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Gautam's 500Mhz 3-3-3 club got me started on this, that thread is a graveyard for brave memory . There are already some threads on this out there and it's generally a common fact that high voltage helps your D9 RAM clock like mad, but it also kills it sooner or later. I just thought I would share my view on the matter as working in the field plus benching/abusing more than a fair share of memory got me some experience.
As you know, Micron DDR2 chips (aka "D9") are extremely responsive to voltage increase, especially at tight timings ... and alot of people are wondering what's the limit. Well, contrary to the old Winbond's BH-5 that were also very voltage responsive but given proper cooling could run 24/7 even at the top of the voltage range, the Micron D9 are much more decieveing. Before I go on though, let me introduce you to ...
It's a phenomena caused by the high current density in a solid material, that alters physically the cristalin structure of the material due to loss of cinetic energy of the electrons. Errr ... a little more explanation maybe ? :P. The solid metals are networks of bonded atoms that have shared a few electrons each. These electrons can move freely trough the metal's structure and transport, of course, electric charge, that's why the metals are so good electric conductors. Theoretically electrons have so small mass compared to the atom's nucleus that can be neglected, but when trough a very small piece of material you pass an immense quantity of current, the bilions of bilions of electrons forced to move at huge speeds hit the atoms in their way and in time they knock out of the otherwise nicely organised structure some atoms, creating voids in the places they were originally.
This phenomena cannot be avoided and it happens in any material the moment current passes trough it; there is a mathematical model called Black's equation that estimates the average life span of a material under known conditions. Of course, under regular conditions failure due to electromigration is very unlikely, but degradation rapidly accelerates under high current density / increased temperature. As manufacturers move to smaller and smaller manufacturing processes, the importance of electromigration grows larger, as the metallic structures are thinner and thinner and thus more sensitive to this kind of damage. It is true though that there are some technological methods to compensate a little the electromigration's effects by wisely placing the material during chip's construction, but they only delay what will happen anyway (hint - does this help to understand hardware burn-in better ? ).
Looking back at our overvolted Micron D9's, it's pretty easy to understand what is happening. High voltage creates high current, too much current starts damaging the internal chip architecture and soon your expensive overclocking memory will start malfunctioning or even stop working. The "voids" created in the material's structure won't let the current pass trough the area anymore (memory will need higher voltage to work) and will force the same current to pass trough an even narrower band of material, increasing the current density and amplifying the effects of electromigration. More and more voids will be created faster and faster (you will need more and more voltage to make the memory to work at all), until it will stop working alltogether.
Enough with the theoretical babble now, let's put some facts and numbers on the table. From what i've concluded so far, for Micron D9 chips 2.4V is the maximum allowable voltage for 24/7 use, considering decent cooling is provided. I know there are manufacturers that are ceritfying memory to work at more than that but hey, higher RMA rate for a highend low volume/high profit model is not a large price to pay if you want to have that PC9000 something to showoff.
From a bencher's point of view, i'd say 2.6V is OK and still safe, as long as you're not trying to break the record for "highest 24h memtest stable 3-3-3" or something. Short benches or those that don't stress memory too much can do OK with even a little more, even if i wouldn't run more than 2.8V not even for short periods trough a kit of expensive memory that i've paid for. And for you hardcore guys out there, 3.2V+ is the land of no return, the D9 that has been there and came back alive should be kissed every night before you go to sleep.
The first sign of electromigration damage is that the memory would not work anymore with very low voltage such as 1.8-2.0V ... if you notice this on your Micron D9 then you know it's already been affected, and using more voltage will shorten it's lifespan even more. A badly damaged chip can require up to 2.5-2.6V just to work properly, even if it can still work OK for a little more time at very high voltages.