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Through-Hole PCB Assembly: The Traditional yet Reliable Method for Electronic Component Mounting
September 3rd, 2025
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Through-Hole PCB Assembly: The Traditional yet Reliable Method for Electronic Component Mounting
In an era dominated by surface mount technology (SMT) for miniaturized electronics, through-hole PCB assembly remains a critical, often underappreciated manufacturing method. Unlike SMT, where components are soldered to the PCB surface, through-hole assembly involves inserting component leads through drilled holes in the PCB and soldering them to pads on the opposite side. This creates a mechanical bond far stronger than SMT’s surface adhesion—making through-hole ideal for applications where vibration, shock, or high power loads demand uncompromising reliability.
While SMT excels at high-density, low-power devices (e.g., wearables, smartphones), through-hole assembly is irreplaceable for industrial controls, aerospace systems, automotive power components, and legacy equipment. This article explores through-hole PCB assembly’s technical workflow, unique advantages, ideal use cases, and how modern PCB Assembly Services integrate it with SMT for hybrid designs. It also highlights ShwayPCB’s PCB Assembly Services as a leader in through-hole assembly, delivering 99.9% first-pass yield for industrial and aerospace clients.
1. The Technical Workflow of Through-Hole PCB Assembly
Through-hole assembly follows a structured, time-tested process that prioritizes mechanical strength and electrical continuity. While less automated than SMT, its steps are optimized for reliability:
1.1 PCB Preparation: Drilling and Pad Plating
- Hole Drilling: High-precision CNC drills create holes in the PCB (diameter = component lead diameter + 0.1–0.2mm) to ensure a snug fit. For high-power components (e.g., 10A connectors), holes may be plated with thick copper (2–3oz) to enhance current-carrying capacity and mechanical stability.
- Pad Plating: Holes and associated pads are electroplated with tin-lead or lead-free solder (per RoHS) to prevent corrosion and ensure solderability. The plating thickness (20–30μm) is critical—too thin, and the joint may fail under stress; too thick, and it may block component insertion.
1.2 Component Insertion
- Manual Insertion: For low-volume or odd-form components (e.g., large transformers, custom connectors), skilled technicians insert leads through PCB holes. This is common for prototypes or legacy equipment with non-standard components.
- Automated Insertion: High-volume through-hole assembly uses axial or radial insertion machines (e.g., Universal Instruments GSM 2) to place components like resistors, capacitors, and diodes at speeds up to 1,000 components per hour. These machines ensure consistent lead length and insertion depth, reducing post-soldering rework.
1.3 Soldering: Wave vs. Selective Soldering
Soldering is the most critical step, as it forms the electrical and mechanical bond between components and the PCB:
- Wave Soldering: The gold standard for high-volume through-hole assembly. The PCB is passed over a molten solder wave (240–260°C for lead-free solder), which wets the exposed leads and pads to form joints. ShwayPCB uses nitrogen-enriched wave soldering machines to reduce oxidation, ensuring solder joint integrity (per IPC-A-610 Class 3).
- Selective Soldering: Used for hybrid PCBs (SMT + through-hole) or components sensitive to wave soldering heat (e.g., sensors). A robotic nozzle delivers precise solder deposits to individual through-hole pads, avoiding thermal damage to SMT components already mounted on the PCB. This is a core capability of ShwayPCB’s Hybrid PCB Assembly, which combines SMT and through-hole for complex industrial designs.
1.4 Post-Soldering Processing
- Lead Trimming: Excess component leads (typically 1–2mm above the solder joint) are trimmed to prevent short circuits and improve mechanical stability.
- Inspection: 100% visual inspection (manual or automated) checks for solder defects:
- Cold Joints: Dull, grainy solder indicating poor wetting.
- Solder Bridges: Unintended connections between adjacent pads.
- Insufficient Solder: Joints with gaps or incomplete coverage.
- Cleaning: Flux residues are removed via aqueous or solvent cleaning (per IPC-J-STD-001) to prevent corrosion and ensure compliance with aerospace/medical standards.
