7.3 Test Process Quality (ICT/FCT & Retest Limits)

Testing is not a value-added process; it is an interrogation. A test station does not fix quality; it merely segregates the survivors from the casualties. In high-volume manufacturing, the most dangerous behavior is "Testing Into Compliance"—the act of re-testing a failed unit until it luckily passes. This is statistical roulette. If a unit fails once, it is suspect. If it passes on the second try without repair, it is a latent defect waiting to fail in the field. The goal of Test Process Quality is to stop the "Freshman Flush" and enforce strict retest logic.

In-Circuit Test (ICT): The Structural Audit

ICT is the electrical equivalent of an X-Ray. It verifies components are present, oriented correctly, and not shorted. It does not prove the device works; it proves the device was built to the schematic.

False Call Management:

ICT fixtures rely on "pogo pins" contacting test points. Dust, flux residue, or slightly warped PCBs cause False Fails.

• Metric: First Pass Yield (FPY) must be > 95%.
• Logic:
  • IF FPY drops below 90%:
  • THEN Stop Line. Clean fixture pins and verify strain gauge stress. Do not adjust test limits to "pass" the noise.

Functional Circuit Test (FCT): The Behavioral Audit

FCT simulates the real-world application. It powers up the board and runs firmware.

The "No Trouble Found" (NTF) Paradox:

A unit fails FCT but passes on the bench.

• Risk: The FCT fixture often imposes mechanical stress or timing constraints different from the final enclosure.
• Action: Correlation analysis. If Bench Pass rate > 90% for FCT failures, the FCT limit set is too tight or the fixture is unstable.

Retest Limits (The "Three Strikes" Rule)

You must define a hard limit on how many times a unit can be cycled through a test station. Infinite loops hide intermittent failures (cold solder joints).

Decision Logic for Retest:

• Pass 1: Unit enters test.
  • Result: FAIL.
  • Action: Check seating/cables. Retest immediately (Pass 2).
• Pass 2 (Retest):
  • Result: PASS.
  • Status: "Conditional Pass".
  • Risk: Intermittent contact.
  • Mandate: Unit must be visually inspected for flux/contamination on pads.
  • Result: FAIL.
  • Status: Hard Fail. Route to Debug/Bonepile.
• Pass 3:
  • PROHIBITED. A unit failing twice is not a "glitch." It is a defect.
  • Action: Tag as defective. Do not re-insert into the line.

Bonepile Management

The "Bonepile" is the graveyard of Failed units. It is also the most valuable source of data in the factory.

Rules of Engagement:

1. Aging Limit: No unit remains in the Bonepile > 3 shifts (24 hours).
2. Disposition: Must be diagnosed (Debugged) or Scrapped.
3. WIP Cap: If Bonepile > 50 units, STOP SMT. You are producing scrap faster than you can fix it.

Pro-Tip: Technicians love to "cherry-pick" easy repairs. Enforce "First In, First Out" (FIFO) on the bonepile. If a board sits for 2 weeks, just scrap it—the troubleshooting cost exceeds the material value.

Final Checklist

Control Element	Parameter	Critical Limit / Rule
Retest Count	Max Attempts	2 (Initial + 1 Retest)
FPY Trigger	Yield Floor	< 90% = Stop Line
Bonepile	WIP Age	< 24 Hours
Fixture Maint	Pin Cleaning	Every Shift (or per counter)
Correlation	Bench vs. FCT	Gap < 5%
Golden Unit	Verification	Run daily to prove tester is sane