Hardware Reference
In-Depth Information
(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 ambi-
ent 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
wiresandcontacts canbreak,andcircuit boardscandevelopstresscracks.Surface-moun-
ted 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. Compon-
ents 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
“
Memory Modules
,”
p.
333
(
Chapter 6
)
.
Thermalexpansionandcontractionaffectnotonlychipsandcircuitboards,butalsothings
suchasharddiskdrives.Mostharddrivestodayhavesophisticated thermalcompensation
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 per-
manently 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
aboutwhatatypicalPCmightconsume,wecandeterminehowmuchitwillcosttorunthe
system annually and what effect we can have on the operating cost by judiciously power-
ing off or taking advantage of the various ACPI Sleep modes that are available. ACPI is
described in more detail later in this chapter.
