What are the Criteria for PCB Inspection?

Introduction

Printed circuit board (PCB) inspection is a critical step in ensuring quality and reliability across the electronics manufacturing process. Both bare boards and assembled PCBs must be carefully examined to validate acceptability before shipment to customers. But what specific criteria constitute a proper PCB inspection? Read on for an overview of inspection objectives, standards, metrics, equipment, and key defect types checked during the PCB inspection process.

Objectives of PCB Inspection

The overall goals of PCB inspection include:

• Screening for assembly errors, defects, and electrical faults
• Verifying all components are properly placed and soldered
• Checking for physical damage like cracks, missing pieces or scratches
• Ensuring design integrity including spacing, hole sizes, feature accuracy
• Validating proper printing and application of solder mask, silkscreen, markings
• Confirming correct board dimensions and hole positions
• Detecting impending reliability risks like thin solder, structural weakness
• Auditing workmanship adherence to acceptability standards
• Providing process feedback to improve manufacturing consistency

Thorough inspection against established criteria minimizes shipping defective boards to customers.

PCB Inspection Standards

Several industry standards guide PCB inspection criteria:

• IPC-A-600 – Acceptability of Printed Boards standard including defect classification
• IPC-A-610 – Acceptability of Electronic Assemblies standard detailing assembly quality requirements
• IPC-6012 – Qualification and Performance Specification for Rigid Printed Boards standard
• IPC-HDBK-830 – Guide for PCBA Process Plating Integrity including inspection criteria
• IPC-AI-641 – User’s Guide for Automated Optical Inspection providing AOI capability guidance
• JEDEC J-STD-001 – Requirements for Soldered Electrical and Electronic Assemblies industry standard

Manufacturers generally define an inspection plan with accept/reject criteria mapped to these standards. IPC standards in particular provide a framework for interpreting the severity of certain defects.

Key Metrics for PCB Inspection

Measurements made during inspection quantify quality:

• Defect Density – Number of defects per area or quantity like defects per square inch or defects per million (DPM) opportunities
• Defect Pareto – Ranking of most frequent defects providing feedback to address root causes
• Process Capability (Cp, Cpk) – Quantitative manufacturing process consistency and centering level
• Yield – Ratio of good boards to total boards inspected, indicating losses
• Escape Rate – Number of defects not caught by inspection reaching customer as fallout failures
• Mean Time Between Failures (MTBF) – Average uptime between board failures, an indicator of reliability

Tracking metrics provides data to continuously improve processes and product quality over time.

Bare Board Inspection

Evaluating bare PCBs prior to assembly involves:

• Automated Optical Inspection (AOI) – Imaging system scans boards at high resolution capturing any defects. Computer algorithms analyze images against CAD data to detect discrepancies.
• Solder Mask Inspection – Validating uniformity and accuracy of solder mask application. Checking for voids, thin areas, misregistration.
• Dimensional Inspection – Measuring critical attributes like board thickness, lengths, hole positions. Confirming within specified tolerances.
• Hole Wall Integrity – Testing plated through-hole conductivity and plating quality using techniques like automated flying probe electrical testing.
• X-Ray Imaging – Captures defects inside boards like cracks, delamination, voiding, drilling errors.
• Thermal Stress Testing – Subjecting boards to thermal shock, cycling, and dwells reveals latent material stresses and workmanship issues through failure analysis.
• Cross-Sectioning – Cutting micro-sections to inspect plating thickness, resin fill, glass weave, and other internal board details under microscope.

Bare board inspection focuses on design, fabrication, and material quality prior to components being mounted.

