I'm not surprised by the temp honestly. Intel Burn Test is not something I personally would use to test an Ivy/Haswell chip, ever. 12 hours of prime and no whea errors is good enough for me.
Futher, you don't provide other information of your BIOS settings. If you have other voltages set on auto, or a high offset voltage, I'm not surprised you are hitting 90° because IBT will make your CPU call for ALOT more voltage than neccessary. How much voltage does CPU-Z say your CPU is getting?
What are your idle and normal temps? If you idle at 30 and normal load at 60 - I'm not convinced you installed the heatsink wrong to start with.
Keep in mind also, after a new install, the temps should drop a few degrees after the thermal paste works its way into the microscopic cavaties of your heatsink and heat spreader. Usually there is a "burn in" time of 200 hours or so.