Inspection LST OQC: Sauvegarder la qualité de chaque PCBA

The PCBA (Assemblage de la carte de circuit imprimé), often referred to as the “heart” of electronic devices, directly determines the reliability and service life of the end product. OQC (Outgoing Quality Control), serving as the final quality gate before shipment, ensures that only qualified assemblies are delivered. Through systematic and standardized inspection procedures, OQC intercepts defective units and minimizes customer complaints. This article elaborates on how OQC safeguards PCBA quality from four key perspectives: inspection objectives, core inspection dimensions, process optimization strategies, and common issue management.

Core Objectives of OQC: Defining the “Protection Boundary”

OQC does not mean inspecting every single detail, but rather focuses on “customer-critical characteristics” and “high-risk process points”, aiming for precise interception and efficient release. Its specific goals include:

• Conformity Verification: Confirm that the PCBA meets customer drawings (Gerber files), BOM lists (material specifications/models), industry standards (Par exemple, IPC-A-610 Acceptability of Electronic Assemblies), and mutual quality agreements (AQL sampling levels, defect classification, etc.).

• Risk Interception: Detect hidden issues from production processes (such as SMT, TREMPER, soldering, cleaning) and potential transportation risks (Par exemple, ESD damage, pin deformation).

• Data Traceability: Record inspection data for each batch (sampling size, defect types/quantities, inspector, date) to support quality analysis, supplier traceability, and process optimization.

Key Inspection Dimensions: Breaking Down the “Quality Control Points”

OQC for PCBA covers five major dimensions — appearance, performances électriques, structure, identification, and packaging. Each must have clear standards, tools, and decision criteria to avoid subjective judgments.

(1) Appearance Inspection: Visual + Assisted Tools to Detect Surface Defects

Appearance is the most intuitive indicator of PCBA quality and forms the customer’s first impression. Focus areas include soldering quality, component condition, and board cleanliness. Inspection tools include the naked eye, magnifying glass (10–20×), and microscope (for fine-pitch components), following IPC-A-610 standards.

(2) Electrical Performance Inspection: Power-On Testing to Detect Hidden Faults

A visually perfect board may still fail functionally. Electrical testing validates circuit continuity, component functionality, et l'intégrité du signal:

• TIC (In-Circuit Test): Uses test probes to verify missing, misplaced, or damaged components (Par exemple, resistance drift, capacitor short, IC open). Detects over 80% of component-level faults — ideal for mass PCBA testing.

• FCT (Functional Circuit Test): Simulates actual working conditions (power-on, peripheral connection) to verify performance — e.g., output voltage stability, signal transmission, button response. It’s the most critical OQC checkpoint.

• Insulation Resistance Test: For high-voltage boards (Par exemple, power PCBA), tests insulation resistance between circuits and housing (≥100 MΩ), ensuring compliance with UL/CE safety standards.

(3) Structural & Dimensional Inspection: Ensuring Assembly Compatibility

PCBA dimensions must fit precisely within the final product. Deviations can cause mechanical stress or assembly failure. Key checks include:

• Board Dimensions: Measure length, width, and thickness with calipers; tolerance typically ±0.1 mm.

• Mounting/Locating Holes: Verify hole diameter and spacing (Par exemple, diameter tolerance ±0.05 mm, spacing ±0.1 mm) to match customer fixtures or screws.

• Hauteur du composant: Measure tallest component (capacitor, connector, etc.) with a height gauge to ensure it doesn’t exceed specified enclosure clearance.

(4) Labeling & Traceability: Ensuring “Accountability and Tracking”

Clear labeling enables complete traceability and accountability. OQC must verify that all markings are accurate and legible:

• Batch Number / Production Date: Printed clearly on designated board area (edge or blank zone) without blurring or erasure, traceable to lot and supplier.

• Customer Logo / Model Number: Must match customer’s drawing for position, font, and color — no offset or missing print.

• Rohs / Environmental Labels: For green-compliance requirements (sans plomb, halogen-free), ensure labeling matches valid third-party test reports (Par exemple, SGS).

(5) Packaging & Protection: Preventing Damage During Transit

Beyond the product itself, OQC ensures packaging effectively protects PCBA during transportation from ESD, humidité, or mechanical stress.

• ESD Protection: Use antistatic bags (surface resistance 10⁶–10¹¹ Ω) for individual packing, plus antistatic bubble wrap to prevent ESD-related IC damage.

• Moisture Protection: For long-term storage or humid regions, include desiccants (Par exemple, montmorillonite) and seal with vacuum or aluminum foil bags to prevent oxidation and pad rusting.

• Stacking & Cushioning: Use antistatic turnover boxes with dividers; label boxes “FRAGILE / ANTI-STATIC / KEEP DRY.” Stacking height must not exceed the limit to prevent deformation of lower layers.

