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Feature List

Sauna at a glance

  • created for the thermal simulation of electronic equipment
  • used to model heat sinks, circuit boards (PCB's) and boxes (with and without internal PCB's)
  • models are composed of the building blocks of plates and boards
  • underlying modeling method is thermal network (thermal nodes and resistors), but modern architecture means that user rarely specifies a resistor value
  • large component library
  • convection and radiation coefficients calculated automatically
  • easy to learn and use, much easier than finite element method (FEM) or computational fluid dynamics (CFD)

Quick links

Assemblies    Circuit board stackups    Heat sources    Component library   
Box creation utilities    Float resistors    Flow networks    Subdividing assemblies   
Join command    Temperature calculation    Control elements    Editing   
Display modes    Reports    Graphics export    Model export    Toolbox    Support   


  • assemblies are the building blocks from which a comprehensive thermal model
    is created
  • assembly types
    • plate
      • material may be metallic, ceramic, plastic, semiconductor
      • many materials in library, including CVD diamond, AlSiC and other new materials
      • user may also enter his own properties
      • fins may be added to one or both sides of plate
      • traces and pads are classified as plates
    • circuit board - ordinary
      • represents a laminate (dielectric) layer in a circuit board stackup
      • material may be FR4, alumina, molding compound, etc.
      • user may also enter his own properties
      • vias are a circuit board property
        • user specifies via diameter, plating, fill material and density
        • via properties can be uniform, or vary on a node-by-node basis
    • circuit board - planar
      • simplified way to represent multilayer board
      • all layers represented with a single board assembly
      • copper coverage (copper density) represents the cumulative copper on all layers
      • copper coverage can be uniform across the board or vary on a node-by-node basis

Circuit board stackups

  • stacked combination of laminate layers (board assemblies) and copper layers (plate assemblies)
  • copper layers can be modeled full-plane or with traces and pads
  • laminate (dielectric) layers have thermal via properties to allow for enhanced heat conduction in approrpriate areas
    • via properties:
      • density
      • diameter
      • plating thickness
      • fill material (solder, epoxy, air, user-defined)
    • via properties can be uniform or vary on a node-by-node basis
  • step-by-step modeling method
    1. create a laminate layer (board assembly)
    2. using library, create component pads:
      • trace options:
        • flareout, straight leads, none
        • detailed or lumped leads
      • duals and quads can be created with/without slug pad
      • option to use oversized heatsinking pad
    3. using library, add enhanced heat sources
      • enhanced source includes heat transfer to lead pads, through body base and from sides/top of component, see "Heat sources" below
      • can use typical data for Rjunction-to-case and Rinterface
    4. create connecting pads and traces
      • important when components are close together
      • widely spaced components can be left unconnected
    5. for multilayer board, create internal layers
      • subdivide command makes this easy
      • typical thickness configurations are available, or can specify thickness of each laminate and copper layer
      • copper layers can be modeled full-plane or with traces and pads
    6. modify via properties
      • vias assigned to an area of laminate, not modeled as individual tubes
      • efficient way to obtain accurate results
    7. create float resistors between board stackup and room
      • for isolated board stackup, simple one-step operation
      • for further details, see "Float resistors" below.
    8. calculate temperatures

Heat sources

  • basic
    • has associated footprint area, but lead conduction is ignored
    • used for heat sinks and quick analysis of PCB's
    • library of standard power packages
    • thermal interface types: greased, dry, mica, silicon pad, solder, special
    • user enters Rjunction-to-case or can use typical value
  • enhanced
    • more comprehensive component model
    • used for circuit board analysis
    • large library, see "Component library" below
    • enhanced source incorporates these thermal resistances:
      • through slug into pad on board
      • through air gap below body into board
      • top and sides of body
      • connection to each lead pad
      • if necessary, can use a different resistance for every lead pad
    • flip chip (heat sink on top) supported
  • besides basic and enhanced heat sources, it is also possible to model a component as a detailed stackup

Component library

  • DPAK (2, 3, 5 leads), D2PAK (2, 3, 5, 7), custom DPAK
  • Dual plastic packages: narrow SOIC, wide SOIC, PSOP
  • Quad plastic packages: MQFP, LQFP, TQFP, QFN, special pitches and lead counts supported
  • SOT-223
  • LED: PLCC-2, PLCC-4, ML, MX, XM, XP
  • Diode: SMA, SMB, SMC, SOD123F

Box creation utilities

  • create six-sided box with a single command
  • "Board In Box" command for easy creation of circuit board within a box
  • "Plate In Box" command for easy creation of internal walls within a box

