Effect of high Vcore and electromigration on CPU failure time
It is known that high temperature and high current (density) have adverse effect on chip behavior due to electromigration, could lead to complete chip failure (not just performance degradation per se). Electromigration may increase the resistance of metal wires and contacts inside a chip (max overclocking degradation before functional failure), and may even lead to open connections and resulted in complete chip failure.
But these are long term effect and would not happen in days or even months (see arguments below). Also the failure is statistical in nature, measured over a large sample of chips, a particular one or few chips may behave very differently within certain statistical deviation. It would not be accurate to simple put a number of max voltage on a particular chip and try to predict its life. Same like trying to put a number of max overclocking frequency on a CPU, ...
Assuming statistical failure is due to electromigration, the statistical failure rate usually measured in terms of the failure time of 50% of the population in a large sample of same chips is given by
T_failure = A exp (E / k T) / (J^2)
where A, k are constants, and E is activation energy of the material, J is current density, and T is temperature (in K). The equation is call Black's equtation and is based on empirical results. There may be some deviations from it (mainly on the exponent of J) for different material in wires and contacts, e.g. Al, Al/Cu alloy, Cu.
So put in simple terms, a rough estimtation, we can say that if
- Vcore is increased by 10%, on the average, the current density in the wires inside a chip would be increased by 10% (assume uniformly distributed current density), keeping temperature the same, so the failure time would be shorten by 17%. So
A 10% increase in Vcore, would shorten the failure time to 83% of nominal failure time.
A 20% increase in Vcore, would shorten the failure time to 69% of nominal failure time.
A 30% increase in Vcore, would shorten the failure time to 59% of nominal failure time.
A 50% increase in Vcore, would shorten the failure time to 44% of nominal failure time.
- An increase of temperature by 20 C over nominal max temperature would roughly result in doubling the electromigration rate, hence shortening the failure time to 50%. But that is measured by the temperature over the max temperature specification (above 85 C), which is not likely to happen in daily overclocking.
E.g. for a TBred B DLT3C, from the estimate, increase Vcore from 1.5 V to 1.95 V (a 30% increase) would shorten the statistical chip failure time defined above to 59%. This number is still in the ranges of 5+ years, assuming the nominal life expectancy of the chip is 10+ years.
Based on the above, if the numbers stand, one can pick and chose the max voltage and max overclocking frequency to trade with the life expectancy of a CPU.
The above only discussed the long term failure rate or life expectancy issues related to electromigration, but the question about short term degradation related to high voltage (if there is any) remains open.
