Silver and microvoids
Here is yet another reason to use NSMD (non solder mask defined) pads for your BGA placements. We generally recommend NSMD pads for your BGA placements because this technique tends to result in a better mechanical connection. I ran across an article in PCB007 that gives another reason. Apparently SMD (solder mask defined) pads are more susceptible to microvoid problems; especially with immersion silver boards.
If you are using immersion silver, ask your board house some difficult questions about their reliability, board shelf life and process control. If their plating process is poorly controlled, you can get uneven deposition and may end up with microvoid problems. Poorly processed silver boards can also tarnish when exposed to air and can give a very short pre-assembly shelf life if not properly sealed.
Another good resource is the website www.microvoids.org. They cover a lot of detail relating to microvoids and surface tarnish. If you need boards fabbed with immersion silver board finish, check out our partner, Sunstone Circuits. They do a pretty good job with the immersion silver process.
Silver keeps the bacteria down too