What is pcb flying probe testing?

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Printed circuit board testing is a critical aspect of PCB assembly, allowing us to identify major circuit issues and address them promptly. There are various methods for PCBA testing, including flying probe testing, AOI testing, fixture testing, etc. This article will delve into the details of flying probe testing for PCBs.

What is Flying Probe Testing?

Flying probe testing is an electronic circuit testing method primarily used for testing printed circuit boards (PCBs). It employs a movable probe system that essentially “flies” over the PCB, making electrical contact with specific test points on the PCB. Ultimately, it identifies defects and verifies the electrical performance of the circuit. The flying probe testing system consists of several key components, including the probes themselves, test fixtures used to secure the PCB in place, and control software to manage the testing process.

Structural Features of PCB Flying Probe Testing

The flying probe tester is an improvement over traditional bed-of-nails testers, utilizing probes instead of a bed of nails. It features four heads with a total of eight testing probes that can move independently at high speeds on an X-Y mechanism, with a minimum testing gap of 0.2mm. During operation, the unit under test (UUT) is conveyed into the testing machine via belts or other UUT transfer systems, where the probes are then fixed to make contact with test pads and vias on the PCB, thereby testing individual components on the UUT. The testing probes are connected to drivers (signal generators, power supplies, etc.) and sensors (digital multimeters, frequency counters, etc.) via a multiplexing system to test components on the UUT. While one component is being tested, other components on the UUT are electrically shielded by the probes to prevent reading interference. The flying probe tester can detect short circuits, open circuits, and component values. Additionally, a camera is used during flying probe testing to aid in locating missing components and inspecting components with clear directional shapes, such as polarized capacitors.

Flying Probe Testing Capabilities

While flying probe testing can easily detect short circuits and open circuits, equipping them with special drivers enables them to test more complex parameters as well. Advanced probes can simultaneously probe and test both sides of multilayer boards, reducing the time needed for separate single-sided testing. Different architectures of flying probes can be used for various solutions, such as:

Signal Integrity Testing: Utilizing Time Domain Reflectometry (TDR) or Time Domain Reflectometer probes along with specialized instruments, various characteristics of PCB traces used for carrying high-speed and high-frequency signals can be tested. This setup typically captures and measures signals in both time and frequency domains to characterize defects in signal paths.

Phase Difference Measurement: Using specially designed probes to send high-frequency signals between reference traces and signal traces, the phase difference between them can be measured. This test eliminates the need for separate isolation testing to measure crosstalk between traces on the PCB.

High Voltage Stress Testing: PCBs may have insulation defects that conventional electrical tests may not detect. The insulation resistance between two traces on a PCB may be high enough to pass conventional resistance testing but still lower than the requirements in specifications. To detect this, high voltage stress testing is required, using a high voltage generator, appropriate probes, and high resistance meters.

Micro Short Circuit Detection: The presence of tiny whiskers can lead to micro short circuits on the PCB. Sometimes, they may burn off during high voltage stress testing, leaving behind carbonized residues on the PCB surface, forming high-resistance conductive paths. Micro short circuit detection probes apply low voltage to check the resistance between two traces on the PCB, gradually increasing the voltage to a level suitable for testing.

Kelvin DC Measurement: This is a highly precise DC measurement technique required for testing BGA and similar densely packed PCB patterns. It involves a force and sense pin in the flying probe. Kelvin connections compensate for losses in the testing probe.

Flying probe testing systems come in different sizes, with the main variable being the number of connectors the system uses. For example, a tester can have up to 16 connector heads, with 8 on the top and 8 on the bottom of the PCB. Of course, the cost of the system increases proportionally with the number of connectors it utilizes.

Advantages of Flying Probe Testing

Compared to traditional bed-of-nails or ICT fixtures, flying probe testing offers several advantages:

No Fixtures Required:Unlike bed-of-nails fixtures, flying probe testing doesn’t necessitate fixture setup. This saves the cost and time typically required for setting up ICT fixtures. In fact, manufacturers can set up flying probes immediately after the PCBs come off the production line, as they have access to Gerber data. On the other hand, designing and installing ICT fixtures can take weeks.

Short and Fast Program Development: Since netlists and CAD data form the basis for generating flying probe test programs, and there are multiple open-source programs to translate this information, program development time is short and requires minimal setup time. This also means that design changes can be easily integrated.

Process Flexibility: Unlike ICT’s bed-of-nails fixtures, flying probe setups are applicable to any PCB, whereas ICT’s bed-of-nails fixtures are specific to individual PCBs and useless for another. Simple modifications to internal programs are all that’s needed to adapt them to another board.

No Need for Test Points: Since flying probe testing is conducted on bare boards, probes can utilize component pads without the need for additional test points.

Controlled Probe Contact: Flying probes can achieve precise connections at closer intervals compared to bed-of-nails. For instance, high-precision flying probes can achieve testing gaps as small as 5 micrometers, while ICT’s minimum gap is 0.5 millimeters. This makes them highly useful for densely populated circuit boards or for achieving broader coverage on small PCBs.

Variable Testing Solutions and Methods: Flying probe systems can offer more testing solutions than ICT or bed-of-nails. This is possible because with programmable integrated test systems, various types of flying test probes can be utilized.

High Measurement Accuracy: Specific flying probes are employed for different tests, with precise probe positioning and complementary test instruments, ensuring high measurement accuracy.

Quick Feedback: Since flying probe test results can be obtained on-site, transmitting information to the production line can help them quickly make appropriate process adjustments. Similarly, PCB designers can receive rapid feedback during prototype design, enabling them to make necessary changes before production.

How Flying Probe Testing Works

Flying probe testing (FPT) is typically the preferred method for small batch and prototype circuit board testing as well as PCB assembly due to its cost-effectiveness and convenience for these smaller quantities.

The main advantage lies in the ability to complete testing at speeds ranging from a few days to a few hours, depending on the complexity of the circuit board, even for larger quantities, and with high coverage of testing.

Let’s break down its operation into steps:

  1. Creating an FPT Test Program

The design aims to test the entire circuit board and is usually completed using an offline computer with an FTP test program generator application. This typically requires Gerbers, BOMs, and ECAD files. On a machine with a motherboard, define the values of the components to be tested, test points, component formats, offsets, debugging, etc., and ultimately finalize the design of the test program.

  1. Uploading the Program to the FTP Tester

The components of the circuit board to be tested are placed on a conveyor belt within the FTP tester and transported to the area where the probes are operated.

  1. Application of Electrical and Power Test Signals

These tests are conducted at probe points and then readings are taken. This process determines whether specific sections of the PCB meet the expected results (components). Any failures or deviations from the established plan and expectations indicate defects within the unit, resulting in a test failure.

Flying probe testing is a crucial technology in the electronics industry, ensuring the quality and functionality of electronic components and systems. These tests utilize specialized equipment to perform various contact and non-contact electrical tests on printed circuit boards (PCBs), printed wiring boards (PWBs), PCB assemblies (PCBAs), individual components, and entire systems. By providing a flexible and efficient method to identify defects and validate performance, flying probe testing has become an essential tool for manufacturers and engineers.