How PCB Moisture Absorption and Desorption Impact PCB Assembly


How Moisture Absorption and Desorption Impact PCB Assembly

Moisture absorption and desorption in printed circuit boards (PCBs) are critical factors that can significantly affect their performance and reliability. In this post, we will explore the interaction of moisture with PCBs, its impact, and the best practices to manage it.


PCB Materials

PCBs are made from composite materials like epoxy resin, glass fiber, and copper layers. These materials, especially the polymeric resin, are hygroscopic, meaning they absorb moisture from the environment.


How Moisture Gets into PCBs

  • Bulk Diffusion: This is the primary way moisture gets into PCBs. Water molecules move through the resin matrix from areas of high concentration to low concentration, a process described by Fick’s Law of Diffusion.
  • Surface Absorption: Moisture can also cling to the surface of PCB materials, particularly on areas not protected by a solder mask.

Factors Affecting Absorption

  • Humidity and Temperature: Higher humidity and temperature levels speed up moisture absorption.
  • Exposure Time: The longer a PCB is exposed to humid conditions, the more moisture it will absorb.
  • Material Properties: Different materials absorb moisture at different rates. For instance, FR-4 laminates absorb less moisture than other resin-based materials.


The Impact of Moisture Absorption on a PCB Assembly

Physical and Mechanical Impact

  • Delamination: When a PCB is heated rapidly during soldering, absorbed moisture can vaporize and cause internal layers to separate.
  • Warpage: Moisture-induced expansion can warp the PCB, affecting its dimensional stability.

Electrical Impact

  • Conductive Anodic Filament (CAF) Formation: Moisture can create conductive pathways within the laminate, leading to short circuits.
  • Dielectric Properties: Increased moisture content raises the dielectric constant, which can affect signal integrity by reducing circuit switching speeds and increasing propagation delays.

Moisture Desorption in PCBs

1. Desorption Mechanisms

  • Thermal Desorption: Heating a moisture-laden PCB causes the absorbed moisture to evaporate and diffuse out.
  • Vacuum Desorption: Lowering the ambient pressure reduces the boiling point of water, enhancing moisture removal.

2. Desorption Processes

  • Baking: Placing PCBs in an oven at elevated temperatures (typically 100°C or higher) drives out moisture.
  • Vacuum Chambers: Using vacuum chambers to lower pressure can assist in moisture removal at potentially lower temperatures.
  • Climatic Chambers: Controlling humidity and temperature in these chambers creates optimal conditions for moisture desorption.

3. Factors Affecting Desorption

  • Temperature: Higher temperatures speed up moisture desorption.
  • Humidity: Lower ambient humidity levels facilitate faster moisture removal.
  • Exposure Duration: Longer exposure times at the right conditions improve the effectiveness of desorption.

Best Practices for Managing Moisture in PCB Assembly

Preventive Measures

  • Proper Storage: Store PCBs in dry, controlled environments with low humidity.
  • Packaging: Use moisture barrier bags and desiccants to protect PCBs during storage and transportation.
  • Pre-Assembly Treatments:
    – Baking: Bake PCBs before assembly to remove any absorbed moisture.
    – Condition Monitoring: Use sensors to monitor humidity and temperature during storage and processing.

Process Optimization

  • Controlled Environments: Maintain controlled environmental conditions during PCB manufacturing and assembly.
  • Regular Checks: Implement regular checks and maintenance schedules to ensure optimal conditions are consistently met.

Understanding the dynamics of moisture absorption and desorption is crucial for ensuring the reliability and performance of PCBs, especially in applications requiring high reliability and environmental resilience. By implementing these best practices, you can significantly enhance the durability and functionality of your electronic products.