How Flying Probe Testing Works for PCB Assembly

by | Mar 21, 2019 | 7 comments

In this article, we will try to cover the whats, the whys, and the hows of a flying probe tester. Before we can understand what precisely a flying probe tester is, we need to understand what makes it so complicated.

So, let’s take down one monster at a time. Starting from what a PCB is!
PCBs are boards made for connecting electronic components together electrically and provide mechanical support. A PCB population is quite with SMT components, processor resistors, capacitors, diodes, LEDs, and so on. These components are connected using conductive traces, pads, and other features etched from one or more sheet layers of copper. This/these layer (s) of copper is/are laminated onto and/or between layers of a dielectric substrate.
But this is something we all know right? What we do not really ponder upon are the problems that occur in those densely populated PCBs!

What is ICT or in-circuit test?

In-circuit tests check the workings of an application, i.e., white box testing. Here, we use an electric probe to check the populated PCB for shorts, opens, resistance, capacitance, and other basic qualities. This will show whether the fabrication of the board is proper. There are two ways of performing ICT, out of which one is our topic: Bed of nails type of fixture and fixureless ICT setup. Traditionally, “bed of nail” testing for a PCB requires a custom fixture to hold the PCBA and the pogo pins that contact the PCBA. Like the real-world bed of nails, these devices contain an array of small, spring-loaded pogo-pins. Each pogo pin connects with one node in the device’s circuit under test.

The Flying Probe Tester

The Flying Probe Tester in our PCB assembly facility

As the trend in the increase of board complexity continues, they challenge the capabilities of traditional testers and even exceed. The flying probe extends in-circuit test benefits to applications where this bed of nail is impossible to use (no access), too long to build (prototype) or too expensive to justify (high mix/low volume).

Fixtureless In-Circuit Test or Flying Probe Tester

The flying probe test originally worked only for bare board testing. But from the above statement we have understood FICT or FPT are efficient in PCBs that pose accessibility issues. Also, in prototypes and low to mid-volume production. In contrast to the bed of nails, it uses software instruction to move the flying probes. electro- mechanical controllers in flying probes access PCB. We equip four headers flying probe tester along X-Y axis and can move at a high speed. We equip a camera on the flying probe tester to inspect component polarity. The flying probe tester can therefore trim down on the number of test fixtures required. It is also becoming much easier to introduce changes, especially to features such as a pad or component positions. It takes just a software change.

Again, flying probes can access component pins directly or through automated test point probing. They don’t require test points. This improves test coverage over in-circuit tests that use beds-of-nails. It uses the vias on the board as embedded test points and uses them as test points. Therefore, the test points are no longer in action.

How does that help? Our sleek to sleeker electronic gadgets cannot really afford these points because board real estate is a precious commodity. Eliminating some, if not all, test points saves substantial amounts of board real estate.

Flying probe testing is a method for low-volume and for prototype circuit boards for several reasons. It’s easy to use in programming even in today’s era of increasing density and complexity. Low-volume production runs or prototypes do not really justify the cost of bed-of-nails in-circuit testing, where fixtures are expensive. Using flying probe technology provides a cost-effective solution. It can also test boards with virtually infinite numbers of nets. Therefore, this will dramatically decrease the product design cycle and will slash the time to market.

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How Flying Probe Testing Works

Here in this part, we will take the working of a flying probe in two different parts: one for the hardware and the other for the test program or software. In case of working of the hardware:

The unit of the device under test will first travel to the internal tester through a conveyor belt. Then, probes will contact the vias and test pads to detect the defects of that unit. It connects probes with drivers such as signal generators and power supply. The connection is through multiplexing system and sensors. Digital multimeter, frequency counters, etc. are such sensors through which it tests components of the defective device. When it tests a component, it shields other components on the same unit from the test.

Wondering why? For the obvious, to stop the reading from being disturbed.

As we already know, flying probe tester can test shorts, opens, and component values. We also equip a camera on the flying probe tester to inspect component polarity. It becomes easier for the tester to test for several aspects in a single test. Hence, looking out for the fabrication of board assemblies.

