| Silver is a widely used surface finish for surface mount
PCB's. Its relatively cheap and works well during surface mount
assembly. Although PCB shelf life is very poor, somewhere in the 3-6
months range, dependent mainly on storage and the condition of the
working solution used to apply it. PCB's that have exceeded their shelf
life normally sill look the same, unfortunately an intermetalic layer
will have formed at the silver copper junction. In extreme cases a small
tap has resulted in all the surface mount components dropping off, in
less extreme cases dry joints result. If multiple heat excursions are
used to assemble a mixed technology board, it's highly advisable if the
boards have been stored to fully assemble a sample first.
Silver is another traditionally banned metal for our industry, but
when used in a thin layer the risk of whisker growth is supposedly
small. One of it's biggest downfalls is the rapid risk of surface
tarnishing, it only takes a splash of tap water containing chlorine or
even simply been handled without protective gloves. Very often an
OSP style coating is also applied on top to
protect the silver, which with the recent developments in
OSP does raise the serious question as to quite
what the silver is actually there for? Best answer is probably
cosmetics, having made the
ROHS leap we still want
everything to aesthetically look the same as before.
It's best to always have full solder masking so that the no exposed
silver coated areas remain after assembly. A recent good example of a
bad application using silver was a car Key Fob, it's once bright battery
connecter pad had turned jet black and turned from conductor to
insulator.
Please note P & M Services recommend prompt assemble if this surface
finish is used. If boards are to be stored for any reason before
assembly keep them tightly wrapped interleaved with acid & chorine free
tissue paper in a cool damp free environment. Many thanks to
Macdermid
for the Storage
and Handling data sheet essential reading if you plan to specify
this finish. Oddly enough eventhough silver is supposedly a green
and ROHS compliant, it is a most unwelcome element in our trade effluent
stream, as dating back to it's mass use in photographic and surface
finishing industries only small amounts are known to poison the good
bacteria in sewage works. From a manufacturing and environmental
perspective we advise skipping the silver +
OSP and using just OSP, as a longer circuit
board shelf life and greener environment result. |