Assembled Board Inspection

Inspecting completed PCB assemblies checks:

• Bill of Materials (BOM) Accuracy – Confirming correct components installed in each designators position.
• Polarity – Verifying part orientation matches footprint polarity.
• Presence/Absence – Detecting missing parts not mounted or incorrect quantities.
• Component Value – Validating correct nominal value parts installed like correct resistor resistance or capacitor values.
• Placement Accuracy – Checking location placement precision against CAD centroid positions.
• Solder Joint Quality – Inspecting that solder fillets, toes, and bonds meet acceptability criteria.
• Foreign Object Debris (FOD) – Checking no loose particles, dirt, debris, or hair exists on the assembly.
• Mechanical Assembly – Ensuring no bent leads, visible damage, board warpage, discoloration, or scorching.
• Electrical Tests – Running in-circuit tests (ICT), boundary scans, and functional tests to confirm proper electrical performance.

Assembled board inspection focuses on components, soldering, and functionality pre-shipment.

Common PCB Defect Types

Inspection targets many potential defect types:

Soldering Defects

• Insufficient, excess, or missing solder
• Solder balls/beads not wetting
• Solder splashes causing bridging
• Cold solder joints lacking wetting

Assembly Defects

• Misloaded components
• Backwards, misaligned or missing parts
• Bent component leads or terminals

Marking Defects

• Missing fine pitch marking. Polarity or ID silks illegible
• Blurred or incomplete text print and fiducials

Board Defects

• Trace spacing violations or neck downs
• Annular ring gaps around plated through holes
• Copper wicking or voids in plating
• Breakouts from drilling or routing operations

Foreign Matter
-Flux residue shorts

• Metal particles causing electrical failure
• Dirt inclusion reducing solderability

PCB Inspection Equipment

Automated tools are available for streamlined PCB inspection:

• Automated Optical Inspection (AOI) – Vision cameras with sophisticated algorithms rapidly scan boards against CAD models detecting discrepancies. Can inspect solder joints, components, PCB features, markings, and more. High throughput inline operation.
• Automated X-Ray Inspection (AXI) – X-ray imaging reveals hidden defects and confirms BOM, placement, and solder joint quality without destructive dismantling. Especially useful for dens chips, BGAs, and QFNs.
• Solder Paste Inspection (SPI) – Validates volume, area, height uniformity, and print registration of solder paste deposits through 3D optical metrology.
• In-Circuit Test (ICT) – Electrical probes test nodes across circuits verifying correct functionality. Fixtures enable repeatable probe alignment.
• Bed of Nails Test – Spring-loaded test pins make electrical contact at testpoints across board checking continuity, shorts, and opens.
• Flying Probe Test – Automated probe mechanism touches down at all test nodes in a programmed sequence without use of a fixture for electrical verification.

Automated inspection replaces manual visual checking with fast, objective, repeatable assessment against acceptance criteria thresholds.

PCB Inspection Checklist

A general PCB inspection checklist would include:

• PCB substrate defects per IPC-6012 standard
• Etching quality, trace integrity, spacing
• Inner layer registration alignment
• Plating quality – voids, nodules, thickness
• Annular ring dimensions around drilled holes
• Solder mask and silkscreen defects
• Soldering defects like cold joints, bridges
• Correct components installed, polarity, placement
• BOM accuracy and component value verification
• Board cleanliness, markings, and appearance
• Electrical functional test passed
• Review of prior defect history and solved issues
• Adherence to specified tolerance ranges

Every product typically maintains a customized checklist based on critical characteristics and risks assessed from prior builds.

PCB Inspection – Frequently Asked Questions

What magnification is used to inspect PCBs?

Solder joints, machined features, and fine pitch components require magnification. Handheld microscopes range from 10x to over 100x. Machine vision cameras provide up to 200x magnification.

What are common PCB inspection failure points?

Typical defects are thin/insufficient solder, missing or incorrect parts, poor solder fillets, pads lifted from board, scorched areas, flexed boards, and debris inclusions.

How does automated optical inspection (AOI) work?

AOI compares images of board features to CAD data using sophisticated algorithms to identify discrepancies in the placement, assembly, and soldering revealing any defects.

Does PCB inspection impact production rate?

Automated inspection integrates inline without slowing overall rate. Manual inspection can become a bottleneck. AOI offers 10x the inspection points per hour compared to manual.

