Reliable printed circuit boards are essential for the consistent operation and long-term performance of electronic products. Although Surface Mount Technology (SMT) has become the dominant assembly method for compact and high-volume electronics, Through-Hole PCB Assembly remains an important manufacturing solution for applications that require high mechanical strength, excellent durability, strong electrical connections, and reliable performance under demanding operating conditions.

Through-Hole PCB Assembly, also known as THT Assembly or PTH PCB Assembly, involves inserting component leads through pre-drilled and plated holes in a printed circuit board (PCB) and soldering them to conductive pads. Because the component leads pass through the PCB, the resulting solder joints provide strong mechanical anchoring and excellent resistance to vibration, shock, thermal cycling, and repeated mechanical stress.

Today, Through-Hole Technology (THT) is widely used in industrial automation, automotive electronics, aerospace and defense, medical equipment, power electronics, telecommunications, and other applications where reliability and mechanical stability are critical.

As an experienced PCB manufacturer and PCB assembly service provider, Kingda provides PCB prototyping, rapid PCB manufacturing, Through-Hole PCB Assembly, SMT assembly, and complete PCBA solutions. Our services support customers from initial design verification and prototype development through low-volume production and volume manufacturing.

What Is Through-Hole PCB Assembly?

Through-Hole PCB Assembly is a PCB assembly process in which electronic components with leads are inserted through plated through-holes (PTHs) in a PCB. After insertion, the leads are soldered to conductive pads to create secure electrical and mechanical connections.

Unlike SMT components, which are mounted directly onto surface pads, through-hole components extend through the PCB. This physical connection provides additional mechanical support, making THT particularly suitable for large, heavy, high-power, or mechanically stressed components.

Typical through-hole components include:

• Connectors
• Terminal blocks
• Transformers
• Relays
• Switches
• Large electrolytic capacitors
• Inductors
• Power resistors
• Power transistors
• Fuse holders
• Heat sinks
• High-current components

Although Through-Hole PCB Assembly generally requires more board space and additional drilling compared with SMT, it remains an effective solution when mechanical strength, serviceability, and high-current handling are more important than maximum component density.

How Does Through-Hole Assembly Work?

The basic THT assembly process consists of several controlled manufacturing stages:

1. PCB and manufacturing data verification
2. Component preparation and insertion
3. Wave soldering, selective soldering, or hand soldering
4. Cleaning when required
5. Visual and automated inspection
6. Electrical and functional testing
7. Final inspection and packaging

The exact process depends on the component mix, PCB design, production volume, soldering requirements, and product reliability requirements.

What Is THT in Through-Hole PCB Assembly?

THT stands for Through-Hole Technology, a component mounting method in which component leads pass through holes in a PCB and are soldered to pads on the opposite side.

THT is one of the traditional PCB assembly technologies and was widely used before SMT became mainstream. While SMT has transformed electronics manufacturing by enabling miniaturization and high-density component placement, THT remains highly relevant for components that require additional mechanical support or higher power-handling capability.

Through-Hole Technology vs. SMT

The main difference between THT and SMT assembly is how components are mechanically attached to the PCB.

SMT components are placed directly onto PCB surface pads, allowing extremely high component density and automated high-speed placement. THT components, by contrast, are inserted through drilled holes, creating a stronger mechanical connection.

For many modern products, the best solution is not choosing one technology exclusively. Instead, manufacturers combine SMT and THT assembly in a mixed-technology manufacturing process.

For example, small integrated circuits, resistors, capacitors, and other compact components can be assembled using SMT, while connectors, transformers, switches, relays, and power components can be assembled using THT.

This hybrid approach allows designers to balance miniaturization, assembly efficiency, mechanical strength, and product reliability.

Types of Through-Hole Components

Understanding the different types of THT components helps engineers select appropriate footprints, optimize PCB layouts, and improve assembly efficiency.

Axial Lead Components

Axial components have leads extending from opposite ends of the component body. They are normally mounted horizontally on the PCB.

