Intel confirmed in their official Coffee Lake briefing that the new processors use a vastly different pin configuration compared to previous generation processors, and as such, there is no backward compatibility with 200 or 100 series motherboards. While Intel did mention it, they didn't go in to much detail but today, David Schor (Engineer and Industry Analyst in the field of Semiconductor & computer architecture) has revealed that the Coffee Lake processors are indeed using a different electrical pin configuration, confirming Intel's claims.
Intel Coffee Lake 8th Generation Desktop Processors Come With Pin Changes, More Ground and Electrical Pins Active Compared To Skylake and Kaby Lake Processors
According to David, the reason we don't have Coffee Lake processors compatible with older series motherboards that feature the LGA 1151 socket is the change in pins. For instance, if the pin config changes on a processor, the sockets on the motherboard need to be configured as such. It's not a process that can be done via software as its more of a hardware level change.
When compared, the Coffee Lake processors have 391 VSS (Ground) pins which is an increase of 14 compared to Kaby Lake, 146 VCC (Electrical) pins which is an increase of 18 pins compared to Kaby Lake and about 25 pins that are reserved and a decrease of 21 pins from the 46 reserved on Kaby Lake.
Kaby Lake -> Coffee Lake
- VSS (Ground): 377 -> 391 (+14)
- VCC (Power): 128 -> 146 (+18)
- RSVD: 46 ->25
Intel LGA 1151 CPU Pin Configuration (Coffee Lake vs Kaby Lake):
So one thing is clear, Intel was in fact telling the truth about electrical changes to the processors and socket in the 300-series platform. Furthermore, it's not just the reserved pins from Kaby Lake that have simply been populated. There are pins aside the reserved ones that were swapped with VCC pins and indicate a design tweak.
While we can put many theories to rest with this new detail, I think much of the confusion could have just been avoided if Intel clarified this themselves. Of course, if you are making the boards with a new PCH and new series of processors on the same socket that ran the previous CPU line, consumers would definitely want to know more why the new platform that has the same socket cannot support their older chips. We previously heard about the LGA 1151 V2 naming scheme and that may have sorted some confusion but as we can tell, all motherboards still use the LGA 1151 naming scheme which may lead to people thinking that their 6th and 7th generation processors can run on the newer boards.