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Understanding the Through-Hole PCB Assembly Process: A Step-by-Step Guide
September 3rd, 2025
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Understanding the Through-Hole PCB Assembly Process: A Step-by-Step Guide
Through-hole PCB assembly remains a cornerstone of reliable electronics manufacturing, particularly for applications demanding mechanical strength, high current capacity, or legacy compatibility. Unlike surface mount technology (SMT), which bonds components to the PCB surface, through-hole assembly creates robust, penetrating connections by inserting component leads through drilled holes and soldering them to pads on the opposite side. This process, though more methodical than SMT, delivers unmatched durability for industrial controls, aerospace systems, and high-power devices.
For manufacturers and engineers new to through-hole assembly—or those seeking to optimize existing workflows—this step-by-step guide demystifies the process, highlighting critical technical parameters, common pitfalls, and how modern PCB Assembly Services like ShwayPCB’s PCB Assembly Services ensure consistency and quality. Each step is grounded in industry standards (IPC-A-610, IPC-J-STD-001) and real-world best practices, making it a actionable resource for both prototyping and high-volume production.
Step 1: Pre-Assembly Design Validation and DFM Review
Before physical assembly begins, the PCB design must be validated to ensure compatibility with through-hole processes—a step that prevents costly rework later.
Key Actions:
- Hole Sizing Verification: Through-hole component leads require holes with precise clearance (lead diameter + 0.1–0.2mm) to ensure a snug fit without damaging the PCB. For example, a component with a 0.8mm lead needs a 0.9–1.0mm hole; undersized holes cause lead bending, while oversized holes weaken solder joints.
- Pad Layout Compliance: Pads must align with IPC-7351 standards (e.g., 1.5x hole diameter for round pads) to ensure sufficient solder coverage. A 1.0mm hole requires a 1.5mm-diameter pad to prevent solder "starving" (insufficient joint strength).
- Component Placement Checks: Ensure through-hole components are spaced to accommodate insertion and soldering—minimum clearance between adjacent components is 2x the component body size to avoid mechanical interference during assembly.
Role of DFM:
ShwayPCB’s Turnkey PCB Assembly includes a free Design for Manufacturability (DFM) review within 24 hours of file submission. This review flags issues like non-standard hole sizes or inadequate pad spacing, reducing design iterations by 70% and ensuring assembly readiness.
Step 2: PCB Fabrication and Preparation
Once the design is validated, the PCB is fabricated and prepared for component insertion—this step lays the foundation for reliable through-hole joints.
Key Actions:
- CNC Drilling: High-precision CNC drills (tolerance ±0.01mm) create through-holes in the PCB. For high-power applications (e.g., 20A connectors), holes are "plated through-hole" (PTH) with a 25–30μm copper layer to enhance current-carrying capacity and corrosion resistance.
- Pad Plating: Pads and hole walls are electroplated with lead-free solder (Sn-Ag-Cu) or tin-lead (for legacy projects) to ensure solderability. Plating thickness is critical—20–30μm ensures proper wetting during soldering, while thinner plating oxidizes quickly, leading to cold joints.
- Cleaning and Inspection: The PCB is ultrasonic-cleaned to remove drilling debris and flux residues, then inspected via automated optical inspection (AOI) to verify hole accuracy and plating integrity. Defects like blocked holes or uneven plating are repaired before assembly.
Technical Note:
For flexible PCBs (used in automotive wiring harnesses), holes are reinforced with nickel-gold plating to prevent tearing during component insertion—ShwayPCB’s Flexible PCB Assembly specializes in this reinforcement, extending flexible PCB lifespan by 50%.
Step 3: Component Kitting and Preparation
Component readiness is critical to avoiding assembly delays—this step ensures all through-hole parts are organized, inspected, and compatible with the PCB.
Key Actions:
- Component Sourcing and Verification: Through-hole components (e.g., resistors, capacitors, connectors) are sourced from authorized distributors (Digi-Key, Mouser) to avoid counterfeits. Each component is checked against the BOM (Bill of Materials) for part number, value, and package type—e.g., confirming a 1kΩ resistor has the correct axial lead length (25mm).
- Lead Trimming (If Needed): Long component leads (e.g., 50mm for transformers) are trimmed to 15–20mm to simplify insertion and prevent lead bending. Trimming is done with precision cutters (tolerance ±1mm) to ensure consistent lead length across all components.
