“The more complex the thermal path, the higher the cost,” says
Paisner. “Then you must figure out how you are going to get heat
out of the system, and what material and layout tradeoffs are you
willing to concede.”
Additionally, component placement plays a major role during
layout from a thermal perspective. “The preference for components
dissipating a lot of heat is to place them near a vent, but
that is not always possible, and other tradeoffs may be necessary,”
Rosato says.
In addition, components that dissipate a lot of power may
generate “downstream” heat, which could easily affect other components.
Another trick of the trade is to place heat-generating
components side by side and normal to the air path. Also, “Diverters
may be used to route airflow where necessary,” notes Rosato.
From a layout perspective, keep your eye
out for stacked-die or stacked-chip configurations,
as taller components tend to
impede heat paths. Also, components that
can be soldered directly to the PCB (and
eliminate any air gap between the component
and PCB) can rely on the PCB to act
as a heat spreader. Furthermore, thermal
vias may be designed in, but typically you’d
like to know that you’re implementing
them before layout.
According to Blackmore, a good layout
rule of thumb is to strive to put the “leading
edge” of any cooling air on the largest
power dissipater. It’s also wise to spread
components out to avoid pockets of hot air
downstream. Lastly, “Tall components and
connectors could cause a dead zone for air
blockage downstream,” he says. Therefore,
any tall components or connectors should
raise a thermal red flag that may require
further analysis.
GARBAGE IN, GARBAGE OUT
Don’t assume the maximum power dissipation
for your component set. The
maximum may be fine during the calculation
stage to get a rough idea of where you
stand. But you must insist on using more
realistic numbers or your design will likely
get over-engineered, adding unnecessary
weight and cost.
If you have an FPGA, is all of the internal
logic going to switch at the maximum
speed all of the time? That’s highly unlikely,
so get the logic engineer to give you a
reasonable estimate for the assumed set of
operating parameters. Then, it’s up to you
to decide if you want to add a fudge factor.
Keep in mind, though, that the FPGA
manufacturer probably already has three
levels of fudge built in by the engineering
team, the testing team, and the sales/marketing
department. If you can get actual
usage data and add some fudge to that, you
will wind up in much better shape.
Companies may then go on to ask you
all-important questions: What is the error
percentage? How do the numbers provided
correlate to “real-life data?” Are the
numbers validated? Were they tested using
real materials in the end environment?
Then, where actual thermal simulation
tools are concerned, you can get a much
better feeling for accuracy. “Thermal-analysis
simulation tools should be able to read
in routing and board design information,
including traces, planes, and via definitions
from other EDA tools,” says Rosato.
Simulations can also include system
packaging, detailed component design
parameters, and so on. “Simulation tools
can predict operating temperatures to see
if rated junction temps will possibly be
exceeded and where your system may have
‘stale air,’” adds Rosato. The simulation may
also take on an iterative approach, where
engineers can play around with various
thermal-management scenarios, add heatsinks,
and rerun the simulation as needed.
Parameters like board outline and size
and the relevant board stackup data, such
as information on the metal layers, are also
read in, says Blackmore. The remainder of
the process involves the systems engineer
describing the environment in which the
system will operate, including information
on the chassis, vents, power supplies, and
other parts. All information is then combined
to provide a thermal simulation.
WHERE TO GO FROM HERE
So you now understand the basic principles
and importance of thermal analysis and
good thermal-management techniques.
But what happens when your design reaches
or exceeds some of these limits, such as
1.5 W/in.2, even after all other precautions
have been considered?
You’re likely aware of the basic tradeoffs
between heatsinks, fans, heatsinks with
integrated fans, and so on. But what about
advanced solutions? Many companies offer
thermal products and solutions.
“Conventional solutions are out of gas,
and thus, there became a need to extend
the performance range by adding other
capabilities,” says Seri Lee, CTO of Nextreme.
For example, heatpipes have solidstate
refrigeration and would be considered
more advanced than heatsinks and fans
alone, yet they’re big, bulky, and expensive
and often must be custom-made.
Nextreme has several chip-level innovations
that actively remove heat using technology
that’s 10 to 20 times thinner and
smaller than typical solutions, yet provides
10 to 15 times greater heat-pumping capability
(Fig. 2). Bergquist manufactures several
different thermal materials and thermal
substrates. Ansys offers tools for
thermal simulation as well (Fig. 3).