Surface Mount Technology (SMT) has become the dominant method for assembling modern electronic products, but through-hole PCB assembly remains essential for many components and applications.
Power connectors, transformers, relays, large capacitors, switches, terminals, inductors, and other components may require stronger mechanical connections or electrical characteristics that make through-hole mounting more suitable than surface mounting.
In a through-hole assembly process, component leads are inserted through pre-drilled holes in the printed circuit board and soldered on the opposite side.
The complete process may include component preparation, insertion, inspection, flux application, preheating, wave or selective soldering, lead processing, AOI, electrical testing, cleaning, and final quality control.
In this guide, we’ll explain how through-hole PCB assembly works, what equipment is required, how THT differs from SMT, and how manufacturers can build a reliable through-hole production process.
What Is Through-Hole PCB Assembly?
Through-Hole Technology (THT) is a PCB assembly method in which electronic component leads pass through holes in the PCB.
The leads are then soldered to conductive pads on the opposite side of the board, creating both:
- An electrical connection
- A mechanical connection
Through-hole assembly is widely used for components that may experience higher mechanical loads or require larger package sizes.
Common THT components include:
- Connectors
- Transformers
- Relays
- Electrolytic capacitors
- Power resistors
- Inductors
- Terminals
- Switches
- Large diodes
- Certain integrated circuits
Many modern PCB assemblies combine THT components with surface-mounted devices.
What Is the Difference Between THT and DIP?
THT and DIP are related terms but do not technically mean exactly the same thing.
THT (Through-Hole Technology) refers to the overall assembly method in which component leads pass through PCB holes.
DIP (Dual In-line Package) originally describes a component package with two parallel rows of pins.
However, in many electronics factories, the term DIP line is commonly used to describe the entire through-hole production area.
Therefore, you may encounter terms such as:
- THT production line
- DIP production line
- Through-hole assembly line
- DIP insertion line
when discussing similar PCB manufacturing processes.
Why Is Through-Hole Assembly Still Used?
Through-hole technology offers several important advantages.
Strong Mechanical Connections
Because component leads pass through the PCB, the resulting assembly can provide stronger mechanical retention than many surface-mounted connections.
This is valuable for:
- Connectors
- Switches
- Terminals
- Large components
that may experience mechanical force during use.
Suitable for Power Components
Large transformers, relays, capacitors, and power components are often more suitable for through-hole mounting.
Better for Certain Harsh Applications
THT components remain common in:
- Industrial electronics
- Automotive products
- Power equipment
- Telecommunications systems
- Heavy equipment
- Energy electronics
where mechanical robustness and long-term reliability are priorities.
Complete Through-Hole PCB Assembly Process
A typical process may follow:
PCB Preparation → Component Preparation → Component Insertion → Pre-Solder Inspection → Fluxing → Preheating → Wave/Selective Soldering → Cooling → Lead Processing → AOI → ICT/FCT → Cleaning if Required → Final Inspection
Let’s examine each stage.
Step 1: PCB Preparation
Before component insertion begins, the PCB should be verified for manufacturing quality.
Checks may include:
- PCB dimensions
- Hole dimensions
- Surface finish
- Contamination
- Warpage
- Damage
- Traceability information
The drilled holes must be compatible with the component leads.
Poor hole-to-lead relationships can make insertion difficult and affect soldering quality.
Step 2: Through-Hole Component Preparation
Some components can be inserted directly, while others require preparation.
Typical processes include:
- Lead forming
- Lead bending
- Lead cutting
- Component sorting
- Polarity verification
- Material identification
Lead forming should match PCB hole spacing without placing excessive mechanical stress on the component body.
Component preparation equipment can help improve consistency in higher-volume production.
Step 3: Component Insertion
Components are inserted into the PCB according to the bill of materials and assembly instructions.
There are several insertion methods.
Manual Insertion
Operators manually insert each component.
Advantages include:
- High flexibility
- Low equipment investment
- Suitable for irregular components
- Easy product changeover
Manual insertion is commonly used in high-mix, low-to-medium-volume production.
