Back at the Embedded Systems Conference in September, I had a number of folks ask me about mixing leaded and lead-free components on a PCB. It's a difficult situation for some people - especially when using old and very new BGA form-factor components.
We generally tell people to follow the BGA. Since the BGA has those little solder balls on it, it's the most sensitive to temperature as far as soldering is concerned. Reflow a leaded BGA at no-lead temperatures and the flux may all burn off and the solder may sag down too far and bridge or dry and crack. Do the reverse and reflow a no-lead BGA at leaded temps and you won't get a good intermetalic mix and the solder joint will be prone to cracking and other bad stuff.
In most cases no-lead components, other than BGAs can be used on a leaded board. Going the other way isn't always so easy though because of the additional 20 degrees C in the no-lead process. Everything's more sensitive to moisture absorption so baking parts or keeping them sealed in moisture-free packaging is more important. Some components may melt, especially chip LEDs. And metal can capacitors can pop.
In a prototype world, where you just need to see if something works, you can sometimes get away with a lot more than you can in production, but it's still not an easy question to answer. Unfortunately if you're in the situation of one of the guys that asked about it and have one leaded BGA and one no-lead BGA, you may have to get one of the BGAs re-balled or you may just need to redesign on of them out. No easy answer there.
My 24 hours is almost come
When I to sulphrous and tormenting flames
Must reflow up myself