Manufacturing PCBs for DDR3 memory
You have some things to consider in the layout process. It is common to use the required narrow trace (you indicated 5 mil) to breakout from the BGA pin field. As soon as you clear the pin field you expand the trace width to achieve the desired impedance for the route over to the DDR3 chip. Of course that routing depends upon your stack-up and calculated trace width to achieve the needed 50 ohm impedance. The other considerations relate to total available area to route and any necessary length matching that may be required. 

Depending upon the DDR3 chip footprint it may be necessary to again neck down the trace a bit of there is a pin field to route through to the final pad.