In today’s world, all products rely on electronic boards that should be tested after manufacturing. Reliability in testing is crucial as a single net failure can cause significant financial losses, equipment, and even lives in some critical applications like ventilator PCBs.
At FC PCBA, we believe in using the efficient and time saving testing methods. In this article by FC PCBA, we will talk about PCB testing with a special focus on flying probe testing method. We will discuss the advantages and limitations, what designers should do to simplify the process, also we will briefly cover other testing methods for comparison. So keep reading to learn something new!
Common PCB Testing Methods
There are several types of PCB testing methods, each of which has its specific applications, advantages, and limitations. We will briefly cover most of them as follows.
In-Circuit Testing (ICT) / Bed-of-Nails
ICT uses a custom fixture as shown in the image, with spring-loaded pins to contact test points. This method is very fast because it tests the board in seconds, and is very effective for high-volume production. But it costs too much and has limited access to high-density designs due to the size of test points.
Automated Optical Inspection (AOI)
AOI uses cameras to inspect component placement, orientation, and thermal defects, also to inspect solder joints. This method provides fast visual inspection, is good for detecting wrongly placed components, and no physical contact is needed. But it can’t test electrical functionality; components may be placed correctly, but with the wrong values.
X-Ray Inspection (AXI)
AXI uses X-rays to examine hidden connections, which detect defects in bare PCBs like voids and cracks, between layer traces, etc. Also used to detect defects in solder joints under components, bridging, and misalignment issues.
But it can’t do an electrical functionality test as AOI, and it’s very expensive.
Flying Probe Testing
Flying Probe Test is a non-contact test technology used to locate the electrical functionality of PCB. It uses high-speed traveling probes to make contact with test points on PCBs to locate defects such as short circuits and open circuits at high speeds. It is particularly suited for small quantity production and prototype testing. It is able to test components such as MOSFETs, diodes, resistors, capacitors, and others by utilizing motorised probes with the test program.
The major components used in the flying probe test include:
- Probe: Electrically conductive and needle-like pins.
- PCB Holder: For holding PCB in desired place.
- Prove Moving System: Helps to move probes with precision.
- Test controller and software: Helps to manage test sequences and data.
- Measuring Tools: For proper electrical measurements.
How Does Flying Probe Test Work?
The flying probe tester uses probes instead of needle beds and uses an array of motor-driven, high-speed electrical probes to contact the device pins and take electrical measurements. The UUT is transported to the tester through a belt or other UUT conveying system when test time comes, and then clamped. The test pads and vias are probed by the tester to test each component of the UUT in isolation. Multiplex transmission is employed to link the test probes to the driver and sensor. The process is employed to test components on the UUT. While a component is being tested, other components of the UUT are electrically shielded from the probe so that the readings would not be disrupted.
Flying Probe Test vs. In-Circuit Test
- Flying Probe Test is flexible and fixtureless, ideal for prototyping or low-volume production.
- In-Circuit Test is faster and more thorough but requires expensive fixtures, making it suitable for high-volume manufacturing.
|
Feature |
Flying Probe Test (FPT) |
In-Circuit Test (ICT) |
|---|---|---|
|
Test Method |
Uses movable probes to test points sequentially |
Uses a fixed bed-of-nails fixture for simultaneous testing |
|
Fixture Cost |
Low (no custom fixture needed) |
High (requires custom fixture per PCB design) |
|
Setup Time |
Fast (programmable, no physical fixture) |
Slow (requires fixture design & fabrication) |
|
Test Speed |
Slower (sequential probing) |
Faster (parallel testing of all nodes) |
|
Flexibility |
High (easily adapts to design changes) |
Low (fixture must be redesigned for changes) |
|
Best For |
Prototypes, low-volume, high-mix production |
High-volume, stable designs |
|
Component Access |
Can test hard-to-reach components |
Limited by fixture probe placement |
|
Power-Up Testing |
Limited (usually passive measurements) |
Supports powered testing (active components) |
|
Fault Coverage |
Good for opens/shorts, basic component checks |
Comprehensive (tests functionality, values, etc.) |
Pros & Cons of Flying Probe Testing
Advantages
- Flying probe testers have several benefits over traditional testing methods like bed of nails; it doesn’t need custom fixtures, which reduces overall test setup cost and testing time.
- They are also easily adapted to different PCB designs. During the research and development period, designs are changed and updated continuously until reaching a stable performance concerning PCB cost, so using the ICT “bed of nails” method will not be applicable because of continuous changes in design and layout. The flying probe method is suitable for this application.
- It can test fine pitch components and detect minute defects like micro shorts also, for signal integrity issues, it can detect impedance mismatches.
Limitations
- Flying probe testing has some limitations. It’s slower than ICT. Testing time increases with board complexity, because probes move sequentially. On the other hand, the bed of nails is constant and fixed; it can test PCB in no time. For the same reason, it’s not efficient for mass production as a bed of nails tester.
Guidelines For Efficient Flying Probe Testing
For PCB designers, there are some considerations that will be good to follow to optimize your board for flying probe testing and improve the efficiency of the testing process. Here are some of these guidelines:
Include Test Points
Test pads must be more than 0.5mm to ensure their accessibility and avoid placing components too close to test points to prevent probe interference.
