Week 9 - PCB Thermal Simulation

AIM:- The objective of the project is to create a PCB model by importing the PCB layout, library files, and traces to Ansys Icepak and set up the physics & solve the thermal model.

Objective:- A PCB board, library files, and traces are imported to create the model. The model is first solved for conduction only, without the components, and then solved using the actual components with forced convection.

The three cases for the following cases:

• The model is solved only for Conduction, without components.
• The model is solved for the forced convection with the actual components.
• The model is solved for the forced convection and traced heating.

Tasks:-

1. The given board file, which contains information about the different components on the PCB, is imported to ANSYS Icepak. The associated library file gets automatically populated.
2. The given ECAD file, which contains information about the trace layers and vias present on the PCB, is imported to ANSYS Icepak.
3. The model is solved for conduction only without the actual components.
4. The model is solved for forced convection with the actual components.
5. The model is solved for forced convection with the actual components and the joule-heating within the trace layers of the PCB is modeled.

Introduction:-

A printed circuit board (PCB) mechanically supports and electrically connects electrical or electronic component electrical or electronic components using conductive tracks, pads, and other features etched from one or more sheet layers of copper laminated onto and between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it. Printed circuit boards are used in all but the simplest electronic products. They are also used in some electrical products, such as passive switch boxes. 

Alternatives to PCBs include wire wrap and point-to-point construction, both once popular but now rarely used. PCBs require additional design effort to lay out the circuit, but manufacturing and assembly can be automated. Electronic computer-aided design software is available to do much of the work of layout. Mass-producing circuits with PCBs is cheaper and faster than with other wiring methods, as components are mounted and wired in one operation. Large numbers of PCBs can be fabricated at the same time, and the layout only has to be done once. PCBs can also be made manually in small quantities, with reduced benefits.

PCBs can be single-sided one copper layer, double-sided two copper layers on both sides of one substrate layer, or multi-layer outer and inner layers of copper, alternating with layers of the substrate. Multi-layer PCBs allow for much higher component density because circuit traces on the inner layers would otherwise take up surface space between components. The rise in popularity of multilayer PCBs with more than two, and especially with more than four, copper planes was concurrent with the adoption of surface mount technology. However, multilayer PCBs make the repair, analysis, and field modification of circuits much more difficult and usually impractical.

 

Input Received:-

1. Create a new project
2. Import board layout *.bdf & *.ldf (The board and the associated library files have to be chosen at this step and the trace file can be imported later.)
3. Form the nonconformal assembly
4. Import ECAD file (*.anf) A1.anf can be found in the folder containing the input files downloaded for his tutorial.
5. Once the import process is completed, you can edit the layer information in the Board layer and via the information panel (metal layers are pure Cu and the dielectric layers are FR-4.)
6. Enter the layer thickness as shown in the table below.

Layer No.	Layer Thickness (mm)
1	0.04
2	0.45364
3	0.062
4	0.467
5	0.055
6	0.442
7	0.045

      7. Specify Grid density by size (0.508 mm X 0.508 mm)

 

 

Task I:- Conduction-only model

1. Geometry

The image attached below shows the geometry of the model.

It can be seen that all the components on PCB are deactivated and the cabinet is rescaled to the dimensions of the PCB.

2. Meshing

The computational domain is discretized using the Hex-dominant grid generated by the "Mesher-HD". The maximum element sizes along the X, Y, and Z directions are set to 2.032 mm, 2.032 mm and 0.05 mm respectively. The minimum gaps along the X, Y, and Z directions are set to 1 mm, 1 mm, and 0.01 mm respectively. The image attached below shows the "Mesh control" window.

It can be seen that the generated mesh yields 137268 elements and 146080 nodes. The images attached below show the computational mesh plotted on three mutually perpendicular planes, each passing through the center of the cabinet.

Visually the mesh appears to be fine. The images attached below show the histograms with the number of elements plotted along the X-axis and the quality measures plotted along the Y-axis.

3. Mesh Quality

1. Values less than 0.05 indicate the presence of severely distorted elements in the mesh. All the elements in the current mesh have a face-alignment of 1 which indicates that they are perfectly aligned to each other.
2. For a double precision solver, the minimum cell volume should not be less than 1e-15 m^3 otherwise the solver may face issues. The minimum cell volume in the current mesh is 1.64139e-10 m^3 and hence the mesh elements aren't small enough to cause problems in the solver.
3. A skewness of 0 indicates that the mesh element is degenerate whereas a skewness of 1 indicates that it is ideal (equilateral/equiangular). All the elements in the current mesh have a skewness of 1 which indicates that they are ideal (equilateral/equiangular).

