There are a whole host of environmental and safety issues with the methods for separating PMs from the motherboard. They make very large, very expensive machines that will do just that, but I've yet to see a home recycling set up that makes it worthwhile and is safe. For the most part, members of the forum here sell their depopulated motherboards to outfits like Boardsort.com or to larger eWaste recyclers in their area.
If you were going to attempt this at home (not recommended), you would need a system that shreds, crushes then incinerates the motherboards. Then you would need a way to separate the metals from whatever plastics are left over. Then you would need to way to extract the PMs from the mass of metals you have. Certainly not as clean as clipping ram fingers and running them in AP.