i have an SD2 and i've been looking into getting a PCB bed heater. has anyone come across a 150x150 pcb bed heater?

It's at the same site I referenced in the above link...just search "PCB" on their website:
http://www.makerfarm.com/index.php/6x6-heat-bed.html

PCB Heaters can be located here or Google for other sources:
http://www.makerfarm.com/index.php/prin … d-mk1.html

The link above is for a 214 x 214mm.  You need to double-check the size so it can mount to your current Alum bed.  As you point out, typically you need to convert from a 3-point mount to a 4-point (corners).  This will mean disassembly and drill press / tapping work to relocate the leveling screws to the four corners; also matching the hole pattern on the PCB.

So the new "sandwich" is (from bottom up):
Original Aluminum Plate altered with new M3 Screw holes
Insulating material - 1/4" cork board panel works well
EDIT - (no need for more springs, just 1/4" or 3/8" spacers to make room for the insulating cork board) Use your current 3-point leveling screws.
PCB Heater
8 x 8 Glass
Glass clipped to PCB with binder clips

If your new PCB heater draws less than 16 amps during heat up, it might be OK for the current mobo, but I am not familiar with the SD3/4.  Others please weigh in here.

Safe bet is to get a separate power supply and run the PCB with a SSR (solid state relay).  The thermistor is mounted on the bottom of the PCB and is connected to the same plug as your current bed thermistor.  The output (power) leads which would normally go to your current SD heater would go to the input side of the SSR.  The SSR would then switch the PCB on and off accordingly using the separate power supply.

I think my bed is warped also. Sucks.

Maybe I can try to increase the bed temp to maxiumum (whats the max possible bed temp, btw?) in hopes that even though outer edges will not be as hot, they will at least be hot enough to prevent unsticking and warping?

Maybe I should try glue sticks? I just dont know which one. I can't find blue glue sticks in Russia. Are they for specific application and what kind of shops can they be found if so?

BTW off topic question, can I replace the fiberglass matting? Its a bit torn up on the edges. And whats the glue I can reapply to hold the new one in place better?

Thanks for the help as always.

EDIT: Maybe I needed to remove the existing kapton tape between the aluminum and glass?

Re warping at the periphery of the bed:

If your SD4 print bed is anything like my later-model SD3, the heater pad is not the whole size of the bed, it's approximately a 150mm square in the middle of the 200mm square bed. The aluminium bed is supposed to conduct the heat and even it up a bit, but especially as heat loss is greatest from the outer parts of the bed (bare edges and the bits where the silicone heater pad itself doesn't provide any insulation, just the fiberglass matting), the temperature tends to drop off as you get to the outside of the bed.

Also, if your bed is a little bowed up in the middle, the glass will contact well in the middle but there will be an air-gap underneath at the edges, so the conduction into the glass won't be as good - this also makes the temps at the outside of the bed rather less than the middle.

There's lots of discussion about improving the bed heating, the goal being perfect flatness and even surface temperature. However, there's only so much you can do, so in reality you have to accept that the outer edges of *any* bed are going to be troublesome. If you're doing multiple pieces, reduce the count and keep them closer to the middle of the bed, and accept it will take more print jobs to complete. If you're doing large pieces then you'll probably have to look at using PLA instead of ABS, consider a split/glue job, or pump up the ambient temp with active heating...

It sticks very well now and actually is sometimes very hard to get off the glass (any ideas how to get it off without damaging? tried freezer, didnt help).
But even though it sticks so extremely well, a bit far fromthe bed center warping still happens.

Ive let my bed warm up very long for the few tests Ive done. Bed temp between 100 and 110.

Devastated. Am I doing something wrong?