PCB Engineers Design Tips for Layout

PCB Best Practices: Six Things to Consider When Transferring PCB Schematics to Layout Design. All examples mentioned in this article were developed using the Multisim design environment, but the same concepts apply when using different EDA tools.

Initial Schematic Transfer

The process of transferring the schematic to the layout environment through the netlist file also transfers device information, netlist, layout information, and initial trace width settings.

Here are some recommended steps to prepare for the layout design stage:

1. Set the grid and unit to appropriate values. In order to achieve more precise layout control of components and traces, the device grid, copper grid, via grid, and SMD grid can be designed to 1mil.
2. Set the board frame blank area and vias to the required values. PCB manufacturers may have specific minimum or nominal recommended values ​​for blind and buried via settings.
3. Set the corresponding pad/via parameters according to the capabilities of the PCB manufacturer. Most PCB manufacturers can support smaller vias with a drill diameter of 10mil and a pad diameter of 20mil.
4. Set up design rules as required.
5. Set up custom shortcuts for commonly used layers so that you can quickly switch layers (and create vias) when routing.

Dealing with errors during schematic transfer

A common error during schematic transfer is non-existent or incorrect footprint assignments. It is important to note that:

If there is a device in the schematic without a footprint, a warning message will pop up indicating that the virtual component cannot be exported. In this case, no default footprint information will be transferred to the layout and the component will simply be deleted from the layout.

If the footprint is transferred but does not correctly match a valid footprint shape, a warning message indicating a mismatch will also be generated during the transfer process.

Correct the footprint assignment in the schematic, or create a valid footprint for any device. After correction, perform the forward annotation step to update and synchronize the design information.

Update the design through annotation

Annotation is the process of transferring design changes from schematic to layout or from layout to schematic. Backward annotation (layout to schematic) and forward annotation (schematic to layout) are key to keeping designs accurate.

To protect the work that has already been done, back up and archive the current versions of the schematic and layout files before any major forward or back annotation steps.

Do not attempt to make changes in both the schematic and layout at the same time. Make changes to only one part of the design (either the schematic or the layout), then perform the correct annotation steps to synchronize the design data.

Renumbering Components

Component renumbering is a function that renumbers the components on the PCB into a specific sequence. Reference designators should be ordered from top to bottom and from left to right on the PCB. This makes it easier to locate the component on the board during assembly, test, and troubleshooting.

Dealing with Last-Minute Component or Netlist Changes

Last-minute PCB component or netlist changes are undesirable, but sometimes they have to be made because of component availability issues or the detection of a last-minute design error. If the component or netlist change needs to be made, it should be done in the schematic and then forward-annotated to the layout tool. Here are some tips:

1. If you add a new device after layout design has begun (such as adding a pull-up resistor to an open-drain output), add the resistor and network to the design from the schematic. After forward annotation, the resistor will be displayed outside the board outline as an unplaced component, and the flying wires will be displayed to indicate the connecting network. Next, move the component inside the board outline and perform normal routing.
2. Backward annotation works well with reference number changes, such as post-layout renumbering.

Locate the device through highlight selection

During the PCB layout process, one way to browse specific components or traces in the schematic is to use the ‘highlight selection’ feature. This feature allows you to select a component or a trace (or multiple objects) and then see where they are located in the schematic.

This feature is particularly useful when matching bypass capacitors and their corresponding IC connections. Conversely, it is also possible to locate specific components or traces in the layout while browsing the schematic.