Semi-Automatic Insertion
Fixtures, conveyors, or assisted workstations help operators improve productivity and accuracy.
Automatic Insertion
Automatic insertion equipment can place compatible through-hole components at higher speed.
This approach is more suitable for stable high-volume production.
Step 4: Component Retention
Some components may need to be held in place before soldering.
Retention methods can include:
- Lead bending
- PCB fixtures
- Mechanical supports
- Component clinching
The objective is to prevent components from moving or falling during board transportation and soldering.
Step 5: Pre-Solder Inspection
Before the PCB reaches the soldering machine, manufacturers should verify that components were installed correctly.
Inspection may check:
- Component presence
- Correct part
- Orientation
- Polarity
- Seating height
- Insertion position
- Bent or damaged leads
Detecting errors at this stage is much easier than repairing them after soldering.
Inspection can be:
- Manual
- Camera-assisted
- Automated
depending on production requirements.
Step 6: Flux Application
If the board is processed using wave soldering, flux is applied to the solder side of the PCB.
Flux helps:
- Remove surface oxidation
- Promote solder wetting
- Reduce oxidation during soldering
- Improve joint formation
Flux application must be controlled carefully.
Too little flux can contribute to poor wetting, while excessive flux may leave unwanted residue or create process instability.
Step 7: Preheating
After flux application, the PCB travels through the preheat section.
Preheating helps:
- Activate flux
- Reduce thermal shock
- Prepare the board for solder contact
- Improve soldering stability
The correct thermal profile depends on:
- PCB thickness
- Board thermal mass
- Component density
- Flux chemistry
- Solder alloy
- Component limitations
Manufacturers should verify actual board temperatures during process development.
Step 8: Wave Soldering
Wave soldering is one of the most common methods for soldering multiple through-hole connections.
The PCB passes over a controlled wave of molten solder.
The solder contacts component leads and PCB pads on the underside of the assembly.
A successful process depends on parameters such as:
- Solder temperature
- Conveyor speed
- Wave height
- Contact time
- Flux application
- Preheat profile
- Board orientation
The objective is to form consistent, properly wetted solder joints without creating bridges or excessive solder.
What Is Selective Soldering?
Not every through-hole assembly is suitable for full wave soldering.
Selective soldering applies solder only to specific THT joints.
This can be useful when:
- The PCB contains many SMT components
- Only a small number of THT components are installed
- Sensitive components are located near solder joints
- The PCB geometry makes wave soldering difficult
A typical selective soldering process may include:
Selective Fluxing → Preheating → Localized Soldering
Selective soldering offers greater process control but may have different throughput and investment considerations than wave soldering.
Step 9: Cooling
After soldering, the assembly cools before downstream processing.
Controlled cooling helps stabilize the solder joints and enables safe board handling.
The required cooling process depends on the solder alloy and product.
Step 10: Lead Cutting
Some component leads may remain longer than required after soldering.
A PCB lead cutting machine can trim excess leads.
This improves:
- Board consistency
- Mechanical clearance
- Finished appearance
- Downstream assembly compatibility
Lead height should comply with the product’s design and quality requirements.
Step 11: Post-Solder Inspection
After soldering, the PCB must be inspected for assembly and solder defects.
Common issues include:
- Solder bridges
- Insufficient solder
- Poor wetting
- Open joints
- Excess solder
- Missing components
- Wrong polarity
- Component lifting
Inspection can include:
- Manual visual inspection
- AOI
- Additional specialized inspection where needed
What Is DIP/THT AOI?
A DIP or THT AOI system automatically inspects through-hole assemblies using cameras and image-processing software.
Depending on the machine and PCB design, it may detect:
- Missing components
- Incorrect orientation
- Wrong polarity
- Component displacement
- Solder bridges
- Visible solder quality defects
AOI provides faster and more consistent inspection than relying entirely on manual checking.
Step 12: ICT Testing
In-Circuit Testing (ICT) evaluates electrical characteristics of the assembled circuit.
