Bare copper oxidizes quickly when exposed to air, and solder does not easily attach to oxidized copper. Therefore, it is common practice to treat the exposed copper (pads and plated holes) with a surface finish. The choice of surface finish is a matter of balancing cost, planarity, shelf life and other characteristics. The most commonly used surface finishes are listed here with a brief overview of their characteristics.
Lead-free HASL
This is probably the most commonly used finish in Europe. Low cost and RoHS compliant. Generally good solderability and long shelf life. Poor planarity; not suitable for fine pitch SMD components. Applied with a high-temperature process that can stress the PCB.
ENIG Gold (Ni/Au)
This finish has good planarity, gold solderability and long shelf life. It is RoHS compliant. High cost.
Immersion silver
Moderate cost, good planarity and RoHS compliant. Limited shelf life.
Electroless tin (Immersion tin)
Moderate cost, good planarity and RoHS compliant. Limited shelf life. Limited availability.
OSP
Moderate cost, good planarity and RoHS compliant. Limited shelf life. Limited availability.
HASL (non-RoHS)
Low cost. Generally good solderability and long shelf life. Poor planarity; not suitable for fine pitch SMD components. Applied with a high-temperature process that can stress the PCB. This finish contains lead and is not RoHS compliant. Most European users may not use this finish.