Common axial components include:

• Resistors
• Diodes
• Inductors
• Certain capacitors

Their straightforward structure makes them relatively easy to insert, solder, replace, and troubleshoot during prototype development and repair.

Radial Lead Components

Radial components have two or more leads extending from the same side of the component body. Depending on the design, they can be installed vertically or horizontally.

Common radial components include:

• Electrolytic capacitors
• LEDs
• Relays
• Sensors
• Certain inductors

Radial configurations can provide efficient mechanical support while reducing the distance between the component body and the PCB.

Specialized Through-Hole Components

Some components are particularly well suited to Through-Hole PCB Assembly because of their size, weight, electrical characteristics, or mechanical requirements.

These include:

• Board-to-board connectors
• Wire-to-board connectors
• Terminal blocks
• Transformers
• Power switches
• Relays
• Heat sinks
• Fuse holders
• High-current terminals
• Large power devices

These components often experience mechanical forces, high current, or thermal stress that make through-hole mounting advantageous.

Through-Hole PCB Assembly Process

A reliable Through-Hole PCB Assembly process requires accurate manufacturing data, controlled component insertion, appropriate soldering parameters, and comprehensive inspection and testing.

1. PCB Design Review and Data Verification

Before production begins, the manufacturing team reviews the customer’s design and production documentation.

Typical manufacturing data includes:

• Gerber files
• PCB fabrication drawings
• Bill of Materials (BOM)
• Schematic files
• Assembly drawings
• Component specifications
• Pick-and-place data when applicable
• Drill files
• Special process requirements

Engineers verify hole sizes, component footprints, lead dimensions, component orientation, PCB clearances, soldering requirements, and assembly constraints.

A thorough DFM and DFA review can identify potential problems before production, reducing rework, material waste, and manufacturing delays.

2. PCB Preparation

The PCB must contain correctly fabricated plated through-holes that correspond to the component leads.

During PCB preparation, manufacturers verify:

• Hole diameter
• Hole position
• Plating condition
• Pad dimensions
• Component polarity markings
• Solder-side accessibility
• Board thickness
• Copper structure
• Clearance around THT components

Accurate PCB fabrication is especially important for THT assembly because incorrect hole dimensions can cause component insertion difficulties or unreliable solder joints.

3. Component Insertion

After PCB preparation, through-hole components are inserted into their designated locations.

There are two primary methods.

Manual Component Insertion

Manual insertion is commonly used for:

• Prototypes
• Engineering samples
• Low-volume production
• Complex assemblies
• Odd-form components
• Specialized components

Experienced operators place components according to assembly drawings, polarity markings, and BOM requirements.

Automatic Component Insertion

For larger production volumes, automated insertion equipment can improve consistency and throughput.

Depending on the component type, automated equipment may be used for axial or radial components. Proper machine programming, feeder setup, component orientation, and first-article verification are essential for reliable production.

4. THT Soldering

After component insertion, the leads must be securely soldered to the PCB.

The main through-hole soldering methods include wave soldering, selective soldering, and hand soldering.

Wave Soldering

Wave soldering is widely used for THT assembly in medium- and high-volume production.

The typical process includes:

1. Flux application
2. Preheating
3. Contact with molten solder
4. Through-hole solder filling
5. Cooling

The molten solder contacts the underside of the PCB and flows around the component leads, forming solder joints between the leads and PCB pads.

Wave soldering provides high throughput and consistent results when process parameters are properly controlled.

Selective Soldering

Selective soldering uses a controlled solder nozzle to solder specific through-hole joints individually.

It is particularly useful for mixed-technology boards where:

• SMT components have already been reflow soldered
• Only selected THT joints require soldering
• Sensitive SMT components are located near THT components
• Wave soldering would expose the entire PCB to unnecessary thermal stress

Selective soldering can provide precise process control while reducing solder consumption and minimizing thermal exposure.

Hand Soldering

Hand soldering remains valuable for prototypes, repairs, engineering changes, low-volume production, and unusual components that are difficult to process automatically.

Experienced technicians can perform localized soldering while controlling solder volume, heating time, and component orientation.

