When we look at where we are today vs. 1980 or earlier with technology, it is truly amazing. Just think about an appliance, for example, the household refrigerator/freezer. It used to be that you opened the door, looked inside, found what was needed, and closed it. I can hear my parents yelling, “shut the door,” like all the cold is coming out and everything will spoil. Settings were cold/coldest on a dial you turned.
Now, our appliances talk to us or beeps at us if we leave the door ajar. The temperature is digitally displayed on the front, along with a host of other applications. We dispense water, ice, crushed ice without opening said door. Amazing! From 2 doors to 4 doors, freezer on the bottom, quick freeze, cooling drawers, what is next?
With smart appliances, obviously, it all starts with the printed circuit board (PCB), and some of these circuit boards are complex, costly, and have expensive components applied. So, what if the circuit board designed is no good? What if all the items are applied to the PCB and it doesn’t work? How will a netlist help?
What is a Netlist?
A netlist is an output format from the design data set comprised of a top and bottom side that will confirm or deny if the PCB is electrically laid out correctly. Each connection, end point, through point, and midpoint is mapped to the netlist. When a net is broken or shorted to another net, the CAM software will report the Gerber files do not match the customer-supplied netlist. The program, when used correctly, will record and report the exact locations of the netlist discrepancies. The shorting or opens will be displayed using different colors for ease of viewing. The number of issues will be recorded for both sides of the PCB, making the netlist an invaluable tool to have. If your PCB supplier finds the netlist invaluable, then why don’t more customers supply the netlist with their data sets?
I would estimate still in today’s world that 90% or greater of PCBs come to Epec without a netlist, and a decent percentage of netlists supplied are proved to have errors within the Gerber files. In some cases, the netlist will prove to show the part has shorts or opens that when reported to the customer, that they are intentionally placed. I would also speculate that although it is a great tool many either do not know how to use it or do not have the software to view it.
Why Is A Netlist Important?
So, what if we do not receive a netlist? Well, it’s a great question, which leaves us to assume that what we received is accurate and we do the next best thing. After import into our CAM software, the files are set according to layer order, copper signal, power/ground planes, mixed, and by polarity. The drill is also labeled as such.
Example of a PCB Netlist showing open and missing drills.
Once the layers are mapped and saved as original format, raw, unmodified, as received the software will extract a golden netlist. This netlist allows engineering to manipulate the files and ready them for production. Once cleanup and initial prep are completed, a new netlist is extracted from the database and compared to the original golden netlist. At comparison, the data should show no electrical changes have been made. The engineer can repeat this process as many times as needed. Editing of data is often a manual process leaving the chance of human error to occur. As long as the steps are completed properly the output files for production will function exactly as designed and laid out.
Can I Build A PCB Without A Netlist?
The impact of a bad file set supplied without a netlist is costly. The PCB supplier can only do so much without the netlist and what we call “looking for issues”. A well-trained CAM operator can sometimes pick out mistakes such as thermal relief isolation or spoke reduction, ground shorts, or broken traces.
Example of a PCB Netlist showing open traces.
Unfortunately, when the errors are found, often the part is fully populated or many parts are populated, prior to discovering the issue. A fully assembled circuit board with shorts or opens even worse could be scrap. This could be prevented at the raw PCB level by supplying a netlist for verification. It has happened that shorts can be cut or opened can be jumped but this is also unsightly and costly for manual labor.
What Is the Impact of No Netlist?
The impact of no supplied netlist opens the door for disaster. The netlist is the roadmap of accuracy for electrical connection within the PCB of all types. When one of the roads is blocked or broken, the netlist guides your supplier to address back to you there is a problem. There are rare occasions when engineering will pick up on an open circuit or missing clearance pads but there is no guarantee that will happen.
Summary
The netlist is your safety net, preventing costly errors from happening to your PCB project. Always supplying a netlist with your data set is the only way to be certain the file set has been created exactly as the schematic requires. Communication with your design team to generate a netlist to be supplied with the data should be part of your standard operation. With a netlist, your PCB supplier will succeed with a 100% review for accuracy every time.
