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Dewetting

Understanding Wetting Issues in PCB Assembly: Non-Wetting vs. Dewetting

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Wetting issues in PCB assembly can significantly impact the quality and reliability of solder joints. These issues are generally classified into two categories: Non-wetting and Dewetting. Both conditions can lead to defective connections and reduced circuit performance if not properly identified and corrected.

What Is Non-Wetting?

Non-wetting occurs when molten solder fails to bond with the base metal surface. During the reflow soldering process, if the solder cannot adhere to the PCB pads or the component leads, it results in incomplete or weak joints. This is often caused by oxidation, contamination, or inadequate heat, which prevents proper metallurgical bonding.

What Is Dewetting?

Dewetting happens when molten solder initially wets the surface but then recedes, leaving behind small, irregular mounds surrounded by thin solder films. Although the base metal is covered, the resulting joint lacks a full solder fillet, weakening its mechanical and electrical integrity. Dewetting is often associated with poor solderability of the PCB pad or component terminations.

Common Causes of Wetting Issues

Several factors can contribute to non-wetting or dewetting problems:

  • Oxidized PCB pads or component pins, preventing solder adhesion

  • Low soldering temperatures, especially with lead-free alloys

  • Expired solder paste, which leads to inactive flux

  • Thin plating thickness (less than 5 microns), reducing solderability

How to Fix or Prevent Wetting Problems

To minimize wetting issues during PCB assembly, follow these best practices:

  • Store PCBs and components in controlled environments (humidity and temperature)

  • Avoid using PCBs that have been in storage for over one year without protective covers

  • Remove oxidation by dipping the board in 2% sulfuric acid for 2–3 minutes before re-soldering

  • Always check the expiration date of solder paste before use

By maintaining strict process controls and quality standards, you can prevent wetting defects and improve overall solder joint reliability.

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