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Applications of CFD tools in PCB, Board-level and Chassis Level Thermal Simulations

Electronics Cooling

There are vast range of applications of thermal simulations in electronics industry dealing with Printed Circuit Board (PCB), Board and chassis level simulation, thermal management of data centres including specialized cooling methods like heat sinks, TEC (Thermo-Electric Cooling), heat pipes, PCM (Phase Change Material), heat spreaders...

Types of Simulations

  • PCB or Package Level: Detail modeling of copper traces, thermal vias and dielectric material inside the PCB panel.
  • Board Level: PCB modeled as simplified orthotropic thermal conductivity.
  • Chassis Level: Similar to board-level simulation, bigger in computational domain.
  • Rack Level such as in Data Centres: Most of the heat sources are modelled as lumped blocks.

Commercial Tools

  • ANSYS ICEPAK: A customized GUI for pre- and post-processing but uses FLUENT as solver.
  • SIEMENS FloTHERM: similar in look and operation like ICEPAK with primitives, SmartPart and attributes.

Some key features of such simulations are usage of Intermediate Data Format (IDF) and Incremental Data eXchange (IDX) files that have been exported from an ECAD package. These files contain informations of traces in PCB. In ANSYS ICEPAK, while importing traces the default materials are Cu-pure for metal and FR-4 for dielectric. PCB construction is a layered design along the thickness direction and hence the thermal conductivity is necessarily orthotropic. Here, the conductivity value along the thickness direction - known as through-the-plane conductivity is far less than in-the-plane conductivity values. These softwares are meant for electronics industry only and hence contains lots of objects related to this applications to fasten the simulation process. They can be summarized as follows:
  • Primitive - Fundamental geometric entities in FloTHERM and ICEPAK: Cuboids, Prisms and Flow Resistances
  • SmartPart - Object parametrically created out of Primitives: e.g. Enclosure, fan, PCB, cylinders, volume or surface heat source, heat sinks (described by base dimension, number of fins, fin width and fin height), perforated plate (fully designated by hole size and arrangement, pitch, free area ratio)
  • Assembly - A group of Primitives, SmartParts and Sub-Assemblies
  • Attribute - A property that can be attached to Primitives and SmartParts (e.g. material properties)
Primitives defined in ICEPAK
ICEPAK Primitives
Reading Mechanical CAD (MCAD) data
  • The MCAD data can be read either in their native format such as ProE, Solidworks or CATIA files (parts and assemblies) or neutral formats such as IGES, STEP or PARASOLID.
  • After initial defeaturing (removal of chamfers and fillets) and simplifications (removal of small holes, branding logos, part identifiers), the MCAD geometry needs to be converted into ICEPAK / FloTHERM entities.
  • Simplification is permissible to the to the extent where the geometry could be created manually using primitives and smart-parts in ICEPAK / FloTHERM!
  • The converion into ICEPAK / FloTHERM entities is a process of replacing a detailed geometry say perforated plate with a plate of same size without perforation and specifying the perforation details as attributes to the ICEPAK / FloTHERM entities.

Mesh generation process and recommedations
  • For naturally convected cases, the computational domain needs to be made bigger than the chassis or the board.
  • For the space above the chassis extend the domain 2~3 × height of chassis. Note that the buoyancy may not be strong enough to establish a flow from lower face to upper face and reverse flow can be observed on both of these faces. In order to reduce the reverse flows, the domain boundaries may need to be kept closer to the heat source.
  • For space below chassis extend the domain equal to the height of chassis.
  • For the remaing sides (front, rear, left and right) of the chassis, extend the domain 0.5 ~ 1.0 × depth and width of the chassis.
  • In case on natural convection, the heat transfer through convectionn is low and hence radiation also contributed significantly. Up to 50% of heat transfer is by radiation and remaining 50% by natural convection.This can be justified by the fact that the HTC value for natural convection in air varies between 5~10 [W/m2.K] whereas equivalent HTC for radiative heat transfer = ε × σ × (TWALL4 - TAMB4) / (TWALL - TAMB) = 4.9 [W/m2.K] for ε = 0.5, TWALL = 100 [°C] and TAMB = 50 [°C].
  • The components are mostly represented at voume with sharp corners such as cuboid and rectangles. A cut-cell method also known as trimmer mesh or Cartesian mesh or snappyHexMesh (in OpenFOAM) is used in ICEPAK and FloTHERM.
  • For printed circuit boards as in any surface with a significant amount of heat flux, 3 cells in the 1st millimeter above the board (air volume) and 3 cells in the 1st millimeter below the board (solid volume).
  • Maintain aspect ratio of cells < 100.
  • For fins, maintain > 5 cells cross a channel for better heat transfer and pressure drop prediction, use two or more cells across the thicness in the solid.

