The flying probe test replaces the “bed-of-nails” test in PCB quality control, offering benefits that range from cost reduction to flexibility. Read on for more about the technique, including how it works and its advantages.
What is the Flying Probe Test?
The flying probe test is a powered testing method in which electrical probes hover above the sample, landing at specific, predetermined locations. The landings allow the probes to make predetermined contact with test points.
The FPT equipment comprises a fixture, moving pins, and signal generation and processing units. It succeeds the “bed-of-nails” fixture, which uses fixed probes. The pins move automatically, controlled by a computer program.
The FPT method is highly flexible and adaptable. You can use it to examine printed circuit boards (PCBs) without needing custom jigs. The following section explains its components.
Flying Probe Test Components
The following are key parts of flying probe test equipment: the fixture, “flying” contact pins, and the control program. Here’s more about their operation and functions.
Testing Fixture
The fixture holds the circuit board, keeping it still to ensure precise probe contact and measurement accuracy. It can be in various sizes or tailored to match the board’s dimensions.
The Probes
These are software-controlled pins. They descend to make contact with the PCB at designated points, completing specific circuits. The machine sends and collects signals using these probes.
The probes attach to a movable platform. This platform can move in X, Y, and Z directions, enabling the probes to land on various points on the circuit board.
Control Program
The FPT equipment is an automated machine controlled by a custom program. Creating the test program requires using the PCB’s BOM and CAD files. These provide information about the board’s structure and parts, including test points.
Testing Unit
This part coordinates the operation of various tools and equipment. The tools include signal generators, frequency counters, and multimeters. The testing unit also receives the signal outputs, processing them to identify errors.
Resource: https://www.youtube.com/watch?v=iaLO5cxztcQ
How Flying Probe Test Works
The FPT process involves the following steps: loading the PCB, moving the probes to contact test points, and taking electrical measurements. Here are the steps involved in the flying probe test explained.
PCB Loading
The PCB goes into the fixture. This procedure is manual or automated, depending on the machine type. It also depends on whether the production scale is small or medium.
Loading involves clamping the board at specific points. The clamping zones depend on whether the board loads vertically or horizontally. Some load and check several boards simultaneously.
Probe Movement
The probes move above the PCB, lowering when needed to make contact with a component or test node. The probe completes circuits, while the machine generates signals and sends them to the PCB’s circuits.
The tester then extracts electrical data and uses computer software to analyze it. Sometimes, the machine takes board images using high-resolution cameras.
Electrical Measurements
The tester measures electrical quantities to detect potential PCB design flaws. It also identifies manufacturing process errors, such as open and short circuits. The measurements include resistance, capacitance, and impedance.
What are the Advantages of Flying Probe Test?
The flying probe tester is an advanced quality control technique. It replaces the earlier method that uses spring-loaded pogo pins to contact the PCB. The following benefits make it better than the traditional bed-of-nails.
No Custom fixtures
You do not need a custom fixture when performing the FPT. Designing such fixtures takes time and effort, sometimes up to several days. It also increases the PCB price, especially in low-volume production.
The FPT process works with existing fixtures. It only requires a custom program. Test program generation is easier and cheaper than creating a custom fixture.
Cost Effectiveness
The FPT is cost-effective when you only have a few boards to check, such as when prototyping or producing low-volume PCBs. You do not need to design a new fixture for every board type. That reduces the time spent on the process and costs.
High flexibility and Adaptability
A change in the control program is easily transferred to the testing probes. This feature makes the technique highly adaptable. In other words, you can use one test fixture to check different board types.
Better Access
FPT probes can land anywhere on the circuit board assembly since they do not make fixed contacts. They are also comparatively tinier with a more extensive reach. That includes probing within the tight spaces of high-density boards.
Key FPT Limitations
The FPT method has a few drawbacks. The downsides range from reduced speed to increased probe wear. Let’s see how each factor impacts the process and its effectiveness.
Low Testing Speed
The test probes must move about in a determined sequence. This sequence reduces FPT test speeds, making it suitable for low to medium volumes. These include design verification, sample creation, and prototyping.
Increased Probe Wear
The probes wear over time, reducing their effectiveness. This problem affects the processes’ accuracy, requiring timely correction. Engineers must, therefore, ensure the pins are in good condition by conducting regular maintenance.
Resource: https://youtu.be/-ZCweE2iUM8?si=HiSSM7QQdmFPnZQb
What Does a Flying Probe Test For?
The FPT process helps electronics manufacturers check circuit boards. It detects opens, shorts, and various other defects, including incorrect component values and placement. In other words, it finds circuit and component errors.
The probes apply electrical signals to designated points, measuring component and circuit parameters, such as resistance, inductance, and capacitance. Other measurable quantities include signal quality and current values.
The FPT procedure is mainly an in-circuit test. Engineers can also use it for some functional tests. It can also apply voltages to stress the board, determining its failure rate.
Flying Probe Test Point Requirements
FPT test points include component pins, exposed pads, vias, and through holes. The designer specifies them when creating the board, enabling their integration into the probing program.
The process must achieve high precision for successful checking. The following are key requirements when testing printed circuit boards with FPT.
Test Point Size
Ensure test points are the correct sizes. Experts recommend a minimum of 0.050 inches. However, that depends on whether the board is low or high-density.
For instance, the tester can probe component pads before assembly. On the downside, the points may become inaccessible after soldering components.
Larger pads allow probing after mounting components. Ensure the points are not too high, as that could lead to signal attenuation and interference.
Clearance Needs
Proper clearance enhances probing accuracy. It also prevents the points from touching and short circuits. Again, designers should ensure a 0.100-inch point-to-point clearance in low-density boards.
The designer may reduce the distance in high-density circuit boards but not below 0.050 inches (1.27mm). Ensure at least 0.100-inch point-to-edge clearance. It helps create space for clamping the circuit board to a fixture, among other needs.
Placement on the Board
Ensure the PCB nets are accessible when testing the board on one side or horizontally. There should be at least a node for each net to enable various electrical tests.
Scatter the points around the board, distributing them properly to avoid clusters and probing difficulties. Avoid placing them near obstructions, such as wires.
Conclusion
The flying probe test is popular in PCB manufacturing. It utilizes hovering pins instead of conventional fixed pogo pins, allowing automated checking. Manufacturers use it to check bare and populated circuit boards cost-effectively.
Venture implements various PCB quality control measures, including conducting the FPT test. The test assesses new designs and prototypes quickly and at a lower cost, reducing your product’s time to market. Contact us for more information about our PCB manufacturing and quality control capabilities.
