The Basics of Through Hole PCB Assembly


When manufacturing a printed circuit board (PCB) there’s no denying that it’s a complex and intricate process. With numerous steps involved, each project brings its unique challenges and requirements. From design to assembly, every stage demands careful attention and expertise. Manufacturing a printed circuit board (PCB) is crucial in bringing electronic designs to life. Once you finish creating the PCB, there’s more work to be done. You’ll have to attach electronic components. As a result, the assembled board is also known as PCBA. The process from conception to design and assembly embodies precision and dedication.

There are two common ways to attach components to PCBs. One of them is the “through-hole” method. As the name indicates, it’s all about drilling holes into a special place on PCB. These holes are strategically placed exactly where the components are supposed to be, following the client’s specific design. It’s a crucial step that ensures everything fits perfectly in the end product. Through-hole technology involves the insertion of the component’s pin in the drilled holes on PCBs. These components have ends called leads. The leads are then attached to pads on the other side of the board using molten metal solder and specialized equipment like wave soldering or re-flow soldering.

Through Hole PCB Assembly
Through Hole PCB Assembly


This method replaced older techniques like point-to-point construction and was widely used from the 1950s. In those days, through-hole components were the only kind found on PCBs. Through-hole PCB assembly remains a cornerstone of electronics manufacturing due to its reliability, mechanical strength, thermal benefits, ease of repair, and versatility in accommodating various component types. Embracing this time-tested technique is essential for delivering high-quality, long-lasting, dependable electronic products across diverse industries.


Through Hole Component Types

The lead wire connection may categorize as through-hole components. They can be divided into active and passive types. They are mounted on the board in holes surrounded by soldering pads. The process involves soldering the component leads to the pads. Mounting these components on the board involves placing them through pre-drilled holes and soldering the leads to the pads on the surface layer.

Mounting electrical components on PCB
Mounting electrical components on PCB


Passive Through Hole Components 

The leads can be long for passive through-hole components, so they are often trimmed before mounting. Radial and Axial are the two passive through-hole components. 

Axial components

They have leads running along the symmetrical axis of components. The resistor’s electrical leads are aligned along its cylindrical axis. Components with axial leads include diodes, resistors, capacitors with axial leads, rectifier diodes, and inductors. Some through-hole components, such as high-power resistors, are available in rectangular packages with a lead wire extending along the package’s length. These components enable designers to create apparatus easily fitting into narrow spaces.

Axial Components Soldered on PCB
Axial Components Soldered on PCB


Radial components

They have leads protruding from one end. Radial leads and components positioned perpendicular to the circuit board take up less space, making them advantageous in high-density applications. Radial lead components are put together and soldered transverse to the circuit board. Examples of such components include large electrolytic capacitors, switches, LEDs, small relays, and fuses. Radial leads are preferred for densely packed circuit boards because they occupy a smaller space than axial lead components flush with the board.

Radial Components
Radial Components


Active Through Hole Components

Active through-hole components are known for their ease of integration into printed circuit boards (PCBs). Manufacturers can create sturdy connections that withstand environmental stresses and vibrations by securely mounting these components through drilled holes on the PCB. Active through-hole components include integrated circuits (ICs) like DIP and PDIP packages, commonly used on breadboards for development. Other components like transistors and higher-power LEDs may come in ZIP or TO packages. These active components mount to the PCB similarly to passive components.

 Dip Component
Dip Component


Advantages of Through-Hole PCB Assembly

  1. Durable: It creates a durable physical connection between PCB and components, making them resistant to environmental stress. 
  2. Great for heavy components: It works well for components like plug connectors, electrolytic capacitors, relays, and semiconductor packages needing extra mounting strength.
  3. Ideal for prototyping: Design engineers like bigger through-hole components because they fit snugly into breadboard sockets.
  4. Power Handling: THA creates a sturdy connection between electrical components and the PCB, allowing them to tolerate high voltage and power safely.
  5. High durability and reliability: Through-hole assembly provides higher environmental stress tolerance due to the solid physical connection, making it preferred in military and aerospace industries with high-reliability requirements. Bright and durable through-hole LEDs are used in giant billboard lights. User-friendly: Through-hole components are easy to replace and adjust, making them widely used in PCB testing and prototyping.
  6. Prototyping: Breadboard sockets make prototyping with more prominent through-hole components easier than with surface mount components.