2. Key Advantages of Through-Hole PCB Assembly
Through-hole assembly’s enduring relevance stems from unique benefits that SMT cannot match, particularly for demanding applications:
2.1 Superior Mechanical Strength
Through-hole joints create a "mechanical anchor" by passing leads through the PCB, making them 5–10x more resistant to vibration and shock than SMT joints. This is critical for:
- Automotive Underhood Components: Sensors and connectors exposed to 10–20G vibration (per MIL-STD-883H).
- Industrial Machinery: Motor controllers and power supplies subject to continuous mechanical stress.
- Aerospace Systems: Avionics components that must withstand turbulence and launch shocks.
2.2 High Power and Current Handling
Through-hole components (e.g., power transistors, terminal blocks) have larger leads and thicker pads, enabling them to handle 10–100A currents—far more than SMT components (typically limited to <5A). For example:
- A through-hole terminal block with 14AWG leads safely carries 20A, while an SMT terminal of the same size is limited to 5A.
- Through-hole capacitors with 10mm leads dissipate heat more efficiently than SMT equivalents, making them ideal for high-power inverters.
2.3 Legacy Compatibility
Millions of industrial machines, medical devices, and aerospace systems rely on legacy through-hole components (e.g., 8-bit microcontrollers, analog amplifiers) that are not available in SMT packaging. Through-hole assembly ensures these systems can be repaired, maintained, or reproduced—extending their lifespan by 10–20 years.
2.4 Ease of Rework and Repair
Through-hole joints are easier to rework than SMT:
- Damaged components can be desoldered with a simple soldering iron (no specialized rework stations needed).
- Leads are visible and accessible, reducing the risk of damaging adjacent components during repair. This is critical for field maintenance of industrial equipment, where downtime costs can exceed $10,000 per hour.
3. Ideal Use Cases for Through-Hole PCB Assembly
Through-hole assembly is not a one-size-fits-all solution—but it is irreplaceable in these key applications:
3.1 Industrial Control Systems
- Applications: PLCs (Programmable Logic Controllers), motor drives, power supplies.
- Why Through-Hole: Withstands vibration (5–15G), handles high currents (10–50A), and supports legacy components (e.g., analog input modules). ShwayPCB’s Industrial PCB Assembly specializes in through-hole for these systems, meeting IEC 61010 safety standards.
3.2 Aerospace and Defense
- Applications: Avionics (navigation systems, communication modules), military radios.
- Why Through-Hole: Complies with MIL-STD-883H (mechanical shock, thermal cycling) and uses hermetic through-hole components that resist radiation and extreme temperatures (-55°C to +125°C).
3.3 High-Power Electronics
- Applications: EV charging stations, industrial inverters, solar inverters.
- Why Through-Hole: Handles 50–100A currents and dissipates heat efficiently. Through-hole power transistors and diodes are critical for converting AC to DC in high-power systems.
3.4 Legacy Equipment Maintenance
- Applications: Medical imaging machines (MRI, CT scanners), older manufacturing robots.
- Why Through-Hole: Reproduces PCBs with obsolete through-hole components, ensuring these high-value systems remain operational. ShwayPCB sources hard-to-find through-hole parts via global component networks, supporting legacy repairs.
4. Integrating Through-Hole with Modern PCB Assembly Services
Modern electronics often require a hybrid approach—SMT for miniaturized components (e.g., microcontrollers, sensors) and through-hole for power/mechanical components (e.g., connectors, transformers). Leading PCB Assembly Services like ShwayPCB optimize this hybrid workflow:
4.1 Sequential Assembly: SMT First, Then Through-Hole
- Step 1: SMT components are mounted on one side of the PCB (e.g., a Bluetooth module, temperature sensor).
- Step 2: The PCB is flipped, and through-hole components are inserted on the opposite side (e.g., a power connector, transformer).
- Step 3: Selective soldering is used to solder through-hole joints, avoiding thermal damage to SMT components. This process is 30% faster than traditional batch assembly and maintains 99.5% first-pass yield.
