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Through hole PCB assembly remains an important manufacturing process for many electronic products that require strong mechanical support and long-term reliability. Compared with SMT, it is often the preferred choice for components exposed to vibration, stress, or higher power demands. In this guide, we will explain how through hole assembly works, compare it with SMT, and help you decide which method is right for your project.
What Is Through Hole PCB Assembly?
Through hole PCB assembly is a PCB manufacturing process in which component leads are inserted into drilled holes on the board and soldered on the opposite side. This creates a strong and stable electrical and mechanical connection, which is one of the main reasons the method is still widely used today.
Compared with some other assembly methods, through hole assembly is especially suitable for components that need extra durability or must handle vibration, heat, or higher power levels. Common examples include connectors, transformers, relays, large capacitors, and other parts that are physically larger or mechanically stressed.
In modern electronics, through hole assembly is often used on its own for specific applications or combined with SMT in mixed-technology boards. That flexibility makes it a practical choice for many industrial, automotive, power, and medical projects.
How Through Hole Assembly Works
The through hole PCB assembly process typically starts with PCB drilling, followed by component insertion, soldering, and inspection. Components can be inserted manually or with automated equipment, depending on board complexity and production volume.
After insertion, the leads are soldered to the opposite side of the board using processes such as wave soldering, selective soldering, or hand soldering. These methods help create a secure joint while supporting different product types, from prototypes and small batches to higher-volume production.
Once soldering is complete, the board is usually inspected to confirm solder quality, lead positioning, and overall assembly accuracy. In many cases, this stage also includes electrical testing or functional testing to make sure the finished board performs as expected.
What Is SMT?
SMT, or surface mount technology, is a PCB assembly method in which components are mounted directly onto the surface of the board instead of inserted through drilled holes. It is widely used for compact, high-density designs because it allows many small components to be placed efficiently on a limited board area.
Compared with through hole assembly, SMT is usually better suited to modern electronics that need smaller form factors, faster production, and high component density. In many real projects, SMT and through hole assembly are used together to balance compact design with mechanical strength.
Through Hole PCB Assembly vs SMT
Through hole PCB assembly and SMT are both widely used, but they serve different design priorities. Through hole assembly is generally preferred when mechanical strength, durability, and support for larger components matter most, while SMT is better for compact, high-density, and fast-turn production.
| Aspect | Through Hole PCB Assembly | SMT |
|---|---|---|
| Component attachment | Leads pass through drilled holes and are soldered on the opposite side. | Components are mounted directly on the surface of the board. |
| Mechanical strength | Stronger, especially for parts under stress or vibration. | Good for many applications, but generally less robust for heavy components. |
| Board density | Lower, because drilled holes take up more space. | Higher, because components can be placed more densely. |
| Production speed | Usually slower, especially for manual insertion or soldering. | Usually faster and better for automated high-volume production. |
| Best use cases | Connectors, relays, power parts, industrial and high-reliability products. | Compact consumer electronics, dense layouts, and mass production. |
| Rework and inspection | Often easier to inspect and rework in some cases. | Efficient in automation, but rework can be more delicate. |
In practice, many products use a mixed technology approach, combining through hole and SMT on the same board. This lets designers balance compact layout with strong mechanical support where it is needed most.
Advantages of Through Hole PCB Assembly
Through hole PCB assembly offers several important advantages, especially for products that need durability and dependable performance. Its biggest strength is the mechanical bond created when component leads pass through the board and are soldered on the opposite side.
- Strong mechanical stability, which makes it a good choice for connectors, relays, transformers, and other heavier components.
- Better resistance to vibration and physical stress, which is valuable in industrial, automotive, and other demanding environments.
- Easier support for high-power or high-current components, where a more robust connection is often preferred.
- Often simpler manual insertion and rework for certain builds, especially prototypes, small batches, and mixed-technology assemblies.
- Suitable for applications where reliability matters more than extreme board miniaturization, such as power supplies and industrial control systems.
Another key advantage is that through hole assembly can fit naturally into a mixed technology process. This allows manufacturers to combine through hole parts with SMT on the same board, helping balance mechanical strength, component size, and design efficiency.
When to Use Through Hole Assembly
Through hole PCB assembly is the better choice when your design needs strong mechanical support, higher durability, or reliable performance under stress. It is especially useful for boards that include larger components such as connectors, relays, transformers, and power-related parts.
You should also consider through hole assembly when the product will face vibration, physical handling, or harsher operating environments. That is why it is commonly used in automotive, industrial control, power supply, and medical applications where reliability matters more than ultra-compact board density.
