Through Hole Technology PCB

What is Through-Hole PCB Assembly Technology?

Through-Hole Technology (THT) is a traditional method for mounting electronic components onto printed circuit boards (PCBs). This technique requires component leads to pass through pre-drilled holes in the PCB, then be soldered and secured on the opposite side. As a professional PCB assembly manufacturer, we understand that through-hole technology still plays an irreplaceable role in modern electronics manufacturing.

Through-hole technology can be divided into manual assembly and automated assembly. Manual assembly is suitable for small-batch production or prototyping, while automated assembly achieves high-efficiency mass production using specialized insertion machines. Although surface mount technology (SMT) has become mainstream, through-hole technology maintains its importance in many applications due to its unique advantages.

Core Advantages of Through-Hole PCB Assembly

1. Exceptional Mechanical Strength and Reliability

The most notable advantage of through-hole assembly is its superior mechanical connection. Component leads passing through the PCB form solder joints that create a three-dimensional connection far more robust than the two-dimensional connection of surface mounting. For applications requiring resistance to mechanical stress, vibration, or impact (such as automotive electronics, industrial equipment, and aerospace products), through-hole components demonstrate unparalleled reliability.

2. Outstanding Power Handling Capability

Through-hole components typically offer higher power capacity. Since leads pass through the board and connect with multiple copper layers, they provide better heat dissipation and can handle larger currents. This makes THT ideal for high-power applications like power supplies, motor drives, and amplifiers.

3. Convenience for Prototyping and Repair

During R&D and repair work, through-hole components’ ease of replacement is invaluable. Engineers can easily desolder and replace components without damaging the PCB. In contrast, replacing surface-mount components (especially fine-pitch BGA packages) is far more challenging and requires specialized equipment and skills.

4. Stability in Extreme Environments

Through-hole solder joints better withstand thermal cycling and harsh environmental conditions. The “columnar” connection formed by solder filling the through-hole is more resistant to thermal expansion stress than SMT’s “meniscus” solder joints, making it more stable in applications with significant temperature fluctuations.

5. Ideal Choice for Large Components

For connectors, transformers, large electrolytic capacitors, and other bulky components, through-hole mounting is often the only viable option. The weight and size of these components make surface mounting inadequate for providing sufficient mechanical strength.

Technical Process of Through-Hole Assembly

1. PCB Design and Drilling

The first step in through-hole assembly is determining component placement and designing the hole pattern during PCB design. Each through-hole component requires a hole with an appropriate diameter, typically 0.1-0.3mm larger than the component lead for easy insertion. Modern PCB design software can automatically generate drill files to guide CNC drilling machines for precise fabrication.

2. Component Insertion

Component insertion can be performed manually or automatically:

• Manual insertion: Operators place components one by one according to the bill of materials and PCB silkscreen markings
• Automatic insertion: Uses axial or radial insertion machines to place components automatically

3. Soldering Processes

There are two primary through-hole soldering methods:

• Wave soldering: The PCB bottom passes over a molten solder wave, with solder rising through capillary action to fill the through-holes
• Manual soldering: Soldering each joint individually with a soldering iron, suitable for small batches or repair work

4. Cleaning and Inspection

After soldering, flux residues must be removed, followed by rigorous quality inspections including:

• Visual inspection of solder joints
• Automated optical inspection (AOI)
• Functional testing

Comparison: Through-Hole vs. Surface Mount Technology

While surface mount technology (SMT) has become mainstream, through-hole technology retains unique value:

In practice, mixed assembly technology (combining THT and SMT) is increasingly common, leveraging the strengths of both approaches.

Top 5 Common Through-Hole PCB Assembly Issues and Solutions

Issue 1: Incomplete Solder Fill in Through-Holes

Root Causes:

• Insufficient solder temperature
• Too short soldering duration
• Mismatch between hole diameter and lead size
• Poor solder fluidity

Solutions:

1. Optimize wave soldering parameters: Increase solder temperature to 250-260°C, extend contact time to 3-5 seconds
2. Ensure the hole diameter is 0.1-0.3mm larger than the lead diameter
3. Use flux with appropriate activity to improve wettability
4. For manual soldering, employ the “feed solder” technique to ensure complete hole fill

Issue 2: Difficult or Damaged Component Insertion

Root Causes:

• PCB drilling position deviation
• The hole diameter is too small
• The deformed component leads
• Improper insertion machine calibration

Solutions:

1. Strengthen PCB manufacturing quality control to ensure drilling accuracy
2. Regularly check and adjust insertion machine positioning systems
3. Perform lead forming on components
4. Implement first-article inspection to identify and correct issues promptly

Issue 3: Solder Bridging or Excessive Solder After Assembly

Root Causes:

• Excessive solder temperature
• Insufficient flux activity
• Inadequate component spacing
• Improper wave height

Solutions:

1. Adjust wave soldering parameters: Lower temperature or reduce contact time
2. Switch to higher activity flux
3. Optimize component layout to increase critical spacing
4. Control wave height to 1/2-2/3 of PCB thickness
5. For existing bridges, use solder wick or rework tools

Issue 4: Loose Components or Misalignment After Soldering

Root Causes:

• Incomplete component insertion
• Excessive clearance between leads and holes
• Unsecured components before soldering
• Wave impact causing displacement

Solutions:

1. Ensure components are fully inserted and flush with the PCB
2. For heavy components, use a temporary adhesive before soldering
3. Optimize wave soldering fixture design to minimize mechanical impact
4. Implement in-process inspection to catch alignment issues early

Issue 5: Heat-Sensitive Component Damage During Soldering

Root Causes:

• Excessive soldering temperature
• No protection for heat-sensitive components
• Prolonged soldering duration

Solutions:

1. Use manual soldering for sensitive components with controlled local heating
2. Apply heat sinks or thermal clamps to protect components
3. Adjust soldering sequence – solder sensitive components last
4. Select low-temperature solder alloys (e.g., Sn-Bi)
5. When necessary, use rework stations for localized heating

Future Trends in Through-Hole PCB Assembly

Although surface mount technology dominates, through-hole assembly continues evolving:

1. High-Density Through-Hole: Smaller holes (0.2-0.3mm) and higher precision drilling increase assembly density
2. Selective Soldering Systems: Precisely solder only through-hole portions of mixed-technology boards, reducing thermal stress
3. Increased Automation: Smarter automatic insertion machines and inspection systems improve throughput
4. Advanced Materials: High-thermal-conductivity PCB materials and novel solders enhance thermal performance

As professional PCB assemblers, we recommend that clients select the most appropriate technology based on product characteristics and application environment. For applications demanding high reliability, strong mechanical connections, and superior power handling, through-hole technology remains indispensable.

Why Choose Our Through-Hole PCB Assembly Services?

• 17 years of through-hole assembly experience with thousands of different designs
• Equipped with high-precision automatic insertion machines and selective soldering systems
• Strict quality control system with defect rates below 0.1%
• Comprehensive services from design support to final testing
• Flexible capacity from prototyping to mass production

Whether your project requires pure through-hole assembly or mixed technology, our engineering team provides expert advice and high-quality manufacturing. Contact us for free technical consultation and quotes.

Recommended Reading

Surface Mount Technology (SMT)