Limitations of Through-Hole PCB Assembly

Lower Efficiency: Due to the extra step of drilling and using leads to hold components, through-hole assembly takes time, leading to higher costs and lower production efficiency.

Limited PCB Design Flexibility: There may be better choices for multi-layer PCBs because the drilled holes need to cover all layers, making layout design and manufacturing more challenging. Additionally, these boards tend to be larger than SMT PCBs, limiting their application areas.

Expensive: The technology behind Through hole is much more expensive than other assembling technology, making it impractical for many applications. Additionally, THT requires wave, selective, or manual soldering methods, which could be more efficient and reliable in some cases.

Soldering Issues: A notable difference is that THT requires back and front board soldering, while SMT only needs soldering on one side. This makes SMT a more favorable choice for many manufacturers.


Steps to Perform Through Hole Soldering

Soldering through-hole components involves two types: radial lead and axial lead components. Axial lead components are easier to solder, but they require more setup. Materials needed for through-hole soldering are pliers, chisel tip, solder flux, wire solder, soldering iron, PCB, cleaning agent, tissues, solder wick, and acid brush. Both axial and radial lead components follow similar soldering techniques. The main difference is that radial lead components have lead wires on the same side.

Soldering components on the PCB backside
Soldering components on the PCB backside


Hole-Through-Lead Components in the Axis

Cleaning the PCB and Components

The first step in through-hole soldering is preparing the hole. Prepare the area so that soldering may take place. This takes little time and improves the efficiency of through-hole soldering overall. Wipe out the circuit boards and component leads with some isopropyl alcohol. Kim wipes that don’t release particles should be used for the wiping. Preheat the soldering iron as well as wipe off the residue with a moist sponge.

Prepare for Soldering

Put a small amount of solder on the tip of the soldering iron and melt it. The iron will be tinned as a result. If any solder is left on the tip, you can remove it with the sponge. This method transfers more heat from the tip to the component being soldered. The pads also need to be tinned by applying solder to them. A solder wick can be used to dissolve the solder. This allows the solder and pads to adhere more readily to one another. When using solder wicks to clean the pads, exercise caution to avoid damaging them. Holding one end of the component leads with a plier, gently press down on the component’s body so that the lead is at a 90-degree angle with the body. Repeat the steps at the other end of the cable.

Assembling components and trimming the lead

Put the wires into the plated holes. After inserting a component, carefully pressing it onto the board ensures it will not fall off. Make sure the part is lying completely flat on the PCB. Reduce the length of the wires so they can connect to the board. Overstretching a lead might cause problems with component placement on a printed circuit board. 

Soldering the Component

The PCB sides should be fluxed so that heat may flow freely. Flux is used to moisten and clean the component before soldering. This is crucial for obtaining successful through-hole soldering. The only face of the PCB that gets soldered is the bottom. When soldering via through-holes, there is a flux rule that must be followed. Both sides can get flux, but only one should be soldered.

Secure the solder to one end of the lead while holding the board steady with the heat-resistant pad, and then touch the tip of the soldering iron to the joint between the lead and the pad. Here is where you should put some solder. Repeat this step with the lead that was left unattached.

Clean and Inspect

Please closely examine the final result and see whether it passes inspection. You can tell that the through-hole soldering is complete when the solder tip has a bright appearance and a hollow fillet. Compared to a tin-lead solder composition, the solder point may look less sharp if you utilize lead-free solder.

Soldering a Lead Radial Component Via a Hole

Before beginning through-hole soldering, clean the printed circuit board with isopropyl alcohol, as was previously stated. Get a tissue and clean it down carefully.

Solder flux

After correctly positioning the components on the PCB, the next step is to apply flux to the underside of the PCB where the leads of the components meet.

