The Ultimate Guide to Flying Probe Test in PCBA

Bare printed circuit boards (PCB) and PCB assemblies (PCBA) undergo specific tests before leaving manufacturing houses. These tests detect circuitry and electrical issues such as opens, shorts, capacitance, and resistance. Passing the tests guarantee high reliability and performance of the end products. The two popular test methods for testing are In-Circuit Test and Flying Probe Test in PCBA, with the former being the first to hit the industry.

This article will highlight the advantages and disadvantages of both testing techniques. At the end of this post, you will fully understand how FPT works and decide which method best suits your PCBA.

What is Flying Probe Test/ FPT?

In simple terms, a flying test probe (FPT) is an automated system with a small number of probes that “fly” or maneuver around the top and bottom of a PCB simultaneously to contact test points. According to the instructions in a program for the specific board undergoing testing, a flying test probe moves from one test point to another according to its instructions.

FPT machines have high-precision needles that perform electrical tests to determine if a printed circuit board is in good order. An FPT test does not require custom tooling or manufacturing of a test fixture. That makes it highly cost-effective and the go-to choice for boards in the early development stage and low to mid-volume production.

What is In-Circuit Testing / ICT?

In-Circuit Test (ICT) is a system that comprises individual probes that make contact with test points on a PCB to check for functionality and assembly defects.

Every printed circuit board assembly requires a custom ICT test fixture consisting of pins that contact the PCBA at test points. Test fixtures can be quite expensive depending on the complexity of the PCBA, making it more suitable for large volume production.

In-Circuit Test vs. Flying Probe: Similarity and Differences

In-circuit test and flying probe test are both excellent systems for testing PCBA and perform their functions in different ways.  Here is a table that summarizes their differences and similarities.

In-Circuit Test vs. Flying Probe Test: Advantages and Disadvantages

How does Flying Probe Testing work?

Flying probe tests are increasingly becoming popular, especially with the current trend of miniaturizing electronics. Small-sized electronics have small circuit boards that are more cost-effective to test using FPT. Besides, programming complex and dense boards are easier with a flying probe tester.

A flying probe test works fairly straightforward. The steps include:

Step 1:

The first step is to create an FPT test program that will test the circuit board assembly. Developing the test program is generally on an offline PC using an FPT test program generating application. The application needs the PCB assembly’s Gerbers, BOM, and ECAD files. The BOM should be in an EXCEL format, and the ECAD file should be an intelligent CAD file.

Step 2:

After creating the test program, the next step is to load it on the FPT tester. The circuit board assembly that requires testing goes on a conveyor belt inside the tester. The circuit board can be a single board or several boards. The board will travel on the conveyor belt to the area where the probe takes place. The test program controls how the probes contact the pads and the exposed vias.

Step 3:

Next, the FPT applies power and electrical test signals to the probe points and then reads the measurements. Further processing of the measurement readings will determine if a specific circuit portion meets the expected results. Any deviation from set programs and expectations will signal a defect in the unit, resulting in a failed test.

Features of Flying Probe Testers

The flying probe tester comes with adequate AC and DC power supplies, digital multimeters, multiplexing systems, frequency counters, various sensors, and signal generators to animate or excite the PCB nodes and make precise measurements. All these elaborate and intricate connections and signals enable the FPT to isolate component segments currently under test from other interconnections on the board. In other words, the measurement for each component is highly accurate since there are no interferences from the rest of the interconnections on the board. By isolating each component segment, the flying probe tester can measure values accurately.

Perhaps, more interestingly, most FPTs currently available can measure pads of ICs through capacitive probing. Apart from testing for opens, shorts, and other component values, flying probe testers have cameras that check for missing components and inspect each component for polarity. Another test the FPT performs is a diode impendence test to check the inputs of integrated devices.

Emerging Advanced Features of FTP

While all these complex processes are great for ensuring accurate and high-quality tests, a few other emerging advanced features make flying probe tests desirable. These include:

1. Phase Difference Measurement Unit: The PDM unit sends signals to estimate the phase difference between different board sections. This feature reduces the need for isolation tests that are not necessary.
2. High Voltage Stress (HVS) Test: If the PDM misses any high resistance isolation defects, the HVS test will run and apply high voltage to the various test points. Doing so will detect the flaws. 250V is the maximum application voltage in keeping with typical measurements. However, HVS provides a lot more voltage than that. It offers options ranging from 500V to 1000V, and at the same time, it ensures the use of the right amount of power to prevent damages to the boards during high resistance tests.
3. Micro Shorts Detection: It is not uncommon for parts containing micro shorts to burn out due to the application of sudden high voltage. For this reason, micro shorts detection allows the application of low voltages and then increasing it gradually to prevent damages.

