CIS CIS Cleaning & Contamination Control 2 — Questions and Answers
Question 1: What is 'electrochemical migration' and how does contamination contribute to it?
- Metal ion movement through the PCB laminate caused by voltage gradients in the presence of moisture and ionic contamination (Correct answer)
- The migration of solder alloy elements due to thermal cycling
- Electrostatic buildup that causes components to shift during reflow
- Copper dissolution into flux during wave soldering
Correct 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.
Question 2: What is the acceptable ionic contamination level per IPC-CH-65 for a Class 2 PCB assembly (ROSE method)?
- 1.0 µg NaCl eq/cm²
- 1.56 µg NaCl eq/cm² (Correct answer)
- 3.1 µg NaCl eq/cm²
- 5.0 µg NaCl eq/cm²
Correct 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.
Question 3: Why is deionized (DI) water used in aqueous PCB cleaning rather than tap water?
- DI water has a lower surface tension, improving penetration under components
- DI water is free of ionic minerals that could leave conductive deposits on the assembly (Correct answer)
- DI water is heated to a higher temperature than tap water in industrial cleaners
- DI water contains surfactants that improve flux removal
Correct 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.
Question 4: Which of the following is NOT a typical source of ionic contamination on PCB assemblies?
- Perspiration from ungloved human hands
- Flux activator residues
- Solder mask pigments (Correct answer)
- Atmospheric contamination in high-humidity environments
Correct 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.
Question 5: What does SIR (Surface Insulation Resistance) testing evaluate on a PCB assembly?
- The mechanical peel strength of solder mask from the copper surface
- The electrical resistance between conductors under controlled temperature and humidity, assessing contamination impact on leakage current (Correct answer)
- The resistivity of the PCB laminate base material
- The adhesion of conformal coating to the board surface
Correct 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.
Question 6: Ultrasonic cleaning is generally NOT recommended for PCB assemblies because:
- It is too slow for production volumes
- Cavitation energy can damage wire bonds, crystal oscillators, and MEMS devices (Correct answer)
- It leaves more ionic residue than aqueous spray systems
- It cannot remove no-clean flux residues effectively
Correct 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.
CIS CIS Cleaning & Contamination Control 2
What is 'electrochemical migration' and how does contamination contribute to it?