The crew over at The Tech Report has discovered BTX may not work on AMDs Athlon 64 processors.
I've spoken with several sources here on the show floor from motherboard and enclosure makers who have all said the same thing: the BTX form factor is a non-starter on AMD systems. The problem seems to be the relative CPU and DIMM placement; the standard requires placement of DIMM sockets too far from the processor. With the Athlon 64's integrated memory controller, following the BTX spec becomes very difficult. Of course, BTX is Intel's spec, but it is also a proposed industry standard. Motherboard and enclosure makers are worrying out loud about the inventory control and design problems that may be caused by the extended coexistence of the ATX and BTX standards.
News source: The Tech Report
