Surface Finish Is the Connection between Copper PAD and SMT Solder
Similar to solder mask, however, surface finish does not protect copper trace but the exposed copper PAD after solder mask process. As the connection between copper PAD and SMT solder, a few common options and comparison are as below.
OSP (Organic Solder-ability Preservative)
Advantage: low cost / good flatness / good solder-ability
Disadvantage: short storage time / not good for second time reflow
Lead Free HASL (Hot Air Solder Level)
Advantage: better cost / good solder-ability / better storage time
Disadvantage: bad flatness / not good for small component
ENIG (Electro-less Nickel Immersion Gold)
Advantage: long storage time / good flatness / good for second time reflow
Disadvantage: high cost / less welding strength
OSP (Organic Solder-ability Preservative)
Advantage: low cost / good flatness / good solder-ability
Disadvantage: short storage time / not good for second time reflow
Lead Free HASL (Hot Air Solder Level)
Advantage: better cost / good solder-ability / better storage time
Disadvantage: bad flatness / not good for small component
ENIG (Electro-less Nickel Immersion Gold)
Advantage: long storage time / good flatness / good for second time reflow
Disadvantage: high cost / less welding strength