2.23 ICT & Fixture Design

In-Circuit Test (ICT) is the fastest, most effective way to eliminate 90% of all common structural defects (opens, shorts, wrong parts) in high-volume production. The success of ICT hinges entirely on the NRE (Non-Recurring Engineering) investment: the fixture. This fixture must achieve a precise, repeatable handshake between the tester's electronics and the PCB's circuitry hundreds of thousands of times without stressing or damaging the board. Failures in ICT are almost always mechanical — dirty pins, worn support plates, or poor Design for Testability (DFT).

2.23.1 ICT Fundamentals and Fixture Requirements

ICT works by powering up the board safely and isolating individual components or nets to measure resistance, capacitance, and basic functionality. The fixture is the mechanism that delivers the measurement probes.

The Fixture's Mandate

1. Contact: Reliably and repeatedly touch every required test point, net, or component lead (the "bed-of-nails").
2. Support: Hold the PCB perfectly flat under immense probing force without bowing (oil-canning) or damaging sensitive parts.
3. Safety: Safely apply and limit current during power-up checks and protect sensitive nets from external noise (guarding).

2.23.2 Design for Testability (DFT) on the PCB

The PCB design determines the cost and speed of the final ICT process. DFT rules are mandatory to ensure fixture viability.

2.23.3 Fixture Mechanics: Force, Support, and Tip Selection

The mechanical design ensures that the high probing force (3-8 ounces per pin) is evenly managed.

Probe Field and Force

• Probe Grid: 100 mil (2.54 mm) is the default for a robust, durable fixture. Tighter grids (75 mil) increase cost and pin wear significantly.
• Total Force Management: The total downward force from hundreds of pins can exceed 200 lbs. This requires extensive use of backup pins placed beneath large components (like BGAs) and vulnerable areas to prevent board bowing or oil-canning under pressure.
• Actuation: Vacuum fixtures are standard for high-volume inline use due to speed and consistent force application. Clamshell/Pneumatic fixtures are preferred for debug benches and odd-shaped boards.

Probe Tip Menu

Tip selection is a maintenance decision; aggressive tips wear faster but ensure contact.

2.23.4 Electrical Design and Maintenance

Clean measurements and fixture longevity are achieved through disciplined electrical routing and maintenance.

• Guarding & Shielding: Guarding nets are routed close to high-impedance nets to shield them from noise, ensuring accurate leakage and high-resistance measurements. Shielded harnesses are used from the fixture to the test matrix.
• Power-Up Strategy: Always run power-off shorts/opens checks first. Power rails must be applied through current-limited power supplies with fast cut-off to prevent catastrophic failure in the event of a short.
• Probe Life Cycle: Pins have a finite life 100k – 500k cycles). Probes must be replaced by refdes group or zone based on hit count tracking, before contact noise (random opens/false failures) starts to appear.
• Self-Test: Implement a fixture loopback coupon (or internal wiring check) that runs daily to verify continuity and probe health before blaming a board.

Final Checklist: ICT Fixture Release

This table summarizes the mandatory checks for both the PCB design (DFT) and the physical fixture before it is released to the line.