If you benched Skylake on LN2 already, I'm sure you had some kind of cracking issues. We already tried a lot to improve the cracking. Mainly by changing the thermal paste.
However, also the black glue between IHS and CPU PCB seems to help. At least it was much worse for me whenever I removed the black glue.
It's just a guess but I think the space between IHS and PCB has some kind of influence on the cracking behavior. I made some spacers out of stainless steel. The thickness is 0.10 mm and you can stack them e.g. if you want to use 0.30 mm spacer. Just fix them with a little bit of TIM.
This is a pure research project - nothing I'm trying to make money on. I have 50 spacers here and paid 3,75 € per spacer. Would be cool if some guys join in and help me investigate further. I can ship a pack of 5 spacer worldwide for 23 €.