Lecture Notes: Dealing with Moisture and Sensitive Devices on PCB

Date: March 20, 2023

Introduction

• Continuous evolution of electronic devices leads to smaller, high-performance PCBs.
• PCBs need protection to avoid corrosion and failures from moisture and contaminants.
• Moisture can cause signal lags and increased propagation delays, leading to circuit failure.

Impact of Moisture

• Moisture Effects on PCBs:
  • Signal transmission lag
  • Increased propagation delays
  • Potential complete circuit failure
• Sensitive Components:
  • Components like BGA and chip scale packages are particularly sensitive.
  • Damage from moisture is often hard to detect.

Sensitive Components and MSD

• Even minimal moisture is harmful during operation or storage.
• Pure water is an insulator, but dissolved substances make it conductive.
• Condensation can form if PCB surface is colder than surroundings.
• Moisture Sensitive Devices (MSD) require protection during assembly and storage.
• Reflow soldering process can cause expansion or cracking of packages due to moisture.
• Delamination and microcavities can occur if moisture is not removed during welding.

IPC Standards for Sensitive Devices

• IPC/JEDEC J-STD-020E: Classification for reliability qualification.
• IPC/JEDEC J-STD-033D: Handling, packaging, and shipping MSDs to increase yield and limit degradation.
• Moisture Sensitivity Levels (MSL):
  • Ranges from 1 to 6, indicating susceptibility to moisture-induced damage.

Protection Techniques

Manufacturing

• Control temperature and moisture during lamination.
• Use clean gloves to avoid contamination.
• Low-pressure vacuum can prevent moisture gaps.
• Meshed copper plates for stronger bonds and moisture prevention.

Assembly

• Soldering at high or low temperatures with controlled humidity.
• Baking boards in oven post-assembly to remove moisture.
• Conformal coating for protection against moisture, fluids, contaminants, UV rays.
• Encapsulation or partial coating with epoxy resins as alternatives.

Storage

• Store boards in MBB protective bags, not just ESD protection envelopes.
• Follow IPC standards for packaging and storage.

Conclusion

• Miniaturization and density increase require new PCB protection strategies.
• Effective protection techniques during production, assembly, storage, and shipping can greatly enhance PCB reliability.

Additional Note:

• Create an account on Proto-Electronics.com for PCBA quotes.