It can help detect:
- Shorts
- Opens
- Incorrect component values
- Connection issues
ICT is particularly useful in higher-volume production where automated electrical verification is required.
Step 13: Functional Testing
Functional Circuit Testing (FCT) verifies whether the completed PCB performs as intended.
Testing may include:
- Power-on testing
- Voltage checks
- Signal testing
- Communication testing
- Functional output verification
FCT can identify issues that visual inspection or AOI cannot detect.
Step 14: PCB Cleaning
Depending on the soldering process and flux chemistry, cleaning may be required.
Cleaning can remove:
- Flux residue
- Ionic contamination
- Dust
- Processing residues
PCB cleanliness may be particularly important if the assembly later receives:
- Conformal coating
- High-voltage testing
- High-reliability qualification
Manufacturers should determine cleaning requirements based on the product and process.
Step 15: Final Inspection
Before the PCBA proceeds to packaging or final product assembly, a final quality check may verify:
- Component condition
- Solder joint appearance
- PCB cleanliness
- Labels
- Traceability
- Test status
Approved assemblies then move to the next manufacturing stage.
Through-Hole PCB Assembly Equipment List
The exact equipment list should be customized around actual production requirements.
Through-Hole PCB Assembly vs SMT
SMT and THT differ primarily in how components are mounted.
How SMT and Through-Hole Assembly Work Together
A modern PCB assembly process may include both technologies.
A typical workflow could be:
Solder Paste Printing → SPI → SMT Pick and Place → Reflow → AOI → THT Insertion → Wave/Selective Soldering → THT AOI → ICT/FCT → Cleaning → Conformal Coating
This hybrid process enables manufacturers to combine:
SMT efficiency and miniaturization
with
THT mechanical strength and power-handling capability.
Common Through-Hole Assembly Defects
Solder Bridges
Solder connects adjacent joints and may create an electrical short.
Insufficient Solder
The joint lacks sufficient solder coverage or fill.
Poor Wetting
Solder does not properly bond to the pad or component lead.
Missing Components
A component was not inserted before soldering.
Wrong Polarity
Polarized components were installed incorrectly.
Open Joints
Electrical connection is incomplete.
Component Lift
The component moves away from the PCB during processing.
Reducing these defects requires good process control throughout insertion and soldering.
How to Improve Through-Hole Assembly Quality
Manufacturers can improve THT quality through several practices.
Standardize Insertion
Use clear work instructions and material identification.
Control Flux
Maintain consistent application.
Validate Thermal Profiles
Measure actual PCB temperatures.
Optimize Soldering Parameters
Control conveyor speed, solder temperature, contact conditions, and wave settings.
Add AOI
Detect visible assembly defects earlier.
Use ICT/FCT
Verify electrical and functional performance.
Maintain Equipment
Preventive maintenance reduces process variation.
Analyze Defect Data
Track defect types and investigate recurring root causes.
How to Improve Through-Hole Production Efficiency
Efficiency can be improved by:
- Balancing operator workstations
- Preparing components before production
- Optimizing conveyor speed
- Using standardized fixtures
- Automating repetitive insertion where practical
- Reducing unnecessary PCB handling
- Integrating AOI
- Improving material supply
- Tracking production data
The highest-speed individual machine does not automatically create the most efficient production line.
Line balance is more important.
Manual vs Automatic THT Assembly
Manual insertion remains highly relevant because many through-hole components have irregular shapes and sizes.
Automatic insertion is best suited for compatible standardized components.
A practical production line may therefore use:
Manual Insertion + Automatic Conveying + Wave Soldering + AOI + ICT/FCT
This hybrid configuration balances investment, flexibility, and productivity.
Which Industries Use Through-Hole PCB Assembly?
THT remains important in:
- Industrial automation
- Automotive electronics
- Telecommunications
- Power supplies
- Renewable energy equipment
- Battery systems
- Home appliances
- Transportation electronics
- Heavy machinery controls
Products requiring large connectors, power components, or mechanically strong joints frequently continue to use THT.
