Automotive PCBA Cold Solder 8D Case — QFN Rework & A...

Case Overview Product: Engine Control Unit (ECU) PCBA for a Tier 1 automotive supplier Component: QFN-32 microcontroller package Failure Mode: Intermittent connection — cold solder joint on pin 15 causing CAN bus communication loss Detection: Customer end-of-line functional test caught 12 failures out of 2000 units (0.6% failure rate) Customer: European luxury OEM, 8-week escalation timeline D2 Problem Description (5W2H) What: QFN-32 pin 15 intermittent open circuit. IMC layer <1μm (specification ≥2μm) Where: ECU PCBA, assembly line #2, reflow oven #3 When: First detected in Lot #ECU-2026-03, manufactured Week 12 Who: Detected by OEM at vehicle assembly end-of-line functional test Why (spec): CAN communication fails when pin 15 resistance exceeds 50mΩ. Joint spec requires IMC ≥2μm How: Functional test detects CAN timeout error How Many: 12/2000 (0.6%, 6,000 PPM). Target <100 PPM D4 Root Cause Analysis 5-Why Chain Why intermittent connection? → Cold solder joint — incomplete IMC formation between solder and PCB pad Why incomplete IMC? → Peak reflow temperature at QFN body only reached 225°C (required 235°C minimum) Why low temperature? → Large ground plane (4-layer PCB, 2oz copper) under QFN acted as heat sink, drawing heat away from solder joints Why thermal mass effect not compensated? →...