Flying Probe Testing 101: Cost-Effective PCB Testing

PCB design can be challenging. The complexity of your project and the potential consequences of PCB failure may require thorough, high-level testing to ensure your product's success before launch.

One of the most dependable methods for testing printed circuit boards (PCBs) is flying probe testing. This guide walks you through the essentials of flying probe testing, its applications, and best practices for setting your contract manufacturer up for success.

Whether you’re a newcomer or looking to optimize your current PCB testing processes, this article will help you understand the value of flying probe testing and the essential documentation for smooth, quick testing.

What Is a Flying Probe Test?

The flying probe test is a standard testing method for inspecting PCBs. Often compared to in-circuit testing (ICT), it remains a popular choice due to its accuracy and flexibility.

Unlike ICT, which requires a specially designed fixture, a flying probe tester uses probes (or “needles”) to test specific points on the board without powering up the circuit. This test evaluates multiple parameters, including:

• Resistance
• Capacitance
• Inductance
• Opens (disconnected circuits)
• Shorts
• Diode checks
• Limited functional tests

For many projects, the flying probe machine can handle a range of testing needs quickly and accurately, making it a valuable alternative to ICT, especially for low- to medium-production volumes.

How Flying Probe Testing Works

Flying probe machines come in various configurations. Simple systems can have 4 probes, testing one side of the board, while complex systems can have up to 20 probes, and can probe both sides of the board at the same time. Tests can include IC programming, boundary scan technology, powered LED diode inspection to determine correct color and polarity, optical tests including bar code reading, and thermal scanning.

The flying probe test relies on data from CAD files, such as ASCII or ODB machine files. Here’s a breakdown of the process:

1. Grid Coordinates: The CAD file coordinates the machine’s probes to match the circuit layout. The probes will touch the leads of parts or pads on the board.
2. Needle Positioning: Probes approach the board at a slight angle, testing each point as programmed.
3. Value Verification: The flying probe tester ensures accuracy by checking actual measured values of a circuit or network against the expected set values.
4. Complex Circuits: If a circuit involves interconnecting sections, components may be thevenized, or isolated for separate analysis, allowing for a deeper look into circuit behavior. Some sections of a circuit may be functionally tested.
   
   

Benefits of Flying Probe Testing

If you’re weighing the benefits of flying probe testing versus in-circuit testing, here’s why the flying probe test might be a better fit for your needs:

Cost-Effective for Low Volumes

Flying probe testing requires no custom fixture, making it cost-effective for low-volume production. The initial investment and setup time for smaller runs are significantly lower than with ICT. A flying probe test can be developed in hours and is easily edited, while an ICT test and fixture take weeks to design and manufacture.

High Accuracy

A flying probe machine delivers reliable results, reducing the risk of issues in the final product. This accuracy is often enough to catch potential problems before they affect functionality.

Design Flexibility

One of the standout features of flying probe testing is its flexibility. Since the test doesn’t require custom fixtures, a redesign of the board does not require expensive modifications to a fixture.

Future Testing is Simple

Once the initial test program is set up, minimal adjustments are required for future tests. Your contract manufacturer can quickly rerun the PCB flying probe tester on additional units.

Drawbacks of Flying Probe Testing

Despite its many benefits, flying probe testing isn’t perfect. Here’s what you should consider:

Not a Functional Test

The flying probe test only looks at a few points at a time, so it doesn’t provide a full real-world view of circuit performance. It is a supplement to functional testing, not a substitute for it.

Limited to Accessible Components

Flying probe machines require two access points per connection, making testing difficult for Ball Grid Arrays (BGAs) and other components with hidden contacts. 

PCB designers should consider creating their PCB to ensure that probes can access the critical points needed for testing.

Slower than ICT for High Volumes

While the flying probe test is excellent for small batches, it may not be ideal for high-volume production. ICT generally offers faster run times for large orders, making it more efficient for high quantities.

Best Projects for Flying Probe Testing

Flying probe testing shines in applications where:

• Low to Medium Volumes: It’s ideal for smaller runs.
• No Functional Test Needed: Flying probe testing works well if your project doesn’t require powered testing after visual inspection.
• Quick Setup with Repeatability: The initial setup is straightforward, and future tests are even quicker, with only minor adjustments needed.
  
  

Documentation You Need for a Successful Test

For a quick turnaround and high test accuracy, be sure to provide your contract manufacturer with essential documentation:

1. CAD Files: ASCII or ODB files are essential for setting up the machine. Without these, the contractor may need to enter values manually, significantly extending setup time.
2. Electrical Schematics: This saves time, as schematics allow technicians to compare and validate component functions without guessing. It is best to have the schematics in a readable .pdf format.
3. Clear Communication: Organized and complete documentation speeds up your test lead time. Incomplete or misaligned data can lead to back-and-forth communication and potential delays.
   
   

Enhancing Your PCB Quality with Combined Testing Methods

Flying probe testing is a valuable alternative to ICT, but it often works best in tandem with other inspection methods, such as:

• Automated Optical Inspection (AOI): Ensures accurate component placement. All SMT boards at Matric receive AOI as part of our manufacturing process.
• X-ray Inspection: Provides visual inspection for projects with hidden leads such as BGAs
• Functional Testing: Tests the function of the entire circuit board assembly.
  
  

Is Flying Probe Testing Right for Your Project?

Flying probe testing is likely the right fit if your project involves low-to-medium volume PCB production and you seek a reliable, accurate test without high upfront costs. Its flexibility, accuracy, and efficiency make it ideal for early production phases, prototyping, and small batch runs.

Matric Group offers tailored solutions, ensuring your PCB flying probe tester is set up for optimal performance based on your specific needs. Contact us to discuss your documentation, volume, and design requirements so we can provide the best testing setup for your project.