Through-Hole Technology (THT) in PCB Explained

Introduction

Through-Hole Technology (THT) is an older way of putting parts onto a circuit board, but it’s still very useful. While Surface-Mount Technology (SMT) is now more common in modern electronics, THT is still used when strong and reliable connections are needed.

THT works by putting the wires (called leads) of a part through small holes in the board and soldering them on the other side. This makes the part stay firmly in place, which is great for devices that handle a lot of power or need to work in tough conditions.

What Is Through-Hole Technology (THT)?

Through-Hole Technology (THT) is a way of placing parts on a circuit board by inserting their metal legs (called leads) into small holes drilled through the board. These leads are then soldered on the back side of the board to hold the parts in place and create an electrical connection.

Even though newer methods like surface-mount technology are more common today, THT is still used when parts need to be extra strong and secure. It’s a good choice for things like powerful components or electronics that work in harsh environments, where the parts might get bumped, heated, or shaken.

How Through-Hole Technology Works

Through-hole assembly might seem simple, but each step plays an important role in making sure the PCB works properly and stays reliable. Let’s break it down step by step:

1. Drilling Holes

The process begins by drilling tiny holes into the PCB based on the component layout. These holes match the size and placement of the component leads.

2. Component Insertion

Next, the component leads are inserted through the holes from the top side of the board. This can be done by hand for small batches or with machines for larger production runs.

3. Soldering

Once everything is in place, the leads are soldered to the copper pads on the underside of the board. This can be done manually with a soldering iron or with a wave soldering machine for faster production. The solder forms a strong mechanical and electrical bond.

4. Inspection & Cleaning

After soldering, the board goes through inspection to check for issues like cold solder joints or misaligned components. It’s then cleaned to remove any flux residue or debris that might affect performance.

5. Final Testing

Lastly, the board is tested to make sure everything is working as expected. This might include electrical checks, functional testing, and sometimes even stress testing for high-reliability products.

This step-by-step process is what makes THT a reliable method, especially when building PCBs that need to handle power, pressure, or extreme conditions.

Key Features of THT Components

Through-hole components have some distinct physical and performance traits that make them different from surface-mount parts. Let’s take a closer look at what sets them apart.

Radial vs. Axial Leads

THT components usually come in two lead styles: axial and radial.

• Axial leads run through the center of the component with one lead on each end. These are often placed flat on the board.
• Radial leads both extend from the same side, allowing the part to stand upright on the PCB.

Each style offers flexibility depending on the layout and space requirements of your design.

Larger Size and Lead Length

Compared to SMT parts, THT components are generally larger and have longer leads. This makes them easier to work with during manual soldering, prototyping, and repairs. The extra size also allows for higher current handling and more robust connections.

Heat and Vibration Resistance

One of the biggest advantages of THT parts is their ability to withstand heat, shock, and vibration. This makes them ideal for demanding environments like automotive systems, military equipment, or industrial machines, where reliability under stress is a must.

Thru-Hole Mounting Advantages and Disadvantages

Advantages of Thru-Hole Mounting:

Even though newer methods like SMT are more popular, through-hole technology still has some great benefits—especially when strength and durability are needed.

• Strong Mechanical Connections: THT parts are held tightly in place because their leads go through the board and are soldered underneath. This gives them a firm and secure connection, which is great for parts that might face movement or stress.
• Great for High-Power Circuits: If a circuit needs to handle a lot of power or high voltage, THT is a better choice. These components can deal with more heat and current than smaller surface-mount parts.
• Works Well in Harsh Conditions: THT is perfect for devices that work in hot, cold, or shaky environments—like in cars, factories, or outdoor equipment. It stays reliable even when things get tough.
• Easy to Test and Fix: Since THT parts are bigger and easier to handle, they’re great for prototypes or repairs. You can quickly swap out a part or make changes without too much hassle.
• Reliable for the Long Run: Because of their strong build, THT components last a long time. That’s why they’re often used in important systems where things just can’t go wrong.

Disadvantages of Thru-Hole Mounting:

While through-hole technology has many strengths, it also comes with some downsides—especially when compared to modern surface-mount methods.

• Slower and Labor-Intensive: THT takes more time to assemble, especially when done by hand. Inserting each lead into a hole and soldering it adds extra steps, which can slow down production and make it less efficient for high-volume manufacturing.
• Takes More Board Space: Because THT components are usually larger and need drilled holes, they take up more space on the PCB. This limits how many parts you can fit on a single board, which is a problem in designs that require compact layouts.
• Higher Production Costs: More steps and more labor usually mean higher costs. Between drilling, inserting, and soldering, THT can end up being more expensive than surface-mount methods—especially in large-scale production.
• Not Ideal for Compact Devices: If you’re building a small product like a smartphone or smartwatch, THT isn’t the best choice. Its larger size and space requirements make it hard to use in tight, high-density designs, where every millimeter counts.

Through-Hole vs. Surface-Mount Technology (SMT)

Both Through-Hole Technology (THT) and Surface-Mount Technology (SMT) are used in PCB assembly, but they serve different purposes. Let’s look at how they compare so you can decide which one fits your project best.

Size, Cost, Speed, and Automation

SMT components are much smaller than THT parts, making them ideal for compact devices like smartphones or wearables. SMT also allows components to be mounted on both sides of the PCB, saving space and enabling more complex layouts.

In terms of production, SMT is faster and more automated, which lowers assembly time and cost—especially for large-scale manufacturing. THT, on the other hand, is more manual, slower, and usually more expensive due to extra steps like drilling and lead insertion.

Mechanical Strength vs. Compact Design

If you need strong mechanical support—for example, in parts that may face vibration or stress—THT is the better choice. It offers deeper, more secure mounting that holds up under pressure.

