How Flying Probe Testing Works for PCB Assembly

Design for testing is crucial in PCB assembly. Various techniques and methods ensure error-free boards. In this article, we explore the essentials of flying probe testers (FPT) in PCB assembly. Initially, in-circuit testers (ICT) were predominant, but FPTs have since become essential.

What is In-Circuit Testing (ICT)?

ICT, or in-circuit testing, checks the functionality of PCB assemblies, using electric probes to verify for shorts, opens, resistance, capacitance, and more. Traditional ICT employs a “bed of nails” fixture with custom pogo pins designed for each PCB assembly, making it costly and time-consuming, especially for complex PCBs. This method suits large production volumes but is neither cost-effective nor time-efficient for smaller or prototype lots.

What is Flying Probe Testing (FPT)?

Flying probe testing uses movable test probes directed by software to contact test points on the PCB. This fixtureless method is highly cost-effective for prototypes and low to mid-volume productions.

Advantages of Flying Probe Testing at CEN Electronic

• Cost-Effectiveness: FPT is ideal for smaller production lots, saving costs on custom fixtures.
• Accessibility: The smaller probe points of FPT access more compact test pads than ICT’s pogo pins.
• Flexibility: Software-controlled probe movements allow easy adjustments and strategy changes.
• Enhanced Coverage: Probes can directly access component pins, providing better test coverage.

At CEN Electronic, we utilize Spa Pilot V8 for flying probe testing. This advanced machine offers high performance, increased speed, and flexibility, making it perfect for prototyping and low to medium volume runs. Simultaneous vertical probing on both sides of the board accelerates the debugging process and enhances prototyping flexibility.

Evolution and Applications of FPT

Initially designed for bare-board testing, FPTs have evolved to measure resistance, capacitance, and inductance, making them valuable for PCB assemblies. The technology’s evolution has led to its widespread adoption for populated boards due to its efficiency and accuracy.

FPT Process and Benefits

Process:

• Programming: An FPT test program is created offline using the PCB assembly’s BOM and ECAD file.
• Setup: The test program is loaded into the FPT tester, and the PCB assembly is placed on a conveyer belt to enter the tester area.
• Testing: Probes contact the pads and vias, applying electrical test signals and power. The tester measures the responses to identify defects.
• Inspection: FPTs perform comprehensive tests, including short, open, and component value checks. They are equipped with cameras for inspecting component polarity and can perform diode impedance tests and capacitive probing for leadless devices like BGAs and QFNs.

Benefits:

• Cost-Effectiveness: Ideal for smaller production lots.
• Accessibility: Smaller probes access more compact test pads.
• Flexibility: Easy software adjustments and strategy changes.
• Enhanced Coverage: Direct access to component pins improves test coverage.

Advanced FPT Features

• Phase Difference Measurement Unit (PDM): Estimates phase differences, enhancing isolation tests.
• High Voltage Stress Test (HVS): Detects high-resistance defects with low power tests.
• Micro Shorts Detection: Identifies and avoids damage from micro shorts with controlled voltage application.

Guidelines for Test Point Design

• Accessibility: Nets linked to active components must be accessible on the same side.
• Contact Points: Prefer test pads without solder resist, vias, through-holes, and PTH/SMD component pads.
• Size: Test points should be 6-20 mils, with a minimum spacing of 10-20 mils.
• Board Clamping: Secure the board with a clamp, providing support every 150 mm.
• Board Dimensions: Follow specific dimensions for Pilot L4 and V8 testers.
• Component Height: Include component height in CAD data.
• Test Point Distances: Maintain proper clearance from components.
• Fiducial Recognition: Use 3 fiducials per board, arranged to eliminate side errors.
• Wire Connection: Avoid collisions with test points.

Flying probe testing has become a preferred and cost-effective method for testing low-volume and prototype circuit boards. Its flexibility, enhanced test coverage, and advanced features ensure high-quality PCB assemblies. At CEN Electronic, our cutting-edge FPT services reduce product design cycles and time to market. For more details on our PCB assembly services, visit our website or contact us today.