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).

5.2.23.1 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).

5.2.23.2 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.

DFT Parameter	Mandate	Why It Matters
Test Pad Size ø	1.0 – 1.2 mm for a 100 mil grid. 0.7 – 0.9 mm for a 75 mil grid.	Provides a large, clean target for the probe tip (crown/cup) without skidding.
Test Point Surface	Round ENIG pads with open solder mask (not tented).	ENIG provides a consistent, corrosion-resistant surface for clean electrical contact.
Component Keepouts	Mark no-crush zones (pressure post keepouts) for crystals, large capacitors, and wire-bonded ICs.	Prevents mechanical damage/cracking when the top plate presses down.
Tooling Holes	At least 3 precision tooling holes for X/Y/θ alignment.	Guarantees repeatable mechanical registration for the fixture frame.
Kelvin Pads	Two adjacent pads on high-current paths (e.g., fuses, shunts, power rails).	Allows for 4-wire resistance measurements, eliminating probe resistance from the reading for high accuracy.

5.2.23.3 Fixture Mechanics: Force, Support, and Tip Selection

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

A) 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.

B) Probe Tip Menu

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

Tip Style	Best Use Case	Note
Crown / Cup	General purpose, standard solder-masked pads, ENIG.	Most common; good balance of contact and wear.
Spear / Conical	Probing into small vias or through slightly oxidized surfaces.	Aggressive; ensure adequate support to prevent board drilling.
Kelvin Pair	Low-ohm measurements (shunts, fuses, power nets).	Requires two probes per net, minimizing measurement error.

5.2.23.4 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.

Status	Item	DFT Check (PCB Design)	Fixture Check (Fabrication)
Contact Access	Test Pads	≥ 95% structural coverage achieved on the bare PCB netlist.	Probe map matches CAD; all required nets are probed.
Mechanical Integrity	Board Support	Keepouts marked for all tall/fragile components; tooling holes present.	Backup pins placed under BGAs/vulnerable areas; pressure plate verified for safe clearance.
Electrical Safety	Power/Sensing	Kelvin pads added for low-ohm/high-current nets.	Power-on sequence is current-limited and interlocked (no power unless fixture is closed).
Maintenance	Probe Type	All required pads are ENIG or defined for acceptable probe tip wear.	Spare probe kits (10% spares for each part number) and cleaning supplies are stocked.
Program Lock	Software	BSCAN (JTAG) pads are chained and verified in the netlist.	Program is debugged and Golden Board verified; sequence runs inside the TAKT time budget.