Previously, Intel has soldered its heatspreaders directly to the die, creating a strong bond at the molecular level that ensures good heat transfer. In its Ivy Bridge family, it moved to using a thermal paste with an impact in heat transfer and a sudden resurgence of interest in de-lidding accessories. Although many considered the move to have been made out of a desire to boost profits, there were sound engineering reasons relating to smaller die sizes causing cracking of the solder and the formation of heat-trapping voids that can damage the chip.
Those issues appear to have been resolved in time for the Haswell-E enthusiast chip family, thankfully. An image published by OCDrift shows a de-lidded Core i7-5960X Haswell-E processor with half the die stuck to the heatspreader - suggesting that solder or an extremely strong thermal epoxy, rather than the weak-adhesive thermal paste of previous chips, has been used to join the two.
Source: Bit Tech