 

4. Solver

The three-dimensional steady-state Navier-Stokes equations for the model are solved within the computational domain for temperature field using the Fluent solver available in ANSYS Icepak. The following settings are applied to the solver:

1. Variables solved: Temperature
2. Radiation: Off
3. Time variation: Steady
4. Solution initialization
   1. Temperature: Ambient (20 deg. C)
5. Number of iterations: 500
6. Convergence criteria
   1. Energy: 1e-20
7. Configuration: Parallel
   1. Number of processors: 4
   2. GPU computing: Enabled
   3. Number of GPUs: 1
8. Discretization scheme
   1. Temperature: First
9. Under-relaxation factor
   1. Temperature: 1
10. Linear solver settings
11. Temperature - Type: F, Tolerance criterion: 1e-6, Residual reduction tolerance: 1e-6, Stabilization: None
12. Precision: Double

 

 

5. Results

In the current section, the simulation results are discussed.

A. Residuals

The residuals for the energy equation are plotted against the number of iterations, as shown in the image attached below.

• Energy equation.

The plot shows that the residuals are quite small quantitatively and don't change appreciably between 300-500 iterations which indicates that the solution has reached a steady state. For the current model, the simulation results after 500 iterations are considered steady-state results.

B. PCB-traces

The images attached below show the PCB traces using different display options.

                                                                   Single color                                                                                                                                                    Color by trace                                                                                                                                                   Color by layer

 

C. Metal fractions

The images attached below show the metal fraction contours plotted over the different layers of the PCB.

It is evident from the contours that a dielectric layer (with no copper traces) is sandwiched between two adjacent PCB layers that contain copper traces.

D. Thermal conductivity contours

The images attached below show the $k_X,k_{\mathrm{Y,}}\text{and}{k}_Z$ thermal conductivity contours plotted over the Z-plane passing through the center of the PCB.

 

It is evident from the contours that the thermal conductivity of the PCB is anisotropic in nature.

E. Temperature contours

The image attached below shows the steady-state temperature contours plotted over the Z-plane passing through the center of the PCB.

It is evident from the contour legend that the maximum temperature on the plane is 64.71 deg. C.

 

Task II - Forced convection model

1. Geometry

The image attached below shows the geometry of the model.

It can be seen that, unlike the conduction-only model, a lot of components are activated in the model to perform forced convection analysis.

2. Meshing

The computational domain is discretized using the Hex-dominant grid generated by the "Mesher-HD". The maximum element sizes along the X, Y, and Z directions are set to 9.5 mm, 5 mm and 0.7 mm respectively. The image attached below shows the "Mesh control" window.

It can be seen that the generated mesh yields 312243 elements and 388830 nodes. The images attached below show the computational mesh plotted on three mutually perpendicular planes, each passing through the center of the cabinet.

Visually the mesh appears to be fine. The images attached below show the histograms with the number of elements plotted along the X-axis and the quality measures plotted along the Y-axis.

3. Boundary condition:

Cabinet inlet wall BC

Cabinet outlet wall BC

Flow BC

Inlet velocity BC

Convergence parameter

3. Mesh Quality

1. Values less than 0.05 indicate the presence of severely distorted elements in the mesh. All the elements in the current mesh have a face-alignment of 1 which indicates that they are perfectly aligned to each other.
2. For a double precision solver, the minimum cell volume should not be less than 1e-15 m^3 otherwise the solver may face issues. The minimum cell volume in the current mesh is 8.1161e-13 m^3 and hence the mesh elements aren't small enough to cause problems in the solver.
3. A skewness of 0 indicates that the mesh element is degenerate whereas a skewness of 1 indicates that they are ideal (equilateral/equiangular). All the elements in the current mesh have a skewness of 1 which indicates that they are ideal (equilateral/equiangular).