What information should PCB inspection reports contain?

Reports convey images/details of each defect found, measurement data, defect counts, process capability metrics, repair recommendations, and pass/fail status relative to acceptance criteria.

Conclusion

Rigorous PCB inspection against established acceptability criteria ensures quality and reliability. Both bare boards and assembled PCBs undergo extensive automated optical, electrical, and functional verification using metrics defined by standards. Passing scrutiny across parameters like soldering, components, board fabrication, and electrical operation provides confidence prior to shipment. By institutionalizing robust inspection practices as a core discipline, electronics manufacturers reduce escapes, field failures, and ensure customer satisfaction.

9 Tips of PCB board Inspection

1. It is strictly forbidden to use the grounded test equipment to contact the TV, audio, video and other equipment on the backplane to detect the PCB board without the isolation transformer

It is strictly forbidden to directly test the TV, audio, video and other equipment without power isolation transformer with the equipment that is grounded. Although the general tape recorder has a power transformer, when it comes to a special TV or audio device that has a large output power or a poor understanding of the nature of the power supply used, it is first necessary to find out whether the chassis of the machine is powered or not. The TV, audio and other equipment with the bottom plate are short-circuited by the power supply, affecting the integrated circuit, causing further expansion of the fault.

2. Pay attention to the insulation performance of the PCB soldering iron

It is not allowed to use a soldering iron for welding. To confirm that the soldering iron is not charged, it is best to ground the soldering iron case. Be careful with MOS circuits. It is safer to use a low voltage circuit of 6~8V.

3. Before testing the PCB board, you need to understand how the integrated circuit and its related circuits work

Before checking and repairing an integrated circuit, you must first familiarize yourself with the functions of the integrated circuit used, the internal circuits, the main electrical parameters, the function of each pin, and the normal voltage of the pin, the waveform and the working principle of the components of the peripheral components. If the above conditions are met, analysis and inspection will be much easier.

4. Test PCB board and do not cause short circuit between pins

When measuring voltage or testing waveforms with an oscilloscope probe, the test leads or probes should not be short-circuited between the pins of the IC due to sliding. It is best to measure on a peripheral printed circuit that is in direct communication with the pins. Any short circuit in any moment can easily damage the integrated circuit, and more care should be taken when testing flat-packaged CMOS integrated circuits.

5. Theinternal resistance of board test instrument should be large

When measuring the DC voltage of the integrated circuit pin, you should use a multimeter with an internal resistance greater than 20KΩ/V. Otherwise, there will be a large measurement error for some pin voltages.

6. Pay attention to the heat dissipation of the power integrated circuit when detecting the PCB board

The power integrated circuit should have good heat dissipation, and it is not allowed to work under high power without a heat sink.

7. Detect PCB board leadsshould be reasonable

If it is necessary to add external electronic components instead of the damaged parts inside the integrated circuit, small components should be selected, and the wiring should be reasonable to avoid unnecessary parasitic coupling, especially to deal with the grounding between the audio power amplifier integrated circuit and the preamplifier circuit.

8. Check PCB board to ensure welding quality

It is welded firmly during welding, and the accumulation of solder and the pores are likely to cause solder joints. The soldering time is generally less than 3 seconds, and the power of the soldering iron is applied to the internal heat type of about 25W. The soldered integrated circuit should be carefully viewed. It is best to measure the short circuit between the pins with an ohmmeter to confirm the solderless adhesion and then turn on the power.

9. Do not easily determine the damage of the integrated circuit by detecting the PCB board

Do not easily judge that the integrated circuit is damaged, because most of the integrated circuits are directly coupled. Once a circuit is abnormal, it may cause multiple voltage changes, and these changes are not necessarily caused by integrated circuit damage. In some cases, when the voltage of each pin is measured to be close to or close to the normal value, it may not be sufficient to indicate that the integrated circuit is good, because some soft faults will not cause a change in the DC voltage.