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What is Through Hole PCB Assembly?


Through hole technology has been used since the inception of the printed circuit board (PCB) industry over 60 years ago. It involves the insertion and soldering of component leads into plated holes on the circuit board to form electrical connections and mount parts. While surpassed in many applications by surface mount techniques, through hole assembly remains an essential process, especially for high reliability, high power, and cost-sensitive electronics.

This article provides a comprehensive overview of through hole assembly technology – its history, processes, design considerations, soldering techniques, defects, and role alongside modern SMT production. For many electronic products, combining both through hole parts and SMT components on mixed technology boards offers the optimal balance of reliability, design flexibility, and manufacturing efficiency.

Brief History of Through Hole Assembly

Through hole assembly has evolved significantly over the decades:

1950s – Introduction of the first automated insertion machines. Manual soldering with irons.

1960sWave soldering adopted for large production runs. Plated through hole (PTH) boards introduced.

1970s – Surface mount packages introduced but very limited use.

1980s – Improved insertion machines for dual-sided boards.

1990s – SMT begins displacing through hole components for miniaturized designs.

2000s – Highly precise chip shooters able to assemble boards with 01005 components.

While revolutionary for its time, through hole assembly offers much lower component density than SMT. However, it still excels for large connector interfaces, high power components, and legacy compatibility.

Process Overview

The core through hole assembly process consists of three main steps:

  1. Component Insertion – Leads are inserted into corresponding holes in the PCB.
  2. Soldering – Solder mechanically and electrically bonds the leads.
  3. Cleaning – Flux residues are cleaned from the board after soldering.

Additional steps like adhesive curing, conformal coating, and testing complete the board assembly process. Through hole components may be assembled alone or mixed with SMT parts.<img src=”” width=”500″ alt=”Through hole assembly process”/>

Component Insertion

Insertion involves temporarily securing components in position by inserting their wire leads into plated through holes on the PCB. A variety of specialized machines have been developed to automate insertion.

Manual Insertion

Manual insertion using hand tools was standard in the early days of circuit assembly. While tedious for large volumes, it is still useful for prototyping or low volume production.


  • Low investment cost
  • Flexible setup changes
  • Allows inspection during assembly
  • Useful for quick-turn prototypes


  • Very slow and labor intensive
  • Inconsistent component positioning
  • Higher defect rates – missing parts, bent leads, variances in orientation

Mechanical/Pneumatic Insertion

Mechanical and pneumatic machines use vibratory bowls or pick-and-place mechanisms to position and insert components sequentially. Single and multi-head insertion machines are common configurations.

For medium and high production runs, mechanical insertion provides increased throughput and reduced labor compared to manual methods. Machines can accommodate a range of component packages and lead forms.


  • Dramatically faster than manual insertion
  • Better consistency in orientation and alignment
  • Handles most axial and radial leaded components
  • No lead forming required in most cases


  • Higher equipment investment cost
  • Additional setup and changeover time
  • Limited capability with non-standard leads

Odd Form Insertion

Specialized mechanisms like lead forming, taping, and custom tooling allow the insertion of more challenging components:

  • Lead forming – Bends axial component leads to match board footprint. Required for dual row connectors, flat packs IC, etc.
  • Taping – Sticks bottom termination components like QFPs onto carrier tape for pickup and insertion.
  • Custom heads – Pick-and-place tools for unique component shapes and lead configurations.

Despite added time and cost, odd form insertion expands the range of insertable component types.

Through Hole Soldering Methods

through-hole assembly process

After insertion, soldering electrically and mechanically bonds component terminations to PCB pads while forming a reliable gas and moisture tight seal.

Wave Soldering

Wave soldering is the predominant technique for high volume through hole assembly. The underside of the populated PCB passes over a pumped wave of molten solder which wets the leads and pads. Parameters like conveyor speed, preheat temperature, and wave shape are tightly controlled.


  • Very high throughput for production
  • Simultaneous soldering of all joints
  • Highly repeatable process
  • Low consumable costs


  • High equipment cost
  • Extended changeover between assemblies
  • Solder skips on tightly spaced leads
  • Shadowed joints may be problematic

Selective Soldering

Selective soldering uses a precision solder nozzle to dispense molten solder only at joints requiring wetting. All others are masked from solder. Useful for rework or mixed through hole and SMT.


  • Avoids disturbing or damaging SMT components with wave process
  • Repairs solder skips selectively
  • Lower equipment cost than wave


  • Much slower than wave solder for high volumes
  • Higher operator skill required

Hand Soldering

Manual soldering remains an option for prototypes, low volumes, or rework. Requires operator skill for quality solder joints. Fume extraction is recommended.


  • Minimal equipment cost
  • Selective application and inspection
  • Repairs accessible without desoldering


  • Very low throughput
  • Joint quality and consistency depends highly on operator skill and conditions
  • Can damage sensitive components with excess heat

Common Through Hole Components

Through hole components encompass a vast range of package styles and lead configurations:

Axial Leaded Parts

Radial Leaded Parts

Dual In-Line Packages

  • Integrated circuits with leads on 0.1” row spacing
  • Includes DIPs, PLCCs, SOICs etc.


