Hardware Reference
In-Depth Information
as it cools off, the components contract. In addition, various materials in the system have different
thermal expansion coefficients, so they expand and contract at different rates. Over time, thermal
shock causes deterioration in many areas of a system.
From a pure system-reliability viewpoint, you should insulate the system from thermal shock as much
as possible. When a system is turned on, the components go from ambient (room) temperature to as
high as 185°F (85°C) within 30 minutes or less. When you turn off the system, the same thing happens
in reverse, and the components cool back to ambient temperature in a short period.
Thermal expansion and contraction remains the single largest cause of component failure. Chip cases
can split, allowing moisture to enter and contaminate them. Delicate internal wires and contacts can
break, and circuit boards can develop stress cracks. Surface-mounted components expand and
contract at rates different from the circuit boards on which they are mounted, causing enormous stress
at the solder joints. Solder joints can fail due to the metal hardening from the repeated stress,
resulting in cracks in the joint. Components that use heatsinks, such as processors, transistors, or
voltage regulators, can overheat and fail because the thermal cycling causes heatsink adhesives to
deteriorate and break the thermally conductive bond between the device and the heatsink. Thermal
cycling also causes socketed devices and connections to loosen, or
creep
, which can cause a variety
of intermittent contact failures.
See the
Chapter 6
section, “
Memory Modules
,
”
p.
346
.
Thermal expansion and contraction affect not only chips and circuit boards, but also things such as
hard disk drives. Most hard drives today have sophisticated thermal compensation routines that make
adjustments in head position relative to the expanding and contracting platters. Most drives perform
this thermal compensation routine once every 5 minutes for the first 30 minutes the drive is running
and then every 30 minutes thereafter. In older drives, this procedure can be heard as a rapid “tick-
tick-tick-tick” sound.
In essence, anything you can do to keep the system at a constant temperature prolongs the life of the
system, and the best way to accomplish this is to leave the system either permanently on or
permanently off. Of course, if the system is never turned on in the first place, it should last a long time
indeed!
Now, I am not saying that you should leave all systems fully powered on 24 hours a day. A system
powered on when not necessary can waste a tremendous amount of power. An unattended system that
is fully powered on can also be a fire hazard. (I have witnessed at least two CRT monitors
spontaneously catch fire—luckily, I was there at the time.)
The biggest problem with keeping systems on 24/7 is the wasted energy. Typical rates are 10 cents
for a kilowatt-hour of electricity. Using this figure, combined with information about what a typical
PC might consume, we can determine how much it will cost to run the system annually and what effect
we can have on the operating cost by judiciously powering off or taking advantage of the various
ACPI Sleep modes that are available. ACPI is described in more detail later in this chapter.
A typical desktop-style PC consumes anywhere from 75W to 300W when idling and from 150W to
600W under a load, depending on the configuration, age, and design of the system. This does not
include monitors, which for LCDs range from 25W to 50W while active, whereas CRTs range from
75W to 150W or more. One PC and LCD display combination I tested consumed an average of 250W
(0.25 kilowatts) of electricity during normal operation. The same system drew 200W when in ACPI
S1 Sleep mode, only 8W while in ACPI S3 Sleep mode, and 7W of power while either turned off or