5. Cleaning

After soldering, flux residues and other contaminants may remain on the PCB assembly.

Depending on the selected soldering process and flux chemistry, cleaning may or may not be required.

When cleaning is specified, manufacturers may use:

• Deionized water
• Aqueous cleaning systems
• Approved cleaning solvents
• Specialized PCB cleaning equipment

The cleaning process must be compatible with the PCB materials and components. Proper cleaning can help reduce contamination-related reliability problems and prepare assemblies for conformal coating or additional testing.

6. Inspection and Quality Control

Inspection is an essential part of Through-Hole PCB Assembly.

Manufacturers may use several inspection methods, including:

Visual Inspection

Operators inspect:

• Component orientation
• Lead insertion
• Solder coverage
• Solder bridges
• Cold solder joints
• Excess solder
• Component damage
• PCB contamination

Automated Optical Inspection (AOI)

AOI inspection uses high-resolution cameras and image-processing algorithms to identify visible assembly defects.

AOI can detect:

• Missing components
• Incorrect component orientation
• Component misalignment
• Solder bridges
• Insufficient solder
• Lead-related defects

AOI is particularly valuable for mixed SMT/THT assemblies.

X-Ray Inspection

X-ray inspection provides non-destructive analysis of solder joints and internal structures that cannot be completely evaluated through conventional visual inspection.

It can be used to identify:

• Incomplete solder fill
• Voids
• Hidden solder defects
• Internal connection problems
• Certain defects in complex assemblies

X-ray inspection is especially useful for high-reliability products and assemblies containing both THT and complex SMT packages.

7. Electrical and Functional Testing

Visual inspection alone cannot confirm whether the finished PCBA operates correctly.

In-Circuit Testing

ICT (In-Circuit Testing) uses test probes to evaluate electrical characteristics and identify manufacturing defects such as:

• Open circuits
• Short circuits
• Incorrect component values
• Incorrect connections
• Certain component-level faults

ICT is particularly effective for repeat production where dedicated test fixtures can be justified.

Flying Probe Testing

Flying Probe Testing (FPT) uses movable probes to contact designated test points without requiring a dedicated bed-of-nails fixture.

It is well suited to:

• Prototypes
• New product introductions
• Low-volume production
• Frequently revised designs

Functional Testing

Functional testing verifies whether the completed PCBA performs according to its intended operating specifications.

Depending on the product, functional testing may include:

• Power-up verification
• Voltage and current measurement
• Communication interface testing
• Sensor testing
• Signal measurement
• Input/output verification
• System-level functional checks

For critical products, functional testing can be combined with burn-in or other reliability testing.

Advantages of Through-Hole PCB Assembly

Despite the rapid development of SMT, Through-Hole PCB Assembly continues to offer several important advantages.

Superior Mechanical Strength

One of the biggest advantages of THT Assembly is its strong mechanical connection.

Because component leads pass through the PCB, the resulting solder joints provide additional anchoring compared with components mounted only on surface pads.

This makes THT suitable for products exposed to:

• Mechanical vibration
• Shock
• Repeated connector insertion
• Physical impact
• Thermal cycling

High Reliability

Through-hole components can provide highly robust electrical and mechanical connections when the PCB layout, component selection, soldering process, and inspection requirements are properly controlled.

This is why through-hole assembly remains common in aerospace, automotive, industrial, power, and other reliability-sensitive applications.

Better Power and Thermal Handling

Large THT component leads can provide robust electrical paths and mechanical support for components that handle relatively high current or dissipate significant heat.

Typical examples include:

• Power connectors
• Transformers
• Power resistors
• Relays
• Large capacitors
• Power transistors

Easier Repair and Replacement

Through-hole components are generally easier to access, desolder, and replace than very small or densely packed SMT components.

This is particularly useful for:

• Prototype development
• Engineering debugging
• Field service
• Maintenance
• Repairable industrial equipment

Excellent for Prototyping

During early product development, engineers frequently need to modify circuits, replace components, and troubleshoot connections.

