I have a customer who wants us to use a stack of fairly rigid Bergquist thermal pad gunk, about the consistancy of used bubble gum, to transfer heat from a pcb, between the bottom of the board and the extruded enclosure. Bergquist wants something like 20% compression, which would bow our board like a potato chip and probably pop the BGA balls.
The customer want us to measure the delta-t from the running board surface (on all production units!) and the enclosure and guarantee 5C max or something. And not stress the board. No thanks.
Extrusions aren't flat or consistant. FR4 cold flows under pressure, so it might take months or years to start cracking caps or BGA balls.