- Moisture Sensitivity Handling: For moisture-sensitive through-hole components (e.g., hermetic connectors), follow JEDEC J-STD-020 guidelines—components stored beyond floor life are baked at 125°C for 8–24 hours to remove trapped moisture, preventing "popcorning" (component cracking) during soldering.
ShwayPCB’s Approach:
Our kitting team uses barcode scanning to track components, ensuring 100% BOM accuracy. For legacy projects requiring obsolete through-hole parts, we leverage a global network of suppliers to source hard-to-find components, reducing lead times by 40%.
Step 4: Component Insertion
Component insertion is the first physical assembly step—its precision directly impacts solder joint quality and mechanical strength.
Key Methods:
4.1 Manual Insertion (Low-Volume/Prototypes)
- Application: Used for odd-form components (e.g., large transformers, custom terminals) or batches <50 units. Skilled technicians insert leads through holes, ensuring:
- Leads are straight (no bending >5°) to avoid pad damage.
- Components are seated flush against the PCB (gap <0.5mm) to prevent vibration-induced stress on joints.
- Tools: Tweezers, lead straighteners, and depth gauges are used to maintain accuracy—ShwayPCB’s technicians are IPC-A-610 certified, ensuring consistent insertion quality.
4.2 Automated Insertion (High-Volume Production)
- Application: Ideal for standard through-hole components (axial resistors, radial capacitors) in batches >100 units. Machines like the Universal Instruments GSM 2 handle:
- Axial Insertion: Components with leads on both ends (e.g., resistors) are fed from reels, cut to length, and inserted at speeds up to 1,000 components per hour.
- Radial Insertion: Components with leads on one end (e.g., capacitors) are inserted with lead spacing matching PCB hole patterns (tolerance ±0.1mm).
- Accuracy: Automated insertion ensures lead depth variation <0.2mm, reducing post-soldering rework by 80% vs. manual insertion.
Step 5: Soldering: Wave vs. Selective Soldering
Soldering is the most critical step in through-hole assembly—it forms the electrical and mechanical bond between components and the PCB. The method chosen depends on production volume and whether the design is hybrid (SMT + through-hole).
5.1 Wave Soldering (High-Volume, Pure Through-Hole)
- Process:
- Flux Application: The PCB’s solder side is sprayed with rosin-based flux (activator content 1–2%) to remove oxide layers from leads and pads.
- Preheating: The PCB is heated to 150–180°C to activate flux and prevent thermal shock to components.
- Wave Contact: The PCB is passed over a molten solder wave (240–260°C for lead-free Sn-Ag-Cu solder) at a speed of 0.5–1m/min. The wave wets leads and pads, forming a fillet (solder joint) with 75–100% pad coverage.
- Cooling: The PCB is cooled to <100°C via forced air to solidify the solder, preventing joint deformation.
- Quality Control: Nitrogen-enriched wave soldering (used by ShwayPCB) reduces oxidation, ensuring solder joint shear strength >50N (per IPC-TM-650 standards).
5.2 Selective Soldering (Hybrid Designs)
- Application: Used for PCBs with both SMT and through-hole components (e.g., industrial sensors with SMT microcontrollers and through-hole power connectors).
- Process: A robotic nozzle (diameter 0.5–2mm) delivers precise solder deposits to individual through-hole pads, avoiding thermal damage to SMT components (which melt at <220°C).
- Advantages: Enables hybrid assembly without compromising SMT joint integrity—ShwayPCB’s Hybrid PCB Assembly uses selective soldering to achieve 99.5% first-pass yield for complex industrial designs.
Step 6: Post-Soldering Processing
After soldering, the PCB undergoes finishing steps to ensure reliability and compliance with industry standards.
Key Actions:
- Lead Trimming: Excess leads (1–2mm above solder joints) are trimmed with automated cutters to prevent short circuits and improve mechanical stability. Trimmed leads are recycled to reduce waste.
- Flux Cleaning: Flux residues are removed via aqueous cleaning (deionized water + detergent) or solvent cleaning (isopropyl alcohol) per IPC-J-STD-001. Residue left on the PCB can cause corrosion over time, particularly in humid environments.
