What is through-hole soldering in PCB assembly

What Is Through-Hole Soldering?

Through-hole soldering is a PCB assembly process in which electronic components with wire leads are inserted into holes drilled through the printed circuit board and soldered to pads on the opposite side. The solder forms both an electrical connection and a strong mechanical bond that anchors the component to the board. This method has been used in electronics manufacturing since the 1950s and remains widely employed today for components that require superior mechanical strength, high current capacity, or easy replaceability.

The term "through-hole" refers to the fact that the component leads pass completely through the board and are soldered on the opposite side, creating a connection that is significantly more robust than surface-mount technology (SMT) joints, which rely solely on solder adhesion to the board surface.

Through-Hole vs. Surface-Mount Technology

To understand where through-hole soldering fits in modern manufacturing, it is helpful to compare it with SMT, the dominant assembly method for most consumer electronics today.

In practice, most modern electronic products use a mixed-technology approach: SMT for the majority of components to maximise density and automation, with through-hole soldering reserved for connectors, power components, large capacitors, relays, transformers, and other parts that experience mechanical stress or carry high current.

The Three Main Through-Hole Soldering Methods

1. Manual Soldering

Manual soldering uses a soldering iron operated by a skilled technician to create individual solder joints. The process involves heating the component lead and the PCB pad simultaneously, then feeding solder wire into the joint to form a strong conductive bond. This method is ideal for:

• Prototype and low-volume production runs
• Rework and repair of assembled boards
• Components that are difficult to access with automated equipment
• Specialised or sensitive components requiring careful heat control

Manual soldering offers maximum flexibility with minimal setup cost, but it is labour-intensive and the quality depends heavily on the technician's skill and consistency.

2. Wave Soldering

Wave soldering is an automated process designed for high-volume through-hole assembly. The populated PCB is passed over a continuously flowing wave of molten solder, which contacts the component leads and pads on the underside of the board. The process typically includes flux application, preheating, the solder wave itself, and a cooling stage.

Wave soldering delivers high throughput and consistent joint quality for large production batches. It is particularly effective for boards with a high density of through-hole components. However, it requires careful control of parameters such as wave height, temperature, and conveyor speed to avoid defects like solder bridging or insufficient wetting.

3. Selective Soldering

Selective soldering targets specific through-hole joints with a programmable solder nozzle, applying flux and molten solder only where needed. This method is ideal for mixed-technology boards where SMT components are already assembled and only certain through-hole joints require soldering. It offers a balance of automation and precision, minimising thermal stress on nearby components and reducing the risk of defects.

The Through-Hole Soldering Process in Professional Manufacturing

At a professional EMS provider like Farway Electronic, the through-hole soldering (DIP) process follows a structured workflow to ensure consistent quality:

1. Component forming and preparation — leads are cut and bent to the required dimensions
2. Manual or automated insertion — components are placed into the drilled holes on the PCB
3. Wave soldering or selective soldering — the board is passed through the soldering process
4. Lead cutting — excess lead lengths are trimmed
5. Repair welding and touch-up — any imperfect joints are corrected by skilled operators
6. Board washing — flux residues and contaminants are removed
7. Functional testing — the assembled board is electrically tested to verify performance

Farway operates 2 dedicated DIP plug-in production lines equipped with Nitto wave-soldering equipment, supported by 24 rear-welding stations and a board-washing machine. This capacity enables the company to handle both prototype and volume production orders efficiently.

Quality Control in Through-Hole Soldering

Quality assurance is a critical element of any through-hole soldering service. Manufacturers that follow IPC-A-610 standards (the industry benchmark for electronic assembly acceptability) implement multiple inspection and testing stages:

• Plug-in visual inspection — verifies correct component placement and lead orientation before soldering
• AOI (Automated Optical Inspection) — detects solder defects, bridging, and insufficient wetting
• X-ray inspection — reveals hidden defects in solder joints, particularly for components with hidden leads
• ICT (In-Circuit Testing) — checks individual component values, opens, and shorts
• FCT (Functional Testing) — verifies the assembled board performs its intended function
• Thermal imaging inspection — identifies hot spots that may indicate poor solder connections
• High- and low-temperature reliability testing — ensures joints survive thermal cycling in the field

Farway Electronic, for example, maintains ISO 9001, ISO 13485 (medical devices), and IATF 16949 (automotive) quality management certifications, and implements IPC-A-610 as its assembly standard. These certifications and standards ensure that every through-hole solder joint meets rigorous industry requirements for reliability and performance.

Common Applications of Through-Hole Soldering

Through-hole soldering remains the preferred method for a wide range of applications across multiple industries:

• Automotive electronics — engine control units, window lift controllers, and infotainment systems that must withstand vibration and temperature extremes
• Industrial equipment — motor drives, power supplies, and PLC controllers requiring robust, serviceable connections
• Medical devices — diagnostic equipment, patient monitors, and imaging systems where reliability is critical
• New energy systems — solar inverters, battery management systems, and EV charging infrastructure
• Security and communications — surveillance equipment, base stations, and networking hardware
• Aerospace and defence — mission-critical electronics where failure is not an option

Choosing a Through-Hole Soldering Partner

When selecting a dip plug-in welding service provider, consider the following factors:

• Production capacity — number of DIP lines, wave-soldering machines, and finishing stations
• Quality certifications — ISO 9001, IATF 16949, ISO 13485, and UL recognition
• Inspection capabilities — AOI, X-ray, ICT, FCT, and thermal imaging
• Component sourcing — ability to manage procurement and BOM verification
• Mixed-technology experience — proven capability in combining SMT and through-hole on the same board
• Order flexibility — willingness to support prototype quantities through to high-volume production

Farway Electronic, based in Shenzhen, China, offers a comprehensive one-stop electronics manufacturing service that includes PCB fabrication, component management, SMT assembly, wave soldering service, conformal coating, PCBA low-pressure injection moulding, PCBA testing, and finished-product assembly. With 2 SMT lines, 2 DIP lines, 2000 square metres of production space, and a track record of serving over 100 customers across more than 20 countries, the company is well-positioned to support through-hole soldering requirements of any scale.

Conclusion

Through-hole soldering is far from obsolete. While SMT dominates in high-density, high-volume consumer electronics, through-hole technology remains indispensable for applications that demand mechanical strength, high-current capacity, thermal performance, and long-term reliability. By understanding the different soldering methods — manual, wave, and selective — and selecting a qualified manufacturing partner with the right certifications and inspection capabilities, engineers can ensure that their through-hole assemblies meet the highest standards of quality and dependability.