How is heatbed PCB bonded to aluminium?

For those factory-made Mk3 Alu heatbeds, where the heater PCB is on one side and the 3mm aluminium on the other: how is the aluminium plate bonded to the PCB for best heat transfer? Double-sided tape? Epoxy? How does it cope with differential expansion?

how is the aluminium plate bonded to the PCB for best heat transfer?
Double-sided tape?
(I guess it's possible lol)

Epoxy?
They use a thermally conductive dielectric [usually epoxy and reinforced with glass(most of the time) or ceramics(occasionally) which varies between manufactures and pcb requirements]

How does it cope with differential expansion?
That would depend on what your requirements of your board. Are you talking about out-of-plane from vias (usually minimized by thicker copper plating, thinner board, materials) or delaminating of the layers?
Heatbeds usually don't have much of vias through the aluminum layer though, or are you just asking about the rest of the pcb?

One common DIY way to do this is 3M double-sided adhesive. Factories might do that as well.

I was really thinking about separation of the PCB from the aluminium, since they would have quite different coefficients of expansion. Anyways, I solved the problem by buying a couple of silicone heat pads instead (they work off the mains, so saving me the cost of a power supply). Since they are soft, the question of differential expansion won't arise, and they come with their own adhesive. I'll update the wiki article (see sig) when I get new photos...

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Aegidius.
[reprap.org]