Protective Coating
Metallic or organic layer applied to the exposed copper pads of a printed circuit board prevents oxidation and ensures reliable component attachment. This protective layer, known as the surface finish, maintains the solderability of the boards during storage and multiple reflow runs. It is selected based on the board’s design requirements, assembly processes, and reliability standards.
Soldering Performance
Ensuring proper wetting of the solder onto the pad requires a flat and stable finish that can withstand reflow temperatures. For instance, an electroless nickel immersion gold surface finish provides both a robust solderable surface and a long shelf life, while organic solderability preservatives offer a lower-cost option but with shorter storage limits. Inadequate or contaminated finishes result in poor wetting, pad de-wetting, and brittle solder joints.
Selection Criteria
Balancing electrical performance, mechanical robustness, and total cost dictates the choice of finish for a given design. For high-density boards with fine-pitch components, flat finishes like immersion tin or electroless nickel immersion gold are preferred over hot air solder leveling. Evaluating the storage environment and the planned assembly process is necessary to select a surface finish that matches the design constraints of the circuit board.
This selection holds a major influence over the assembly yield, as it dictates the required flux type and reflow temperature profiles.
