Got my two boards, they look great, but both heatsinks immediately fell off. I thought that maybe they just shook loose during shipping, so I pressed them back on as firmly as I could -- but one of them fell off with a clunk just as I got hashing a few minutes later. Now I'm debating whether to drill some holes or just run them heatsink-side-up.
It's not a huge problem, but was there a reason you didn't screw the heatsinks in place?
Despite earlier photos showing the heatsink below, Barntech later suggested to position the board with the heatsink on top, to avoid the 'separation' that you described. I started with the heatsink below, later turned the boards so it is on top on my 4 Av16 boards. No separation so far on mine. Drilling holes might be risky, as you could bridge internal ground and power layers. The first and easy step is to turn the boards so the heatsinks face up. You may want to consult Barntech (check out the drillbit forum at
http://drillbitsystem.com/forum/, you may want to register there so that you may post your issue) about any adhesive or way of re-attaching the heatsinks to ensure good thermal transfer and avoid pre-mature thermal death of your ASICS.
Cheers