HEAT SPREADERS
Heat spreaders are used in die-level packaging
to spread heat from a microprocessor
chip into its associated heatsink. One type of
heat spreader is a natural graphite sheet with
anisotropic thermal properties: it exhibits a
high thermal conductivity in the plane of the
sheet and a much lower thermal conductivity
through the thickness of the sheet.
This allows a natural graphite sheet
to function as both an insulator and heat
spreader that eliminates hot spots in
microprocessor chips. Also, because of their excellent flexibility, natural graphite materials
can conform well to surfaces under low contact pressures. This
combination of properties makes natural graphite a potential substitute
for aluminum and copper materials as heat spreaders.
Made from natural graphite, Graftech’s Spreadershield products
distribute heat evenly while providing thermal insulation.
They offer a variety of in-plane thermal conductivities, from
300 to 500 W/m-K. By eliminating heavy thermal solutions,
these products slim down product design and reduce product
weights by up to 50%.
THERMOELECTRIC MODULES
Thermoelectric modules (TEMs) must be placed between the
MPU die or package and a heatsink (Fig. 5). Power dissipated
by the TEM must be dissipated by the heatsink, which can result
in higher ambient temperatures at the heatsink that may impact
downstream components. TEMs have low efficiency because they
consume more power than they transport.
TEMs employ the Peltier effect, which produces rapid heating
or cooling of electronic components. These solid-state devices have
no moving parts, making them reliably maintenance-free. They’re
often used to eliminate hot spots on an MPU.
Applying a low-voltage dc to the TEM causes one side to cool
down and the other side to heat up. Cooling is proportional to
the amount of current applied. Varying the current applied and
the direction of current provides tight temperature control in
cooling applications.
A typical module has two wires for the application of power,
which must be the correct polarity for cooling. The wrong polarity
will heat rather than cool. If the TEM cooling fails, the results can
be disastrous. Also, don’t apply power to a TEM without a heatsink,
or it may overheat and fail.
A TEM can damage an electronic circuit with condensation
because it’s possible to cool components below ambient temperature.
The exact temperature at which condensation occurs depends
on the ambient temperature and humidity.
THERMAL-ANALYSIS SOFTWARE
Before committing a design to production,
it’s a good idea to evaluate its thermal
characteristics. Several software programs
can perform this evaluation. For
instance, Flomerics’ Flotherm 3D simulation
software for thermal design of electronic
components and systems enables
the creation of virtual models of electronic
equipment (Fig. 6).
Flotherm also performs thermal analysis
and test design modifications quickly
and easily in the early stages of the design
process well before any physical prototypes
are built. It uses advanced computational
fluid dynamics (CFD) techniques
to predict airflow, temperature, and heat
transfer in components, boards, and complete
systems.
In evaluating thermal-analysis software
for electronic systems, it’s imperative
for the user to have readily accessible
technical support from the supplier.
The user should consider the modeling
methodology, definition of a system for
analysis, creation of a computational grid,
solution and control features, and presentation
of the results.
DYNAMIC POWER MANAGEMENT
Heatsinks and fans can only go so far to
cool microprocessors. However, sleep and
suspend modes can also reduce power
consumption. This has led to new circuit
techniques, called dynamic power management
(DPM), that reduce a microprocessor’s
average power dissipation by
dynamically reconfiguring a system to
lower power consumption during lowworkload
periods.
In principle, DPM identifies low-processing-
requirement periods and reduces
operating voltage (voltage scaling) and/or
frequency (frequency scaling) to reduce
operating power consumption. This technique
is called dynamic voltage and frequency
scaling (DVFS). Furthermore,
during these low-power-requirement
periods, idle circuits can be turned off to
provide even lower power consumption.
Proposed DPM solutions can be categorized
as either predictive or stochastic.
Predictive schemes attempt to predict a
device’s usage behavior in the future, based
on past experience. Stochastic techniques
make probabilistic assumptions based on
usage-pattern observations. To be effective,
DPM must account for the time it
takes to change a power-supply voltage.
Plus, the processor must be able to operate
reliably when its supply voltage or clock
rate changes.
REFERENCES
1. 2007 International Technology Roadmap
for Semiconductors (ITRS).
2. R. Mahajan, et al, “Cooling a Microprocessor
Chip,” Proc. Of IEEE, August 2006,
p. 1476-1486.