Through-Hole (TH) Technology: An Ultimate Guide

What is Through-Hole Technology?

Through-Hole Technology (THT) involves mounting electronic components onto a PCB by inserting their leads through drilled holes and soldering them to pads on the opposite side. This method, which predates SMT, was the industry standard until the 1980s. While SMT now dominates mass production, THT retains critical importance in sectors where robustness and longevity are non-negotiable.

---

A Brief History of Through-Hole Technology

THT emerged in the 1950s with the rise of consumer electronics. Early PCBs relied on manual component insertion and hand soldering. The introduction of automated axial inserters and wave soldering in the 1970s streamlined production, making THT the backbone of electronics manufacturing for decades. Though SMT’s efficiency eventually overshadowed THT for small, high-volume devices, through-hole components remain indispensable in industries like aerospace, automotive, and industrial machinery.

---

Through-Hole Components: Design and Characteristics

TH components are distinguishable by their wire leads, which are inserted into pre-drilled PCB holes. Common examples include:

• Passive Components: Resistors, capacitors, and inductors with axial or radial leads.

• Connectors and Headers: Used for board-to-board or cable connections.

• Electromechanical Parts: Relays, switches, and transformers.

• High-Power Devices: Voltage regulators and heat-sinked semiconductors.

These components are typically larger than SMDs, with leads that withstand mechanical stress and thermal cycling.

---

The Through-Hole Assembly Process: Step-by-Step

The TH assembly process combines precision and durability. Here’s how it works:

1. PCB Design and Drilling

• Hole Drilling: Holes are drilled into the PCB based on the component lead diameters.

• Plated Through-Holes (PTH): Copper plating inside the holes ensures electrical connectivity between layers.

Why It Matters: Accurate finished-hole sizing and tolerances allow insertion without excessive clearance. Confirm the lead dimensions, plating allowance and accepted hole tolerance with the fabricator.

2. Component Insertion

• Manual Insertion: Used for prototypes or low-volume runs.

• Automated Insertion: Axial (horizontal) and radial (vertical) inserters place components at high speeds.

Why It Matters: Automated insertion can reduce manual variability, while manual insertion remains useful for unusual parts and small runs. Confirm the method planned for your board.

3. Soldering

• Wave Soldering: The PCB passes over a molten solder wave, which wicks into the holes to form joints.

• Selective Soldering: For boards with mixed SMT/THT components, targeted nozzles solder only the through-hole parts.

• Hand Soldering: Used for rework or delicate components.

Why It Matters: A suitable soldering process helps prevent insufficient wetting and bridging. Control flux, preheat, solder contact and handling; no atmosphere or machine alone guarantees defect-free joints.

4. Inspection and Testing

• Visual Inspection: Checks for solder defects or misaligned components.

• Automated Optical Inspection (AOI): Checks visible assembly and solder features within the equipment’s coverage; it does not prove every internal hole is fully wetted.

• In-Circuit Testing (ICT): Checks selected circuit nodes and component characteristics according to fixture access and the test program; it does not replace a complete functional test.

Why It Matters: Specify inspection, electrical tests and acceptance criteria for the actual assembly. An IPC class must be agreed for the product; selecting AOI or X-ray alone does not establish compliance.

---

Advantages of Through-Hole Technology

1. Mechanical Strength: Through-hole leads can provide strong mechanical anchoring, useful for connectors and heavier parts. Vibration and shock performance still require product-specific design and validation.

2. Ease of Prototyping: Simplifies manual adjustments and replacements during R&D.

3. Thermal Performance: Some through-hole packages accommodate substantial leads or external heatsinks; actual heat removal depends on the package and complete cooling path.

4. High-Power Handling: Suitable for circuits requiring high voltage/current.

---

Challenges and Limitations

• Size Constraints: TH components occupy more board space than SMDs.

• Slower Production: Drilling and insertion increase assembly time.

• Higher Costs: Labor and material expenses rise for complex designs.

How to Discuss These Challenges with PCBPit: Provide the component list, quantities, insertion requirements and test plan. Ask the project team to confirm the assembly route, engineering changes and quotation scope.

---

Applications of Through-Hole Technology

THT thrives in demanding environments:

• Industrial Equipment: Motor controllers, power supplies.

• Aerospace and Defense: Avionics, radar systems.

• Automotive: Engine control units (ECUs), LED lighting.

• Medical Devices: MRI machines, patient monitors.

For these industries, discuss the required coating, mechanical protection and acceptance requirements with PCBPit. Confirm applicability to the specific project rather than assuming that every service or qualification is included.

---

Through-Hole vs. SMT: Choosing the Right Technology

Factor	Through-Hole	SMT
Size	Larger components	Compact, high-density
Durability	Can provide strong mechanical anchoring	Depends on package, mounting and product design
Cost	Higher for mass production	Lower for high volumes
Applications	High-power, harsh environments	Consumer electronics

Hybrid Approach: Many boards combine THT and SMT. Discuss the process sequence, solder compatibility and inspection requirements with PCBPit before release.

---

Quality Control in Through-Hole Assembly

• IPC-A-610 Compliance: Defines acceptability criteria for solder joints.

• Thermal Stress Testing: Evaluates the assembly under defined temperature cycles; test conditions and acceptance criteria must match the product.

• Cross-Sectional Analysis: Inspects PTH copper plating integrity.

Verify any required certification by its issuing body, scope, validity and covered site. Do not treat a logo or an unspecified laboratory claim as proof that a particular order meets every standard.

---

What to Confirm with PCBPit for Through-Hole Assembly

1. Experience: Ask about comparable component types, production volumes and reliability requirements.

2. Process: Confirm which insertion, soldering and inspection methods will be used for your board.

3. Support: Define the required DFM review, component sourcing and testing scope.

4. Scalability: Agree on prototype acceptance and the controls needed before increasing production volume.

5. Industry Requirements: Identify any application-specific documentation, traceability and qualification obligations.

Whether the project is an industrial controller or a mixed-technology design, provide PCBPit with the requirements that matter to the actual product.

---

The Future of Through-Hole Technology

Through-hole technology remains useful when component selection and mechanical interfaces justify it. Review the available parts and assembly route with a manufacturing partner such as PCBPit, rather than selecting a technology solely because it is newer.

---

Conclusion

Through-hole technology remains relevant for applications that need its mechanical interfaces, package options or accessible prototyping. Reliability is a result of design, materials, process control and verification, not the mounting method alone. Discuss these requirements with PCBPit when preparing your assembly order.

For a through-hole assembly quotation, prepare your design files, BOM, quantities and acceptance requirements, then contact PCBPit to confirm the project scope.