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PCB Assembly Process - Ultimate Guide Is Here - NextPCB

Posted:02:58 PM November 10, 2022 writer: NextPCB



Printed Circuit Board Assembly  is an essential step in every PCB design and PCB manufacturing. Your consumer electronics would only function properly with a proper circuit board assembly. There is an urgent need to understand PCB Assembly and its steps.

Printed Circuit Board Assembly is putting together essential components on a fabricated circuit board to create a functional circuit board. The more components on the circuit board, the more complex the assembly process becomes.

NextPCB provides an ultimate guide to the step-by-step Printed Circuit Board Assembly process in this post. Also, it discusses the kinds of PCB mounting technology in the Assembly process and essential components that are common in PCB Assembly process.

Printed Circuit Board Assembly Step-By-Step 

Printed Circuit Boards are the heart of all electronic devices regardless of their operations and functions. As such, there is a need to carefully follow the outlined steps involved in the PCB Assembly Process.

Printed Circuit Board Assembly takes place after PCB Manufacturing; that is, a bare circuit before placing any component on it. The circuit board will have the required layers (single, Double-sided, multi-layer) and copper traces.

pcb assembly process flow chart

pcb assembly process flow chart

Here are the required steps in the PCB Assembly Process:

Step 1: Solder Paste Stenciling

The first PCB assembly process is solder paste application to the bare circuit board. This solder pasting aims to fill the circuit board with enough paste to fix electrical components quickly.

This process requires a stainless-steel stencil with the same shape, width, and size as the bare Printed Circuit Board. It is usually made of brass or common material, stainless steel.

The Stencil follows the set PCB design and has holes that will be filled with solder paste. This Stencil is then carefully fit on top of the bare circuit board, which covers areas that do not require solder paste.

After fitting the stencil on the circuit board, a solder paste applicator will distribute the solder paste evenly to the uncovered spaces. Then, the PCB assembly team will check if the solder paste evenly distributes on the circuit board via the Stencil.

For a double-sided circuit board, each side will have a stencil for even distribution of solder paste

Step 2: Pick and Place components.

Once the solder paste evenly distributes on the bare Printed Circuit Boards, the next step is picking and placing components on the circuit board. The required components undergo thorough inspections to check for defects or malfunctions before placement.

With advanced technology, this process is automated as Pick & Place machines will pick specific components and place them on their required paste. The automation ensures maximum efficiency and accuracy.

Upon placement of the components, the Circuit board assembly team will inspect the PCBs of the solder paste enough to hold the components in place. This process takes place several times to ensure that no component is loose on the circuit board.

For a double-sided PCB, the step is the same; the components fixed on one side will undergo thorough checks then the other side with its components will also go through similar inspections.

Some pick-and-place machines put small amounts of adhesive to secure the components further. Yet, it can be disadvantageous, especially if the components’ placement is misaligned from the original PCB design specification.

Regardless of the Assembly technology, this pick-and-place process is necessary and automated for better accuracy.

Step 3: Reflow Soldering

The third step is Reflow Soldering. This process involves passing the circuit board through varying temperatures to melt and resolve the solder.

The circuit boards with their required components will pass through an industrial oven or furnace that heats the solder and then melts it to ensure a proper connection between the components and the board.

The PCB temperature could be as high as 245. A primary concern regarding this process is that the components affixed to the circuit boards must withstand these high temperatures for some time.

After, the printed circuit boards moves to an industrial cooler, which lowers the circuit board’s temperature and solidifies the liquid solder paste, further fixing the components on the board.

For double-sided circuit boards, it is necessary to use a unique adhesive to hold the other side components in place to prevent them from falling off. This process takes about ten minutes (usually less).

                                                Reflow soldering working principle diagram

Reflow soldering working principle diagram

Step 4: X-ray Inspection and Quality Control

After the Printed Circuit Board is at room temperature, it undergoes two inspections.

  1. The manual inspection checks the circuit board for any misalignment of components during the third stage or the circuit board’s fault.
  2. The x-ray inspection is where the circuit boards pass through an x-ray machine to check for inner layer defects.

The manual inspection could cause eye fatigue and strain, especially if a circuit board holds hundreds of components.

The x-ray inspection, however, enables easy detection as the machine captures an image of the solder joints to determine the quality of the solder joints is comparable to industry standards. Also, x-ray inspection can help identify if the solder past is insufficient, there are misaligned components, or there is an incomplete reflow process.

Step 5: Final inspection and Functional Testing.

The first final stage of the Printed Circuit Board Assembly Process is the final inspection. The final inspection ensures that the circuit board, the solder paste points, and the components conform with the desired PCB specifications. Also, thorough cleaning of the printed circuit boards to remove debris or excess solder.

After the final inspection, functional tests are necessary to test the Printed Circuit Boards and their soldered components for functionality.

Employing Automated Optical Inspection can subject your PCB to real-world testing via simulations to ensure that it provides quality operations to consumers in electronic devices.

