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Simple Tips for Quickly Reworking a PCB Layout

Published Date
Author Cadence PCB Solutions

PCB layout rework

PCB layouts and prototypes may need some kind of rework at multiple stages in the design and development process. Rework normally refers to on-the-line changes or corrections being applied to a PCB as it is moving through assembly. But in addition to this, prototypes may need to be reworked on your lab bench during testing.

Even before your board comes out of assembly and is shipped to you for testing, you just might need to rework your PCB layout. Quick changes to design files before production can cut down the development time and get your project past any production stalls. While quick layout rework does not always follow the traditional workflow, it can get you to production sooner than if you were to restart in the schematics.

PCB Layout Rework

Design changes may be identified in the engineering review phase, such as footprint changes, drill changes, or net connection changes. This will almost always cause a delay during manufacturing, and it will require changes to the PCB layout in order to move the project forward. To get the changes done as fast as possible, some changes can be applied directly to the PCB layout data without going back to the schematic.

Quick Layout Updates

Layout updates most commonly arise before the PCB has to go into fabrication, and sometimes right before assembly. If the board has been fabricated then it may need to be scrapped or you will need to plan for board updates during bench testing. However, if required layout changes are identified prior to fabrication, there is still a chance to update the board before it enters production.

Some of the most common instances of layout updates that may need to be applied before fabrication are found below. Some of these occur by simply updating the layout, while others require changes beginning in the schematic.

Problem

Solution

Incorrect mask expansions
  • Change footprint rule and export updated layout

Incorrect pad size/location
  • Fast way: change pads directly in the layout, export the footprint for later update
  • Slow way: update the original footprint (if it exists) and follow the workflow from the schematic

BOM mismatch: component swap for existing package
  • Copy the package to your mismatched component
  • If package is not in your database, apply update to the schematics and update BRD file

Duplicate drill symbols
  • Update drill table and symbol definitions, export new drill data and drill layer Gerber

Vias too small to meet fabrication requirements
  • Use selection filter to resize small vias
  • Resize via land pads directly in layout
  • Export new Gerbers and drill data

Board cutout or slot routing changes
  • Editable directly in the PCB layout, requires no schematic update
  • Update in the fabrication drawing

Pin naming mismatch leading to wrong net connections
  • Update net assignments on pads directly
  • Apply routing updates as needed

Among these updates, those that can be the most irritating are simple changes to footprints. This could include pad locations, pad sizes, silkscreen, or assembly data. Updating the layout directly is one way to quickly resolve problems and provide new fabrication data, but make sure to keep a changelog so the database can be finalized once the board has been tested and validated.

Design For Rework?

Once you get a prototype onto your bench and it’s ready to test, what’s the best way to ensure your device is easy to rework? If you start with an eye towards potential rework during the design phase, you’ll have a much easier time testing circuitry, identifying layout errors, making quick fixes on the bench, and taking measurements. Some simple strategies to make a board easier to rework include:

  • Include scope test points on any nets that have dynamic signal behavior
  • Include exposed pads or vias on important signals so wires can be attached
  • If nets require configuration resistors, leave open pads around discretes
  • Pull solder mask from vias on important signals when there is no room for test points
  • Use solderable pads where a net can be bridged with a wire to quickly change a connection
  • Consider using headers or board-to-board connectors with a module if a peripheral has not been tested

Finalize Your Design Database

Quick updates that get you past a delay are just fine, but eventually you will eventually need to totally update the design database to match your rework. This means going back to the schematics, pointing to your updated footprints, exporting a new netlist, and applying the updates in the PCB layout. As long as you keep a thorough changelog of your layout updates and test results, your final database update can be quick and error-free.

No matter how you need to edit or build your PCB layout and libraries, you can take full control over your design data package using the best set of PCB design features in OrCAD from Cadence. If you’re ready to take even more control over net logic and board layout, you can graduate to Allegro PCB Designer for a more advanced toolset. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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