How to Choose Between Wave and Selective Soldering
Choose wave soldering when:
- Many THT joints need soldering
- PCB layout supports wave processing
- Production volumes are relatively high
Consider selective soldering when:
- Only limited THT joints exist
- SMT components are close to THT areas
- PCB geometry is complex
- Localized process control is important
The correct selection should be made after reviewing the actual PCB design.
How Much Does a Through-Hole Production Line Cost?
There is no universal price.
Investment depends on:
- Manual or automatic insertion
- Number of assembly stations
- Conveyor length
- Wave soldering configuration
- Selective soldering requirements
- AOI
- ICT/FCT
- PCB cleaning
- Automation
- Production capacity
- Factory layout
A basic manual insertion line with wave soldering may require significantly less investment than a highly automated THT line.
For accurate pricing, suppliers need actual PCB and capacity information.
What Information Is Needed to Configure a THT Line?
When requesting a production line proposal, provide:
- PCB dimensions
- PCB photos
- Through-hole component list
- Component quantity per PCB
- Required capacity
- Product mix
- PCB thickness
- Soldering process preference
- Quality requirements
- Inspection requirements
- Testing requirements
- Factory layout
- Automation target
- Destination country
This information allows engineers to recommend appropriate equipment and line balance.
Why Choose 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 delivering dependable equipment, professional technical support, and complete solutions for PCBA production lines.
Our product portfolio includes SMT pick and place machines from internationally recognized brands such as Fuji, Panasonic, ASM, Yamaha, JUKI, and Hanwha, along with ERSA reflow ovens from Germany, MagicRay SPI/AOI inspection systems, automatic solder paste printers, high-quality pre-owned SMT equipment, SMT spare parts, equipment leasing, and maintenance services.
For DIP/THT manufacturing, Fuliu Electronics can support customers with:
- Through-hole insertion conveyors
- DIP assembly workstations
- Component preparation equipment
- Wave soldering machines
- Selective soldering solutions
- DIP/THT AOI inspection
- PCB handling equipment
- ICT/FCT integration
- PCB cleaning equipment
- Conformal coating solutions
- Factory layout planning
- Automation integration
- Installation and commissioning
- Operator training
- Long-term technical support
Rather than recommending a fixed production line, our team evaluates the customer’s PCB dimensions, component types, production volume, product mix, quality requirements, automation target, factory layout, and budget before configuring the solution.
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
So, how does through-hole PCB assembly work?
The process begins by preparing and inserting component leads through PCB holes. The board is then inspected and soldered using wave, selective, or manual soldering methods. After soldering, manufacturers may perform lead processing, AOI, ICT/FCT testing, cleaning, and final quality verification.
A typical production flow can be summarized as:
Component Preparation → Insertion → Inspection → Fluxing → Preheating → Soldering → AOI → Testing → Cleaning → Final Inspection
Although through-hole technology is older than SMT, it remains essential in modern electronics manufacturing.
Rather than replacing THT, modern factories combine SMT and through-hole technologies to achieve the right balance of compact design, production efficiency, mechanical strength, and electrical performance.
For manufacturers building a new PCB assembly facility, planning SMT, DIP/THT, inspection, testing, cleaning, and coating as one integrated production system can significantly improve overall efficiency and product quality.
Frequently Asked Questions
Through-hole PCB assembly is a process in which component leads are inserted through drilled PCB holes and soldered to conductive pads on the opposite side of the board.
They can be soldered manually, with a wave soldering machine, or using selective soldering equipment depending on PCB design and production requirements.
Yes. THT remains widely used for connectors, transformers, relays, power components, terminals, and other parts requiring strong mechanical connections.
SMT mounts components directly onto the PCB surface, while THT inserts component leads through PCB holes. SMT supports higher component density, while THT is often used for larger or mechanically stressed components.
Yes. Many modern PCB assemblies use both SMT and THT components. Typically, SMT processes are completed first, followed by through-hole insertion and soldering.
Depending on the factory, equipment may include insertion conveyors, component preparation machines, wave or selective soldering equipment, DIP AOI, lead cutters, ICT/FCT systems, PCB cleaning equipment, and board handling systems.