Thermal analysis of a PCB with a chip

Estimated time to complete: 15–20 minutes

In this tutorial, you will simulate the heat transfer from an electronic chip to the printed circuit board (PCB). This tutorial presents Simcenter 3D Thermal.

1: Open the part and start Pre/Post

  1. On your desktop or the appropriate network drive, create a folder named pcb-w-chip.

  2. Click the link below:

    Download the part

  3. Extract the part files to your pcb-w-chip folder.

  4. Start Simcenter 3D or NX.

File

  • Open (Standard toolbar)

  • Files of type

    Part Files (*.prt)

  • File name

    pcb-w-chip.prt

  • OK

     
2: Reset dialog box memory

The options you select in dialog boxes are preserved for the next time you open the same dialog box within a given session. Restore the default settings to ensure that the dialog boxes are in the expected initial state for each step of the activity.

File

Preferences→User Interface

  • Options

  •   Reset Dialog Memory

  • OK

     
3: Create FEM and Simulation files

Application

  • Pre/Post (Simulation group)

Simulation Navigator

  • pcb-w-chip.prt

  • New FEM and Simulation

  • Solver

    Simcenter 3D Thermal/Flow

  • Analysis Type

    Thermal

  • OK

     

  • Name

    PCB_chip_solution

  • OK

      Solution dialog box

4: Display the FEM file

Simulation Navigator

  • pcb-w-chip_fem1.fem

  • Make Displayed Part

5: Create an orthotropic material

Start by creating an orthotropic material you will use for the PCB.
A value of 1 is used for non-thermal properties as these are required by the interface but not used during the analysis.

The material density is 2700 kg/m3, the specific heat is 396 J/kg·K, and the orthotropic thermal conductivities are 0.009 W/mm °C, 0.041 W/mm °C, and 0.00055 W/mm °C in the X, Y and Z directions, respectively.

Manage Materials (Home tab→Properties group)

  •  

  • Type

    Orthotropic

  • Create material

  • Name

    PCB_ortho
  • Note:

    Verify the units of each property. To modify the units use the list beside the entry box.

    Mass Density (RHO)

    2700 kg/m3

  • Thermal/Electrical

  • Specific Heat (CP)

    396 J/(kg·K)

  •  

  • Thermal Conductivity (K)

    0.009 W/(mm·°C)

  • Thermal Conductivity (K2)

    0.041 W/(mm·°C)

  • Thermal Conductivity (K3)

    0.00055 W/(mm·°C)

  • OK

     Orthotropic Material dialog box

  • Close

     Manage Materials dialog box

Note:

This is a simplified example. NX PCB Exchange and Simcenter 3D Electronic Systems Cooling let you create complex orthotropic board conductivities with information imported from ECAD files.

6: Create a 2D mesh for the PCB

Create a 2D mesh for the PCB with an element size of 7 mm, and apply the orthotropic material you just created. The thickness of the board is 1.8 mm.

2D Mesh (Mesh group)

  • Type

    QUAD4 Thin Shell

  •  

  • Element Size

    7 mm

  •  

  • New Collector

  • Type

    Thin Shell

  • Material

    PCB_ortho

  •  

  • Material Orientation Method

    Coordinate System

  • Material Orientation Type

    Absolute

    Note:

    The coordinate system you select defines the orientation of the orthotropic material properties. See the Pre/Post Help for more information.

  • Create Physical (Thin Shell Property)

  • Name

    PCB_thickness

  • Thickness

    1.8 mm

  • OK

     Thin Shell dialog box

  • Radiation

    None

  • Name

    PCB

  • OK

     both dialog boxes

Simulation Navigator

  • 2D Collectors→PCB

  • 2d_mesh(1)

  • Rename

  • Change the name to PCB_mesh.

  • Enter

     

  • Esc

     
7: Create a 3D mesh for the chip

To model the chip, use a 3D tetrahedral mesh and apply an isotropic material with the following values: density 2700 kg/m3, thermal conductivity 383 W/m·K, and specific heat 380 J/kg·K.

3D Tetrahedral Mesh (Mesh group)

  • Type

    TET4

  • Element Size

    5 mm

  •  

  • New Collector

  • Choose Material

  •  

  • Type

    Isotropic

  • Create material

  • Name

    Chip_iso

  • Mass Density (RHO)

    2700 kg/m3

  • Thermal

  • Thermal Conductivity (K)

    383 W/(m·K)

  • Specific Heat (CP)

    380 J/(kg·K)

  • OK

     Isotropic Material dialog box

  • OK

     Material List dialog box

  •  

  • Radiation

  • Name

    Chip

  • OK

     both dialog boxes
Tip:

Change the name from 3d_mesh(1) to Chip_mesh.

