Mixing Leaded and lead-free BGAs
gCan you mix lead and lead-free when using BGAs? Well, no, and yes, sometimes or maybe. It would be nice if there were an easy answer, but there really isn't.
At first glance, it's pretty safe to just say no, you can't mix. The processing has to go with the BGA. Lead-free BGA means the whole board has to me lead free. Leaded BGA means the whole board needs to be leaded. That's the safe way to go, but it isn't always possible.
You can take a lead-free BGA and put it on a board with leaded solder paste and lead-free temperatures (below 217C). You will be putting the board at risk for cracks at some of the solder balls due to the stresses caused by uneven heating and cooling.
If the rest of your componentry can handle the extra heat of a lead-free reflow profile and you need to put a lead-free BGA in a leaded build, you can use either a lead-free paste or leaded paste and run it through a lead-free profile. Industry studies have shown (1, 2) that if the two alloys are allowed to fully mix, the BGA will reliably adhere to the board, at least for prototype purposes.
Going the other way (leaded BGA on a lead-free board) may not be possible. The leaded balls might over heat, over sag and end up cracking. In this case, since the board would be out of compliance anyway, it would probably be better to just run the whole board as a leaded project.
(1) Fay Hual, etal, Intel Corporation, "Solder Joint Reliability Assessment of Sn-Ag-Cu BGA Components Attached with Eutectic Pb-Sn Solder"
(2) P. Snugovsky, etal, Celestica, "Theory and Practice of Lead-Free BGA Assembly Using Sn-Pb Solder"