Although Surface Mount Technology (SMT) has become the dominant PCB assembly method for many modern electronics products, Through-Hole Technology (THT) remains an important manufacturing process.
THT assembly is widely used for components that require strong mechanical connections, higher durability, or specific electrical and thermal characteristics. Common examples include connectors, transformers, relays, terminals, switches, large capacitors, and power components.
For PCB assembly factories, understanding what THT assembly is and how it works is essential when designing a complete PCBA production line.
This article explains the THT assembly process, common equipment, advantages, applications, and the differences between THT and SMT.
What Is THT Assembly and How Does It Work?
THT assembly, or Through-Hole Technology assembly, is a PCB assembly method in which component leads are inserted through holes drilled in a printed circuit board.
Unlike surface-mount components, which are mounted directly onto PCB pads, THT components use leads that pass through plated or non-plated holes in the PCB.
After insertion, the component leads are soldered to the corresponding pads, creating both an electrical connection and a mechanical connection.
THT assembly is also commonly associated with DIP assembly, although the terms are not completely interchangeable.
DIP generally refers to Dual In-line Package components or the manufacturing process commonly called Dual In-line Package assembly, while THT is the broader technology category covering many different through-hole components.
How Does THT Assembly Work?
A typical THT assembly process can include the following stages:
PCB Preparation → Component Preparation → THT/DIP Insertion → Inspection → Flux Application → Preheating → Wave Soldering → Cooling → Inspection → Testing
Depending on the product, some processes may be performed manually, semi-automatically, or fully automatically.
The exact process depends on PCB design, component type, production volume, quality requirements, and factory automation level.
Step 1: PCB Preparation
Before THT components are inserted, PCBs should be inspected to confirm that they meet production requirements.
Important checks may include:
- PCB dimensions
- Hole locations
- Pad condition
- Surface finish
- PCB cleanliness
- Board orientation
- Panelization
- Component identification
Correct PCB preparation helps prevent downstream insertion and soldering problems.
For high-volume production, PCBs may be supplied in panels to improve handling efficiency.
Step 2: Component Preparation
THT components are prepared according to the bill of materials and production requirements.
Components may arrive in different packaging formats, including:
- Bulk
- Tape
- Tray
- Tube
- Reel
Some components may require lead forming or cutting before insertion.
Proper component preparation is particularly important when automated insertion equipment is used.
Component dimensions, lead spacing, lead length, and packaging must be compatible with the selected insertion process.
Step 3: THT Component Insertion
The next stage is inserting component leads through the appropriate PCB holes.
Depending on production volume and component type, insertion can be:
- Manual
- Semi-automatic
- Automatic
Manual insertion is often suitable for low-volume or high-mix production.
Automatic or semi-automatic insertion can improve consistency and productivity for suitable high-volume products.
During insertion, component orientation and polarity must be controlled carefully.
Incorrect insertion can lead to electrical failure even when the solder joint itself appears acceptable.
Step 4: Insertion Inspection
After components are inserted, the PCB should be inspected before soldering.
Typical inspection points include:
- Component presence
- Component orientation
- Polarity
- Insertion position
- Lead length
- Component height
- Missing components
- Incorrect components
Identifying insertion problems before soldering can reduce unnecessary rework later in the process.
Step 5: Flux Application
Flux is commonly used before wave soldering to help remove surface oxides and promote solder wetting.
In an automated THT production line, flux may be applied using a controlled spraying system.
The objective is to achieve appropriate and consistent coverage rather than simply applying a large amount of flux.
Flux performance depends on the selected material, PCB design, component condition, application method, and soldering process.
Step 6: Preheating
After flux application, the PCB enters the preheating section.
Preheating helps prepare the assembly for contact with molten solder and supports appropriate flux activation.
The required thermal conditions depend on factors such as:
- PCB thickness
- Copper distribution
- Component thermal mass
- Flux characteristics
- Solder alloy
- Conveyor speed
- PCB construction
A suitable thermal profile should be developed for the actual product rather than using the same settings for every PCB.
Step 7: Wave Soldering
Wave soldering is one of the most important processes in high-volume THT assembly.
The PCB passes over a controlled wave of molten solder. The exposed component leads and PCB solder pads come into contact with the solder, forming electrical and mechanical connections.
A typical wave soldering process needs control of:
- Solder temperature
- Conveyor speed
- Flux application
- Preheating
- Wave height
- Solder contact
- PCB orientation
- Solder condition
Poor process control can result in defects such as solder bridges, insufficient solder, poor wetting, solder icicles, and incomplete hole fill.