Another good use case is mixed-technology production, where some components are mounted with SMT and others require the stronger attachment of through hole assembly. This approach is often practical for complex boards and can help reduce supplier switching and shorten lead time when handled by one assembly partner.
If the project involves prototypes, small batches, or products that may need easier manual rework, through hole assembly can also be a very practical option. In those cases, the process gives manufacturers more flexibility without sacrificing robustness.
When SMT May Be a Better Choice
SMT is often the preferred method when your design requires high component density, compact size, or fast production for large volumes. It allows engineers to place smaller components directly on both sides of the board, which leads to sleeker, lighter products.
Choose SMT for applications like smartphones, wearables, tablets, and other consumer electronics where board space is limited and miniaturization is a priority. The process also supports better high-frequency performance and signal integrity, thanks to shorter electrical paths between components.
For high-volume manufacturing, SMT offers faster production cycles, lower costs, and easier automation scaling compared to through hole assembly. This makes it ideal for mass-produced devices in automotive infotainment, industrial IoT, and portable medical equipment.
Even so, SMT is not always the only choice—many designs use a mix of SMT and through hole to get the best of both worlds.
Mixed Technology Assembly
Mixed technology assembly uses both through hole and SMT on the same PCB, allowing designers to combine the best of both methods for better overall performance. SMT handles compact, high-density parts, while through hole provides the mechanical strength needed for larger or stressed components like connectors, transformers, and power devices.
This hybrid approach is common in real-world designs because it offers flexibility, enhanced reliability, and durability for complex applications. For example, SMT can fill most of the board with small components for efficiency, while through hole secures parts that need extra anchoring against vibration or heat.
Benefits of Mixed Assembly:
- Design flexibility: Use SMT for space-saving and through hole for robustness on one board.
- Higher reliability: Ideal for automotive, industrial controls, power supplies, and medical devices where both density and strength are key.
- Cost and production efficiency: Supports prototypes, small batches, and volume runs without switching suppliers.
The process usually starts with SMT soldering (reflow), followed by through hole insertion and wave or selective soldering. This order helps avoid damaging sensitive SMT parts while ensuring a clean final assembly.
Quality Control in Through Hole Assembly
Quality control in through hole PCB assembly focuses on ensuring solder joints are reliable, properly filled, and free from defects that could cause failures over time. Manufacturers follow standards like IPC-A-610 and J-STD-001 to check hole fill, lead protrusion, and joint formation.
Key inspection steps include:
- Solder joint quality: Visual and automated optical inspection (AOI) for smooth wetting, no bridges, and no cold joints.
- Hole fill and barrel inspection: X-ray or cross-section analysis to confirm at least 75% fill for Class II/III assemblies.
- Lead positioning: Verify leads are properly trimmed and centered without excessive protrusion.
- Electrical testing: In-circuit testing (ICT), flying probe, or functional tests to detect opens, shorts, and poor connections.
For high-reliability applications, additional checks like thermal cycling or vibration testing help confirm long-term performance. These steps reduce defects and ensure the assembly meets demanding industry requirements, such as automotive or medical standards.
How to Choose the Right Assembly Partner
Selecting the right partner for through hole PCB assembly is important to ensure quality, reliability, and efficient production. Look for a manufacturer with proven experience in through hole, mixed technology, and your target industry, such as automotive, industrial, or medical.
Key factors to consider include:
- Technical capabilities: Ability to handle manual insertion, wave soldering, selective soldering, and mixed SMT/through hole assemblies. Confirm they support prototypes, small batches, and volume production.
- Quality standards and certifications: IPC-A-610 compliance, ISO 9001, and industry-specific certs like IATF 16949 for automotive. Ask about inspection methods like AOI, X-ray, and functional testing.
- Engineering support: Design for manufacturability (DFM) review, DFM feedback, and collaboration to optimize your design early.
- Lead time and flexibility: Reliable delivery, supply chain management, and scalability from prototypes to production without switching suppliers.
- Communication and track record: Responsive support, clear quoting, and references from similar projects. Volume should not exceed 30-40% of their capacity for better service.
A strong partner will help shorten time to market, reduce risks, and deliver high-reliability boards that meet your specs. Start by requesting a DFM review and samples to test their fit.
Conclusion
Through hole PCB assembly excels in applications needing mechanical strength, reliability, and support for larger components, while SMT is ideal for compact, high-density designs. Mixed technology often provides the best balance for complex modern boards, and proper quality control ensures long-term performance.
If your project requires robust through hole or mixed assembly, contact our team for a free DFM review, fast prototyping, and reliable production. We specialize in high-reliability solutions for automotive, industrial, and medical electronics—let’s discuss your next project today.
