The rule is, if a circuit board is manufactured with leaded solder, you shall use leaded solder in the repair process. If lead-free solder is used in production, lead-free solder shall be used in repair process.
The two types of solder shall not be mixed.
When lead-free solder started to appear on the market, we recieved an informational pamphlet at work. It basically said that the lead-free solder is better for the environment, stated the stuff I mentioned above, and also said that it is actually more hazardous to inhale the fumes from lead-free solder than from leaded. I wouldn´t worry too much, though. Just avoid inhaling it, and if you are going to solder on a regular basis, you might want to get a fan which sucks the fumes away.
