OSP; Organic Solderability Preservative's have
made great progress in recent years. They are now capable of
multiple solder passes, and have probably the longest shelf life of all
the std ROHS compliant
leadfree finishes. Unlike
silver or tin the
OSP doesn't form an intermetalic layer that results in the short shelf
life and poor solderability of the other metallic finishes.
It's also extremely green environmentally in comparison with the
other common leadfree finishes, which suffer from either more toxic
waste or substantially higher energy consumption.
After much consideration
Macdermid's
M-Coat OSP is currently our
default and favourite leadfree finish. OSP is projected to be used
on 558 million sq ft of circuit boards in 2008 second fiddle to
HASL at 737 million sq ft.
It's best thought of as a an almost invisible
protective lacquer on the surface of the board. In reality the surface
of the board is treated with an acidic cleaner followed by a
persulphate microetch to render to clean and
increase the surface area, at which point it is then given it's
organic coating. The process is selective whereby a reaction take
place with the copper on the PCB to form the coating, only a very minimal
amount is deposited onto the base laminate.
