Through Hole PCB Assembly: A Complete Guide to Processes, Techniques, and Applications

13 Aug, 2025

By 管理

In the realm of electronics manufacturing, through hole PCB assembly remains a cornerstone technology for creating robust, reliable connections in high-stress environments. Unlike surface mount technology (SMT), which attaches components directly to a PCB’s surface, through hole assembly involves inserting component leads into pre-drilled holes and securing them with solder—creating mechanical and electrical bonds that withstand extreme conditions. This guide explores the intricacies of through hole PCB assembly, from its step-by-step processes to its unique advantages, applications, and quality standards, helping engineers and buyers make informed decisions for their projects.

What Is Through Hole PCB Assembly?

Through hole PCB assembly is a manufacturing process where electronic components with axial or radial leads are inserted into precision-drilled holes in a printed circuit board (PCB). Once inserted, the leads are soldered to copper pads on the opposite side of the board, forming strong mechanical and electrical connections. This method has been a staple in electronics production for decades, valued for its durability and suitability for high-power, high-voltage applications.

Unlike SMT, which relies on solder paste and reflow ovens, through hole technology (THT) emphasizes physical anchoring—making it ideal for components that undergo vibration, thermal cycling, or mechanical stress. Examples include transformers, connectors, capacitors, and switches, where secure mounting is critical to performance.

Key Takeaways

  • Through hole PCB assembly uses drilled holes to anchor component leads, creating robust connections ideal for harsh environments.
  • The process involves precise drilling, component insertion, soldering (manual or automated), and rigorous quality inspection.
  • It excels in high-power applications, heavy components, and products requiring easy repair or modification.
  • Industries like aerospace, automotive, and medical devices rely on through hole technology for its reliability under stress.
  • Compliance with standards such as IPC-A-610 ensures consistent quality across military, industrial, and consumer electronics.

The Through Hole PCB Assembly Process

The through hole assembly workflow demands precision at every stage to ensure reliability. Below is a detailed breakdown of each step, from design to final inspection.

1. Design and Drilling

The process begins with PCB design, where engineers map out hole locations, sizes, and spacing to accommodate component leads. Drill holes must match lead diameters with minimal tolerance—typically ±3 mil for plated through holes (PTH) and ±2 mil for non-plated through holes (NPTH). This precision prevents loose fits (which cause electrical instability) or tight fits (which damage leads during insertion).

Drilling Techniques

Manufacturers use various drilling methods based on PCB complexity and hole size:

Drilling MethodDescriptionIdeal ApplicationsKey AdvantagesLimitations
Mechanical DrillingHigh-speed carbide bits rotate to cut holesStandard PTH/NPTH in single-layer PCBsCost-effective, mature technologyNot suitable for microvias (<0.1mm)
CNC Automated DrillingComputer-programmed machines with precision controlsMultilayer PCBs, HDI designsHigh accuracy (±0.01mm), efficientHigher setup costs, complex maintenance
Laser DrillingFemtosecond lasers vaporize material to create microviasHDI boards, fine-pitch componentsUltra-precise, minimal material stressSlow for large holes, high equipment cost
Manual MillingHand-operated machines for small-batch productionPrototypes, low-volume runsFlexible, low initial investmentLow precision, labor-intensive

LTPCBA, a leading manufacturer, uses CNC drilling for most projects, ensuring consistency across high-volume orders while employing laser drilling for HDI boards requiring microvias.

2. Component Insertion

After drilling, components are inserted into the holes. This step requires careful handling to avoid bending leads or damaging PCBs:

  • Manual Insertion: Suitable for prototypes or low-volume runs, technicians use tweezers or vacuum tools to place small components. Polarized parts (e.g., diodes, capacitors) must align with silkscreen markings to prevent reverse installation.
  • Automated Insertion: High-volume production uses AI-driven insertion machines that feed components from tape-and-reel or trays, positioning leads with ±0.02mm accuracy. Robotic arms bend leads post-insertion to secure components before soldering.

Best Practices:

  • Clean component leads to remove oxidation, ensuring strong solder bonds.
  • Position heavy components (e.g., transformers) near the PCB’s center to distribute weight evenly.
  • Use tape or temporary adhesives to secure components during soldering, preventing shifting.

3. Soldering

Soldering creates the permanent electrical and mechanical bond between leads and PCB pads. Through hole assembly uses two primary methods:

Manual Soldering

Technicians use soldering irons (25–60W) to apply solder to individual joints. The process involves:

  1. Heating the pad and lead simultaneously (350–400°C) to melt solder.
  2. Applying 60/40 or 63/37 tin-lead solder (or lead-free alternatives like SAC305) to the joint, not the iron tip.
  3. Allowing the joint to cool naturally to form a smooth, concave fillet.

Manual soldering is ideal for:

  • Prototyping and small batches
  • Rework or repair
  • Components with heat sensitivity

Note: Skill-dependent quality makes manual soldering unsuitable for high-volume production.

Wave Soldering

Automated wave soldering is the workhorse of large-scale through hole assembly. The process unfolds in stages:

  1. Preheating: PCBs pass through infrared heaters (80–120°C) to activate flux, remove moisture, and prevent thermal shock.
  2. Flux Application: A spray or foam applies flux to clean pads and promote solder flow.
  3. Wave Immersion: The PCB’s bottom side contacts a molten solder wave (250–270°C), which fills holes and forms joints as the board exits the wave.
  4. Cooling: Forced air or water cooling solidifies solder quickly, maintaining joint integrity.

Wave soldering offers:

  • High throughput (up to 500 boards/hour)
  • Consistent joint quality
  • Compatibility with lead-free solder

For mixed-technology PCBs (SMT + through hole), selective soldering is used. Robotic nozzles apply solder only to through hole joints, avoiding damage to SMT components.