Also, in-circuit Testing is another intensive test that ensures that each component is functional and offer no errors on the Printed Circuit Boards. However, the in-circuit test is time-consuming and could add to the manufacturing costs. Still, clients can request this test for their PCB, fully aware that they will pay the extra charges.

The functional test determines the success or failure rate of the Printed Circuit Board assembly. Hence, a Printed Circuit Board will not get to packaging and shipment if it fails in any required functional tests.

Printed Circuit Assembly Technology

The PCB industry is always dynamic in providing quality PCB Manufacturing and Assembly processes. When it comes to the Assembly process, there are two mounting technologies that PCB Manufacturers employ: Surface Mount Technology (SMT) and Through-Hole Technology (THT). Let’s examine each of these mounting technologies and their differences.

Surface Mount Technology

SMT is undoubtedly one of the best Printed Circuit Board Assembly technologies in the PCB Industry. With surface mount Technology, the manufacturing process of Small-sized yet functional PCBs is achievable.

Also, the Surface Mount Technology allows small components like resistors, diodes, and integrated circuits carefully mount on the surface of the PCB. Surface mount technology is low-cost, faster, and offers higher accuracy in the production process.

However, Surface Mount Technology has a significant drawback. It is not ideal for circuits prone to frequent vibrations and stress, which would lead to the components falling off the circuit boards.

The Assembly Process involved in Surface Mount Technology includes:

  1. Surface Mount Technology Printing: This process is vital in Surface Mount Technology. SMT printing involves printing solder paste on the circuit board using a stencil.
  2. Pick and Placement of Components: The components are first picked and placed on the solder paste on the Printed Circuit Board.
  3. Reflow Soldering: This process involves passing the Printed Circuit Board to high temperatures to ensure a tight connection between the circuit board and its electronic components.

Standard production line for smt

 Surface mount standard assembly line

Through-Hole Technology

Through-Hole technology shines excellently in circuits subjected to frequent mechanical stress and vibrations. This technology requires precise drilling of holes on the Printed Circuit Board for components with leads firmly attached.

Through-Hole Technology is great for large components such as capacitors, relays, plug connectors, and electromechanical coils that fit into the board. These components use the holes to pass signals from one side of the board to the other.

However, This Technology is more expensive than SMT. It is a less reliable method as the holes need to be precise, or the assembly process could lead to design and functional errors during testing.

The Assembly process involved in the Through-Hole Technology includes:

  1. Drilling: drilling of the circuit board to create holes for easy fixing of the components' leads into the circuit board.
  2. Pick and Place components: This process ensures the attachment of the components' leads into the circuit boards
  3. Wave soldering: Since holes are on the circuit board, the conventional solder paste method will not work. Instead, THT components require a unique soldering technique called wave soldering.      The circuit board firmly attaches on a conveyor belt that runs into an industrial oven where molten solder washes over the bottom of the PCB. This process ensures permanent fixing of all the leads of the components on the other side of the circuit board.
  4. Final inspection and packaging: After the soldering process, the next process is final inspection of the PCBs to ensure no defects on the circuit board and its components. Also, the Printed Circuit Boards pass through the necessary test before packaging and shipment to their customers.

What Are the Essential Components In PCB Assembly?

The components on a circuit board work together to function perfectly on our electronic devices. Here we have highlighted ten essential components required in PCB Assembly.

Electronic Components Classification Chart


Resistors are essential components in a printed Circuit Board. It controls the flow of electric current from one part of the circuit board to the other. On the printed circuit board, Resistors denotes as “R.”

Resistors also resist the flow of current and dissipate them as heat. The standard kind of resistors is the axial style resistors with a pair of long leads  at both ends. They are often thin and come in different materials.

Also, there are color rings on the resistors’ bodies. These rings are color codes that indicate resistance value. Calculating the color codes gives you an idea of the resistor value you add to your circuit boards.


Capacitors are essential components in printed circuit board assembly. The function of capacitors is to store electric charges temporarily, before releasing it back to the circuit board.

Capacitors found in printed circuit boards measure in microfarad (uF) or picofarad (pF). The capacitance of the capacitors is the ability of this electronic device to store charges.

Capacitance depends on the surface area of the capacitor, the distance between the plates, and the nature of the dielectric constant. The typical appearance of a capacitor is radial, with two leads attached from the same end.

The types of capacitors include electrolytic, ceramic disc capacitors, and diverse polymer capacitors.


An inductor is another essential component in printed circuit board assembly manufacturing. Like resistors and capacitors, the inductor is a passive component that stores electric charge in a magnetic field. Inductors are used to filter out or block specific signals to prevent interference.

The inductors are coils of wire wrapped around a magnetic core; with more coil windings, the greater the inductance. Also, increased coil windings amplify the magnetic field and the stored energy.


Diodes are essential components that allow the electric current to flow in one direction from the positive terminal, called the anode, to the negative terminal, called the cathode. They offer no resistance in this unidirectional flow of electric current in the printed circuit board.

