So this thread is about installing *a* heavier heatsink like NH-D15S.
Forgive my posting slightly off-topic about installing /two/ heavier heatsinks like NH-D15S…

It's a dual Broadwell-EP system which I built in 2016. The case is a so-called compact ATX tower; it's just 40 cm (15.6 inch) deep. It wasn't compatible with SSI-EEB out of the box, but I made it so by drilling the required mounting holes into the motherboard tray and by bridging some gaps at the top of the tray with aluminum rails.
I believe I first mounted the coolers and their fans — and attached the CPU cooler fan cables to the mainboard — before I put the board into the case. Otherwise it would have been difficult to plug in the fan cables.
This case is one of the rare ones with the possibility to remove the top panel. This helped quite a bit to put everything into place, notably the ATX power cable and EPS12V cables.

This system survived a move just fine which was a ~3 h drive. The computer was in a moving box with some padding. I don't remember anymore whether or not I stuffed anything into the voids within the computer to secure it some more for the move. I suspect I did not; certainly not to a degree which would hold the coolers rigidly.
Later I removed and re-attached the coolers once more in order to upgrade the CPUs from 14 to 22 cores, at which occasion I also doubled the amount of RAM. I don't recall anymore, but I believe I left the motherboard mounted in place during this upgrade.
Forgive my posting slightly off-topic about installing /two/ heavier heatsinks like NH-D15S…

It's a dual Broadwell-EP system which I built in 2016. The case is a so-called compact ATX tower; it's just 40 cm (15.6 inch) deep. It wasn't compatible with SSI-EEB out of the box, but I made it so by drilling the required mounting holes into the motherboard tray and by bridging some gaps at the top of the tray with aluminum rails.
I believe I first mounted the coolers and their fans — and attached the CPU cooler fan cables to the mainboard — before I put the board into the case. Otherwise it would have been difficult to plug in the fan cables.
This case is one of the rare ones with the possibility to remove the top panel. This helped quite a bit to put everything into place, notably the ATX power cable and EPS12V cables.

This system survived a move just fine which was a ~3 h drive. The computer was in a moving box with some padding. I don't remember anymore whether or not I stuffed anything into the voids within the computer to secure it some more for the move. I suspect I did not; certainly not to a degree which would hold the coolers rigidly.
Later I removed and re-attached the coolers once more in order to upgrade the CPUs from 14 to 22 cores, at which occasion I also doubled the amount of RAM. I don't recall anymore, but I believe I left the motherboard mounted in place during this upgrade.