Hardware Reference
In-Depth Information
manufacturers specify some type of thermal interface material to be placed between the processor and
heatsink. This typically consists of a ceramic, alumina, or silver-based grease but can also be in the
form of a special pad or even a type of double-stick tape. Some are called
phase-change
material
because they change viscosity (become thinner) above certain temperatures, enabling them to better
flow into minute gaps between the chip and heatsink. In general, thermal greases offer higher
performance than phase-change materials, but because they always have a lower viscosity, they flow
more easily, can be messy to apply, and (if too much is used) can spill from the sides onto the socket
and motherboard.
Figure 3.54
illustrates preapplied thermal material on typical AMD and Intel
heatsinks packaged with processors.
Figure 3.54. Preapplied thermal material on an Intel heatsink for a Core i5-2500 processor (a)
and an AMD heatsink for an A10-5800K processor (b).
No matter what type you use, a thermal interface aid such as thermal grease or phase-change material
can improve heatsink performance dramatically compared to installing the heatsink dry. Thermal
interface materials are rated by thermal conductance (in which case higher is better) or thermal
resistance (in which case lower is better). Unfortunately, several industry-standard rating scales
measure performance, often making product comparisons difficult. Some measure the thermal
conductivity; others measure the thermal resistance; and the scales used can vary greatly. The most
commonly reported specification is thermal resistance in degrees centigrade per watt (°C/W) for an
interface layer 0.001-inch thick and 1 square inch in size. For a given material, the thinner the
interface layer or the larger the area, the lower the resistance. In addition, due to other variables such
as surface roughness and pressure, it is often impossible to directly compare different materials even
if they appear to use the same ratings scale.
I've seen actual tests of multiple brands of thermal greases, and in most cases the differences in
temperature readings between different brands are insignificant. For that reason, I generally don't get
too excited about different brands of thermal grease; most of the premium products on the market have
surprisingly similar performance.
Liquid Cooling
One of the more extreme methods for cooling a PC is to use some form of liquid cooling. Liquids