However, suppose the current flows from the cathode to the anode. In that case, there will be high resistance and no current flow, which is quite effective as the reverse current flow can damage the diode.

The Light Emitting Diode or LED is the standard diode used in printed circuit board assembly. PCB LEDs emit light, and it is essential to connect them to the PCB correctly, or they will not function in the circuit board.


Transistors are the building blocks of modern \electronic devices. These components essentially amplify or electrically switch electronic signals in a printed circuit board. The standard kind of Transistor is the Bipolar Junction transistor or BJT.

The Bipolar Junction Transistors come with three pins which are the base (B), collector (C), and Emitter (EE). BJTs are further categorized into PNP (positive-negative-positive) and NPN (negative-positive-negatives) types.

Another type of Transistor is the Field effect Transistor or FET, which activates the circuit via an electric field. Transistors are often marked as “D” or “Q” on the Printed circuit board during the manufacturing process.

Integrated Circuits

Another essential component in the printed circuit Assembly process is the Integrated Circuit or IC. Integrated circuits combine several electronic components (resistors, capacitors, amplifiers, timers, and inductors) into a single chip.

Integrated Circuits are in different shapes and sizes and encased in black plastic housing. The development of Integrated Circuits led to the evolution of modern technology from smartphones to supercomputers.

Switches and Relays

Switches make or break circuitry on the circuit board. A switch could take the shape of a slider, push button, key switch, toggle, etc. Relays are electromagnetic switches that act as temporary magnets when current flows through them. They can amplify small currents to larger currents on the circuit board.


Sensors are sensitive electronic devices that detect environmental changes and generate electric signals in response to such changes. They are transducers on a circuit board that converts a physical phenomenon into electrical energy.

Sensors are applicable in various modern technology and are used in sensing changes in physical or environmental conditions like light, air, wind, humidity, sound, moisture, and motion.


A transformer consists of a soft iron core with two coils of wire around it; the primary and secondary coils. Transformers are smaller when applied in the Printed Circuit Board Assembly process. Their function is to “step up” or “step down” voltage.

The primary coil is the source circuit, where current flows first, while the secondary coil is where electric current is transferred. Also, at the secondary coil, voltage either increases or decreases as it powers the circuit.

Crystal Oscillator

Crystal Oscillators are used in electronic PCBs that require precise timing. This component's design is to vibrate at a particular frequency and produce a periodic electronic signal causing the crystal to oscillate.

Crystal Oscillators are commonly found in micro-controllers and quartz watches. They are marked with the letters “X” and “Y” on the circuit boards during the assembly process.

Printed Circuit Board Assembly Process: Factors That could affect Quality 


Moisture is threatening PCB electronics assembly. It may seep into the PCB manufacturing process or during the assembly phase. In any case, moisture can affect the lifespan of the circuit board and its components due to condensation.

This phenomenon, condensation, occurs when these circuit boards are in containers with lower temperatures than the surroundings. To ensure quality PC Board Assembly Processes, practical anti-moisture evaluation should be available during components’ selection.


It is a known fact that PCB electronic devices are widely used in various sectors such as automotive, medical, industrial, and transportation. These printed circuit boards operate in harsh conditions, especially high temperatures. These high temperatures could prevent the components from functioning correctly.

PCB Manufacturers must note the effects of high temperatures on their electronics. Also, they can specify the temperature range of these products during packaging and shipping.

Due to advancements in PCB trends, thermal-resistant materials are employed in designing active and passive PCB components to ensure they operate within temperature ranges in their respective industry.

Mechanical Stress

As earlier stated, PCBs are useful in various applications and sectors, often requiring operating them at high vibrations or frequencies. It is noteworthy that high vibrations or shocks could break the circuit board hence the circuitry and mechanical components found within the printed circuit board.

The through-hole technology effectively deals with PCBs often associated with shocks and high vibrations to combat this problem. This PCB assembly technology ensures that components are permanently fixed into the circuit board.

The period cycle of components

Due to rapid changes in modern electronic technology, components have limited lifespans, becoming obsolete when new components hit the PCB market. These old components become obsolete and have reached their End of Life stages.

When components become obsolete, there is an increased chance that the PCB starts to malfunction due to mechanical and physical stress, thereby reducing its efficiency and reliability. Also, when new PCB technologies arise, these obsolete components may have difficulty meeting the new performance requirements and hence have a greater chance of failure.

To ensure consumers know their PCB expiration date, PCB manufacturers must carefully explain to them the new alternative devices to opt for when their PCB exceeds the expiry date.


The Printed Circuit Board Assembly process involves putting all components in a circuit board for the proper functioning of the board. This guide provided five straightforward steps in the PCB assembly process. It also gave two types of PCB Assembly Technology.

At NextPCB, we are committed to offering a comprehensive range of services from PCB designing, manufacturing, PCB Assembly, and more. By developing PCBs with state-of-the-art machines and equipment, you can receive high-quality PCBs suited to your needs.

If you have any information regarding some PCB services and solutions, kindly quote now and enjoy an unbeatable customer experience from us.

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