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 phase:
1. Set Grid and Units to appropriate values. In order to achieve finer layout control of components and traces, the device grid, copper grid, via grid and SMD grid can be designed to 1mil.
2. Set the blank area and vias of the board frame to the required values . PCB manufacturers may have specific minimum or nominal recommendations for blind and buried via settings.
3. Set the corresponding pad / via parameters according to the PCB manufacturer's capabilities. Most PCB manufacturers can support smaller vias with a drill diameter of 10mil and a pad diameter of 20mil.
4. Set design rules as required.
5. Set customized shortcut keys for commonly used layers, so that you can quickly switch layers (and create vias) when routing.
Handling errors during schematic transfer
A common mistake during schematic transfers is non-existent or incorrect package assignments. have to be aware of is:
● If there is a device in the schematic without a package, a warning message will pop up, indicating that the virtual component cannot be exported. In this case, no default package information is passed to the layout and the component is simply removed from the layout.
● If a footprint passes through but does not correctly match a valid footprint shape, a warning message indicating a mismatch will also be generated during the pass.
● Correct footprint assignments in the schematic, or create a valid footprint for any device. Correction is followed by forward dimensioning steps to update and synchronize design information.
Updating Designs with Callouts
Annotation is the process of communicating design changes from schematic to layout or vice versa. Backward annotation (layout to schematic) and forward annotation (schematic to layout) are key to maintaining design accuracy.
To protect the work already done, backup and archiving of the current version of the schematic and layout files is required prior to any significant forward or back annotation steps.
Don't try to make changes in schematic and layout at the same time. Make changes to only one part of the design (either the schematic or the layout) and perform the correct annotation steps to synchronize the design data.
Renumber the device
Device renumbering refers to the function of renumbering the components on the PCB in a specific order. The reference designators should be sorted from top to bottom and left to right on the PCB. This makes it easier to locate components on the board during assembly, test, and troubleshooting.
Handles last-minute device or netlist changes
Last-minute PCB component or netlist changes are not desirable, but are sometimes necessary due to component availability issues or detection of last-minute design errors. If it is a component or netlist that needs to be changed, it should be done in the schematic, and then marked to the layout tool through the forward direction. Here are some tips:
1. If adding a new device (such as adding a pull-up resistor to an open-drain output) after the layout has started, add resistors and networks to the design from the schematic. When positively annotated, the resistor will appear outside the board outline as an unplaced component, with flying leads indicating the connected net. The components are then moved inside the board outline and routed as normal.
2. Backward annotation works well with reference designation changes, such as post-layout renumbering.
Locate the device by highlighting it
During PCB layout, one way you can browse for a specific component or trace in the schematic is by using the 'highlight selection' feature. This feature allows you to select a component or a trace (or multiple objects) and see where they are on the schematic.
This feature is especially useful when matching bypass capacitors to their corresponding IC connections. Conversely, it is also possible to locate specific components or traces in the layout while browsing the schematic.