But if your priority is to keep the design small, light, and fast to assemble, then SMT is a smarter option. It’s perfect for space-constrained applications where every millimeter counts.

When to Use Each One

Use THT when:

• Your components will face high power, heat, or vibration
• You need easy prototyping or repair
• You’re building for industrial, military, or medical environments

Use SMT when:

• You want to minimize board size
• You need fast, high-volume production
• You’re designing consumer electronics or wearable tech

Design Considerations for THT PCBs

When working with through-hole technology, getting the design right from the beginning is key to building a strong and reliable PCB. Here are a few important things to keep in mind.

• Hole Size and Pad Diameter: The holes should match the size of the component leads. If the holes are too small, the parts won’t fit. If they’re too big, the solder joint may be weak. The pads around the holes also need to be big enough to hold the solder and make a strong connection.
• Lead Length and Placement: Make sure the leads are long enough to go through the board but not so long that they get in the way. Good component placement also helps prevent parts from overlapping or crossing each other, which makes soldering easier and safer.
• Thermal Management: Some THT parts, like power components, can get hot during use. To keep your board safe, plan for cooling, like using thicker copper traces or adding heat sinks.
• Mixed Assembly Designs: Many PCBs use both THT and SMT components. This is called mixed assembly, and it lets you enjoy the benefits of both. Just make sure the layout works for both types so your board is easy to build and test.

Manual vs. Automated THT Assembly

When it comes to assembling through-hole components, there are two main ways to do it: by hand or using machines. The right choice depends on your project size, timeline, and budget.

Prototyping with Hand-Soldering

For small projects or early-stage development, manual soldering is the go-to method. It’s perfect for prototyping because it lets you place components by hand and make quick changes as needed. Hand-soldering is also useful for repair work or when you're only building a few boards.

Mass Production with Machines

When it’s time to scale up, automated THT assembly is much faster and more consistent. Machines insert the components into the PCB, and then wave soldering is used to solder all the leads at once. This method is ideal for large production runs where speed and repeatability matter.

Cost vs. Volume

Manual assembly is more affordable for small batches, but it’s slower and takes more labor. Automated assembly has a higher setup cost, but it becomes more cost-effective as volume increases. If you're producing hundreds or thousands of boards, automation usually makes the most sense.

Testing & Quality Control in THT Assembly

Making sure your through-hole PCB assembly works as expected isn’t just a final step—it’s a must for building reliable electronics. Here’s how testing and quality checks are usually done in THT assembly.

Visual Inspection

The first step is often a visual inspection, either by hand or using cameras. This helps catch obvious issues like misaligned components, poor solder joints, or leftover flux. It’s a simple but important way to ensure your board is clean and ready for more detailed testing.

Continuity & In-Circuit Testing

Next, we check if all the electrical connections are working properly. Continuity testing confirms that each circuit path is complete, while in-circuit testing (ICT) looks at components one by one to spot short circuits, open circuits, or wrong values. This step ensures your board works as designed.

Functional & Stress Testing

To go a step further, functional testing powers up the PCB to check if it performs correctly in real use. In some cases, stress testing is done to simulate tough conditions like high temperatures or vibrations. This is especially useful for products used in automotive, aerospace, or industrial settings.

IPC Compliance Standards

For professional-grade results, most manufacturers follow IPC standards, especially IPC-A-610 for acceptability of electronic assemblies.

These guidelines cover everything from soldering quality to component placement, helping ensure your boards meet industry expectations for safety and performance.

Real-World Applications of THT

Even though many modern devices use surface-mount parts, through-hole technology is still the best choice in certain industries—especially where strength and reliability are important.

• Military & Aerospace: In military and space equipment, things need to work in tough conditions like heat, vibration, and shock. THT is great for these jobs because it makes strong, stable connections that don’t easily fail.

• Automotive Electronics: Cars and trucks use many electronics that face heat and constant movement. THT helps parts like engine controls and safety systems stay firmly in place, even after years of use.

• Industrial Machinery: In factories and heavy-duty machines, THT is a solid option. It can handle high power and rough environments, making it perfect for equipment that runs nonstop.

• Power Supplies: Power supplies and converters need parts that can carry lots of current. THT components work well here because they stay cool and handle heat better than smaller surface-mount parts.

• Medical Devices: Medical equipment must be safe and reliable. THT is often used in devices like monitors and diagnostic machines because it keeps parts steady and helps prevent errors.

• Consumer Electronics: You’ll also find THT in things like home appliances, TVs, and speakers—especially where large parts need extra support inside the product.

Future of Through-Hole Technology

Even though Surface-Mount Technology (SMT) is more common today, through-hole technology (THT) is not going away. It’s still needed for special cases where strength and reliability are more important than size or speed.

• Why THT Is Still Useful: THT is the better choice for parts that face heat, vibration, or high power. That’s why it’s still used in aerospace, military, medical, and industrial equipment. These systems need connections that are strong and built to last.

• A Niche Technology That Matters: THT may not be used in every product, but it still matters for the right jobs. It’s great for power supplies, large components, and anything that needs a secure hold on the board.

• Mixing THT and SMT: Many modern PCBs use both THT and SMT together. This is called a hybrid design, and it gives the benefits of both—compact size from SMT and strong support from THT. It’s a smart way to build reliable and flexible products.

FAQs About Through-Hole Technology (THT)

Conclusion

Even though many modern devices use SMT, Through-Hole Technology (THT) is still very important. It’s the best choice when your project needs strong, reliable connections—especially for high-power parts or harsh working conditions. Choosing THT helps make sure your product works safely and lasts longer.

If you’re not sure which type of assembly is right for your design, we’re here to help. Contact GlobalWellPCBA to get expert advice and support. We’ll help you find the best solution for your project—whether it’s THT, SMT, or both.

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