TL;DR

  • Bare-board testing (continuity, isolation) happens before assembly; assembled-board testing (ICT, FCT) happens after.
  • Method choice depends on volume and design: flying probe for prototypes, in-circuit for high-volume runs, X-ray and boundary scan for hidden or dense connections.
  • Designing for testability (adding test points early) makes every method faster and cheaper down the line.

Printed circuit board testing is crucial, both for new PCBs and the ones already in use. It enables you to troubleshoot your PCBs, locate issues, and fix them, without needing to replace the whole board or build it again. Nevertheless, it is a complex process. How do you do it? Here's our guide!

The Most Common Issues

Before we delve into fixing PCBs, it is essential to understand what to look for. Certain issues occur more frequently. Knowing them will help you solve problems more easily and allocate your time and energy more effectively. So, what issues are most likely to haunt your printed circuit board?

  • Component issues - specific, individual components malfunctioning and impacting the overall performance of your PCB.
  • Short circuits - usually caused by solder bridging, copper traces touching due to poor insulation, or plating voids.
  • Broken connections - caused by physical damage or poor soldering.
  • Design issues - problems caused by a faulty design.

Each of these problems requires a different type of inspection. Hence, we recommend starting your printed circuit board testing process by looking for the most common issues. If neither of these occurs, you can inspect more thoroughly to find the culprit.

Bare-Board vs. Assembled-Board Testing

A bare board is an unpopulated PCB, tested for continuity and isolation before any components are soldered on. An assembled board, also called a PCBA, carries its components and gets tested for functional and component-level correctness instead.

Bare-board testing catches manufacturing defects in the board itself: broken traces, shorts, and bad vias, before expensive components ever get placed on a faulty board. Assembled-board testing, covered in the methods below, catches issues introduced during component placement and soldering, or in the components themselves.

How to Test a Printed Circuit Board?

Whether it's during the PCB design process or for troubleshooting, you can apply different methods to test your printed circuit board. What are they? How to test your PCB effectively? Take a look below.

Visual Inspection

You should always start with a visual inspection—it will allow you to detect component issues and some circuit-related problems. What should a visual inspection look like?

  • Component damage indicators - cracked, discolored, or swollen components. Also, look for any signs of heat damage.
  • Circuit issue indicators - inspect the pins to ensure that they aren't bent, broken, corroded, or contaminated.
  • Connectors and socket issues - check whether connectors and sockets are soldered steadily and not loose.

You can also expand your visual inspection with technology. This is called automated optical inspection (AOI)—a practice in which cameras and microscopes are used to scan the PCB, followed by specialized software that looks for any, even subtle, anomalies that may indicate damage or issues. However, both traditional visual inspection and AOI do not test for all potential defects—they're a good starting point, which you need to follow up.

In-Circuit Test

In-circuit testing is perhaps the most common method to troubleshoot printed circuit boards. It utilizes an in-circuit tester, a fixture that connects it to the PCB, and specialized software. This method enables you to test out components separately, without them being impacted by other components = you can locate faulty components if that's the issue.
On the other hand, in-circuit testing can get costly, especially if we factor in the cost of equipment. Therefore, it is best to perform it on stable PCBs, rather than prototypes.

Flying Probe Testing

Flying probe testing uses moving probes controlled by software to contact test points on the board, rather than a fixed bed-of-nails fixture. This makes it well suited to prototypes and low-volume runs, since it skips the cost and lead time of building a custom fixture.

The trade-off is speed: flying probe testing runs slower per board than in-circuit testing, which makes it less practical once volume climbs high enough to justify a fixture.

Functional Testing (FCT)

Functional testing powers up the assembled board and runs it under conditions that mimic real operation, checking that it performs its intended function rather than just checking individual components or connections.

This catches problems that in-circuit or continuity testing can miss, such as issues that only appear when components interact under load. Functional testing typically runs later in the process, after a board has passed in-circuit or flying probe testing, since it validates the finished product rather than isolating individual faults.

X-Ray Inspection (AXI) and Boundary Scan

X-ray inspection, or AXI, uses X-rays to see inside the board and check solder joints that are physically hidden from view. This makes it the method of choice for components like BGAs, where the solder connections sit underneath the package and can't be inspected optically or with flying probes.

Boundary scan tests the digital connections between chips using built-in test circuitry on the components themselves, rather than external probes. It works well on densely packed boards where physical test-point access is limited, since it tests through the chip's own boundary-scan cells instead of needing a probe to touch every pin.

Continuity Testing

You can also test the signal paths and traces in your printed circuit board. For that, use a multimeter or a dedicated continuity tester. This will help you locate any signal continuity issues - then, all you have to do is determine what exactly causes them and whether the problem can be fixed easily.

Which Testing Method Should You Use?

The right method depends mainly on volume. For prototypes and low-volume runs, flying probe testing avoids fixture costs, while visual inspection and continuity testing catch obvious defects cheaply at any volume.

For high-volume production, in-circuit testing pays off despite the fixture cost, since it tests every board quickly and consistently once that cost is spread across many units. Functional testing typically runs alongside in-circuit testing at any volume, since it validates real-world performance that in-circuit testing alone can't confirm.

X-ray inspection and boundary scan get added when the board's design demands them, specifically for hidden solder joints (BGAs) or limited physical test-point access, regardless of volume.

Design for Testability (DFT)

Design for testability means planning for testing during the PCB design stage, not after the board is built. Adding dedicated test points, keeping components accessible for probes, and following standard test-point spacing all make later testing faster and cheaper.

A board designed without testability in mind often needs a custom, more expensive fixture, or can't be tested by flying probe or in-circuit methods at all. Considering DFT early, during PCB design, avoids that cost later.

The Takeaway

The above are just a few of the ways you can test your printed circuit board. In practice, different types of tests are applied when you design and manufacture a PCB, and different ones when you troubleshoot an existing one. Some additional testing methods worth mentioning include:

  • Solderability testing checks that a board's surface accepts solder cleanly, so joints form reliable connections rather than weak or cold ones.
  • Peel testing measures how much force it takes to peel copper traces away from the board, confirming the copper is properly bonded to the substrate.
  • Burn-in testing runs a board at elevated temperature and voltage for an extended period, catching early-life component failures before the board ships.
  • Stress testing exposes a board to extreme conditions, such as thermal cycling, to confirm it holds up under real-world wear rather than just initial use.

Need help testing your PCB design? Contact us at Conclusive Engineering - we'll be happy to help!

FAQ

What is the best way to test a PCB?

The right method depends mainly on volume and board design. Flying probe testing suits prototypes, in-circuit testing suits high-volume production, and X-ray or boundary scan get added when the design requires them.

What's the difference between bare-board and assembled-board testing?

Bare-board testing checks an unpopulated PCB for continuity and isolation before components get soldered on. Assembled-board testing checks a fully populated board for functional and component-level correctness.

Do I need a fixture to test a PCB?

Not always. Flying probe testing uses moving probes and needs no fixture, while in-circuit testing requires a custom bed-of-nails fixture built for that specific board.

What does X-ray inspection catch that other methods miss?

X-ray inspection, or AXI, sees solder joints that are physically hidden, such as those under BGA packages, which visual inspection and flying probes can't reach.

How can I make my PCB easier to test?

Plan for testability during the design stage by adding dedicated test points and keeping components accessible for probes, rather than addressing it after the board is built.