Float resistors

  • float resistors are used for modeling convection and radiation
  • nonlinear characteristics of convection and radiation are automatically taken into account when temperatures are calculated
  • convection
    • natural or forced air convection
    • internal and external flows
    • air temperature range: -60C to +300C (-76F to +572F)
    • specify altitude or air pressure, convection resistors are automatically updated
    • automatic calculation of transition between laminar and turbulent flow
    • calculation of developing flow regime
    • isolated plates, parallel plates, parallel gaps
    • fin array (heat sink) convection
      • natural
        • vertical baseplate/vertical fin channels, Van de Pol & Tierney equation
        • vertical baseplate/horizontal fin channels, equation based on Bilitzky data
        • horizontal baseplate, faces up or down, equation based on Bilitzky data
      • forced: parallel plate with developing flow
    • any number of different convection modes are allowed in one thermal model
  • radiation
    • blackbody
    • gray for the inside of an enclosure
    • automatic calculation of view factors
      • algebraic method for non-obstructed view factors
      • ray tracing for obstructed view factors
      • fast and accurate
      • completely integrated within software, no interfacing to external programs
  • in addition to modeling with float resistors, can also model using constant value resistors
  • automatic connection routines for simultaneous creation of ambient nodes and float resistors

Flow networks

  • flow resistors
    • "one way" resistors
    • fluid types: air, water, special fluid
    • create a single flow resistor or chains of flow resistors
    • error checking insures flow balance and fluid type continuity
  • multiple flow paths and flow branches allowed
  • easy to modify flow networks
    • when editing flow, can specify a flow value or use a scale factor
    • when the flow rate is modified, associated channel convection resistors are automatically updated
    • reverse flow direction
    • extend a flow path
  • heat sink flow network
    • specify fan or assign flow
    • for fan, flow is calculated automatically based on fin characteristics
    • obtain temperature rise at intermediate points in flow path
  • liquid cooling (cold plates)
    • obtain pipe thermal resistance from Toolbox
    • use generic resistors to incorporate pipe thermal resistance into Sauna model

Subdividing assemblies

  • slice
    • subdivide assemblies at an arbitrary coordinate
    • 4 types of slices (single line, double line, 2 point, 4 point)
    • use slice to align mesh between adjacent model elements: boards and walls in enclosure, silicon chip and substrate in semiconductor module, etc.
    • subdivide assemblies to increase mesh density in key heat flow areas while using low mesh density elsewhere
    • Sauna understands that subdivided assemblies are part of a larger entity (superassembly); when temperatures are calculated, overall dimensions are used
  • create stackup
    • subdivide into layers
    • stack joins between layers are created automatically
    • existing joins to adjacent assemblies are maintained
    • boards
      • used to create alternating layers of copper and laminate
      • typical thickness configurations are available, or can specify thickness of each laminate and copper layer
    • plates
      • user chooses number of layers in final stackup, Sauna creates uniform thickness layers
      • heat loads are placed on user-specified layer

Join command

  • used to provide a thermal connection between assemblies
  • join types: stack and edge
  • joins have an interface type associated with them, so it isn't necessary to create a separate interface layer
  • checks performed for interference and air gaps
  • edge join
    • any two assemblies which share an edge can be joined
    • interface types: zero resistance, insulated card guide, conductive card guide, special
  • stack joins
    • join layered assemblies
    • interface types: zero resistance, die attach epoxy, flat/dry, flat/greased, solder, air gap, special

Temperature calculation

  • steady state, simple transient, duty cycle transient
  • for simple transient and duty cycle transient, the Sauna Modeling System version is required
  • highly optimized and efficient numerical solvers
  • steady state calculations
    • handled with direct sparse matrix solver
    • automatic iteration to calculate convection and radiation coefficients
    • uses equivalent gray method to accelerate radiation calculation
  • transient calculations
    • two solver methods: implicit or explicit
    • in automatic mode Sauna analyzes model characteristic and chooses either implicit or explicit solver, Sauna may switch between methods during the course of the calculation
    • user can also directly specify the solver method to use
    • convection and radiation coefficients updated at each output time step
  • Rmax/Rmin ratio of 1015 allowed
  • analyze models with small time constants
    • time steps as small as 0.1 microseconds
    • feature size down to 0.1 micron
  • comprehensive error checking
    • interference
    • nodes without path to ambient
    • wattage and fixed temperature inconsistencies
    • out of range on convection correlations
    • incomplete joins
    • islands in model

Control elements

  • time base
    • use these control elements for duty cycle problems where a predetermined set of on/off and environmental conditions exist
    • wattage can vary with time (q vs. time control element)
    • fixed node (boundary temperature) can vary with time (temperature vs. time control element)
    • simulate square waves, ramps, pulses and arbitrary user-defined waveforms
    • characteristics specified with menus or by writing a script
    • with scripts, virtually any waveform can be defined
  • temperature base
    • wattage vs. temperature
      • control types
        • simple linear shutback
        • thermostat, used for simple control
        • complex, fully defined by user
      • remote reference temperature allowed
      • calculate component operating point during partial shutback
      • obtain plots of wattage vs. time
      • can be combined with time-based control
    • temperature vs. temperature
      • control types
        • on/off, use to activate a cooling mode, such as a temperature controlled fan
        • ΔT, use to establish a ΔT between parts of the model, as for a thermoelectric application
        • complex, fully defined by user
      • absolute temperature and ΔT options
      • can be combined with time-based control
  • wattage base
    • wattage can vary based on heat flow into a node (q vs. q control element)
    • control types
      • simple scale
      • complex
    • use these control elements for thermoelectric applications
    • can be combined with time-based control and temperature-based control
  • since Sauna's solver is very fast, it is ideally suited for handling complicated control element problems


