When I first started thinking about printed circuit board (PCB) testing, my immediate reaction was boredom. Let’s be honest, it doesn’t have the hands-on excitement of building the circuit board itself. You're not designing the PCB; you're just running it through a tester, sorting it into a pass or fail bin, and moving on. I’ve done the job myself, so I know how repetitive it can feel.
But the more I thought about it, the more I realized how critical this step really is. Testing isn't just a final checkbox before shipping; it plays a key role in ensuring product quality, minimizing defects, and maintaining consistency in production.
So, while it may not be the flashiest part of PCB manufacturing, it’s definitely one of the most important. Let’s take a closer look.
What is Data Testing?
So, we’ve determined testing is more than just final PCB, so much more, in fact. It truly starts in CAM. That’s right (you know how I love data)! After processing thousands of sets of data over the past 40-something years (yes, I said 40), it still amazes me how little we pour into a great data set.
With all the capabilities of design software compared to the old photo resist processing, we still receive <10% of data that includes a netlist. It boggles the mind of CAM personnel. So why a netlist? Well, it proves out that the data set is electrically sound as provided. It allows for processing files through software for manufacturing and is the safety net to check against when an edit goes sideways. Without it, you could be left with a bad set to production and creating a bad product.
In most cases, this doesn’t happen, however, it is nice to have a netlist to compare data to as you work through edits. As a comparison, CAM software will let you pull a netlist from the data and store it for comparison; however, this is not 100% fail-proof. It is only as good as the data provided. This is test #1.
In Process, Visual, and AOI Testing
As layers are processed, there is additional “testing” or comparing that is performed to be sure everything is as it should be. For one example, visual inspection is performed throughout processing, where machine operators are looking for operational checks to be sure equipment is functioning properly. And, if you are running a top-notch facility, quality control is popping in to do an acceptable quality limit (AQL) as well. This visual is, in most cases, accurate, or as accurate as your people, but what comes next?
Automated optical inspection (AOI). Initially developed in the 1990s due to a technological push, this piece of equipment was quickly denounced as extremely valuable and necessary. After all, we were pushing technology, and everything was shrinking, making it impossible to visually inspect everything by humans.
AOI testing PCBs should always be 100% and certified.
With this new technology, we could inspect more processes on the floor to ensure that quality was spot on. Bare board inspection or “testing” for line width, spacing, excess copper, pin holes, missing features, shorts, breaks, hole breakout, missing holes, the list goes on and on. The only problem is the equipment time; it's just not possible to look at every single layer for every process. Although AOI is great, in-process inspection will always be critical for manufacturing success.
Shorts and Opens
Back in the day when I started working in PCB manufacturing, there was no data. We had a large photography department with film, cameras, micro-modifiers, a darkroom, developer, and all the best equipment at the time. Silver film, diazo, red tape, days of processing layers, and if you were lucky at the end, you’d get a great set of films, an accurate bombsighting program off an original film to create a drill program and a rout to supplied dimensions. After the initial run, you could retain a golden board to test future orders, too.
From all this pre-production work, you could create a testing fixture for electrical testing to be used at the final test. Electrical testing (ET) was one of the largest test areas. Storage for testing plastics alone was a large area. You had buckets of nails for the testers with tiny tubes to load the testers. The manpower alone to make a fixture was crazy. Bed of nails are still used for testing today. Shorts and opens (ICT test) are fast and accurate, but on average can only test 80-90% of the PCB. Testing one side of the board at a time, the tester cannot possibly test all points due to access, limitations or hidden test points.
Types of Testing Equipment
Clam shell testing is another form of ET equipment that is still used today. An improvement on bed of nails, this equipment (like its name) features a hinged shell to access both sides of the part at one time increasing the test area decreasing the time but still not 100%. Like the bed of nails, still the average access is 80-90%, and if the planets align on a great day, maybe 95% testing. We were still striving for 100% ET for PCB as technology raced to 2-side surface mount, blind and buried vias, fine lines and tight spacing, the flying probe hit the market (also in the 80s) but never really increased on sight until the mid-80s to early 90s. It was quickly accepted as the testing equipment needed the most in production, but still a slower method, and time was not always built into delivery.
Many production facilities use a combination of bed of nails and flying probe or have multiple probes on site as well as testing facilities that do nothing but test PCBs. Either way, a 100% test with today’s technology is what is needed the most. ET time is still lapsed in quote time largely due to it is an unknown amount of time needed. Test points end-to-mid, end-to-end, end-to-smd, end-to-test points, these can add up to thousands of testing points per board, causing several minutes to some of the time hours of testing.
Summary
There is no doubt we have come a long way in the electrical testing world of PCBs, and the improved process of 100% testing leaves us with comfort that the bare board is good going out the door. However, not to rain on the parade of success, it is difficult to celebrate from the land of engineering knowing that the data is key to all the success and without netlist provided from the start leaves us wondering: is it as good as supplied? Fortunately, designers are proactive and good at the art of layout and routing and know their stuff. I am most confident that the Gerbers not supported by netlist, that they are good as supplied. So, with all that being said, it is not a high alert worry but rather it would be a nice benefit to have it supplied.
Know your supplier and what they do for checks in-process and ET and receive certification for ET and stamps on parts when the room is available. As always, take advantage of Epec’s always free DFM process and talk to my engineering team about your data prior to production.
Key Takeaways
- Testing Begins with Data, Not the Final Board: Effective PCB reliability testing starts in the CAM department, where having a complete data set, including a netlist, helps verify electrical integrity before production even begins.
- AOI and In-Process Inspections Are Essential: While visual checks help catch obvious defects, automated optical inspection (AOI) is critical for detecting fine-pitch errors like shorts, breaks, and missing features that human eyes often miss.
- Electrical Testing Still Isn’t 100%, But It’s Close: Traditional methods like bed-of-nails and clamshell testers offer 80–95% coverage. Newer flying probe systems help close the gap, but a full 100% electrical test (ET) is still time-consuming and not always guaranteed.
- Test Equipment Must Evolve with PCB Complexity: As designs push toward finer lines, blind/buried vias, and double-sided SMT, test point access becomes limited, requiring a mix of ET systems and sometimes dedicated external test houses to ensure reliability.
- Partnering with the Right Manufacturer Makes the Difference: Knowing your supplier’s testing protocols, including their DFM review and netlist validation process, can help catch potential issues early and ensure long-term board reliability before your product hits the market.