For this reason, wave soldering should be treated as a controlled manufacturing process rather than simply a heating and soldering operation.
Step 8: Cooling
After soldering, the PCB is cooled under controlled conditions.
Cooling allows solder joints to solidify and helps prepare the board for subsequent inspection and testing.
Cooling conditions should be appropriate for the PCB assembly and the manufacturing process.
Step 9: Post-Solder Inspection
After wave soldering, the PCB is inspected to identify potential assembly or soldering defects.
Depending on the product and quality requirements, inspection may include:
- Visual inspection
- AOI where applicable
- Through-hole solder joint inspection
- X-ray for selected applications
- Electrical testing
- Functional testing
Inspection requirements vary depending on component type, PCB design, product reliability requirements, and applicable industry standards.
Step 10: Testing and Downstream Assembly
Following inspection, the assembled PCB may proceed to electrical or functional testing.
Some products may also require additional processes such as:
- Rework
- Conformal coating
- Programming
- Final assembly
- Labeling
- Packaging
The THT process is therefore normally part of a larger PCBA production line rather than a standalone manufacturing operation.
What Components Are Used in THT Assembly?
THT components are often selected when mechanical strength, power handling, thermal performance, or specific component construction is important.
Common examples include:
Connectors
Connectors frequently use through-hole mounting because the mechanical connection may need to withstand repeated insertion and removal.
Transformers
Certain transformers and magnetic components use through-hole mounting because of their size, construction, and mechanical requirements.
Relays
Relays can require strong PCB connections because of their mechanical structure and application requirements.
Large Capacitors
Large electrolytic capacitors and other high-profile components may use through-hole mounting.
Terminals and Switches
Terminal blocks, switches, and similar components may benefit from the mechanical strength of through-hole connections.
Power Components
Certain power-related components can use THT packages to support mechanical stability and thermal requirements.
The appropriate mounting technology depends on the component and product design.
What Equipment Is Used in a THT Production Line?
A typical THT PCB assembly line may include:
PCB Loader → Component Preparation → THT/DIP Insertion → Inspection → Fluxer → Preheater → Wave Soldering Machine → Cooling → Inspection → Testing
Depending on the factory, additional equipment may include:
- PCB conveyors
- Automatic insertion machines
- Lead forming machines
- Lead cutting machines
- Magazine loaders and unloaders
- AOI systems
- Testing equipment
- Rework stations
- PCB handling systems
The configuration should be based on production volume, component mix, PCB dimensions, labor availability, and automation requirements.
THT Assembly vs SMT Assembly
THT and SMT are two major PCB assembly technologies, but they use different component mounting methods.
In modern electronics manufacturing, THT and SMT are often used together.
A PCB may use SMT for most components and THT for selected connectors, terminals, transformers, or other components requiring through-hole mounting.
Why Is THT Still Important?
Despite the growth of SMT, through-hole PCB assembly continues to have several advantages.
Strong Mechanical Connection
Because component leads pass through the PCB, THT components can provide strong mechanical attachment.
This can be valuable for connectors, switches, terminals, and components subject to mechanical stress.
Suitable for Large Components
Some large or heavy components are more practical to mount using through-hole technology.
Suitable for Certain Power Applications
THT components can be appropriate for selected power and high-current applications, depending on product design and component specifications.
Long-Established Manufacturing Process
THT has been used in electronics manufacturing for decades, and many manufacturers have established processes, equipment, and technical expertise for it.
Easy Visual Identification
Many THT components are relatively large and accessible, making manual identification and visual inspection comparatively straightforward.
What Are the Limitations of THT Assembly?
THT is not suitable for every PCB.
Potential limitations include:
- Larger component footprint
- PCB holes consume routing space
- Generally lower placement density
- More manual labor for certain products
- Potentially lower production speed than SMT
- Additional soldering processes
- More limited suitability for very small components
For high-density consumer electronics, SMT is often the primary assembly method.
For products requiring larger mechanical components or strong through-board connections, THT can remain highly valuable.
Can SMT and THT Be Used on the Same PCB?
Yes. This is extremely common.
A mixed-technology PCB may use SMT for small and high-density components while using THT for components that require stronger mechanical connections.
A typical mixed assembly process could be:
Solder Paste Printing → SPI → SMT Placement → Reflow → AOI → THT/DIP Insertion → Fluxing → Wave/Selective Soldering → Inspection → Testing
The exact process sequence depends on PCB design and component requirements.
This combination allows manufacturers to take advantage of both SMT and THT technologies.
How to Improve THT Assembly Quality
Stable THT production depends on controlling the entire process.
Manufacturers should pay attention to:
Component Preparation
Make sure component dimensions, lead lengths, and packaging are suitable for the insertion process.
