixed assembly and a double-side mixed assembly. Also, it could be one-sided THT and one-sided SMT.

Differences Between SMT and THT Assembly

The SMT and THT assembly are the two major assembly techniques used in the PCB industry. However, there are some differences between these two techniques.

Component placement

In SMT assembly, an automated machine places component on a bare circuit board. Also, SMT assembly doesn’t require any drilling of holes. However, THT assembly requires hole drilling. Also, these holes are necessary for insertion of component lead. SMT PCB assembly mounts components to surface pads.


SMT assembly makes use of surface mount devices. These devices feature shorter leads and high reliability. Also, they are smaller than THT components. THT components have leads and are larger in size. The use of SMDs helps to maximize space in circuit board. Therefore, this results in the production of smaller size circuit board.


This is one of the major differences between these assemblies. SMT uses reflow soldering while THT uses wave or manual soldering. The use of reflow soldering is an advanced method of soldering. However, wave or manual soldering are great options in some cases.

Pin count

SMT can hold higher pin count than THT. Pin count refers to the lead count of a component. Also, it refers to the total number of the part leads a PCB can fit. SMT component offer better performance because the pins are more interconnected.  


SMT is a much cheaper option. This is because THT requires drilling holes on the board. This task can be time and money consuming. Also, THT limits the routing areas in any multilayer PCB. SMT parts are more cost effective since they are less expensive to manufacture.

PCB Assembly Failures and Prevention Methods

During PCB assembly process, some failures are likely to occur. Some of the errors are common. However, there are ways to prevent these errors from occurring.

Bent or cracked PCB

During PCBA, a circuit board may bend or crack. Also, this could be a result of physical and mechanical stress. A board may fail if exposed to vibrations or thermal shocks. Also, if a flexible board bends beyond its capability, it can crack. To prevent bends and cracks, ensure the board is stored in humidity controlled places.

Joint solder fracture

The thermal coefficient of the circuit board can cause solder joint fracture. Therefore, the PCB can fail if subjected to high temperature environments. Another cause of this problem is manual soldering. It is advisable to utilized automatic equipment during soldering. Also, use the appropriate solder amount. This will prevent pieces of the PCB from coming off during vibrations.

PCB corrosion

Solder flux is commonly used in PCB manufacturing. However, when improperly cleaned, the residue can cause corrosion to a PCB. Flux may decay on a board if it comes in contact with moisture. PCB manufacturers can avoid corrosion by thoroughly cleaning their printed circuit boards.

Unsuitable copper weight

The appropriate copper weight is important for efficient electricity flow on a circuit board. Also, if the value of the copper is too thin or thick, the board might malfunction. Therefore, ensure you use the ideal weight for copper foil. PCB assemblers must adhere to the standard specifications for copper weight.


De-wetting is commonly associated with problems with solder pad and electric components.  Also, de-wetting makes solder joint loose. Therefore, these solder joint fail to create secure connections.

Soldering Processes for Printed Circuit Board Assembly

Soldering is a crucial aspect in printed circuit assembly. There are different soldering methods for PCB assembly.

Reflow soldering

This is the most commonly used form of soldering. Also, reflow soldering offer reliability for various components and pad sizes. This soldering is very easy to control and monitor. In reflow soldering, the printed circuit board goes through controlled heat. This helps the solder paste to melt. Also, the heat ensures that the components are well secured to the board.

Reflow soldering is suitable for use in surface mount components. Therefore, it is suitable for mass production of electronics devices. Reflow ovens are crucial for reflow soldering. These machines offer soldering capability for PCB assembly areas.

Wave soldering

This is a large-scale form of soldering that uses waves of molten solder to attach components. Here, the printed circuit board goes through a pot of molten solder. Wave soldering helps to produce joints that are electrically and mechanically stable.  Also, this soldering is ideal for both surface mount components and conventional THT components.

Wave soldering involves the use of a heated tank of solder. Assemblers keep this tank at the required temperature. There is a wave of solder within this tank. Therefore, PCBs pass over this tank so that the underside of the PCB contacts the solder wave. Metal fingers hold the boards firmly on a conveyor.

Manual soldering

This soldering form is a bit outdated. However, it is still a form of soldering in PCB assembly. Manual soldering is suitable when wave or reflow soldering isn’t a good option. For instance, surface mount PCB that includes few through-hole components. Manual soldering will take care of such cases.

Printed Circuit Board Assembly Applications

The printed circuit board is very important in the manufacturing of electronic devices. However, the bare PCB isn’t functional without the assembly of electronic components. Therefore, the assembled board is widely used in several applications.

Industrial application

PCB assembly is widely used in several industries. The automation systems in the industrial sector help to automate the manufacturing process. Also, these systems help to control a manufacturing unit. Therefore, this automation system features complex PCB assembly. Due to its operating power, industrial PCBA can operate such complex automated systems.


The aerospace sector solely depends on printed circuit board assembly. These assemblies are commonly used to design different devices. The PCBA used in the aerospace sector feature high performance properties. This is because this circuit board assembly functions in more hostile environments. Also, it is more complex to assembly a board for this application.