  • Board-to-board, cable-to-board, power connectors
  • Wide spacing suits wave soldering

New through hole package development has declined in favor of SMT. However, many traditional axial, radial, and DIP components remain in wide use today.

Key Design Considerations

Through hole assembly imposes certain PCB layout constraints and best practices:

Component Spacing

  • Sufficient clearance between component bodies to allow insertion and minimize tombstoning.

Lead Spacing

  • Minimum hole spacing of 0.1” required for most wave soldering applications.

Annular Rings

  • Allow 0.25mm clearance around pad for solder fillet formation.

Thermal Relief Pads

  • Use for heat sensitive parts to reduce conducted heat.

Solderable Surfaces

  • Avoid pads under large components which prevent wave access.

Mixed SMT Placement

  • Position SMT components after solder joints to avoid reflow damage.

Test Points

  • Include pads/vias for test probes to access nodes difficult to probe.

Adhering to through hole assembly design guidelines ensures manufacturability and reliability.

Common Defects

Like all assembly techniques, through hole manufacturing can experience a range of quality defects if not properly controlled:


  • Component stands vertically during soldering. Mitigate by:
    • Sufficient lead spacing
    • Proper clinching after insertion
    • Modifying wafer flow

Solder Skipping

  • Missing solder fillet with exposed base metal. Causes are:
    • Insufficient heat or contact time
    • Contaminated or oxidized surfaces
    • Excessive lead spacing

Solder Bridging

  • Unintended solder joining between terminals. Reduce with:
    • Sufficient pad spacing and solder masking
    • Modified thermal profiles
    • Additional cleaning

Cold Solder Joints

  • Dull, granular looking joint with poor wetting. Fix by:
    • Increasing soldering temperature/time
    • Removing surface oxidation
    • Verifying proper thermal profiling

Overheated Joints

  • Burnt flux residues indicate excess heat exposure which can weaken solder or damage components.

Rework and Repair


When defects occur, through hole assemblies can be reworked by:

  • Manual soldering – Apply heat locally to remove and replace components
  • Solder suckers – Vacuum extrudes molten solder for lead removal
  • Desoldering stations – Simultaneously heat and vacuum desolder joints
  • Clip-off leads – Cut faulty component leads and bypass with wires

However, excessive rework can damage boards through pad lifting, track damage, or overheating.

Process Control

Consistent quality through hole assembly requires process control of key parameters:

Solderability Testing

  • Verify acceptable hole wall wetting to confirm process settings.

Lead Clinched Testing

  • Measure clinch height and pull force to avoid tombstoning.

Thermal Profiling

  • Use thermocouples to monitor preheat and contact times within tolerances.

** destructive Testing**

  • Cross-section samples to inspect internal joint quality.

** AOI Inspection**

  • Monitor critical parameters like placement accuracy and fillet formation.

Controlling these factors is essential to minimize defects and avoid costly rework. Statistical process control provides added oversight.

Mixed Technology Assembly

For many products, the ideal solution is assembling through hole and SMT components on the same PCB:<img src=”” width=”500″ alt=”Mixed technology PCB”/>

This allows combining the benefits of both technologies:

Through Hole

  • Mature, reliable packaging and leads
  • Superior thermal and mechanical performance
  • Capable of higher currents and voltages
  • Ease of inspection, repair, and rework


  • Ultra-compact packaging
  • Extremely high density assembly
  • Lower assembly costs at high volumes
  • Improved signal integrity at high frequencies

Integrating SMT assembly into existing through hole production typically follows a gradual transition plan. New SMT designs may be introduced via drop-in replacements for equivalent through hole parts. Other boards can function as hybrids, with SMT used only for the most space and weight critical functions.

Over successive product generations, SMT content increases until reaching high density SMT assembly with just the necessary minimum of through hole components remaining. This evolution allows a smooth upgrade path while leveraging investments in existing through hole production.


While the influence of through hole technology has decreased relative to SMT, it remains a viable production technique for many applications. Combining the economy, flexibility, and process maturity of through hole assembly with the space savings of SMT delivers an optimized manufacturing solution. With careful process control and discipline, through hole manufacturing continues to deliver quality and reliability for current and legacy product designs.

Frequently Asked Questions

Q: What are the challenges of high-density through hole assembly?

A: Dense through hole assembly requires managing challenges like sufficient hole spacing, complex insertion, tombstoning, solder bridging, rework difficulties, and testing access. Careful process control is essential.

Q: What causes tombstoning defects?

A: Insufficient lead spacing, uneven clinching, misaligned insertion, and uneven soldering heat are common tombstoning causes. Modifications to the assembly process can reduce instances.

Q: How are through hole components inspected?

A: Inspection checks for correct assembly, orientation, clinched leads, solder fill, fillet shapes, markings, spacing, and the absence of defects like solder bridges. Automated optical inspection is commonly employed.

Q: What is the typical defect rate for through hole assembly?

A: With proper process controls, through hole assembly processes can routinely achieve defect rates lower than 10 parts per million. Continuous improvement further reduces defect occurrence.

Q: Can wave soldering reliably solder 0.4mm pitch components?

A: Yes, with a properly configured wave soldering system 0.4mm pitch is achievable. Higher density down to 0.3mm pitch is possible with mini-wave or specialized soldering equipment.




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