To improve conductivity, enhance adhesion, or for other reasons, manufacturers often adopt specific surface treatments for PCB. While these treatments offer many benefits, they can also pose challenges during subsequent PCBA processing. One common issue is discoloration of ENIG surfaces after reflow soldering, especially on selective OSP ENIG boards. In this article, we’ll explore the reasons behind this phenomenon and how to prevent it.
Understanding the Causes
In the processing of selective OSP ENIG boards, the ENIG surface treatment is typically completed before the OSP treatment. However, the ENIG surface may not receive special protection during the OSP treatment, leading to discoloration. This issue is closely related to the OSP treatment process, with the following primary causes:
- OSP Chemical Reaction: Some OSP chemicals may react with the gold surface, forming complexes that result in discoloration after reflow soldering. This is similar to the discoloration observed after OSP processing.
- Process Oversights: PCB cleaning is an essential part of the manufacturing process to avoid residue buildup. Some manufacturers may overlook this step, allowing residual impurities to cause discoloration after reflow.
Preventive Measures
To address the issues outlined above, consider the following preventive measures:
- Choose Compatible Chemicals: Ensure that the OSP chemicals used are compatible with the gold surface treatment to avoid chemical reactions that lead to discoloration.
- Follow IPC Standards: Establish and adhere to production specifications that meet IPC standards, ensuring that every step of the manufacturing process maintains quality and reliability.
Ending
ENIG surfaces are not easily oxidized, and the observed “discoloration” is often due to organic residues on the surface undergoing changes. In most cases, pure ENIG boards that exhibit discoloration do not require special treatment. However, if discoloration occurs on selective OSP ENIG boards, it is advisable to perform a cleaning treatment. This not only helps eliminate the discoloration but also ensures that any OSP films affecting electrical contact are removed.
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