Medical industry

The advent of PCB has positively impacted the medical sector. Machines like ECG and X-rays feature high performance circuit boards. These machines solely depend on printed circuit board assembly. Also, the PCB used for medical devices need to be perfect and accurate. Therefore, it is important to use high-performance and high-frequency boards in this application.

Automotive industry

Printed circuit board assemblies are widely used in the automotive industry. You would not only find these PCBAs in headlights or wipers, there are now used in several parts of an automobile. Also, the PCBAs used in automobiles are for advanced operations. PCBAs are widely used in regulator systems and navigation systems.

Consumer electronics

Consumer electronics such as refrigerators and smartphones feature PCBs. Most consumer electronics depend on a printed circuit board assembly.

Methods of PCB Assembly Testing

Testing is an important aspect of the printed circuit board assembly process. Also, it is a method of maintaining quality control in PCB assembly. There are several tests a PCB can undergo during the assembly process.

Automatic optical inspection (AOI)

This test method works according to optical principles and image analysis. Also, it uses image analysis and other technologies to identify problems during the assembly process. It is a method of detecting any defects on  a printed circuit board. AOI is often utilized before and after reflow soldering. Also, it is ideal before electrical testing.

AOI incorporates two 3D cameras and a 2D camera to capture images of the board. Then it compares this image to a detailed schematic. The board must match the detailed schematic to a certain extent. Otherwise, the board will need further inspection.

X-ray inspection

This type of testing detects defects at an early stage of the assembly process. Also, X-ray inspection views internal traces and solder connections. There are 3D and 2D x-ray inspections. X-rays helps to view through the parts that need testing.  Also, X-ray is ideal for some PCBs with concealed solder joints. X-rays feature powerful penetrability. Therefore, it can easily identify defects in a printed circuit board assembly.

In-circuit test

This test is primarily meant for testing the PCBA for short circuit and open circuit. Also, in-circuit test uses several ICT applications. ICT can greatly enhance production efficiency. Also, it can reduce production costs. The in-circuit test is widely used in the PCB industry.

Manual visual inspection

This manual testing depends on visuals to verify the component placements on the board. Also, this method is widely used in printed circuit board assembly. However, this method isn’t really ideal due to small and large number components.

Guidelines for Printed Circuit Board Assembly

Having understood what printed circuit board assembly entails. Some guidelines will help you to achieve a successful circuit board assembly.

Carefully select electronic components

The rule of thumb here is to carefully select components in the design phase. This will help you to prevent conflicts between the design and the assembled components. Also, use readily available components for your assembly.

Component spacing

Ensure you adhere to the appropriate specifications when spacing electronic components. Also, this will help to avoid any issues during assembly and circuit design. For example, placing components too close to each other can cause further issues. Therefore, this will result in lose of time and money.

Place electronic components evenly

It is important you place large components across the PCB during layout. Also, this ensures optimal thermal distribution during reflow soldering. Don’t allow components to come too close to the board edges on smaller PCBs. Be careful of the copper layer when placing components.

Avoid combining technologies

To achieve the best results, don’t combine technologies. Also, the use of a single through-hole can’t surpass the extra time and money spent on PCBA. Therefore, it is advisable to use a single assembly technique.

Use a singular pad for every connection

Make sure all connections to components have their individual pads. Also, these pads should match their sizes. It becomes difficult to align components when they share a single pad. Also, when a pad is bigger than a component, it could result in component tombstoning. Therefore, it is advisable to mask-define pad with the appropriate size.

Ensure the availability of protracted lead-time components

Before starting the assembly process, ensure all components and parts are available. Therefore, order components that require long lead times early enough.

How to Choose a Good Printed Circuit Board Assembly Service

Printed circuit board assembly is a critical process. Therefore, it is important to choose a professional PCB assembler. There are several PCB assemblers. However, it is crucial to consider some factors when choosing these assemblers.


This is the first factor you should consider. The quality of your PCBA determines a lot. Therefore, choose a PCB assembler that offers quality PCBA. Also, you can access quality by checking the reviews of past customers. The quality of the materials used determines the quality of your PCBA.

Also check out the type of substrate material and integrated circuits. Materials like solder mask and copper traces should be of good quality.

Assembly Technologies

The method of assembly techniques used is a factor to consider. Some assembly services offer only SMT assembly. However, some assemblers still offer THT assembly. The type of assembly you want will determine the assembly service to opt for.

Quality control standards

A good assembler should integrate quality control measures. Also, assemblers without good quality control measures can jeopardize your PCBA project. Ensure the assembly service you opt for carry out good quality inspection. Also, the assembler must ensure careful selection of all materials used.


The success of electronics manufacturing depends on circuit board assembly. Also, PCBA allows electronics to perform well. Therefore, it is important to carry out this assembly process carefully. The assembly of a board is completely different from PCB manufacturing. In this article, we have thoroughly discussed what circuit board assembly entails.

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