8: Create a heat load

A heat load defines the power or energy per time applied into the selected geometry. Apply a 5 W heat load to the top surface of the chip.

Simulation Navigator

  • pcb-w-chip_fem1.fem

  • Display Simulation→pcb-w-chip_sim1.sim

  • pcb-w-chip_fem1.fem

  •   2D Collectors (hide)

  •   3D Collectors (hide)

    Only the polygon geometry is now visible.

Thermal Loads (Loads and Conditions group→ Load Type list)

  • Type

    Heat Load

  •  

    Name

    Heat_Load_5W

  •  

  • Heat Load

    5 W

  • OK

     

The thermal load is applied to the chip.

9: Create a temperature constraint

A temperature constraint defines known temperatures of a heat source or heat sink within the thermal model. Apply a temperature constraint of 25°C on the edge of the PCB opposite the edge closest to the chip.

Temperature (Loads and Conditions group→ Constraint Type list)

  • Type Filter

    Polygon Edge

  •  

  • Name

    Edge_at_25C

  • Temperature

    25 °C

  • OK

     

The temperature is applied to the edge.

10: Create contact thermal coupling

A thermal coupling models conduction between the entities of components that are physically or thermally in contact. Create a thermal coupling between the chip and the PCB.

Thermal Coupling (Loads and Conditions group→ Simulation Object Type list)

  •  

  • Name

    Chip_to_PCB

Simulation Navigator

  • Polygon Geometry

  •   Polygon Body (1) (hide)

  • Primary Region

  • Polygon Body (1) (show)

  •  

  • Secondary Region

  • Assume that the chip is connected to the PCB with a Ball Grid Array (BGA) solder connection with a total area that is half of that of the chip.

    Assume a Pb-Sn weld with a thermal conductivity of 80 W/m-°C and a gap of 1 mm. You can now calculate a heat transfer coefficient:
    Heat Transfer Coefficient =
    ksolder * Asolder / (lgap * Achip)
    Heat Transfer Coefficient =
    80·0.5/ 0.001·1 =40000 W/m2·°C

  •  

  • Type

    Heat Transfer Coefficient

  • Coefficient

    40000 W/(m2·°C)

  • OK

     
11: Create a convection constraint for the chip

A convection constraint simulates convection for one or more surfaces by implicitly modeling the movement of fluid at a specific temperature in contact with one or more surfaces. Assume that the convection coefficient on the top and side faces is averaged to a single value of 24 W/m2°C.

Convection to Environment (Loads and Conditions group→ Constraint Type list)

  •  

  • Name

    Chip_fan

  • (5 polygon faces)

  •  

  • Type

    Convection Coefficient

  • Convection Coefficient

    24 W/(m2·°C)

  •  

  • Temperature

    Specify

  • Temperature Value

    30 °C

  • Apply

     
Note:

Do not leave the Convection to Environment dialog box.

12: Create a convection constraint for the PCB

Now create a convection to environment constraint for the PCB with a lower heat transfer coefficient of 19 W/m2°C from the PCB surface to air at ambient temperature of 25 °C.

  •  

  • Name

    PCB_convection

  •  

  • Type

    Convection Coefficient

  • Convection Coefficient

    19 μW/(mm2·°C)

  •  

  • Temperature

    Specify

  • Temperature Value

    25 °C

  • OK

     
13: Set up a run directory

The run directory determines which directory is used to save the simulation files.

Simulation Navigator

  • PCB_chip_solution

  • Edit

  • Solution Details

  • Run Directory

    Solution Name

  • Observe the other options available for Run Directory.

  • OK

      Solution dialog box

14: Solve the model

Simulation Navigator

  • PCB_chip_solution

  • Solve

  • OK

     

  • Wait for Completed to display in the Analysis Job Monitor dialog box, before proceeding.

  • Yes

     Review Results dialog box

  • Review the messages in the Solution Monitor dialog box.

  • Solution Monitor dialog box

  • Information window

  • Cancel

      Analysis Job Monitor dialog box

15: Display the results

Post Processing Navigator

  • Thermal

  • Load

  • Thermal

  • New Post View→Contour

You can see the effects of the orthotropic material in the temperature distribution. Heat is better conducted in the Y direction. Temperatures vary from approximately 25 ºC to 60 ºC.

16: Display the temperature gradient on the chip

Observe the temperature variation on the chip and change the post processing display to banded. Each uniform band of color corresponds to the value limits shown on the color bar.

Post Processing Navigator

  •   Post View 1

  •   1D Elements (hide)

  •   2D Elements (hide)

  • Banded Display (Results tab→Display group)

  • Feature (Display group→Edge Style list)

The post processing display updates. The average temperature is 60 ºC.

When you finish looking at the results, return to the model.

17: Save and close

Save and close your files when you are finished.

File

  • Save→Save All

File

  • Close→All Parts