CIS Cleaning & Contamination Control Flashcards
6 cards from real CIS practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 CIS Cleaning & Contamination Control flashcards as text
What is 'electrochemical migration' and how does contamination contribute to it?
Answer: Metal ion movement through the PCB laminate caused by voltage gradients in the presence of moisture and ionic contamination
Electrochemical migration (ECM) occurs when metal ions dissolve and migrate across the surface under a voltage bias in the presence of moisture, often seeded by ionic contamination, potentially forming conductive dendrites that cause shorts.
What is the acceptable ionic contamination level per IPC-CH-65 for a Class 2 PCB assembly (ROSE method)?
Answer: 1.56 µg NaCl eq/cm²
The traditionally referenced ionic cleanliness limit is 1.56 µg NaCl equivalent per cm² (10.06 µg/in²) as established by older MIL-P-28809 criteria, still widely referenced for Class 2.
Why is deionized (DI) water used in aqueous PCB cleaning rather than tap water?
Answer: DI water is free of ionic minerals that could leave conductive deposits on the assembly
Deionized water removes mineral ions, preventing ionic contamination deposits on the board; using tap water would deposit minerals (calcium, magnesium, etc.) that are conductive.
Which of the following is NOT a typical source of ionic contamination on PCB assemblies?
Answer: Solder mask pigments
Solder mask pigments (the materials that give solder mask its color) are not a significant source of ionic contamination; human perspiration, flux residues, and atmospheric contamination are common ionic sources.
What does SIR (Surface Insulation Resistance) testing evaluate on a PCB assembly?
Answer: The electrical resistance between conductors under controlled temperature and humidity, assessing contamination impact on leakage current
SIR testing measures the electrical resistance between specific conductor patterns under elevated temperature and humidity, providing a direct measure of how contamination affects leakage current and insulation integrity.
Ultrasonic cleaning is generally NOT recommended for PCB assemblies because:
Answer: Cavitation energy can damage wire bonds, crystal oscillators, and MEMS devices
Ultrasonic cleaning can damage sensitive components like wire bonds, crystal resonators, and MEMS through cavitation-induced mechanical vibration, making it unsuitable for populated PCB assemblies.