Using Printed Circuit Board Materials

To Assign a Printed Circuit Board Material

1. Open the Material quick edit dialog. There are several methods:
   • Left click on the part, and click the Edit icon on the context toolbar.
   • Right click on the part, and click Edit...
   • Right click on the part name under the Materials branch of the Design Study bar, and click Edit....
   • Click Edit in the Materials context panel.
2. Select one or more parts. PCBs should be modeled as three dimensional volumes having the same physical size and shape as the actual PCB. No internal layers should be modeled within the PCB.
3. Select the database from the Material DB Name menu.
4. Select Printed Circuit Boards from the Type menu.
5. Select the material from the Name menu.
6. Click Apply.

No direction data is required when assigning a PCB material. Autodesk Simulation CFDautomatically determines the through and planar directions based on the relative dimensions of the part.

To Create a Printed Circuit Board Material

1. To open the Material Editor, click Material Editor on the Materials context panel.
2. Click the List button.
3. Right click on a custom database, and select New material. Select Printed Circuit Boards. Specify a Name.
4. Click the property button that is to be defined.
5. For each property, enter the appropriate value and units, and click Apply:

   Step 5-a: Total PCB Thickness

   Specify the Total PCB Thickness. In this step, the total physical thickness of the circuit board is specified.

   1. Click the Total PCB Thickness button.
   2. Enter the thickness of the printed circuit board in the Thickness field as well as the units. Constant is the only variation method for PCB thickness.
   3. Click Apply to save the value.

   The thickness can easily be obtained from the CAD model or from the actual device. Using this value and the sum of the trace layer thicknesses, the thickness of the dielectric layer is automatically computed.

   Step 5-b: Traces and Planes

   Define the trace layers. In this step, the solid material that makes up the traces is selected from the Solid Material library. This material is typically copper, and is available by default in the Solid Material library. Additionally, the thickness and percentage of metal of each layer are specified.

   1. Click the Traces and Planes button.
   2. Select the trace material from the Material drop-down menu. This menu lists all of the solid materials stored in the material database. Copper is the most commonly used material for PCB trace layers. If a material that has variable properties is selected, a median value will be used for the PCB material. This property value will be constant throughout the analysis. (See note below.)
   3. Enter a line for each layer, and specify the Thickness and the Percent Metal Content. For example, if the 35% of the layer is copper, enter 35 in the % Metal column.
   4. Add additional rows by clicking the Insert button; remove rows with the Delete button.
   5. A two-column table of data in “.csv” format can be imported by clicking the Import button. Likewise, input data can be saved to a “.csv” file by clicking the Save button.
   6. The Coverage Exponent is a weighting function used to account for the effect of the configuration and concentration of copper within the board on the in-plane conductance. The default value is 2. A value of 1 is most applicable for strips or grids; a value of 2 is applicable for spots or islands.

      An example describing the meaning of the Coverage Exponent:

      Consider a PCB in the XY plane.

      It has one layer of parallel copper traces running in the X direction. The traces all have the same width and are evenly spaced with the spacing equal to the trace width. The coverage ratio is thus 50%.

      In the X direction, the conductivity of the trace layer is half of the value it would be if copper covered the entire board. The effective coverage exponent in the X direction is equal to 1.

      In contrast, in the Y direction, the conductivity is approximately equal to twice the in-plane value of the FR4 laminate since resistances in series are always dominated by the higher values (and there is a 3 order of magnitude difference between the copper and FR4 conductivities). The effective coverage exponent in this case would actually be equivalent to about 4.5.

      In real PCBs the situation is never as bad as in the Y direction. Because there are usually cross traces, ground planes, vias, etc., the conduction paths are typically better. Consequently, several authors have used an empirical formulation with a coverage exponent of 2 which has been found to work quite well on a range of typical multi-layer PCBs with random trace length and orientation.

      Therefore, the value of 2 should be used for typical boards with multiple layers and random traces.

      A value of 1 should be used for a board with a regular grid/array of traces (common for memory cards etc).

   7. Click Apply to save the values and to activate them with the equivalent properties calculation.

   To use a material not in the list, close this dialog, and switch to Solid Materials on the Material task dialog. Create the desired solid material using the Solid Material Editor. This material will then be available on the PCB Material drop-down menu.

   Step 5-c: Dielectric

   Define the dielectric material. The solid material that makes up the dielectric is selected from the Solid Material database. This material is listed in the Solid Material database under the name: PCB Plastic for Laminate.

   The dielectric layer is typically a glass-reinforced polymer that gives the PCB its rigidity, and surrounds the copper layers. In this step, the solid material that makes up the dielectric layer is selected from the Solid Material database.

   1. Click the Dielectric button.
   2. Select the trace material from the Dielectric Material drop-down menu. This menu lists all of the solid materials stored in the material database. FR4 is the most commonly used material for PCB trace layers.
   3. Click the Apply button to activate the material.
   4. Click OK to finish.

   If a material that has variable properties is selected, only the value for the x-direction conductivity will be used for the PCB dielectric material. This property value will be constant throughout the analysis. Note: anisotropic dielectric conductivity is not supported.

   To use a material that is not in the list, close this dialog, and switch to Solid Materials on the Material task dialog. Create the desired solid material using the Solid Material Editor. This material will then be available on the PCB Material drop-down menu.

6. Optionally, click Save.
7. Click OK. The new material is available when the Materials quick edit dialog is opened.

The Default material database contains at least one instance of every material type. A convenient way to create a new material is to use a Default material as an example. Because these materials are read-only, use the Material Editor to copy the original into a custom database, and modify the copy. For more about creating a material from an existing material...

Related Topics

PCB Modeling Guidelines

Example showing creation of a PCB Material

Example showing assignment of a PCB Material