OQC Process Optimization: From “Passive Inspection” to “Proactive Prevention”

Efficient OQC is not just about intercepting defective products — it’s about reducing defect rates through process optimization and improving inspection efficiency. The core strategies are as follows:

1. Develop a “Graded Sampling Plan”: Balancing Efficiency and Risk

PCBA production batches are often large (thousands of units per lot). Full inspection is costly and inefficient. Donc, AQL (Acceptable Quality Level) sampling should be defined based on customer requirements and product risk level:

• High-risk PCBA (Par exemple, medical device boards): Adopt AQL 0.65 for strict sampling, allowing minimal defect tolerance. For critical items (Par exemple, FCT testing), increase the sampling rate (Par exemple, à 20%).

• General consumer PCBA (Par exemple, toy electronics): Use AQL 1.0–2.5 for moderate sampling. Apply AQL 1.0 for appearance inspection and AQL 0.65 for electrical testing.

• Special requirements: If the customer demands 100% essai (Par exemple, small customized batches), both appearance and FCT must be performed on every unit to ensure full compliance.

2. Establish a “Defect Classification Standard”: Ensuring Consistent Judgment

Defects vary in severity and impact on product performance. Clear classification prevents inconsistencies among inspectors. Typiquement, defects are divided into four levels:

Defect Type	Definition	Action
Critical (CR)	Causes complete failure or safety risk (Par exemple, short circuit, leakage, FCT failure).	Even one defect leads to batch rejection; full reinspection required.
Major (MA)	Affects primary function (Par exemple, button nonresponsive, voltage out of range).	Allow 0–1 defect per AQL 0.65 sampling.
Minor (MI)	No functional impact but affects appearance or assembly (Par exemple, 30% misalignment, slight flux residue).	Accept 3–5 defects per AQL 2.5 sampling.
Trivial (MIN)	Negligible impact (Par exemple, faint surface scratches).	May be released upon customer confirmation; not counted as batch failure.

3. Strengthen the “Defective Product Handling Process”: Building a Closed-Loop System

When nonconformities are detected, a closed-loop control process prevents them from reaching customers:

• Isolation & Labeling: Mark defective PCBA with red “Defective” labels and store them separately in a designated area with batch, defect type, and discovery date clearly noted.

• Root Cause Analysis: Collaborate with production (SMT/DIP) and R&D departments to identify causes — e.g., false soldering due to low flux activity or insufficient placement pressure.

• Rework / Réparation:

  • Repairable items (Par exemple, minor solder bridges): rework using hot air gun or soldering iron; then perform full OQC reinspection.

  • Unrepairable items (Par exemple, IC damage, board crack): scrap and record.

• Preventive Measures: Update upstream production parameters (Par exemple, flux ratio, pick-and-place calibration) and prioritize revalidation in subsequent OQC batches to prevent recurrence.

4. Enhance Inspector Competence: Reducing “Human Error”

OQC depends heavily on operator judgment (appearance evaluation, equipment use). Inadequate skills can cause false passes or misjudgments. Improvement methods include:

• Standardized Training:
  Develop a detailed OQC Inspection SOP specifying steps, tools, and criteria. New staff must pass both theory and practical exams (Par exemple, identifying 10 common defects) before certification.

• Regular Skill Assessment:
  Conduct monthly “blind tests” using PCBA samples with hidden defects; inspectors must achieve ≥95% detection accuracy, or undergo retraining.

• Cross-Department Learning:
  Allow OQC personnel to participate in SMT/DIP production observation sessions to understand process-related defect sources (Par exemple, fine-pitch IC solder risks), improving inspection focus.

Common OQC Issues and Countermeasures

Issue	Root Cause	Solution
Hidden cold solder missed	Defect occurs under stress/temperature change; not visible; ICT lacks test coverage	1. Add vibration test (simulate transport stress) and retest via FCT. 2. Optimize ICT program and include high-risk components.
Unstable FCT results	Probe wear or poor contact; fluctuating test environment	1. Calibrate fixtures daily (clean probes, check pressure). 2. Maintain stable temperature (20–25 °C) and voltage (±0.5 V).
Blurry / erased batch markings	Low-adhesion ink or friction during shipping	1. Use high-adhesion ink and cure at 80 °C for 30 min. 2. Apply transparent protective film over marking.
Bent component pins after shipping	Inadequate packaging cushioning	1. Use custom ESD trays with positioning grooves. 2. Fill boxes with ESD foam to limit movement.

Four Core Advantages: The “Zero-Defect” DNA of LST

Behind LST’s OQC system lie 18 years of experience and four strategic strengths:

1. Engineering Support
   ◇ Provides PCB design optimization and DFM manufacturability analysis to reduce defects at the source.

2. Rapid Service
   ◇ 24/7 technical response; quotations within 24 h; MES system enables real-time production tracking.

3. Zero Defect Commitment
   ◇ 19 quality control checkpoints; 100% OQC inspection; defect rate <0.01%.

4. One-Stop Solution
   ◇ From component sourcing to finished assembly — full process traceability, minimizing supply chain risks.

Conclusion

OQC inspection for PCBA is not an isolated final step but an extension of production quality control and the realization of customer quality commitment. By defining clear standards, optimizing inspection workflows, enhancing personnel capability, and enforcing closed-loop defect management, manufacturers can not only intercept nonconforming products efficiently but also use OQC data to drive upstream process improvements — ensuring that every PCBA delivered meets customer expectations and underpins the reliability of the final product.