Applications of CFD tools in PCB, Board-level and Chassis Level Thermal Simulations
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.
Construction Features on IC Packages
Die: It designates the piece of semiconductor on which all the active circuits lie. Dies are made of Silicon (110 – 150 W/m.K) or Gallium Arsenide (GaAs, 45 – 60 W/m.K) is used in special applications such as microwaves. The circuitry is present within a thin layer on one side only, known as active surface. The Die is often attached to the substrate or the die pad by an adhesive known as the "die attach" which is made of an epoxy based compound having thickness 0.025 - 0.050 [mm] and thermal conductivity in the range 1 - 2 [W/m.K]. The semiconductor material of a die has a conductivity approaching that of a metal. If the active layer is assumed to generate constant power per unit area (an assumption that may not always
be valid), the die will be practically isothermal. In practice, applications exist for which the heat flux varies significantly across the die, in
which case a temperature gradient may exist on its active surface. For most packages, the thermal resistance offered by the die is small in comparison with that offered by the rest of the package.Referecnce: FloTHERM Pack User's Guide
Due to low thickness of the "die attach", it causes very little heat spreading. At the same time, it offers significant thermal resistance in out-of- the-plane direction due to its poor thermal conductivity value.
Die Pad or Die Flag: The Die is placed in insulated boxed often plastic packages, on a thin metal plate (made of copper and larger than the die) known as the die flag or die pad. It helps both in the manufacturing and thermal (heat dissipation) function. The metallic die flag acts as an effective heat spreader due to very high thermal conductivity of copper which can reduce the thermal resistance of a package by up to 15%.
Bond wires, made of Gold or Aluminum having diameter of the order of 0.025 [mm], are characteristics of wire-bonded packages. The number of bond wires in an IC package are of same order as the number of external leads/pins. Excerpts from "FloTHERM Pack User's Guide": In most ceramic packages, a negligible portion of the heat from the Die flows to the substrate through the bond wires. However, in plastic packages this may not be the case. Bond wires play a significant role in the heat transfer within peripheral leaded packages such as the PQFP. In area-array plastic packages such as the PBGA, bond wires can be important, especially for a 2-layer substrate.
Encapsulant: Also known as 'Overmold', it is an epoxy based compound with a thermal conductivity 0.6 - 0.7 [W/m.K].
- 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:
Primitives defined in ICEPAK
- 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)
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 conversion 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 recommendations
- 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 remaining 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 convection 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 as volume 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 first millimeter above the board (air volume) and 3 cells in the first millimeter below the board (solid volume) is recommended.
- 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 thickness 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.
- 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 define operation of heat pipes. Due to phase change from liquid to vapour, the heat transfer coefficient for heat pipes is extremely high - of the order of 40,000 [W/m2.K]. Originally invented by NASA for space application, it has gained widespread application in electronics industry.
- 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 value||Remark|
|Melting point, TMP ||As per temperature control||Selection of material will depend on temperature to be maintained|
|Specific heat capacity, Cp ||High||Energy storage capacity ∝ Cp. Higher the Cp, lesser the mass required to store a given amount of energy.|
|Density, ρ ||High||Energy storage capacity ∝ ρ and the volume required is also less as m = ρ * V|
|Thermal Conductivity, k ||High for energy storage purpose||Low value is required for insulation where heat is to be maintained near the source itself|
|Coefficient of volume expansion, γ ||Low||This governs flexibility or void space required in the storage container|
|Chemical compatibility ||Non-corrosive||Should not react with the container and other materials in case of leaks|
|Thermal cycling (heating-cooling) stability ||No degradation||The 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 parallel arrangement 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 %.
- CFD mesh vs. background mesh:
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|
|Job ||ICEPAK ||ICEPAK ||job|
|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 &rtarrow; 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 post-processing. All solutions that exist for the current project are listed by solution ID.|
|Log ||ICEPAK ||ICEPAK ||.log || |
|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
Basic Solver Setting Note that ICEPAK uses FLUENT in background as solver.
Mesh Generation The default option in ICEPAK is to generate a Cartesian mesh similar to snappyHexMesh in OpenFOAM, trimmer mesh in STAR-CCM+
Fan, Fins and Grilles There are many in-built feature to model flow resistances and momentum sources / sinks.
Fan swirl: tangential component of flow at exit of a fan.
Grilles and Louvres:
Boundary Conditions Wall boundary condition:
PCB Gerber File Import
PCB Stack Data
PCB Stack-up Data in Detail
Monitors and Runs Settings
Micro-Channels Cooling systems in the field of microelectronics and microelectromechanical systems (MEMS) use small and microscale flow passages. The transport phenomena in microscale channels are very different as compared to conventional size channels or macroscale channels. There are several dimensionless numbers used to represent the feature of fluid flow in microscale channels, similar to Reynolds number in macro-chnnels. The distinction between macro- and microscale channels may also be classified According to these dimensionless numbers.