The flying probe test is one of the most used testing methods in modern PCB manufacturing. It presents a more advanced testing process, replacing the traditional bed of nails. Find out why below, including how it works and its advantages and disadvantages.
What is a Flying Probe Test?
As the name suggests, the flying probe test is a circuit board testing method that uses hovering pins to check a PCB’s circuits – the pins descend at designated points, completing circuits to send and receive test signals.
The test derives its name from the “flying probes,” an array of pins mounted on a mobile arm or platform. The arm moves under the control of the test program, making pre-determined contacts with established test nodes.
On the other hand, the traditional version, or bed-of-nails test, uses fixed pins to probe circuits. Each method has good and bad sides, as we will see later.
Parts of a Flying Probe Tester
The flying probe test machine is a simple construction with only a few distinct sections: the probes, the testing fixture or jig, the control program, and a few other components. Let’s understand the function of each part.
Flying Probes
These are test pins arranged on a movable arm. They are thin and loaded with springs. The springs make them less destructive by reducing contact pressure, while the thin size enables them to reach tighter spaces of high-density circuit boards.
Test Fixture
The test is the testing station or the part that holds the PCB. It usually has a design that matches the board’s size and other requirements. Some fixtures are adaptive and capable of accommodating different circuit board assembly sizes.
Control Software
The control software or program issues instructions that drive the tester’s operation. Based on the PCB’s design and layout, it tells the pins when to descend and where.
Electrical Equipment and Interfaces
Electrical parts include power supplies, signal generators, processors, and measuring tools. The processing unit analyzes the signals and determines if the test is a fail or a pass. The interfaces connect the equipment to the PCB under testing.
Optical Components
Some flying probe testers include high-resolution cameras to improve their function. In other words, they combine electrical testing with optical inspections for a more rounded analysis.
How the Flying Probe Test Works
The FPT process involves a series of steps. Understanding its completion stages should give you a better overview of how it works, including the defects it detects.
Program Creation
The test begins with creating custom software. This program is unique to the specific PCB and is written using its CAD data or design files. The files allow the programming expert to create instructions that guide the process based on test point locations.
Loading Program
The next step is for the testing expert to load the custom program onto the tester using appropriate means. With the program ready, the actual FPT process can now begin.
Testing Steps
This part involves placing the PCB on the testing station. Depending on the production scale, the loading is either manual or automated. Starting the machine causes the probes to hover above the board, lowering to make electrical contact.
The tester sends signals through test nodes and takes electrical measurements, including resistance, inductance, and capacitance values.
The signal processor receives the output or feedback signals and analyses them to detect defects. The defects detected are electrical. They can be open circuits, short circuits, or incorrect component values.
Flying Probe Test Point Requirements
The unique testing of the FPT process has various requirements. PCB designers must understand these needs to facilitate the procedure and avoid encountering problems. They include the following:
- The test points must be correctly placed for easy access by the FPT equipment
- They must also be evenly distributed across the board and not clustered
- Optimized test point sizes (not less than 0.05 inches) for easier and reliable probing
- Minimized test point height to avoid signal issues
- Proper clearance (about 0.1 to 0.5 inches) to allow room for clamping and other steps
Flying Probe Test Advantages and Limitations
Like any other circuit board test method, the FPT has advantages and disadvantages. Let’s examine them, starting with the advantages.
Advantages
The test offers various benefits, making it a better choice in some situations. The advantages of flying probe tests are mentioned below.
- It requires no custom fixture, making it highly adaptable and reducing development time
- The fixtureless nature makes it one of the most cost-effective test solutions for low volumes
- It has better access, given the flexibility of the flying probes, allowing it more reach
- Its reduced cost makes it suitable for frequent PCB design changes when prototyping
Limitations
The disadvantages of the flying probe test mainly concern its speed, coverage, and probe durability. Let’s look at each drawback below.
- The testing speed is slow and unsuitable for high-volume QC needs
- Reduced test coverage per session compared to the bed-of-nails method
- High probe wear rate due to their increased motion and frequent contact with test points
When to Use the Flying Probe Test
The flying probe test is a cost-effective electrical testing method that doesn’t require a custom fixture. However, its low speed limits its use. Here’s when to choose it over other technologies.
- When checking high-density circuit boards. It has thinner probes that go into smaller spaces.
- When you only have a few boards to test, such as low to medium-volume projects
- In the early board development stages, such as manufacturing prototypes, when you expect various design changes
- Specialized electrical tests where other methods may not apply due to cost, flexibility, or accessibility
The testing method suits many circuit boards, from general electronics circuit boards to specialized types for medical, automotive, and other branches of the electronics industry. Electronic manufacturers mainly benefit from its reduced cost.
Conclusion
The flying probe test is an automated PCB testing method for electrical checks. It generally replaces the earlier method called a bed of nails, offering greater versatility and reducing testing costs. This guide has helped you understand how it works and its benefits.
