Although there are zillions of tales of damaged CPUs and other system components, in most cases overclocking is completely harmless. There are, however, a few things to take into consideration both fatal and non fatal :
Heat is number one enemy of CPU
The non-fatal one is due to timing. Processors are designed and tested so that their internal parts will all be ready about the same time, according to specifications published by their manufacturer. As the heat of the processor increases above specifications, the internal paths slow down. Some paths slow down more than others, and eventually there becomes a significant difference between when something is expected to happen and when it actually does happen. At this point you may get false information (such as 0+0=1), system lock-ups, or spontaneous resets. This behavior is usually a signal for you to decrease processor speed or temperature (see next section).
One fatal possibility is called electromigration. Overstressed ICs can be slowly destroyed by electromigration. The combination of heat and electric fields cause metal atoms to wander around under the passivation layer.
Electromigration takes place on the actual silicon chip of your CPU in areas which operate at a very high temperature, and can cause permanent damage to the chip. Before you start to panic, you should first realize a few things. CPUs are designed to run at temperatures between -25 and 80 degrees Celsius. To give you an idea, 80 degrees Celsius is a temperature that nobody is able to touch for longer than 1/10 second. I have never come across a CPU at this temperature. There are plenty of ways to keep the CPU case at less than 50 degrees Celsius which increases the probability of keeping the chip inside at less than 80 degrees. Also, electromigration does not immediately damage your chip. It is a slow process, which more or less shortens the life span of a CPU running at a very high temperature. A normal CPU is meant to live for about 10 years. However, in ten years nobody is going to be using a CPU with today's technology. If you want to be kept free from this electromigration scare, you have to do as much as possible to cool the CPU. Cooling is the number one requirement in overclocking!! Never ever forget that these terms don't necessarily apply for Cyrix, IBM, and AMD CPUs. Because of the already high rate of heat production at their original clock rate, you must work extra hard to keep them cool in overclocked conditions. I've come across several dead Cyrix 6x86 CPUs so far, so be careful or just let it be!
Due to electromigration CPU tend to grow little whiskers at any sharp corner or irregularity along a trace. Whiskers at different potentials tend to grow towards each other, much as stalactites grow towards stalagmites in a limestone cave, because the sharp point accentuates the potential gradient. In a cave, limestone columns eventually form from floor to ceiling. On a chip, you get a short circuit.
(Heat is only a secondary factor in electromigration. The primary factor is current density in the presence of an electric field. In most digital ICs, the internal clock signal is distributed by a conductor, usually aluminum, which is sized carefully for its load. If the clock switches more often than the designer sized it for, then the clock "trunk" is overloaded and subject to premature failure due to electromigration.)
The other fatal possibility is simply burning out the bond wires that run from the pins on the outside of the package to the silicon die of the processor.
Some other problems
Just because your processor will run satisfactorily at the speed it is running doesn't mean that the rest of your system will. In fact, it is more likely that, if your system becomes unreliable, it is because other devices are overstressed by the higher clock speed. Relatively items such as video / disk controllers and memory, may fail because there is not enough time for them to do their work when bus frequencies are increased. You can try decreasing bus speeds, and increase memory wait-states, or you can get faster devices. Generally, if you get lots of strange errors, such as "No ROM BASIC installed", try slowing down memory and bus speed.
The other device-based problem is with your motherboard. Every cheap little component on the board must function reliably at the higher speed or you will likely experience reliability problems. The only reasonable solution to this problem is to get a faster motherboard.
Nobody likes system crashes or hangs, but in a professional business environment, avoiding a system crash or hang can be most crucial. It certainly is a fact that you are increasing the probability of system faults by overclocking your CPU. But this is only the probability !! If you have just overclocked your system and the first thing you do is use it to start writing your dissertation, don't be surprised if a system crash occurs which causes you to lose all your data. After finishing the overclocking process you have to put your system through a tough and thorough testing procedure. If the system passes all the testing, only then can you talk of successful overclocking and feel confident everything is working well.
The third debate against overclocking is that your father, brother, best friend, neighbour, or boss thinks it's immoral. (Is this morality important today when we have paid many times price and obsolescence monster is catching up ultra fast).
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