If you don't own an MRI (I did check ebay and dealextreme just in case!) then this is a fairly easy, effective, but very slow way of recreating PCB artwork from a multilayer board.
I imagine a Dremel would rip through this in seconds... but that's probably going to cause problems when you only want to remove a few microns at a time. Instead I went for a combo of 120grit alox and 400grit carbide paper. I also nearly got screwed over by HP and their bullshit claims of 1200dpi! Take a look; on the left is a corner scanned on a HP at "1200dpi". On the right, the same corner scanned on a £50 Canon I got from PC World, also set to 1200dpi:
The phone was broken by the way, it just buzzed from somewhere on the back of the board... although I did get a complete eMMC dump by jamming it in a vice with a bit of an IDE connector putting pressure on whatever was broken. It took a few goes as when it warmed up it must have moved just enough to switch off. I also forgot that without a battery you only ever get 13.5 minutes in bootrom mode before it switches off (MT6592). Finally with a PSU pretending to be a battery, I got it imaged in about 17-18 hours.
The concept is straightforward:
While you have remaining layers,
1. Scan a layer
2. Remove half* a layer
*Micro vias are impossible to see if you only scan the copper layers. Take a scan part-way through the epoxy as well. They are pretty small at around 100μm in diameter:
(yes, I was too lazy to rotate one of these)
(Lesson I learnt is I should have put scrap PCBs around the sides to prevent this from happening)
2 more layers! On the left, remaining 2 copper layers. On the right, sheet of standard 80gsm printer paper. Photo through a 30X lens.
Chinese magic phone dust is probably highly toxic and definitely stains your hands green, brown or grey. It also ended up in my ears somehow!
Then with a set of scanned layers, use your favourite image editing software to isolate the copper coloured bits. Combine all of those, align them all, job done:
Further improvements that I'd do if this wasn't just a practice would be to neaten up the few areas the colour processing got wrong, and square off the tracks / non-round pads.
I'm still trying to find a program to visualise the layers in 3D. If it was a vector image then it would be straightforward but I couldn't find the equivalent for raster. There isn't even a good file type to save it in... Adobe psd seems to be the most popular :/
If I find something appropriate then I'll upload the whole thing, but at the moment it's 1.2GB and not very friendly.