Minimize Probe Travel Distance
To minimize overall testing time, try to group test points logically, which reduces probe movement time. And make it easy to test manually, even after fabrication.
Avoid High-Density Probe Areas
If test points are very close to each other in tight spaces, this may cause interference and may lead to an incomplete test due to the inability to probe adjacent points. The designer should include tester capabilities in design rules before placement of the testing pad in the layout.
Provide Clear Netlist & Gerber Files
Including component values for diodes, transistors, resistors, inductors, and capacitors will help in automated verification. Also, accurate Gerber files, BOMs, and netlists can serve the process a lot and help generate efficient test programs.
Testability for Power & Ground Nets
Power and ground nets must have multiple test points around the board; this is required to test ground bouncing and power delivery network (PDN). Also, each power level must have at least one test point to measure the voltage level.
FPT in Various Application
Semiconductor and AI/HPC fields
- 3D stacked chip package testing
- Conductivity of through-silicon via (TSV) detection (resistance value < 0.5Ω) to prevent micro short circuits in HBM high-bandwidth memory stacking
- Example: In the CoWoS package process, flying probe testing detected 0.8μm TSV copper residue for an AI chip manufacturer, improving the yield from 78% to 93%
- Aging test board (BIB) verification
- Compare the DDR5 interface capacitance value variation (tolerance ±3%) after 150℃/1000-hour aging test to identify the failed termination resistor
- Technical highlights: Use phase difference method to verify the impedance matching of 10GHz high-speed signal network
Automotive Electronics
- 800V high-voltage platform detection
- Utilize 1500V DC to detect the IGBT module insulation resistance (＞500MΩ) to prevent silicon carbide device creepage failure
- New application: Bosch utilizes a six-axis flying probe tester to detect the contact resistance fluctuation of the BMS sampling harness in vibration conditions (<2mΩ)
- Interconnection of smart cockpit multi-screens
- Check LVDS differential pair impedance (100Ω±5%) to ensure the integrity of the 4K automotive display screens signal
- Data comparison: One automobile firm reduced the center control black screen failure rate from 3% to 0.2% with flying probe testing
Consumer Electronics
- Folding screen mobile phone FPC detection
- Detect 5μm micro cracks (capacitance change>10%) of CPI cover film to prevent circuit breakage after 100,000 dynamic bending
- Process breakthrough: Huawei uses laser compensation flying probe test to achieve accurate testing of 0.1mm curvature radius area
- TWS headset miniaturization test
- Detect LCP antenna impedance in 4x4mm space (2.4GHz band 50Ω±10%)
- Efficiency improvement: Goertek cuts down AirPods Pro motherboard detection from 120 seconds to 45 seconds with 8-pin synchronous test
Aerospace Application
- Satellite equipment radiation resistance test
- Test gate leakage current of GaN devices under a vacuum environment (<1nA@100V) for assurance to resist single-particle upset
- Special process: China Electronics Technology Group Corporation 38 Institute studied tungsten carbide probe withstanding the -196℃ liquid nitrogen environment
- Airborne wiring harness reliability verification
- Simulate 10g vibration conditions to identify contact resistance of MIL-DTL-38999 connectors (<5mΩ)
- Information recording: AVIC established a 2000-hour vibration-temperature cycle test database
Medical equipment sector
- Testing of the implantable device
- Flex circuit insulation resistance test of the pacemaker (more than 1GΩ @37℃ body fluid environment)
- Safety standards: 0 defect detection strategy in accordance with the requirements of ISO 13485
- High-voltage test of imaging devices
- Withstand voltage performance test of X-ray tube driver board of the CT machine (leakage current below 10μA at 40kV DC)
- Innovation of technology: United Imaging Medical develops non-contact electric field detection flying probe to avoid high-voltage discharge danger
Conclusion
In summary, we can say that flying probe testing is very essential for any PCB manufacturer to test their boards with high flexibility for design modifications. It’s also the perfect choice for high-density designs because of its high accuracy in detecting defects.
At, FC PCBA, we specialized in providing the high precision PCB testing and manufatciring services. Contant us for expert guidance and quality you can trust.
FAQs about Flying Probe Testing
What is flying probe testing?
Flying probe testing is a common method for circuit testing using automated probes that can check proper component placement and orientation, electrical continuity, short circuit, and open circuit tests.
What is the difference between probe testing and ICT (in-circuit testing)?
ICT requires fixed fixtures, which are called a bed of nails; these fixtures are not required in the flying probe technique, but on the other hand, flying probes are slower but more flexible, suitable for medium volume production.
What types of defects can flying probes detect?
Flying probes can detect open circuit, short circuit, and incorrect component values, like resistances, capacitances, and inductors.
Can flying probes test BGAs and QFN packages?
Yes, it can, but it has limitations. for BGA chips, probes can’t test under component pads, but designers can make test points for them, or they can be visually inspected using X-ray inspection.
What design features help flying probe testing?
Simply, test pads must be more than 0.5mm in diameter to ensure accessibility, and the space between test points should be at least more than 1mm or as per tester capabilities. Also, try to keep test points away from high components. At least you should provide ground and power test points.