In-Circuit Test (ICT)

In-Circuit Test (ICT)

• ICT identifies defects and component failures in PCBAs.
• It occurs post-soldering, pre-functional testing.
• The purpose of ICT is to ensure correct component placement, orientation, soldering, and board functionality.

How ICT Works

• ICT testers use a bed of nails fixture with spring-loaded pins.
• Pins contact specific test points, corresponding to circuit nodes.
• Electrical signals are applied to test points.
• The system measures responses to verify circuit integrity.
• Measurements include resistance, capacitance, inductance, and diode/transistor junctions.
• Measured values are compared to pre-programmed values; deviations indicate faults.

Types of ICT

• Bed-of-Nails Testing uses custom fixtures with spring-loaded pins for direct contact with PCBA test points.
• Flying Probe Testing uses moving probes to sequentially contact test points, eliminating custom fixtures.

Bed-of-Nails Testing

• It's a traditional ICT method that requires a custom fixture specific to the PCBA.
• A bed of nails consists of spring-loaded pins positioned to contact test points.
• When the PCBA presses onto the fixture, pins make electrical contact.
• The ICT system runs tests via these contact points.
• This method is fast and efficient for high-volume production, but fixture costs can be high.

Flying Probe Testing

• Flying probe testing doesn't need a custom fixture, but uses moving probes.
• Probes sequentially contact test points.
• It is more flexible, suiting low-volume production, prototype testing, and design changes.
• The process is slower because probes move individually to each test point.

Advantages of ICT

• High fault coverage detects many manufacturing defects and component failures.
• Fast testing, especially with bed-of-nails, is suitable for high-volume production.
• Diagnosis accurately pinpoints fault location and type.
• Early fault detection reduces rework costs.
• Improved product quality ensures that only functional boards ship.

Disadvantages of ICT

• Bed-of-nails testing has high fixture costs.
• Limited access means that some components/test points may be inaccessible.
• ICT needs dedicated test points on the PCBA, potentially increasing board size.
• Developing the test program is time-consuming and requires expertise, and flying probe testing is slower.

Common Defects Detected by ICT

• Open circuits are breaks in the circuit path.
• Short circuits are unintended connections.
• Missing components were not placed during assembly.
• Incorrect components have the wrong value/type.
• Misoriented components are placed in the wrong direction.
• Wrong polarity means component polarity is reversed.
• Solder bridges are unintended solder connections between pads/pins.
• Component value errors include incorrect resistor, capacitor, or inductor values.

External Visual Inspection (EVI)

• EVI detects PCBA defects through visual examination.
• It's a non-destructive technique, often performed at various manufacturing stages.
• EVI can identify issues missed by automated testing.

Objectives of External Visual Inspection

• Defect Detection identifies visual defects affecting functionality/reliability.
• Process Monitoring identifies and addresses defect-causing manufacturing issues.
• Quality Control ensures that PCBAs meet quality standards before shipping.

What to Look for During External Visual Inspection

• Component placement: Ensuring correct positioning.
• Orientation: Verifying correct orientation like capacitor polarity and IC pin 1.
• Solder joints: Inspecting for proper wetting, smooth fillets, and defect absence.
• Solder bridges: Identifying shorts between pads/pins.
• Solder balls: Looking for solder balls that can cause reliability issues.
• Missing components: Verifying the presence of all required components.
• Damaged components: Looking for cracks, chips, or broken leads.
• Board cleanliness: Ensuring there's no flux residue, dirt, or contaminants.
• Trace defects: Looking for breaks, scratches, or corrosion on traces.
• Labeling: Confirming correct labels/markings.

Tools Used for External Visual Inspection

• Magnifying glasses improve visibility for small details.
• Microscopes provide higher magnification for detailed inspection.
• Controlled lighting is essential for detecting defects.
• Inspection mirrors help inspect hard-to-reach areas.
• Measurement tools (rulers, calipers, gauges) measure component placement/dimensions.
• ESD protection (wrist straps, grounding mats) protects against electrostatic discharge.

Advantages of External Visual Inspection

• EVI is low-cost compared to automated methods.
• Simple implementation requires minimal equipment/training.
• It catches visual defects that automated testing may miss.
• Immediate feedback allows for quick corrective actions.

Disadvantages of External Visual Inspection

• EVI is subjective and reliant on inspector skill/experience.
• It can be time-consuming, especially for complex boards.
• Limited fault coverage means that it may miss internal component failures or subtle electrical issues.
• Results can vary depending on the inspector and conditions, and human error can occur.

Best Practices for Effective External Visual Inspection

• Thorough training for inspectors on defect recognition, procedures, and standards is vital.
• Clear, objective inspection criteria for each defect type should be established.
• Use standardized procedures to ensure consistency and repeatability.
• Provide adequate and controlled lighting to improve visibility.
• Use magnifying glasses or microscopes to inspect small details.
• Schedule regular breaks to prevent fatigue and maintain accuracy.
• Give feedback to the manufacturing process on defects found during inspection.
• Document all defects, including type, location, and severity.
• Continuous improvement should be based on feedback and data analysis.

Complementary Nature of ICT and EVI

• ICT is valuable for detecting electrical faults.
• EVI is critical for identifying visual and mechanical defects.
• Combining both methods provides thorough quality control.
• EVI can catch errors that ICT might miss, such as bent leads or misaligned components.
• ICT confirms electrical connectivity and component functionality.
• The combined approach enhances product reliability.
• It prevents defective products from reaching customers.