The larger package sizes and accessible leads of THT components can make through-hole prototypes easier to inspect and modify.

Disadvantages of Through-Hole PCB Assembly

Although THT has important advantages, it also has limitations that designers should consider.

Lower Component Density

Through-hole components require drilled holes and generally occupy more PCB area than equivalent SMT components.

This can limit miniaturization and routing density.

Additional Drilling Requirements

THT assembly requires holes to be drilled and plated during PCB fabrication. This adds manufacturing steps compared with surface-mount-only designs.

Slower Assembly

Automated SMT equipment can place very large numbers of components at high speed. THT insertion and soldering generally have lower throughput, particularly when manual insertion is required.

Higher Manufacturing Cost for Some Designs

Additional drilling, plating, component insertion, soldering, and inspection requirements can increase manufacturing costs, particularly for highly dense designs with many through-hole components.

For this reason, engineers often use a mixed SMT and THT assembly process to optimize overall manufacturing efficiency.

Through-Hole vs. SMT Assembly Comparison

The choice between Through-Hole Assembly and SMT Assembly depends on the mechanical, electrical, environmental, and manufacturing requirements of the product.

Feature	Through-Hole Assembly	SMT Assembly
Mechanical Strength	Excellent	Good
Component Density	Lower	Very High
Board Miniaturization	Limited	Excellent
Manufacturing Speed	Moderate	Very High
High-Volume Efficiency	Good	Excellent
Power Handling	Excellent for many large components	Good to Excellent depending on package
Repairability	Generally Easier	More Difficult for fine-pitch parts
Vibration Resistance	Excellent	Good
Component Size	Medium to Large	Small to Very Small
Automation	Moderate to High	Very High
Typical Applications	Industrial, power, automotive, aerospace	Consumer, communications, computing

The comparison does not mean that one technology is universally better than the other. In modern electronics manufacturing, SMT and THT are often used together.

Applications of Through-Hole PCB Assembly

Through-Hole PCB Assembly remains important across many industries where reliability, mechanical strength, power handling, and serviceability are priorities.

Industrial Automation

Industrial control systems, motor drives, power supplies, controllers, and factory equipment may use THT components because they must withstand continuous operation, vibration, and demanding environmental conditions.

Automotive Electronics

Automotive electronics can experience significant vibration, temperature cycling, and mechanical stress. THT is therefore useful for selected connectors, power components, relays, and other mechanically demanding components.

Aerospace and Defense

Aerospace and defense electronics require high reliability and stable performance under demanding conditions. Through-hole components can provide strong mechanical connections for selected critical assemblies.

Medical Equipment

Medical electronic systems often require high reliability and consistent manufacturing quality. THT may be used for connectors, power components, transformers, and other components where mechanical robustness is important.

Power Electronics

Power supplies, energy conversion equipment, motor controllers, industrial drives, and related systems often contain large components that benefit from through-hole mounting.

Telecommunications and Audio Equipment

Professional audio equipment, communication systems, and certain telecommunications products may combine SMT and THT to achieve both compact circuit layouts and robust mechanical mounting.

When Should You Use Through-Hole Assembly?

Through-Hole PCB Assembly is particularly appropriate when one or more of the following conditions apply:

Harsh Operating Environments

THT is suitable for products exposed to vibration, mechanical shock, thermal cycling, and other demanding operating conditions.

High-Power Applications

Large power components, transformers, high-current connectors, and power devices can benefit from the mechanical support and robust electrical connections provided by through-hole mounting.

Frequent Maintenance or Repair

For field-serviceable equipment, THT components can be easier to access and replace.

Prototyping and Product Development

Through-hole components are often easier for engineers to handle, inspect, measure, replace, and modify during early-stage product development.

Mixed-Technology PCB Designs

A combination of SMT and THT assembly is often the most practical solution for modern electronic products.

SMT can handle compact and high-density components, while THT can be used for connectors, switches, transformers, relays, and power components.