4. Inspection and Quality Control

Rigorous testing ensures through hole assemblies meet performance standards. Leading manufacturers like LTPCBA adhere to IPC-A-610, the industry benchmark for electronic assembly acceptability, which classifies assemblies into three tiers:

IPC-A-610 ClassDescriptionThrough Hole Relevance
Class 1General electronics (e.g., toys)Basic solder joint integrity, minimal inspection
Class 2Industrial equipmentModerate testing; rejects critical defects
Class 3Aerospace, medical, militaryStrict criteria for solder fillets, lead trim, and mechanical strength

Inspection Techniques:

  • Automated Optical Inspection (AOI): High-resolution cameras check for missing components, misalignment, and solder defects (e.g., bridges, cold joints).
  • X-Ray Inspection: Penetrates PCBs to verify solder flow in hidden joints (e.g., multi-pin connectors).
  • In-Circuit Testing (ICT): Uses probes to validate electrical continuity and component functionality.
  • Mechanical Stress Testing: Simulates vibration (per IEC 60068-2-6) and thermal cycling (-40°C to 125°C) to ensure joint durability.

Advantages of Through Hole PCB Assembly

Through hole technology retains its relevance due to unique benefits that complement modern SMT:

  1. Mechanical Strength: Leads anchored through the PCB resist vibration and shock—critical for automotive and aerospace applications.
  2. High-Power Handling: Thicker leads and larger solder joints dissipate heat better, supporting high-voltage components (e.g., power supplies).
  3. Ease of Repair: Components can be desoldered and replaced manually, reducing maintenance costs.
  4. Prototype Flexibility: Manual insertion and soldering simplify design iterations in R&D.
  5. Reliability in Extreme Environments: Resists corrosion and thermal expansion, making it ideal for industrial and outdoor electronics.

Limitations and Considerations

While versatile, through hole assembly has drawbacks to weigh:

  • Space Constraints: Drilled holes reduce routing space, limiting component density compared to SMT.
  • Higher Costs: Drilling and manual labor increase production expenses for high-volume runs.
  • Slower Turnaround: Automated insertion is faster than manual work but lags behind SMT’s speed.
  • Signal Integrity: Long leads introduce inductance, making through hole less suitable for high-frequency designs (>1 GHz).

Applications Across Industries

Through hole assembly excels in sectors demanding durability and power handling:

IndustryUse CasesReason for Preferring Through Hole
AerospaceAvionics, radar systemsWithstands extreme G-forces and temperature swings
AutomotiveEngine control units, sensorsResists vibration and under-hood heat
IndustrialMotor controllers, power invertersHandles high currents and mechanical stress
MedicalMRI machines, patient monitorsEnsures reliability in life-critical devices
MilitaryCommunication equipment, weapon systemsMeets MIL-STD-883H standards for ruggedness
PrototypingR&D boards, test fixturesEnables quick component swaps and redesigns

LTPCBA’s Through Hole Assembly Capabilities

LTPCBA combines decades of expertise with advanced technology to deliver high-quality through hole assemblies:

  • Certifications: ISO 9001, IPC-A-610 Class 3, UL, and RoHS compliance ensure adherence to global standards.
  • Advanced Equipment: CNC drilling machines, selective soldering robots, and 3D AOI systems guarantee precision.
  • End-to-End Support: From DFM (Design for Manufacturability) reviews to post-delivery testing, LTPCBA offers turnkey solutions.
  • Scalability: Handles prototypes (1–100 units) to mass production (100,000+ units) with consistent quality.

LTPCBA’s engineers collaborate with clients to optimize hole placement, component selection, and soldering parameters—reducing defects and lowering costs.

FAQ

Q: What’s the main difference between through hole and SMT assembly?

A: Through hole uses drilled holes to anchor component leads, while SMT attaches components to surface pads. Through hole offers stronger mechanical bonds, while SMT enables higher component density.

Q: Can through hole and SMT be used on the same PCB?

A: Yes. Mixed-technology PCBs use SMT for small, low-power components and through hole for heavy or high-power parts. Selective soldering ensures compatibility.

Q: What solder types are used in through hole assembly?

A: Lead-free options (SAC305: 96.5% Sn, 3% Ag, 0.5% Cu) comply with RoHS, while tin-lead (63/37) is still used for military/aerospace applications requiring lower melting points.

Q: How long does through hole assembly take for prototypes?

A: LTPCBA delivers prototypes in 5–7 days, with expedited options (2–3 days) for simple designs.

Q: What design files are needed for through hole assembly?

A: Gerber files, a Bill of Materials (BOM) with lead diameters, and assembly drawings indicating polarized component orientations.

Conclusion

Through hole PCB assembly remains an indispensable technology for applications demanding durability, power handling, and repairability. Its ability to withstand harsh conditions makes it a top choice for aerospace, automotive, and industrial sectors, while ongoing advancements in automation (e.g., AI-driven insertion) continue to bridge efficiency gaps with SMT.

By partnering with a manufacturer like LTPCBA—equipped with certifications, advanced equipment, and end-to-end support—you can leverage through hole technology to create reliable, long-lasting electronics. Whether prototyping or scaling to mass production, prioritizing quality control and compliance with IPC standards ensures your assemblies meet the highest performance benchmarks.

See Also

  • Through Hole vs. SMT: Choosing the Right Assembly Method
  • IPC-A-610 Compliance for Through Hole Electronics
  • Optimizing Through Hole PCB Design for Manufacturability
  • Selective Soldering Techniques for Mixed-Technology PCBs
  • Industrial Applications of Through Hole PCB Assembly
Contact

Write to Us And We Would Be Happy to Advise You.

    l have read and understood the privacy policy

    Do you have any questions, or would you like to speak directly with a representative?

    icon_up