I using a old harddrive as a example. You can salvage most any surfacemount, BGA or even through hole parts using this method.
Step 1: Remove the PCB from anyother casings.
Here I have just a few screws to remove.
Step 2: Heat up area using heatgun.
Here I am going to heat the area around the yellow SMT parts in the top left corner.
After heating the area. Watch the solder to turn shiny to show that its flowing, You can then remove the parts using tweezers or needle nose pliers. Then place in a safe place to cool off.
Be careful especially with the smaller parts or the parts that might be heat sensitive. The air from the heatgun can blow small parts around. You also don't want to burnout the parts you are trying to save.
Step 3: Parts are removed.
This picture shows the parts removed.
I have removed through-hole, BGA, SMT parts using this method. For some parts heating the backside of the PCB and letting the parts fall off can be faster. This only works with parts big enough to fall off.
Also I have seen some parts seem to be glued to the board and are more difficult to remove. So be warned.