Surface Preservative
Printed circuit board finishes use thin organic coatings applied directly to exposed metal to prevent oxidation during storage and transport. In this process, osp copper pads receive a protective coating of benzotriazole or imidazole compounds that chemically bonds with the copper. This layer prevents corrosion while remaining thin enough to dissolve during the soldering process.
The underlying copper is preserved until it makes contact with flux and molten solder during reflow.
Assembly Performance
During assembly, the applied solder flux dissolves the organic preservative layer, allowing the molten solder to wet directly to the pristine copper surface. This reaction occurs quickly during the first reflow cycle, producing high-strength intermetallic bonds. Because the finish has no metal-on-metal boundary, it does not suffer from defects like black pad that are associated with nickel-based finishes.
This finish remains a cost-effective choice for fine-pitch surface mount devices and multi-layer assemblies. However, the organic layer must withstand multiple thermal cycles if double-sided assembly is required, as exposure to heat can degrade the protective film before the second side is soldered.
Handling Limit
Protective organic coatings are susceptible to degradation from humidity and fingerprints, requiring strict handling procedures. Boards with this finish have a limited shelf life and should be stored in sealed vacuum-moisture barrier bags to prevent premature degradation. Physical contact with bare hands destroys the protective layer and can lead to dewetting during assembly.
Chemical exposure to aggressive solvents will also strip the organic film, exposing the copper to immediate oxidation.