- Inspection: 100% inspection is performed to detect defects:
- Visual Inspection: Automated optical inspection (AOI) checks for solder bridges (unintended connections), cold joints (dull, grainy solder), and insufficient fillet size.
- Electrical Testing: In-Circuit Testing (ICT) verifies continuity and resistance of through-hole joints—e.g., ensuring a power connector’s joint has resistance <1Ω to avoid voltage drops.
Step 7: Final Quality Assurance and Packaging
The final step ensures the assembled PCB meets all performance and reliability requirements before shipping.
Key Actions:
- Reliability Testing: For critical applications (aerospace, medical), PCBs undergo:
- Thermal Cycling: -40°C to +125°C (1,000 cycles) to test joint durability under temperature fluctuations.
- Vibration Testing: 10–20G acceleration (20–2,000Hz) per MIL-STD-883H to simulate industrial or aerospace conditions.
- Hi-Pot Testing: 2–5kV AC for 1 minute to verify insulation integrity between through-hole joints.
- Traceability Documentation: Each PCB is labeled with a unique serial number, linking to component lot numbers, soldering parameters, and test results. This documentation is retained for 10+ years to support regulatory audits (e.g., FDA QSR for medical devices).
- Packaging: PCBs are packaged in anti-static bags or trays to prevent ESD damage during shipping. For high-volume orders, ShwayPCB uses custom packaging to minimize transit damage, achieving a 99.9% on-time, defect-free delivery rate.
FAQ: Through-Hole PCB Assembly Process
1. How long does the through-hole assembly process take for a typical PCB?
- Prototypes (5–50 units): 3–5 days (including DFM review, manual insertion, and wave soldering).
- High-Volume (1k+ units): 7–10 days (automated insertion, wave soldering, and full testing).
ShwayPCB offers expedited services for urgent projects, reducing lead times by 40% (e.g., 2 days for prototypes).
2. What is the maximum component size that can be inserted via through-hole assembly?
Through-hole assembly handles components up to 50mm×50mm×30mm (e.g., large transformers, power modules). For larger components, ShwayPCB’s engineers design custom fixtures to ensure stable insertion and soldering.
3. Can through-hole assembly be used for PCBs with blind or buried vias?
Yes—but with limitations: Blind vias (drilled from one surface) can accommodate through-hole components if the hole extends to the opposite side. Buried vias (between inner layers) cannot be used for through-hole components, as they do not penetrate the entire PCB. Our DFM review identifies via compatibility early in the process.
4. What is the difference between lead-free and leaded through-hole soldering?
- Lead-Free (Sn-Ag-Cu): RoHS-compliant, melting point 217°C, requires wave soldering at 240–260°C. Ideal for consumer, industrial, and medical electronics.
- Leaded (Sn-Pb): Melting point 183°C, wave soldering at 220–240°C. Used only for legacy aerospace or military projects with non-RoHS requirements.
ShwayPCB supports both, with dedicated equipment to prevent cross-contamination between leaded and lead-free processes.
5. How do you fix a defective through-hole solder joint (e.g., cold joint)?
Defective joints are repaired via:
- Desoldering: A soldering iron (350°C) or desoldering pump removes old solder.
- Cleaning: The pad and lead are cleaned with flux to remove oxides.
- Resoldering: Fresh solder is applied to form a new fillet (75–100% pad coverage).
ShwayPCB’s rework technicians achieve a 99% success rate for defective joints, minimizing scrap.
Conclusion
The through-hole PCB assembly process is a structured, standards-driven workflow that delivers unmatched reliability for high-power, high-stress applications. By following these 7 steps—from design validation to final testing—manufacturers can ensure consistent quality, reduce rework, and meet the strict requirements of industrial, aerospace, and legacy systems.
ShwayPCB’s PCB Assembly Services specialize in every stage of through-hole assembly, leveraging automated equipment, IPC-certified technicians, and global component sourcing to deliver solutions tailored to your project’s needs. Whether you’re prototyping a legacy repair or scaling high-volume production, our team ensures your through-hole PCBs meet performance and durability targets.
To discuss your through-hole assembly project, request a DFM review, or get a customized quote for Industrial PCB Assembly, contact ShwayPCB at yuan@shwaypcb.com. For detailed process videos, compliance documents (IPC-A-610, RoHS), and case studies, 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