It is possible to assemble Lead Free BGA with tin/lead HASL but, as mentioned in previous compatibility issues, this is not recommended. The issue is the segregation of the lead from the board finish. Is the BGA the only lead free device on the board and is your intention to assemble with Sn/Pb or Lead-free solder? It is difficult to answer with out having a better picture of the overall assembly and final requirement for this assembly.

One of many issues raised by the transition to Pb-free manufacturing is mixed metal assembly ( http://www.meecc.com/pres2006/Track%203%20-%20Poole.pdf). For a situation like this, when using Pb-free solder paste (e.g. SAC alloy based), higher reflow temperatures are used. These higher temperatures alloy for greater chance of homogenous mixing of the SAC and Sn/Pb alloys.

When using a Sn/Pb based solder paste, there will be incomplete mixing of the alloys leaded to a solder joint with severely impacted reliability. (D. Hillman, et al., The Impact of Reflowing a Pb free Solder Alloy Using a Tin/Lead Solder Alloy Reflow Profile on Solder Joint Integrity, International Conference on Lead-free Soldering, CMAP, Toronto, Ontario, Canada, May 24-26, 2005). In either case the addition of Pb to the Pb-free solder joint will impact the reliability of the final solder joint and should be tested. The more homogenous the overall solder joint, the better the overall reliability (J. Pan, et al., IPC/JEDEC Pb-free Conference, San Jose, 2006.).