4.2 Design for Manufacturability (DFM) for Hybrid PCBs
- Component Placement: Through-hole components are placed away from SMT components to avoid soldering interference.
- Hole Sizing: Holes are sized to match component leads (0.1–0.2mm clearance) to ensure a tight fit without damaging the PCB.
- Thermal Management: Through-hole power components are placed near heat sinks, while SMT components are positioned in cooler areas of the PCB.
ShwayPCB’s DFM team reviews all hybrid designs to optimize this balance, reducing rework by 40% and ensuring compliance with IPC-A-610 Class 3.
5. FAQ: Through-Hole PCB Assembly
1. Is through-hole assembly more expensive than SMT?
Yes—through-hole typically costs 20–50% more per unit due to:
- Manual labor for odd-form component insertion.
- Slower processing (wave soldering vs. high-speed SMT placement).
- Higher material costs (larger through-hole components).
However, the cost is justified for applications requiring mechanical strength or high power—rework costs for failed SMT joints in industrial systems often exceed through-hole’s upfront premium.
2. Can through-hole components be used on flexible PCBs?
Yes—but with limitations:
- Flexible PCBs require reinforced holes (plated with nickel-gold) to prevent tearing during component insertion.
- Through-hole components should be lightweight (≤10g) to avoid flexing the PCB.
ShwayPCB’s Flexible PCB Assembly supports through-hole for flexible designs used in automotive wiring harnesses and wearable medical devices.
3. What is the maximum current a through-hole joint can handle?
It depends on lead size and pad design:
- 22AWG lead (0.64mm diameter): ~5A.
- 14AWG lead (1.63mm diameter): ~20A.
- Thick-copper pads (3oz): Up to 50A for short-duration loads (e.g., EV charging).
ShwayPCB uses IPC-2221 calculations to design through-hole joints for specific current requirements.
4. How does through-hole assembly comply with RoHS standards?
Through-hole assembly uses lead-free solder (Sn-Ag-Cu, Sn-Cu) and RoHS-compliant components (no lead, cadmium, or mercury). ShwayPCB’s wave soldering machines are calibrated for lead-free solder (250–260°C peak temperature) and provide RoHS compliance documentation for every batch.
5. When should I choose through-hole over SMT?
Choose through-hole if your application requires:
- Mechanical robustness (vibration, shock resistance).
- High current handling (>5A).
- Legacy component compatibility.
- Easy field repair.
Choose SMT for miniaturization, high density, or low-power devices (e.g., smartphones, IoT sensors). For hybrid needs, ShwayPCB’s Turnkey PCB Assembly combines both methods.
6. Conclusion
Through-hole PCB assembly is far from obsolete—it is a reliable, technically critical method for applications where SMT’s limitations (mechanical weakness, low current capacity) are dealbreakers. From industrial control systems to aerospace avionics, through-hole’s ability to withstand stress, handle high power, and support legacy components ensures its place in modern electronics manufacturing.
ShwayPCB’s PCB Assembly Services honor through-hole’s legacy while integrating it with modern workflows—offering wave soldering, selective soldering, and hybrid SMT/through-hole assembly for industrial, aerospace, and legacy projects. Our team of IPC-certified technicians ensures every through-hole joint meets strict quality standards (IPC-A-610 Class 3), delivering reliability that lasts decades.
To discuss your through-hole assembly needs, request a DFM review for a hybrid PCB design, or get a customized quote for Industrial PCB Assembly, contact ShwayPCB at yuan@shwaypcb.com. For detailed case studies (e.g., through-hole assembly for a 50A industrial inverter) and compliance documents (MIL-STD-883H, RoHS), visit our dedicated PCB Assembly Services page.
Copyright © 2005-2025 ShwayPCB : PCB Fabrication and Assembly Manufacturer. All Rights Reserved. POWERED BY UEESHOP
Copyright © 2005-2025 ShwayPCB : PCB Fabrication and Assembly Manufacturer. All Rights Reserved. POWERED BY UEESHOP