Why Heat Kills CPUs

Written: 10/30/99
Written by: Lee Winde Severn, MD

There are several areas which, I believe, are lacking or need further explanation. My background in electronics is EE in '67 and considerable R/D development for ten years in audio circuitry. I went on to professional school and have not worked as a EE in years; however, I've kept fairly current. All my comments are pre-computer yet are exactly what your CPU thermal lowering or degradation article describes. IC voltage regulators and several case styles or series pass regulators were a big problem in the early 1970's both in mil spec gear and the areas I was involved in. My comments follow:

I've read a number of recent "descriptions" concerning the ill effects of temperature on life expectancy; however, I do not recall two important considerations being explained or raised:

What does excessive heat do anyway? Probably less than you imagine, since a CPU's failure increase is a direct function of the number of thermal cycles and the CHANGE in temperature above ambient. One of the two mechanisms of failure is called "thermal fatigue" a misnomer since it is metal fatigue.

The second failure mechanism deals with absolute temperature. For each ten degrees C, chemical reactions double in their speed. This doubling is roughly accurate when referring to leakage currents. Leakage currents represent heat sources and further increase junction (CPU) temperatures. With a CPU you have the following three mechanisms of degradation:

1. Temperature Range: Delta T (a techie way of saying a change in temperature) is a function of ambient and Steady State or equilibrium conditions. Too high a delta T, the temperature goes up yielding increased leakage. As IC current flows increasing junction temperatures and an increase in the total delta T per cycle. Minimizing the magnitude of Delta T is one of the most important design parameters. The degree by which the CPU increases in temperature beyond ambient is a function of heat sink efficiency and ability of heat transfer. In an ideal world the heat sink and junction temperatures are equal.

2. Thermal Cycles: Degradation due to thermal cycling (turning the computer on and off) is one reason there is some validity for 24/7 operation. You have to balance mechanical fatigue against how many Kwh per year are spent running 24/7. Once a critical temperature is reached, leakage currents become a progressively larger and self "feeding" problem.

3. Voltage: When applying more voltage than what is recommended, the internal heat generated is considerably greater since increases in voltage normally yield an increase in current flow: Elementary my dear Watson - Volts x Amps equals Watts. More watts means more heat (light bulbs are a good example).

Lee Winde Severn, MD

PS: Freezing CPUs also constitutes a delta T but at a reduced current flow due to the lower temperature reached with active cooling. Temp ambient is still the starting spot and the magnitude of delta T is directly proportional to failure rate.

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