Friday, December 09th
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Designing a printed circuit board poses many challenges. It can be exhilarating to complete a design and send it off to manufacturing but later deflating to receive boards and identify a failure that could have been prevented. Was a mistake made in the design or did the manufacturer make an error?

Providing the manufacturer with a ‘netlist’ in addition to your design files and fabrication notes is one way to prevent unexpected errors from occurring, as it serves as an extra verification for the design.

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Examples of Netlist Errors

What is a netlist?

Wikipedia defines a netlist as “…a description of the connectivity of an electronic circuit.”  The netlist identifies the actual connections in a circuit and allows the data from the final output to be compared all the way back to the schematic.  When all of the data files have been exported, the final output should match the netlist, which will match the schematic.

How is a netlist generated?

One of the available export options, the netlist can be generated from the PCB design software.  It may be referred to as IPC-D-356, as IPC (www.ipc.org) has defined the netlist in a specific way in order to standardize the netlist format, especially in reference to the final electrical test of the boards. But before a netlist is generated, it is recommended to run all final DRCs (design rule checks) on the design in order to prevent missing any connections.

Why should a netlist be generated?

A netlist should be generated because:

1. It provides an additional level of verification of the design for the manufacturer during the CAM (computer-aided manufacturing) review process.
2. It will ensure that when the manufacturer is completing the electrical test of the circuit board, the process will be accurately testing against the intended design.

What happens if a netlist is not generated?

If a manufacturer is not provided with a netlist, key design elements may be missed in the circuit board. To illustrate, we’ll provide some examples from recent projects.

In this first example, several of the copper pours did not import correctly.  This can happen in some cases when importing Gerber data into the manufacturing CAM software.  In this case, if we had a netlist to compare to the missing copper pours, it would have been flagged when running a ‘netlist compare check’ and any discrepancies resolved prior to manufacturing.

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Click to Enlarge – In this image the areas that areas outlined with white lines show sections of the board that should have copper pours.

 

 

 

 

 

 

 

 

In the next example, the highlighted pins were not connected to the inner plane in the data.

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Click to Enlarge – In the right side of the component you will see a white outlined area. These pins where not connected to the inner planes in the data. A netlist comparison would have found that.

 

 

 

 

 

 

 

 

 

 

 

 

Here, the CAM operator noticed overlapping holes and requested a customer netlist.  When the netlist compare check was completed, it was discovered that these would have been shorts and the boards would have been defective.

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Click to Enlarge – The overlapping holes are circled in yellow. These would have caused shorts.

 

 

 

 

 

 

 

 

 

 

In a similar situation the CAM operator noticed overlapping traces.  With a netlist, this could have been immediately verified without placing the job on hold to contact the customer.

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Click to Enlarge – See the traces inside the white circle. This error would have been caught with a netlist comparison.

 

 

 

 

 

 

 

 

When a netlist is not provided, one is created from the design (Gerber) data and used to create the electrical test program to test the boards.  If there are no problems with the Gerber data, then this will work fine.  However, if data is missing, as in the case of our examples, the boards may actually pass electrical test but then fail to work correctly based on the original design.

Providing a manufacturer with a netlist is an additional, yet simple, step to ensure the manufacturing process goes smoothly. Its seconds of additional work to help prevent manufacturing delays and ensure your next printed circuit board project will be manufactured to your exact specifications.

If you have questions about the netlist and how we use it, please contact us.  If you have examples of how you use a netlist and how it has helped you prevent design and manufacturing problems we would love to hear your comments.

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