The issue with any BGA is mass related, and profile to the circuit. Getting enough energy into the device is the challenge and managing the energy transfer rates at the edges of the Part to the centre of the part are also difficult. One alternative methodology being used is Vapor Phase. The energy management issues are overcome and the parts can be successfully removed and replaced in an Oxygen free environment.

We are possible the manufacturer of your rework system - PDR. We use a 'black conductive tape' which does two things . . . it better conducts and distributes the heat . . . and secondly, it corrects the emmissivity of the component surface . . . so that you can get an accurate temperature reading. Metal components should be no problem. Any further problems contact me.

The problem you are facing is due to the IR energy bouncing off the reflective surface of the metal topped BGA. Your easiest solution is to switch to a convection rework system that uses hot air from the bottom of the board and the top of the component to melt the solder. A convection rework machine will provide even heating regardless of the reflectivity of the surface being reworked. In addition, the hot air provides a gentler ramp up to reflow, better emulating the oven reflow process.