Quality Control and Testing for THT Assembly

A reliable Through-Hole PCB Assembly process requires quality control throughout the entire manufacturing cycle rather than relying only on final inspection.

At Kingda, quality management can be integrated into each stage of the manufacturing process, from engineering review and PCB fabrication to component procurement, assembly, inspection, testing, and final shipment.

Incoming Quality Control

Components and materials are checked before entering production to verify specifications, quantities, packaging condition, and other customer requirements.

Process Quality Control

Critical process parameters such as soldering temperature, solder contact time, flux application, insertion accuracy, and equipment conditions are monitored to maintain consistent production quality.

Final Inspection

Finished PCB assemblies are inspected for component placement, solder quality, workmanship, contamination, labeling, and other customer-defined requirements.

Electrical Testing

Depending on the project, Kingda can support ICT, Flying Probe Testing, functional testing, and other electrical verification methods to identify potential assembly and electrical defects before shipment.

Traceability

For projects requiring detailed production records, manufacturing information can be managed through appropriate documentation and traceability procedures to support quality analysis, troubleshooting, and future production.

Why Choose Kingda for Through-Hole PCB Assembly?

As an experienced PCB manufacturer and PCB assembly service provider, Kingda provides integrated manufacturing support for customers requiring reliable PCB and PCBA solutions.

Our capabilities are designed to support the complete product development cycle, from initial engineering verification to production.

Integrated PCB and PCBA Services

Kingda can support customers with:

• PCB Prototype Manufacturing
• Rapid PCB Prototyping
• Through-Hole PCB Assembly
• SMT PCB Assembly
• Mixed-Technology PCBA
• Component sourcing and procurement
• PCB fabrication
• Assembly inspection
• Electrical testing
• Small-batch production
• Volume manufacturing

This integrated approach can simplify supplier management and improve coordination between PCB fabrication and assembly.

Engineering Support

Kingda provides engineering support during the manufacturing preparation stage, helping identify potential PCB design, component, assembly, and manufacturability issues before production.

DFM and DFA reviews can help reduce unnecessary redesigns and improve the transition from prototype to production.

Flexible Production for Different Project Stages

Different electronic products require different production volumes.

Kingda supports customers through multiple stages, including:

PCB Design → DFM Engineering Review → PCB Prototype Manufacturing → PCB Assembly → Inspection & Testing → Design Optimization → Small-Batch Production → Volume Manufacturing

This allows customers to work with one manufacturing partner as their product moves from development to commercial production.

Complete Manufacturing Solutions

Rather than treating PCB fabrication and assembly as isolated processes, Kingda provides coordinated PCB and PCBA manufacturing solutions.

This approach helps customers manage:

• PCB fabrication
• Component procurement
• SMT assembly
• THT assembly
• Mixed assembly
• Quality inspection
• Electrical testing
• Packaging
• Production delivery

Reliability-Focused Manufacturing

For products that depend on mechanical strength, stable electrical connections, and long-term performance, Kingda can help select an appropriate combination of PCB materials, component mounting methods, soldering processes, inspection methods, and testing procedures.

Conclusion

Through-Hole PCB Assembly remains an important technology in modern electronics manufacturing. Although SMT dominates highly miniaturized and high-volume electronic products, THT Assembly continues to provide significant advantages when mechanical strength, durability, power handling, repairability, and long-term reliability are critical.

Through-hole components are particularly suitable for connectors, transformers, relays, switches, large capacitors, power devices, and other components that require strong mechanical support.

The most effective manufacturing strategy is often not to choose between SMT and THT, but to combine both technologies according to the requirements of the product. A well-designed mixed-technology PCB assembly can achieve high component density while maintaining strong mechanical connections and reliable power-stage components.

As an experienced PCB manufacturer and PCB assembly service provider, Kingda offers PCB prototyping, rapid PCB manufacturing, SMT assembly, Through-Hole PCB Assembly, and complete PCBA manufacturing solutions. By supporting customers from initial design verification through prototype development, small-batch production, and volume manufacturing, Kingda helps transform PCB designs into reliable electronic products.