Testing your PCB is a critical step, if you’re interested in the quality and consistency of your product (which you should be!). When it comes to testing your boards, there are only a few broad methods to choose from. If you’re producing only a few boards or prototyping, manually testing the board by hand is cost-effective and tells you exactly what you need to know. However, past prototyping or limited productions, testing by hand would be too cumbersome. For large scale testing, Blue Clover Devices (BCD) most frequently uses In-Circuit Test (ICT) fixtures custom to the board design. ICT fixtures typically use a static bed-of-nails (or pogo pins) to make physical contact with key points on the circuit board for validation testing. Another common method is Flying Probe Test (FPT). For this post, I will be summarizing everything I learned about Flying Probe Testers and their uses.
The first ever flying probe tester was designed by Takaya in 1987. According to their website, they were motivated to create something that combined the advantages of advanced electrical tests and high precision mechanics. The result was the FPT, which was such a big game changer that Takaya decided to offer it as its own product to be sold to other manufacturing firms.
While I could not find published research on FPTs and their impact on the electronics manufacturing and testing space, I was fortunate enough to speak with Ray Rattey (Senior Applications Engineering Manager at Takaya). According to Ray, the time ratio from his experience is about 10:1, meaning that it would take on average about 10 times longer to test a board using FPT than ICT. However, it's important to note that the development lead time for FPT is much less substantial than for ICT fixtures. Unlike ICT fixtures, which have to be custom designed and built for every single board iteration (which can take up to 6 weeks), FPTs can be adjusted for each board iteration. This means that FPT is great for short-run and quick turnaround products, but the time required to test an individual board is longer than ICT fixtures.
Figure 1: FPT probing from a Takaya FPT
Image retrieved from https://www.takaya-itochu.com/technology/
The idea of an FPT is relatively simple, one can think of the machine as just an automated multimeter, or as Ray told me, “a multimeter on steroids”. It is a machine that has a clamping mechanism that holds on to the board while software controlled probes accesses components or test points on the board to measure values or check for opens or shorts. So the question comes up, why would anyone choose this method of testing if it takes 10x longer than an ICT? Because at the end of the day time is money right?
Well the answer is quite simple, the FPT requires no fixtures. The fixture is the part of the ICT that interacts directly with the board that is being tested and is what makes the ICT so much faster. The ICT is able to test every net and component on the PCB simultaneously while the probes on a FPT tests every point sequentially, meaning it is only able to perform one test at a time. But because the FPT uses probes it requires no static fixtures (which ICTs require). Additionally, product design changes like pad positioning or component changes on the board are much easier on the FPT because all it requires is a change in the test program, while in the ICT it might require a whole new fixture. So this means that the FPT is an excellent choice if the board that is being made is still in its prototyping stage.
At this point of the article you might be wondering how does one go about setting up an FPT to test a specific board? I’m glad you asked! The overall process is very similar for each FPT. In general, to create the test data it requires a Bill of Materials (BOM) and gerber file, which is also just a CAD file. For example Hioki (another firm specializing in FPT machines) is able to break down setting up test files into 5 steps:
- Import gerber data
- Import BOM
- Register components (ICs, resistors, capacitors, and etc)
- Generate test data
- Export test data
Although it might depend on the machine that is being used, at the end of the day the gerber file and BOM is almost always needed to generate a test file for the FPT. Most of the time is spent registering the components on the PCB especially if the BOM is incomplete, which makes creating the test file even more time consuming.
There a few prominent companies that are currently creating and selling flying probe testers. I’ve mentioned Takaya and Hioki already, but some others include Acculogic, Seika, atg, SPEA, and digitaltest. The special features vary with each company’s focus and design. If you’re an expert in FPTs or have worked with different companies for different purposes, I’d love to hear about your experience! If you’re interested in purchasing an FPT machine, I recommend reaching out directly to the firm and asking for specific details. However, here are some of my notes and observations from my research on FPT and conversations with experts.
When choosing which company to buy a FPT from, you should first list all your company’s technical needs as well as constraints for things like budget and time. Some prominent technical capabilities that some providers may have over others are JTAG/boundary-scan, automatic optical inspection (AOI), Statistical Process Control (SPC). A boundary scan is a method of testing interconnections, so if the product you want to test is something like Ball Grid Array ( BGA) devices, which have connections that are hard to access. Usually the only other way to test BGAs is by using X-ray technology, which in most cases can be very costly. AOI is just a visual test that is able to inspect defects on the board. SPC software helps to analyze test results by plotting distributions of measurements that are being made, inspecting deviation, and essentially just does further analysis on the data that is gathered by the test reports.
Additional features like number of probes, the types of flying test tools, and dual side probing are also things one can look out for when researching which machine to buy. For example the flying scorpion test system that is offered by Acculogic is scalable, meaning that customers can purchase what they need initially and then add more over time.
Figure 1: Ball Grid Array
Image retrieved from: 300px-Solder_ball_grid.jpg
Additionally, the operating system (OS) and programs being used on these machines determines how easy it is to use and setup the FPT. Ray pointed out that Takaya machines, while cutting edge, require some training to use and are better suited for more advanced users. However, in my conversation with Mark Lionbarger from Hioki, I was told that Hioki’s software interface is easy to use and requires very little training. Additionally customer support is another important factor to consider when purchasing these types of machines in the scenarios when the machine is not working or you just want extra support. For example, Hioki offers complimentary technical support for their FPT machines, while others require support packages.
While I mentioned before that one of the major drawbacks of the FPT is its speed relative to the ICT, some manufacturers have worked to improve the FPT’s performance speed. For example. Takaya and Hioki have options to add an in-line feeder to their FPTs to improve efficiency. An in-line feeder eliminates the need for an operator to have to manually place a board in the machine each time the FPT is finished and to sort out the passed or failed boards. Instead PCBs are gathered by the FPT through a feeder mechanism and then tested. Once the board is tested, some FPTs are set up in such a way that there is an additional feeder that sorts out failed and passed boards. This helps to reduce the amount of time that is spent loading the boards onto the FPT and then sorting the boards. Ray informed me that this practice is not as common for American manufacturers, while European firms have more readily embraced this system addon.
To sum up my findings, I made this reference chart:
I gathered this information based off the individual firms’ websites and conversations with representatives. I wasn’t able to get quotes from all of the companies, but typically an FPT machine will cost upwards of $250,000, with additional costs for various packages and versions. But if you are in the manufacturing business, these prices are quite standard for high-tech machinery. I was even told by Mark that he has seen oscilloscopes cost twice as much.
Hopefully, this article has helped to provide a broad overview of flying probe testers and their place in PCB manufacturing. Again, if you’re interested in purchasing these machines, you should reach out to the firms to see which machine would best fit your needs! I would like to give a special thank you to Ray Rattey and Mark Lionbarger for taking the time to share their insights on the FPT.
This post was written by Andy Ma, BCD's 2019 Summer Intern.
About the Author
Andy is due to graduate in December from San Jose State University, with a B.S. in Electrical Engineering. Fun fact: he loves coffee!