Meaning
Thin layers of metal are deposited onto copper pads of a circuit board to facilitate bonding and prevent the base metal from corroding. Applying a surface finish plating ensures that the pads remain solderable even after months of storage in a warehouse. This process involves chemical baths where gold, silver, or tin is layered over the copper surface through electrolytic or electroless reactions.
Barrier Layer
Nickel is frequently used as a middle coating to stop the copper from migrating into the top layer of gold. This prevents brittleness. The separation stops the formation of weak intermetallic compounds that would otherwise cause the joint to fail under mechanical stress.
Surface Energy
Different finishes alter the way molten solder spreads across the pad during the reflow process. Wetting must be even. A high-quality surface finish plating provides a flat and clean area that allows the metal to bond without leaving gaps or creating cold joints.
Storage Lifecycle
Organic coatings offer a low cost alternative but have a shorter shelf life than metal-based options. Tin finishes are susceptible to the growth of microscopic whiskers that can cause short circuits if the board is stored in a humid environment. Manufacturers choose the specific plating based on the expected assembly method and the final operating environment of the device.
Documentation for each batch tracks the thickness of the deposit to ensure it meets the technical specifications of the customer.