Special applications in Electronic Cooling
  • Thermo-Electric Cooling (TEC): This device is based on Peltier effect where thermal energy is absorbed at one dissimilar metal junction and discharged at the other junction when electric current flows within a closed circuit. It comprises of p-type and n-type semiconductors sandwiched between ceramic electrical insulators. TEC are solid state heat pumps for applications where cooling below ambient are required. The cold junction acts as 'evaporator' and hot junction as 'condenser' of a refrigeration cycle.
    TEC - Thermo-Electric Coolers
    • Seebeck Effect: ΔV = α × (TH - TC) where α [V/K] = differential Seebeck coefficient or (thermo electric power coefficient) between the two materials, positive when the direction of electric current is same as the direction of heat flow.
    • QH [W] = β [V] × I [A] where β is differential Peltier coefficient between given two materials
    • β < 0: Electric current and heat flow in opposite directions
    • β > 0: Electric current and heat flow in same directions
    • COP = coefficient of performance of the thermoelectric device = QC / J, which typically is between 0.4 and 0.7 for single stage applications.
  • Heat Pipe: Capillary effect and phase change (evaporation and condensation) are the phenomena which defines operation of heat pipes. Due to phase change from liquid to vapour, the heat transfer coefficient for heat pipes is extermely high - of the order of 40,000 [W/m2.K]
  • Heat Spreader: This is an application similar to heat sink. The purpose of a heat spreader is to use material with a very high thermal conductivity such as graphite with k = 1400 [W/m-K] to make the heat flow in-plane over a larger area so that it can be further dissipated into ambient using heat sinks.
  • PCM - Phase Change Material: These are energy storage and release mechanism based on change of phase (typically solidification and melting). The material can be used to keep temperature fluctuations low in case of heating and cooling cycles. Sometimes, the PCM can also act as an insulator to heat dissipation based on thermal conductivity. Some key characteristics required for a energy storage and release type PCM are tabulated below.
    Characterisitcs Desired valueRemark
    Melting point, TMP As per temperature controlSelection of material will depend on temperature to be maintained
    Specific heat capacity, Cp HighEnergy storate capacity ∝ Cp. Higher the Cp, lesser the mass required to store a given amount of energy.
    Density, ρ HighEnergy storate capacity ∝ ρ and the volume required is also less as m = ρ * V
    Thermal Conductivity, k High for energy storage purposeLow value is required for insulation where heat is to be maintained near the source itself
    Coefficient of volume expansion, γ LowThis governs flexibility or void space required in the storage container
    Chemical compatibility Non-corrosiveShould not react with the container and other materials in case of leaks
    Thermal cycling (heating-cooling) stability No degradationThe micro-structure and material properties should not degrade with heating-cooling cycles
  • Anisotropic or Orthotropic Thermal Conductivity: The printed-circuit boards are formed by many layers of copper wires known as traces and dielectric material (say FR4). They are so thin that they cannot be modeled (meshed and boundary conditions applied) separately. Hence, the thermal conductivity of board can be simplified using equivalent uniformed value along the thickness and in-the-plane direction. This simplification is done using series and parallell arragement of thermal resistances analogous to electrical resistances. Orthotropic conductivity based on lumped block assumptions:
    • kIN_PLANE = VFCu/100 × kCu + (1-VFCu/100) × kDie
    • 1/kX_PLANE = VFCu/100 / kCu + (1-VFCu/100) / kDie
    • Here, VF = Volume Fraction of copper in %.

ICEPAK is a GUI for pre- and post-processing. It uses FLUENT as solver and in this process many files get created. Following is a list of files and its owner (ICEPAK or FLUENT?).

File Type Created by Used by Suffix / Filename Remark
Model ICEPAK  ICEPAK  model
Problem ICEPAK  ICEPAK  problem
Mesh input ICEPAK  mesher grid_input Inputs for the mesh generator.
Mesh output mesher ICEPAK  grid_output Output from the mesh generator; that is, the mesh file
Case ICEPAK  FLUENT  .cas Contains all the information that is needed by ICEPAK to run the solver
Data FLUENT  FLUENT  .dat, .fdat Files when it has finished calculating: *.dat and *.fdat. These data files can be used to restart the solver
Residual FLUENT  ICEPAK  .res Information about convergence monitors: Solve -> Solution monitor or select Convergence plot in Post menu
Script ICEPAK  ICEPAK .SCRIPT or _sc.bat Runs the solver executable, and can also be used to run the solver in batch mode.
Solver input ICEPAK  FLUENT  .uns_in The solver input file (projectname.uns_in) is read by the solver to start the calculation.
Solver output FLUENT  .uns_out Information from solver that is displayed on screen during calculation - this file is written only on Linux systems
Diagnostic ICEPAK  .diag Contains information about correspondence between object names in model file and object names in case file
Optimization ICEPAK  optimizer .log, .dat, .tab, .post, .rpt Optimization of field variables
Postprocessing FLUENT  ICEPAK  .resd Used by ICEPAK for postprocessing. All solutions that exist for the current project are listed by solution ID.
Geometry External ICEPAK  .igs, .stp CAD geometry - input to ICEPAK
Packaged ICEPAK  ICEPAK .tzr Project archive

Few keyboard short-cuts and special topics in ICEPAK
  • Move legend with Ctrl and the middle mouse button
  • Edit levels and set orientation with shift right-click on legend
  • Move the cut-plane in the domain with shift and the middle mouse button
  • Shift middle-click on CAD objects to graphically move the CAD geometry
  • Thermal Chokepoint: The dot product of heat flux and temperature gradient, The Thermal Chokepoint shows regions of high heat flux experiencing large thermal resistances
  • Thermal Cross: The cross product of heat flux and temperature gradient - The Thermal Cross shows regions where large heat flux vectors not aligned with high thermal gradients
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