 

4. Solver

The three-dimensional steady-state Navier-Stokes equations for the model are solved within the computational domain for flow and temperature field using the Fluent solver available in ANSYS Icepak. The following settings are applied to the solver:

1. Variables solved: Flow (velocity/pressure), Temperature
2. Radiation: Off
3. Time variation: Steady
4. Solution initialization
   1. X velocity: -1 m/s
   2. Y velocity: 0
   3. Z velocity: 0
5. Temperature: Ambient (20 deg. C.): 1e-20
6. Number of iterations: 500
7. Convergence criteria
   1. Flow: 0.0001
   2. Energy: 1e-20
   3. Joule heating: 1e-7
8. Configuration: Parallel
   1. Number of processors: 4
   2. GPU computing: Enabled
   3. Number of GPUs: 1
9. Discretization scheme
   1. Pressure: Standard
   2. Momentum: First
   3. Temperature: First
10. Under-relaxation factor
    1. Pressure: 0.7
    2. Momentum: 0.3
    3. Temperature: 1
    4. Viscosity: 1
    5. Body forces: 1
    6. Joule heating potential: 1
11. Linear solver settings
    1. Pressure - Type: V, Termination criterion: 0.1, Residual reduction tolerance: 0.1, Stabilization: None
    2. Momentum - Type: flex, Termination criterion: 0.1, Residual reduction tolerance: 0.1
    3. Temperature - Type: F, Termination criterion: 1e-6, Residual reduction tolerance: 1e-6, Stabilization: None
    4. Joule heating potential - Type: W, Termination criterion: 1e-6, Residual reduction tolerance: 0.1, Stabilization: None
12. Precision: Double

5. Results

In the current section, the simulation results are discussed.

A. Residuals

The residuals for the following equations are plotted against the number of iterations:

1. Continuity equation
2. X-momentum equation
3. Y-momentum equation
4. Z-momentum equation
5. Energy equation

It can be seen that the residuals are quite small quantitatively and don't change appreciably between 300-500 iterations which indicates that the solution has reached a steady state. For the current model, the simulation results after 500 iterations are considered steady-state results.

B. Temperature Monitor Point

Temperature monitor points are created within the computational domain not only to monitor the steady-state temperatures of the key objects in the model, such as heat sources and heat sinks but also to inform the advent of a steady state. The image attached below shows the data recorded by the temperature monitor points plotted against the number of iterations.

It can be seen that after 100 iterations there is no appreciable change in the recorded temperatures which indicates that the energy equation has possibly converged. According to the plot, the objects attain the following steady-state temperatures:

• U8: 73.31 deg. C
• BOARD_OUTLINE.1: 73.62 deg. C
• Heatsink.1: 47.92 deg. C
• Heatsink.1.1: 46.04 deg. C

C. PCB traces

The images attached below show the PCB traces using different display options.

                                                                       Single color                                                                                                                                                    Color by trace                                                                                                                                                   Color by layer

 

 D. Metal fractions

The images attached below show the metal fraction contours plotted over the different layers of the PCB.

It is evident from the contours that a dielectric layer (with no copper traces) is sandwiched between two adjacent PCB layers that contain copper traces.

E. Thermal conductivity contours

The images attached below show the  thermal conductivity contours plotted over the Z-plane passing through the center of the PCB.

It is evident from the contours that the thermal conductivity of the PCB is anisotropic in nature.

F. Temperature contours

The image attached below shows the steady-state temperature contours plotted on the Z-plane passing through the center of the PCB.

It is evident from the contour legend that the maximum steady-state temperature on the plane is 85.35 deg. C.

G. Object face temperature contours

The image attached below shows the steady-state object face temperature contours plotted on the PCB.

It is evident from the contours-legend that the maximum temperature on the PCB is 89.91 deg. C.

H. Velocity contours

The image attached below shows the steady-state velocity contours plotted on the Z-plane passing through the center of the cabinet.

It is evident from the contour legend that the maximum steady-state velocity on the plane is 5.91 m/s.

  

TASK III - Forced convection model with trace heating

In order to model trace heating in one of the traces present in the second trace layer of the PCB, a solid trace is created out of it using the following settings in the "Trace heating" window:

1. Min area: 4124 mm^2
2. Max angle: 135
3. Min length: 1 m

Once the solid trace is created two voltage/current type sources are created on it using the following settings:

Source 1

(a) Geometry                                                         (b) Properties

Source 2

(a) Geometry                                                         (b) Properties

1. Geometry

Apart from the addition of a solid trace and two voltage/current type sources, the geometry of the current model remains the same as that used for the forced convection model. The image attached below shows the solid trace and the two sources created using the settings described above.

2. Meshing

A non-conformal assembly is defined around the trace to reduce the mesh count and the mesh settings applied for the forced convection model are applied again for the current model. The image attached below shows the "Mesh control" window.

