Hi,
Just quoting...
"The contaminant is basically naturally occurring
lead, which has decay particles in the uranium sequence. There are six
decay products that emit an alpha, with half lives from 4.5 billion
years (uranium itself) to tens of hours (some of the polonium decay
products. No other emission causes any problem (only the alphas).
The rate of upset is one in every 88 days for a 2V6000, as measured in
my lab, on 100 devices that are being monitored now for more than
200,000 device hours. The alphas in this sequence range from about 4
MeV, to about 8.5 MeV, and penetrate to the devices from the solder
bumps in the flip chip package. They can not be detected outside the
package, as they are absorbed by the various packaging materials, and do
not have enough energy to go all the way through the silicon die itself."
You can also find an document on xilinx regarding the subject (sorry, but I could't find it on my computer....)
AMCC