Insertion Accuracy
Control component position, polarity, orientation, and lead insertion.
PCB Design
Consider hole size, pad design, component spacing, solder drainage, and wave soldering direction during PCB design.
Flux Control
Maintain consistent flux application and use materials compatible with the soldering process.
Thermal Control
Develop appropriate preheating and soldering conditions for each PCB.
Wave Soldering
Monitor solder temperature, conveyor speed, wave stability, and solder condition.
Inspection
Use appropriate inspection and testing methods to detect insertion and soldering problems.
Equipment Maintenance
Regularly maintain insertion, transportation, fluxing, preheating, and wave soldering equipment.
A standardized process reduces variation and helps maintain consistent production quality.
THT Assembly in a Complete PCBA Production Line
THT assembly is often integrated into a larger SMT and PCBA manufacturing process.
A complete production flow may look like:
PCB Loading → Solder Paste Printing → SPI → SMT Placement → Reflow → AOI → THT/DIP Insertion → Wave Soldering → Inspection → Testing → Conformal Coating → Final Assembly
Not every factory requires every stage.
The correct configuration depends on the product and manufacturing requirements.
For example, a PCB containing only SMD components may not require THT insertion or wave soldering. A PCB with connectors and large through-hole components may require both SMT and THT processes.
This is why production line planning should begin with the product’s PCB and BOM rather than a standard equipment list.
THT and DIP Production Line Solutions from Fuliu Electronics
Established in 2014, Fuliu Electronics is dedicated to providing customers with high-quality PCBA intelligent manufacturing solutions and services.
With extensive experience in the SMT industry, we specialize in dependable equipment, professional technical support, and complete solutions for SMT, DIP/THT, and PCBA production lines.
Our product portfolio includes SMT pick and place machines from Fuji, Panasonic, ASM, Yamaha, JUKI, and Hanwha, along with ERSA reflow ovens from Germany, MagicRay SPI/AOI inspection systems, automatic solder paste printers, supporting SMT equipment, pre-owned SMT equipment, SMT spare parts, machine leasing, and maintenance services.
For manufacturers building or upgrading a DIP production line, Fuliu Electronics can help evaluate the relationship between SMT, THT insertion, soldering, inspection, PCB handling, and other production processes.
Rather than selecting equipment individually, customers can develop a production line configuration based on PCB dimensions, BOM, component types, production capacity, factory layout, automation requirements, and investment plans.
As part of our global growth strategy, Fuliu Electronics is actively expanding into international markets, with key focus areas including India, Vietnam, the Philippines, Indonesia, the Middle East, South Africa, and Europe.
Conclusion: Understanding the Role of THT Assembly
THT assembly remains an important part of modern PCB manufacturing, particularly for components that require mechanical strength, larger package sizes, or specific electrical and thermal characteristics.
While SMT is often the preferred technology for high-density electronic assemblies, THT provides capabilities that complement surface-mount technology.
For many PCBA manufacturers, the most effective solution is not choosing between SMT and THT, but integrating both technologies into a balanced manufacturing process.
If you are planning a new THT/DIP production line, upgrading an existing PCB assembly factory, or developing a mixed SMT and THT production line, Fuliu Electronics can help evaluate your PCB specifications, component requirements, production capacity, and automation goals.
Provide your PCB dimensions, BOM, component types, target production capacity, factory layout, existing equipment, and automation requirements to start planning a suitable SMT and THT PCBA production line solution.
Frequently Asked Questions
THT assembly, or Through-Hole Technology assembly, is a PCB assembly method in which component leads are inserted through holes in a PCB and then soldered to form electrical and mechanical connections.
A typical THT assembly process includes PCB preparation, component preparation, through-hole insertion, inspection, flux application, preheating, wave or selective soldering, cooling, inspection, and testing.
The exact process varies by product and production requirements.
THT is the broader technology category for components mounted through PCB holes. DIP commonly refers to Dual In-line Package components or the associated production process.
Therefore, DIP assembly can be considered a type of THT assembly, but not all THT components are DIP components.
THT components have leads that pass through PCB holes, while SMT components are mounted directly onto surface pads.
SMT generally supports higher component density and automation, while THT is often preferred for components requiring strong mechanical connections or specific power and mechanical characteristics.
Yes. THT remains widely used in products containing connectors, terminals, relays, transformers, switches, large capacitors, and certain power components.
Many modern PCBs combine both SMT and THT technologies.
Wave soldering is widely used for high-volume THT production. Selective soldering and manual soldering may also be used depending on PCB design, production volume, and component requirements.