  • many advanced editing commands
  • very easy to perform "what-if" modifications
  • any assembly property, such as material or surface type, is modifiable with a single command
  • re-mesh assembly
    • use fine mesh in regions of high heat flow, low mesh density elsewhere
    • can re-mesh in one or both axes
    • when re-meshing, Sauna maintains basic sources, joins and float resistors
  • align mesh
    • aligns mesh to match a heat source or assembly
    • efficient way to obtain accurate mesh in most important areas
  • enlarge/reduce assembly
    • extend to limit point, enter new overall dimension, enter delta dimension, trap delta dimension
    • retain origin, center or endpoint
    • when enlarging/reducing, Sauna maintains basic sources, joins and float resistors
  • trace/pad dimensions
    • reshape pad
    • edit trace width (entire trace or one end)
    • extend trace (shift endpoint)
    • edit copper layer weight (thickness)
  • surface assembly
    • create on face or edge
    • assembly can be "thin" or with a user specified thickness
    • material can be the same or user-specified
    • useful for many situations
      • define boundary conditions
      • quickly convert a board to a metal-backed structure
      • quickly add insulation to the outside of a box
      • add a gap filler pad between a board and a box wall
      • allows for conversion of a board to a plate
      • useful when defining chip stackups
  • float resistors
    • modify dissipation mode and air velocity
    • specify special flow length
    • specify laminar or turbulent flow (default is Sauna calculates transition)
    • h vs. length (use average or h varies with length)
    • convert between certain float resistor types
  • merge command to undo the effects of slicing
  • rotate
  • mirror
  • append one model into another
  • easy to edit fin length, spacing, thickness and orientation to obtain optimum fin dimensions

Display modes

  • orthogonal, perspective and oblique projections
  • wireframe display mode to see node and resistors
  • shaded display mode to show surfaces
  • contour mode to display temperature contours


  • "English language" reports give a complete description of thermal model and temperature results
  • assembly reports provide dimensions, cooling modes, material properties, etc.
  • heat load reports
    • provides detailed information about the position and characteristics of heat sources and distributed wattage
    • input report: heat load before considering the effects of control elements
    • final report: heat load at the end of the analysis, which includes duty cycle and power shutback effects
    • transient report: heat load vs. time (text, graph or export spreadsheet data)
  • board stackup report
    • weight and area for each copper layer
    • thickness and via properties for laminate layers
  • temperature reports
    • indicate maximum, minimum, and averages for all heat sources, internal ambients, and assemblies
    • current report: steady state temperatures or final step in transient analysis
    • transient report: temperature vs. time (text, graph or export spreadsheet data)
  • mass reports provide weight, mass and thermal capacitance

Graphics export

  • Postscript files
  • BMP files
  • Windows meta files
  • "temperature vs. time" data in tab-delineated spreadsheet format

Model export

  • SINDA thermal and fluids modeling program
    • SINDA-85 or SINDA/G format
    • SINDA results can be imported into Sauna for post-processing
  • Saber circuit simulation program
    • perform advanced electrothermal modeling


  • perform "classic" heat transfer calculations to obtain h, Gr, Re, etc.
    • isolated plates
    • parallel plates and channels
    • cylinders
    • natural or forced convection
    • handles laminar and turbulent regimes
    • corrects for entrance effects
  • fin arrays
    • calculate total heat transfer
    • calculate pressure drop
    • optimize fin spacing and thickness
    • calculate fin efficiency
  • pipe heat transfer calculations
    • fluid types: air, water, special fluid
    • automatically switches between laminar and turbulent
    • corrects for entrance effects
    • analyzes smooth or rough pipes
  • calculate layer-to-layer board resistance which is based on via density and via characteristics.
  • calculate conduction resistances
    • rectangular
    • cylindrical/radial
    • cylindrical/axial
    • tube/axial
    • end-to-end trace resistance
  • radiation shape factors
  • calculate fluid temperature rise
  • obtain properties from Sauna's libraries


  • comprehensive telephone and email support provided, including assistance with modeling strategies and assumptions
  • user manual
    • paper manual provided at no extra cost
    • very complete (400+ pages) with many tutorials
  • context sensitive help
    • complete description of every menu
    • one-click access
    • all acronyms are defined
    • reference pictures available for many operations
  • technical support area on website for technical notes
  • training sessions available (one day training session included at no charge for Sauna Modeling System)

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