The Flying probe tester

The hardware of a flying probe to test for shorts, opens and component values.

Before we move into the software section, let’s look into some exclusive stuff about testing through the probes.

Phase Difference Measurement Unit

The phase difference measurement unit is one such a test. Today we can afford anything but unnecessary time. PDM sends a high-frequency signal between the reference line and signal line to measure differences between these phases. This will cut down unnecessary isolation tests within a net.

High Voltage Stress Test

The high voltage stress test is for the high resistance undetected isolation defects by PDM. We conduct HVS by applying high voltage pulses between signal lines to detect high resistance defects. HVS and PDM are similar on the aspect that both inspect each net once. Thus, is rapid on time.

When we say high it includes the range from high to higher. Thus, next we shall discuss this high to the higher range. The maximum applicable voltage with standard isolation resistance measurement is 250V. This offers the option of applying a maximum of 500V/1000V. Now, for the super high resistance isolation defect. It applies a low power test voltage to do this. The usage of low voltage allows users to conduct a super high resistance test without adding high stress on the tested boards.

Micro Shorts Detection

Next on our platter is Micro Short detection. Sudden application of a high voltage in isolation tests may cause burning out of parts containing micro shorts. This is for carbonized residue possibly reacting with moisture in the air or gases in the factory. This causes the element to relapse as a high resistance short error after the test is complete. Micro-short tests can avoid such damage and trouble by applying a low voltage before gradually increasing the applying voltage. Sometimes, high resistance short between multiple layers of a board possesses the characteristics of semiconductors and capacitors. The reversing of polarity of high voltage could also detect these defects.

As for the software or test program working:

Flying probe tester carries out programming more easily and more quickly than traditional ICT.
Why? Let’s find out…

For implementing flying probe test programming, the tester first transform CAD data provided by engineers into an applicable file. After that the newly generated file runs through a test program. They run it with new files with corresponding formats generated. Last, all the files created are exactly catering to that units’ test demands and requirements.

The test probe gets ready for action as soon as the program is ready. Then, as obvious as determining an item of testing, open resistance, for example. Then, reference data points conforming to the defective or something to test that unit should be picked up from CAD data. As soon as a unit gets fixed on the platform, the programming will be carried out to inspect the fabrication or assembly issues. It’s a must keep in mind that debugging needs be done prior to official testing.

What’s good news; it’s that, we can now do debugging of flying probe test way early when compared to conventional ICT test.

Flying Probe Tester

Our Expert working on the tester software

Our Promise

The flying probe tester takes a long and advanced study (which by the way is in our top list right now). But we think this article would help you set your foot on this topic. We will keep you updated and keep on updating ourselves with this heavy loaded task only for you.


Visit our PCB assembly page and learn more about our services. Try our free BOM Checker to quickly identify issues in your bill of materials.



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  1. Avatar

    This makes me think of the other page I was seeing.

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    When using the flying probes for testing, what is the smallest size test pad and annular ring that can be used at Sierra Circuits?

    • Avatar

      At Sierra Circuits, 50 microns i.e. 2 mils is the smallest size test pad and annular ring.

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    Can flyinging probe you test opens on a populated board. I have a transformation board with solder balls soldered to a large board on one side and the flat lga pads on the other side. I believe that I have opens from the soldebsll side. Can you test for opens in this type of application

    • Avatar

      Yes, flying probe can test opens! Mention the board size to check the bed area. At Sierra Cirucits we do not take up this type of job work. You need to find out a subcontracting agency that performs flying probe test for populated PCBs.

  4. Avatar

    The flying probe test (FPT) is an electrical test method, which makes a simultaneous in-circuit test (ICT) of the top and bottom of a module easily and flexibly.

  5. Avatar

    When checking component values, many components are connected to other components. As an example a power regulator could have a 10K load resistor but circuit resistance might measure less than 1K due to capacitor leakage values. How is this accounted for?


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