Apply a dab of solder to the leads to secure them. Check that the component’s body is flush with the PCB to ensure a secure connection.

Join all of the leads with solder. If you place it next to the lead, you may melt the solder wire with a soldering iron. When soldering the axial lead components, follow the same steps you used to create the soldering bridge.  

Clean and Inspect

Like what we reviewed with axial components, ensure the solder joint is up to par. It needs to seem sleek and polished. Remove any isopropyl alcohol from the board and clean it well. Keep in mind that lead-free solder flows better when the flux is employed. Lead-free solder requires careful removal of flux residue after through-hole soldering. 


Types of Through Holes

Printed circuit boards contain numerous holes that serve different purposes. Some larger holes are stable bases for mounting brackets, connectors, and switches. These holes may lack metal plating to ensure durable mechanical anchoring for the bolted parts. However, most of the holes on a circuit board have metal plating, facilitating soldering through-hole component leads and allowing for intra-layer signal connectivity. PTH (Plated Through Hole) and NPTH (Non-Plated Through Hole) are two types of holes used in PCBs.

Difference between Non-Plated Through Hole and  Non-Plated Through Hole
Difference between Non-Plated Through Hole and  Non-Plated Through Hole

Plated Through Hole 

PTHs are holes with conductive material plated on their walls, allowing components to be mounted on both sides of the PCB and providing electrical connections between the layers. They are commonly used for components requiring strong electrical connections. They are more complex to manufacture than NPTHs, making PCB fabrication costs higher for designs with many PTHs.

Non-Plated Through Hole

NPTHs are holes without conductive plating, used for mounting components on only one side of the PCB or as positioning holes for mechanical purposes. They are often used for components that don’t require a strong electrical connection or when there is limited space on the PCB. They can also be used when electrical connections between layers are not needed.


Through Hole PCB Assembly Design Considerations

Selection of Lead Components: 

To achieve successful soldering, it’s crucial to understand the differences between axial lead and radial lead components. Components with radial leads take up less room on the board but have a set distance between them, making them preferable to those with axial leads with flexible distances.

Solder Joint Quality

Inspecting the quality of solder joints is a critical step in ensuring the reliability and functionality of electronic components. The quality of this joint can significantly impact the overall performance and longevity of the circuit.

Drill Hole Sizes

When it comes to drilling holes, one important factor to consider is the size of the hole. The size of the hole can have a significant impact on the overall outcome of the drilling project. The first thing to consider is the purpose of the hole. It is essential for PCB manufacturers to bore holes per the aspect ratios of the board.

Size of Annular Ring 

Annular ring diameters must be large enough to make strong solder connections.

Size of Pad

A specific formula should determine the through-hole components to ensure proper assembly.


Through-hole Component Placement
Through-hole Component Placement

Solder mask relief

It refers to the optimal spacing between the solder mask and the annular rings or pads on the PCB. This tolerance parameter plays a crucial role in ensuring the reliability and functionality of the PCB assembly. The solder mask is a protective layer applied to the PCB to prevent solder bridges and short circuits during the soldering process. Maintaining a consistent solder mask relief value of 4 mils is crucial in ensuring uniform component placement.

Solder Mask Relief from Hole and Edge
Solder Mask Relief from Hole and Edge

Solder Fill 

While a full solder fill is ideal, it is not always required. Class 2 and 3 boards, for instance, routinely allow fill rates of 75%.

Importance of Adequate Clearance

  • Soldering from both sides of the board is optional if there is adequate clearance inside the hole.
  • If plated and non-plated through-holes (PTH and NPTH, respectively) are to be drilled, unique drill files must be made available for each kind.
  • You should always have two different drill files if you need to drill pilot and non-pilot through holes.
  • Start with the smaller and flatter parts, and work up to the larger and higher ones.
  • If the component leads are too short to bend, you can secure them using masking tape.
  • Apply a conformal coating for corrosion protection. And potting to secure the components mechanically and electrically.
  • The component’s terminal should be curved upwards, past the solder joint. This method requires less energy to heat and solder.


