Hardware Reference
In-Depth Information
market don't include this figure in their specifications. Without knowing the thermal resistance, you
have no easy way to compare the performance of one heatsink to another.
Heatsink Ratings and Calculations
When cooling a processor, the heatsink transfers heat from the processor to the air. This capability is
expressed in a figure known as
thermal resistance
, which is measured in degrees Celsius per watt
(C/W). The lower the figure, the lower the thermal resistance and the more heat the heatsink can
remove from the CPU.
To calculate the heatsink your processor requires, you can use the following formula:
R
total
= T
case
- T
inlet
/P
power
T
case
is the maximum allowable CPU case temperature, T
inlet
is the maximum allowable inlet
temperature to the CPU heatsink, and P
power
is the maximum power dissipation of the processor. For
example, the Intel Core i7-3960X Processor Extreme Edition is rated for a maximum case
temperature of 68°C and has a thermal design power of 130 watts. Intel recommends a maximum
heatsink inlet temperature of 43.4°C, which means the heatsink required to properly cool this chip
needs to be rated 0.18°C/W, or (68°C - 43.4°C) / 130W. Chips with lower TDP need less powerful
cooling, which explains why chips with higher TDPs use larger heatsinks, and why overclocking
enthusiasts sometimes use liquid cooling with the fastest processors.
Another useful formula can describe processor power:
P
power
= C × V
2
× F
P
power
is the maximum power output of the processor, C is the capacitance, V
2
is the voltage squared,
and F is the frequency. From this you can see that if you double the frequency of a processor, it
consumes twice as much power, and if you double the voltage, it consumes four times as much power.
Consequently, if you lower the voltage by half, it consumes only one-fourth the power. These
relationships are important to consider if you are overclocking your processor because a small
increase in voltage has a much more dramatic effect than a similar increase in speed.
In general, increasing the speed of a processor by 5% increases the power consumption by only the
same amount. Using the previous heatsink calculation, if the processor speed was increased by 5%,
the 103W processor would now draw 108.15W and the required heatsink rating would go from
0.34°C/W to 0.32°C/W, a proportional change. In most cases, unless you are overclocking to the
extreme, the existing heatsink should work. As a compromise, you can try setting the voltage on
manual and dropping it a small amount to compensate, thereby reducing the power consumption. Of
course, when you drop the voltage, the CPU might become unstable, so you need to test it. As you can
see, changing all these settings in the interest of overclocking can take a lot of time when you consider
all the testing required to ensure everything is working properly. You have to decide whether the
rewards are worth the time and energy spent on setting it up and verifying the functionality.
Note that most professional heatsink manufacturers publish their °C/W ratings, whereas many of what
I call the “boutique” heatsink vendors do not. In many cases the manufacturers of the more extreme
heatsinks don't do the testing that the professional manufacturers do and are more interested in the
looks than the actual performance.
Installing a Heatsink
To accomplish the best possible transfer of heat from the processor to the heatsink, most heatsink
