Immersion Silver seems to be gaining popularity as a RoHS board finish. It is less expensive than Electroless-Nickel Immersion Gold (ENIG), safer then Immersion Tin, easier to use than Organic Solder Preservative (OSP) and has a more level surface than Hot Air Surface Leveling (HASL).
Early versions of the process tended to create boards with a very short shelf-life due to silver migration, microvoids and tarnish. Current processes are much better but we have found that silver boards still need better handling and closer inspection to maintain solderability.
Here is an example of simple tarnish. This board has been exposed to the air for about three months. This board could probably still be used but would need some careful cleaning. At first glance, the pattern looks a bit like the uneven surface that is common with HASL boards, but the surface is perfectly flat. The dark area is tarnish, not a shadow. Click on the image to see it in close up.
In the second example, the silver has migrated leaving a completely unusable patch-work board surface. This is more of a worst case, having been exposed to air for nearly a year. We would never attempt to assemble parts on a board like this. It also illustrates why solder mask is so important on Immersion Silver surfaced boards.
If you do choose to use Immersion Silver boards,
- keep them sealed and avoid storing them in bright areas.
- Use them as soon as you can.
- Carefully inspect them before use.
As time goes by and the industry gets more experience with this surface, I'm sure it will become as reliable and robust as we all want and need it to be, but until then, use it but use caution.
And silver kills salmonella bacteria too.