Comparing Through-Hole Technology (THT) and Surface Mount Technology (SMT)

Surface Mount Technology (SMT) and Through-Hole Technology (THT) are two important types of components assembling technologies used in printed circuit boards (PCBs). SMT is more commonly used than THT because it offers better reliability and cost-effectiveness. However, THT still possesses unique advantages that ensure its relevance in the foreseeable future.

Surface Mount Technology (SMT)

SMT components use small metal pads on one side, enabling direct soldering onto the PCB during assembly. SMT is the prevailing component package technology today.

Surface Mount Technology
Surface Mount Technology

Through-Hole Technology (THT)

Through-hole technology installs components with pin-through-hole (PTH) contacts onto the PCB. The pads on the other side are connected to the leads with molten metal using re-flow or wave soldering equipment.

Through Hole Technology (THT) Surface Mount Technology (SMT)
  • This technology includes easy prototyping, making it simpler and more efficient to test new ideas and designs. 
  • Additionally, it offers solid physical connections, ensuring a reliable and stable circuit. 
  • Its good heat tolerance makes it suitable for applications with higher temperature requirements. 
  • It can handle higher power loads, making it suitable for power-intensive applications.
More minor size results in denser boards, which can benefit specific applications. 

The reduction in size leads to making the board more reliable at higher speeds.

Faster and cheaper assembly due to the reduced complexity. 

The absence of drilling in the manufacturing process makes board fabrication more affordable.

Disadvantages One of the main drawbacks is the higher board cost, primarily due to the need for drilling during the manufacturing process. 

This technology takes up more broad real estate, which might be a concern in space-constrained applications. 

The PCB assembly process is more involved, which could increase production time and complexity. 

This technology has slower speeds than other alternatives, which might not be suitable for high-speed applications.

The physical connections to the PCB may weaken due to the reduced space. 

Smaller boards have lower heat tolerance and may not be suitable for high-power applications.

They may have lower power handling capabilities and encounter issues like tombstones and pop cornering.

Cost Higher cost due to manual drilling and soldering requirements. Less expensive due to automation and fewer holes.
Assembly Time Longer assembly time, especially for large THT components. Faster assembly due to automated procedures.


Applications of Through-Hole Technology 

Thru-hole technology suits products needing high reliability in challenging conditions like high voltage, power, and heat. It’s commonly used in large units where cost reduction through smaller components isn’t a significant concern. Thru-hole is helpful for testing and prototyping due to the ease of manually adjusting and replacing leads for different layouts.

Certain products such as transformers, semiconductors, electrolytic capacitors, and plug connectors favor thru-hole technology. Industries like industrial, aerospace, and military equipment use it for its reliability in harsh conditions. Even LED lights in outdoor applications may utilize this technology due to their ability to withstand outdoor conditions.

Applications of Surface Mount Technology

Surface Mount Technology (SMT) is widely employed in electronic hardware today. It’s becoming more dominant as it becomes cost-effective compared to thru-hole technology. The trend towards smaller electronic devices also contributes to the increased use of SMT.

Electronic products like smartphones, tablets, laptops, and IoT devices typically use surface-mount components. Telecom, transportation, lighting, medical devices, and commercial and industrial hardware also rely on SMT.



In conclusion, manufacturing printed circuit boards (PCBs) using Through-Hole Technology (THT) is a vital process in the electronics industry. THT offers several advantages, including physical solid connections, high heat tolerance, and excellent power handling capabilities. It remains a cornerstone of electronics manufacturing, especially in industries that require high reliability, such as aerospace and defense. Surface Mount Technology (SMT) has become more prevalent due to its cost-effectiveness, smaller size, and faster assembly. However, the choice between THT and SMT depends on the specific requirements and application of the electronic product, with both technologies serving essential roles in the electronics industry.

Ready to bring your PCB designs to life? 

Contact MorePCB today and experience top-quality manufacturing, 

quick turnaround times, and exceptional customer service. 

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