Before reading this section, we recommend that you look through
Some people think that thermal simulation starts with a completed
board layout. It doesn't. Before PCB layout, some basic decisions
need to be made. Will an external heat sink be required? Is a fan
necessary? How should components be positioned? Will a metal core
board be needed? Some of these are go/no-go decisions. After all,
there's no point in doing a full PCB layout if the project is unfeasible.
Companies need a thermal tool for quick analysis.
Sauna is the thermal simulation package which provides for both "quick
and dirty" analysis and detailed models.
Quick and dirty components
Detailed and simplified heat sources
Sauna provides two different ways to model heat generating components:
basic heat sources and enhanced heat sources. Both heat source types
allow for heat flow through the bottom of the component. However, only
enhanced sources allow for lead connections. For quick and dirty modeling,
you would use either basic sources or an enhanced source with simplified
In addition to individual heat sources, you can also apply
distributed wattage to plate and board assemblies. You can apply the
wattage to the entire assembly or you can assign wattage to a specific
Quick and dirty heat sinks
Fan applied to heat sink flow network
Heat sinks are rather easy to model with Sauna. So to some extent,
all heat sink modeling is "quick and dirty". Actually, it's more like
"quick and clean", because the accuracy is excellent.
As described in Sauna Overview, heat sink
modeling is a simple process. After creating the
baseplate, you can add fins to one or both sides with a single
command. Complex shapes are created by combining plates in different
planes. For heat sinks you can use basic sources and obtain accurate results.
Several options are available for forced air cooling. You can specify
a linear flow rate (m/sec of ft/min), a volume flow (m3/hor or CFM) or
you can define a fan. Heat sink modeling is fast and accurate with
Quick and dirty boards
For quick and dirty board applications, there are two different
approaches. First, the simplest method, is to use a "planar" board
assembly. For planar boards, you do not define individual traces
and pads. Instead, you adjust the copper coverage (sometimes know as
"copper density"). The planar board is a great tool for obtaining
quick results, particularly for components with heat slugs.
Dual and quad components with lumped leads
The second quick and dirty approach is to create a layer by layer
stackup, but with simplifed enhanced sources and fewer traces. While
the examples in Sauna Overview show
individual traces for each lead, you would use "lumped leads" for a
simplified model. With lumped leads, a large pad of reduced copper
coverage is created to combine traces. Also, in a simplified model,
most components will not be connected to other components. In fact,
for heat slug components which are connected to an internal plane,
you could skip traces entirely.
All of the components in Sauna's library include typical data for
Rjunction-to-case and Rcase-to-sink. So
you can get an idea of thermal performance without using a
datasheet, which is quite useful for initial analysis.
Quick and dirty boxes
Finned box, easy to model
In Sauna, a box is created with a single command. It's very easy to add
fins to any of the outer surfaces. To add a circuit board, Sauna provides
the "board in box" command. For internal convection and radiation, Sauna has
specialized commands for defining internal networks with either
natural or forced air cooling.
So you can create a model of a box with one or more boards
in just minutes.
As you have seen, Sauna is a great "quick and dirty" tool
for everything from heat sinks to boxes with boards.
Another approach: Sauna's Toolbox
With all of the techniques described above, you will create a Sauna model.
But it's also possible to obtain thermal results without a model.
Sauna's Toolbox gives you access to a variety of thermal calculations.
For example, if
you want to calculate the wattage dissipated by a
100 mm x 100 mm
plate with a temperature rise of 35°C at an altitude of 4,000
meters, you can use the Toolbox to get an answer. The Toolbox gives
access to a variety of convection and radiation equations, including
radiation view factors. You can also calculate the thermal
resistance of a variety of shapes (bars, wires, tubes, etc) and there
are electronics-oriented resistances as well (thermal via resistance,
end-to-end trace resistance and more). Although people don't buy Sauna for
the Toolbox, most users use it frequently.
Interacting with FEM and CFD
Your company may already own an FEM (finite element method) or CFD
(computational fluid dynamics) software package. And, in fact, you
may be the user of one of these packages. So does it still make
sense to purchase Sauna?
The answer is "yes". Users with access to multiple packages simply
choose the best package for a particular need. Sauna is well suited
for many thermal problems but there are times that CFD
is useful for a complicated air flow pattern. And FEM packages can
predict mechanical stress, which Sauna can't do.
Actually, Sauna can be used to complement a CFD or FEM analysis.
A quick Sauna model is very useful as a check on the
CFD or FEM results. Also, FEM packages only have limited ability to
calculate convection and radiation boundary conditions. So you can
use Sauna's Toolbox to calculate the necessary boundary heat
transfer coefficients. This is another common use of Sauna.
Sauna is an excellent thermal package for a wide variety