Feb 2, 2021|Product Innovations and Design

What is a Netlist in PCB Design?

It is imperative to ensure any code’s accuracy to create printed circuit boards (PCB) in electronic design. One way to manage this code is by using a Netlist. What is a netlist? It lays out each component and its connections on the PCB. 

What is a Netlist File?

To answer the question of what is a Netlist file, there are a few basics to cover. For starters, ‘net’ is a term that describes a connection between two or more components. 

These components constitute the items on the circuit board. It can also refer to the association of capacitors, resistors, and transistors concerning analog simulation tools.

Therefore, a Netlist is simply a textual catalog of components and the connections that go with them. Each Netlist has a preceding list of parts and their designations, PINs, and signal keywords for context and reference. 

Netlists are incredibly helpful during the testing process, even before starting any physical work on the PCB. They allow engineers to carry out these tests to locate any incorrect or missing connections. 

Netlists also provide nodes, instances, and might even show specific attributes of the components involved. With various structures, complexity, and multiple Netlist formats, no Netlist is the same, but they all play the same integral role in electronic design.

Understanding how and why Netlists are beneficial can be advantageous when creating a PCB. Not only will it help identify errors, but making adjustments is a straightforward process.

How the Netlist is Used Before Editing

What is Netlist in PCB design? Netlists have multiple instances where they come in handy during PCB building. Before any editing and during the standard Gerber data transfer file loading, the program also reads the Netlist. 

After this initial load, the program organizes the components logically from top to bottom and gives each type a layer. With these defined layers, Gerber, along with the drill data, creates another Netlist to compare to the one provided. 

What is a gate level Netlist? This generated Netlist then allows designers to find and fix any discrepancies in the file. Locating a divergence from how things should be is essential for any product’s function, especially in such an early stage.

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Possible Causes of Discrepancies

lvs design results dont match

Source: University of Virginia 

While most modern PCB manufacturing formats are, for the most part, infallible, things can still happen. Most inconsistencies occur due to one of two reasons.

An incorrect description of a feature imported into Gerber can cause the system to misinterpret the data. This discrepancy includes the faulty resolution of self-intersecting polygons. 

It is best to export Gerber files with the maximum resolution possible to solve or prevent this issue. Also, using line fill instead of polygons can minimize errors.

Operator errors during setup are the other potential cause of a discrepancy. Something as simple as inaccurate layer assignment can cause raw files to import erroneously.

To prevent CAM (Computer-Aided Manufacturing) from misunderstanding these errors, use positive polarity whenever possible and provide a Netlist with the PCB Gerber files. Because of how useful and practical Netlists are, it is always best to provide one.

During CAM Editing

Netlist files remain active during CAM testing. The computer performs another Netlist check for any possible openings or electrical shorts created during the other editing steps.

After this testing and performing any reconciliations, CAM creates a file for one of the next phase’s electrical test fixturing methods.

Electrical Test Fixturing

When all of the obvious technical details are in agreement, the next step is to test the PCB implementation. There are a few ways to do this, and it depends on the individual construction elements.

Hard test fixture plates made from a polycarbonate or similar material allow for physical testing of the PCB. These plates have holes drilled to match the locations of testing circuit endpoints. These holes must accept conductive probes to allow for the testing of the necessary connections.

Non-components and midpoints usually show as false opens. Because of this and the limited number of grid positions available, testing skips over them to avoid misinterpretation. 

Endpoint probes will always be more important than testing every single node. Even without the omitted elements, tests are 100 percent valid. 

What is  Netlist in VLSI? When one plate doesn’t make contact with every net, another panel comes into play. This ‘clamshell’ method involves parallel plates with one facing up and the other facing down. Very large-scale integration processes require extra materials, but the Netlist concept is the same.

After loading with matching stationary probes, the plates compress together, providing signals that might or might not match the Netlist. The PCB receives passing marks if they do turn up to be equivalent.

Another method, the flying probe method, does not use such fixtures. Instead, it uses a mounted board between two fast-moving probes to perform the test. 

The tester has a wired controller with the Netlist file loaded into it, which it uses to compare the results. 

During the actual test, one of the probes makes contact with an endpoint pad on the top layer and the other to the bottom layer endpoint pad. If the results match the provided Netlist, it passes the test, and the probes move onto the next set.

Sometimes, there is no provided IPC Netlist. It’s still possible to do testing, but testers must make some assumptions from what they do have. While it isn’t difficult to generate a Netlist, making assumptions about electrical components may not always be the best approach.

CAM software can extract a reference Netlist from the drill data and Gerber information. The Gerber files’ data are under the assumption that they are correct and accurate for any electrical requirements.

This curated Netlist is usable for electrical test fixturing and CAM editing checks, though it isn’t as reliable as an official one. 

Conclusion

Netlists are valuable information sources for the entire electronic design process. They allow designers and manufacturers to determine a printed circuit board’s viability. 

With the data included in a Netlist, those testing the code have an easy reference to everything going on. When something doesn’t add up, it should show in the Netlist and is fixable from there.

Netlists come in handy regardless of the design or testing phase. Without them or a way to generate them, it would be challenging to determine where errors are and how to address them.

Author Profile

Sunny Patel
Sunny Patel is the Engineering and Sales Manager at Candor Industries. Sunny is trained as a IPC-A-600 trainer, AS9100 Lead auditor, IPC CID and got his Engineering degree at the University of Toronto.

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