Thanks to Prusajr I've got a prototype PCB Heatbed running on my ShaperCube.
This heatbed is an etched, single sided 220x250mm big PCB, ~1,5mm thick in total with 35ym coppper thickness.
The PCB itself is not flat, but can be bent slightly. Similar to Prusajr's design, I utilized magnets to keep the sides down.
The Magnets I use are 9mm in diameter and 5mm thick NdFeB ones. They have a catch: The ones I use can only withstand temperatures up to 80°C. Because of that, I didn't glue them onto the PCB, but I glued some strips of steel tape onto it with temperature resistant glue.
I made a pile of 2 magnets each on four sides of an MDF board hoping to keep the temperature down a bit. The magnets are glued to the MDF using some expoy glue. So far, this works nicely.
After a couple hours of operation, the glue on the middle piece failed. The ones on the side don't get that hot and are still working.
As it turns out, magnets are not the most elegant way to mount it but probably the easiest non-permanent "do-it-yourself" way.
The heatbed needs a lot of power. It draws 11A @ 12V when cold. Using this kind of power from a ATX PSU requires to use two plugs and probably a very good PSU. I had problems that the voltage on the +12V rail dropped too much on my 400W PSU (which is appearantly rated for 16A on the +12V rail). Currently I'm using an external power supply for the heatbed.
Heating up & Polyimide Tape
To stick ABS onto the heatbed, I use Polyimide tape. Those can be ordered in the same size as the PCB is. Large strips of Polyimide tape are a bit tricky to get on to the PCB without air bubbles. The factory actually suggested a machine to roll it onto, but from $350 a bit pricey.
Anyone else use large tape and have different experience?
Apart from some air bubbles I had, the parts come out perfectly flat. A bit of warping still appears on the sides of the heatbed. Nophead wrote about this and has a nice solution for his Mendel.
The bigger problem I still see is in adjusting the heatbed. If the nozzle height is not set correctly, it can easily snap traces on the PCB. Once snapped, they can be repaired with solder and some sanding, but the results are far from perfect.
I was thinking of putting a thin sheet of aluminium on top of the heatbed to protect it. But this would need a better way of mounting it.