In a nutshell, a flying probe test carries out a comprehensive test on the board as much as it is feasible, depending on how easy it is to access a board’s design.

FPT Guidelines and Recommendations

The following guidelines and recommendations are important during a flying probe test to reduce the risk of board damage and increase test accuracy.

Cleaning:

Ensure that the probe areas and the assembly are clean before testing. You can do this by removing any flux or ensuring the use of pin-probeable flux during production. Cleaning the assembly will guarantee quick contact, shorter testing time, and reliable results. Not cleaning the assembly can lead to a false fail, and test time can increase if the tester has to move its position to ensure better contact repeatedly.

Board Rail:

The PCB should have at least a 3 mm wide border edge along the opposite sides free of any component. This empty margin will provide room for handling the PCB inside the machine. The border edge can be space in the board’s design or panel waste.

Fiducials:

Testing machines need fiducial markers or reference points to position the probes properly. These points are usually on the panel waste, but putting them on the printed circuit board can be helpful, especially if the panel waste is not present.

Component Legs:

There should be ample space on the “toe” to probe near the component legs. Doing so will allow for a good solder joint. However, it is best not to probe the component legs themselves during tests, as the probe pressure can push the component legs onto the pad and correct any potential open circuits.

Vias:

Probing on the edge of PCBvias is possible. However, exposing the vias rather than covering or tenting it is likely to give better results. The ideal thing is to add the “non-tented” specification in the PCB design.

Probe Points:

The recommendation is to have accessible probe points at the bottom side of the PCB for the ground and power rails. Easily accessible probe points can significantly speed up test time and reduce overall cost.

Component Height:

The recommendation is to keep the heights on both sides of the PCB to approximately 40 mm and 90 mm, respectively. Exceeding that height can cause a “No Fly” zone that makes it difficult to access the assembly.

Test Access:

It is best to assign at least one side of the assembly for testing on single-sided machines, as this will reduce the cost of turning the board over and testing from both sides.

Size:

Flying probe tests are relatively slow because it involves moving probes between measurement points, which takes a lot of time. You can cut back this time by keeping access points close together, especially on a large PCB.

Which Suits Your PCBA?

Deciding to choose one testing system over the other might not be the easiest thing to do. However, your decision will depend on some crucial factors, such as:

1. Product Design

Consider the complexity of your PCB, and how easy (or otherwise) it would be to access it. FPT machines are a great choice when it comes to probing the ends of component pads. They can also access electrical networks in exposed vias.

On the other hand, ICT needs a minimum of 50 thou wide test pad per net. The PCB has to include this requirement upfront to use it for fixed test probes.

2. Lead Development Time

FPT has a short lead development time since it does not require designing custom test fixtures. Typically, development lead time for FPT is less than a week. Conversely, ICT lead development time can take as much as six weeks. This period covers programming and fixture manufacturing.

3. Expected Volumes

ICT is ideal for assemblies in large volumes. Usually, machine test time takes a few seconds to complete, making it possible to go through high volumes as quickly as possible. That is the fastest way to recover the cost of test fixtures and programming.

On the flip side, if you are testing prototypes or low to mid-volume production, your best bet would be an FPT. Actual machine tests can take up to several minutes, making production far much slower than ICT.

4. Cost

The cost of programming for both testing systems falls within the same range. The difference is in the associated charges that come with each system.

There is zero cost of a test fixture for a flying probe test. On the other hand, an ICT fixture can cost twice as much as the programming cost of both the FPT and ICT systems. Also, moving test pads or changing components in ICT attracts additional costs. In some cases, the cost will be for a new fixture. However, ICT is relatively inexpensive when considering its “per unit” speed during actual board testing compared to FPT.

Conclusion

A flying probe test in PCBA has many advantages, including minimal lead time and optical inspection. There is also no extra costs for fixtures. It is an excellent choice, especially for low volume productions and products in the early development stages.

Although the cost per unit is considerably higher than ICT, a flying probe test is the most cost-effective option among the two. It gives room to make corrections that are not expensive or require additional time to manufacture new test fixtures.

FPT is increasingly becoming popular, and with emerging advanced features, it can only get better.