It can be seen that the mesh settings yield 429902 elements and 543948 nodes. The images attached below show the computational mesh plotted on three mutually perpendicular planes, each passing through the center of the cabinet.

3. Mesh Quality

Visually the mesh appears to be fine. The images attached below show the histograms with the number of elements plotted along the X-axis and the quality measures plotted along the Y-axis.

1. Values less than 0.05 indicate the presence of severely distorted elements in the mesh. For the current mesh, the minimum value of face alignment is 0.10775 which indicates that the mesh elements aren't severely distorted.
2. For a double precision solver, the minimum cell volume shouldn't be less than 1e-15 m^3 otherwise the solver may face issues. For the current mesh, the minimum cell volume is 1.55448e-13 m^3 and hence the mesh elements aren't small enough to cause problems in the solver.
3. A skewness of 0 indicates that the mesh element is degenerate whereas a skewness of 1 indicates that it is ideal (equilateral/equiangular). For the current mesh, the minimum value of skewness is 1.86593e-5 which indicates that no element in the mesh is degenerate.

 4. Solver

The three-dimensional steady-state Navier-Stokes equations for the model are solved within the computational domain for flow and temperature field using the Fluent solver available in ANSYS Icepak. The solver settings used for the forced convection model are reused for the solver in the current model. BCGSTAB is chosen as the stabilization criterion for both temperature and Joule heating potential.

5. Results

In the current section, the simulation results are discussed.

A. Residuals

The residuals for the following equations are plotted against the number of iterations:

1. Continuity equation
2. X-momentum equation
3. Y-momentum equation
4. Z-momentum equation
5. Energy equation

It can be seen that the residuals are quite small quantitatively and don't change appreciably between 300-500 iterations which indicates that the solution has reached a steady state. For the current model, the simulation results after 500 iterations are considered steady-state results.

B. Temperature Monitor Points

Temperature monitor points are created within the computational domain not only to monitor the steady-state temperatures of the key objects in the model, such as heat sources and heat sinks but also to inform the advent of a steady state. The image attached below shows the data recorded by the temperature monitor points plotted against the number of iterations.

It can be seen that after 100 iterations there is no appreciable change in the recorded temperatures which indicates that the energy equation has possibly converged. According to the plot, the objects attain the following steady-state temperatures:

1. U8: 74.26 deg. C
2. BOARD_OUTLINE.1: 74.43 deg. C
3. Heatsink.1: 48.45 deg. C
4. Heatsink.1.1: 47.38 deg. C
5. BOARD_OUTLINE.1.layer-3-trace-A3V3_2784: 75.48 deg. C

C. PCB traces

The images attached below show the PCB traces using different display options.

 

D. Metal fractions

The images attached below show the metal fraction contours plotted over the different layers of the PCB.

It is evident from the contours that a dielectric layer (with no copper traces) is sandwiched between two adjacent PCB layers that contain copper traces.

E. Thermal conductivity contours

The images attached below show the thermal conductivity contours plotted over the Z-plane passing through the center of the PCB.

 

It is evident from the contours that the thermal conductivity of the PCB is anisotropic in nature.

F. Temperature contours

The image attached below shows the steady-state temperature contours plotted on the Z-plane passing through the center of the PCB.

It is evident from the contour legend that the maximum steady-state temperature on the plane is 85.91 deg. C.

G. Object face temperature contours

The image attached below shows the steady-state object face temperature contours plotted on the PCB.

It is evident from the contours-legend that the maximum steady-state temperature on the PCB is 87.33 deg. C. The image attached below shows the steady-state object face temperature contours plotted on the solid trace.

It is evident from the contour legend that the maximum steady-state temperature on the solid trace is 86.09 deg. C.

H. Velocity contours

The image attached below shows the steady-state velocity contours plotted on the Z-plane passing through the center of the cabinet.

It is evident from the contour legend that the maximum steady-state velocity on the plane is 6.07 m/s.

I. Electric potential contours

The image attached below shows the steady-state electric potential contours plotted on the solid trace.

It is evident from the contour legend that the maximum electric potential on the solid trace is 93.58 mV.

Conclusion

In the current project, the following tasks were performed:

1. The given board file and ECAD file were imported to ANSYS Icepak.
2. All the components on the PCB were deactivated and the resulting model was solved only for conduction.
3. Key components on the PCB are activated and the resulting model was solved for forced convection.
4. In order to model trace heating, a solid trace was created using the identified trace and two voltage/current type sources were created using appropriate settings. The resulting model was solved for forced convection and trace heating.