With the rapid development of the PCB industry, especially with the growing demand for high-frequency, high-speed circuits, precision components, and multilayer PCB, quality control in the production process has become increasingly crucial. Testing equipment, as a key tool to ensure production quality and product yield, has also seen continuous innovation and improvement. Against this backdrop, the demands on testing equipment have expanded beyond stability and efficiency, now emphasizing precision and reliability under complex process conditions.
However, false open issues during flying probe testing remain a common and unavoidable problem, particularly when there are more than 10 instances of false opens, which not only affects the accuracy of test results but also severely impacts production schedules. Therefore, production staff and testers must pay close attention, performing a comprehensive analysis of the equipment, process data, PCB products, and other related factors to identify the root cause and implement effective solutions.
This article takes the Seica series flying probe testing equipment as an example, drawing from the author’s actual experience in equipment maintenance and process feedback analysis. It explores the causes behind frequent false open issues during flying probe testing and suggests corresponding solutions. Through an in-depth analysis of this issue, the article aims to provide valuable insights for industry peers and promote further optimization and improvement of testing processes.
Equipment Malfunctions
Evaluating Equipment Accuracy
If you suspect that the false open issue is caused by a malfunction in the testing equipment, it is best to perform an inspection. The most direct method is to test with previously qualified original data and corresponding qualified PCB (ensure the surface is free of oxidation, etc.). If the false open issue still persists, it is reasonable to suspect that the equipment may be at fault. On the other hand, if the test is normal, the problem is likely to lie with the process data or the PCB.
Software Maintenance
First, use the self-diagnostic program provided by the equipment to perform a comprehensive check to confirm if there are any damaged components or system malfunctions. If any damaged parts or error messages are detected, they should be immediately replaced and repaired, and the necessary calibration should be carried out according to the results of the self-test.
Next, use DMC software to check whether the feedback system for each axis is functioning properly. The specific steps are as follows:
- Minimize the test system interface.
- Open the DMC program (via Start\Program\DMC directory or desktop shortcut).
- In the DMC interface, press the emergency stop switch.
- Manually move each axis and observe the changes in the digital readings and sensitivity of the grating feedback system to ensure they fall within the specified range.
- Run the TESTXY.DMC and TESTZ.DMC programs to check if each axis can return to the zero position (i.e., axis position reading shows “0”).
Hardware Failures
Hardware malfunctions are more common in testing equipment, especially due to the operational environment (e.g., temperature and humidity) and the wear and tear from long-term usage. The following hardware components are particularly prone to issues:
- Z-Axis Linear Motor
Due to the long-term and high-frequency operation of the linear motor, dirt can accumulate on the moving parts, causing uneven movement, increased load, and even precision issues.
To avoid this, it is recommended to dismantle the linear motor every six months and clean it with anhydrous alcohol. Care should be taken during operation to prevent the dislodging of the guide rail’s ball bearings.
- Grating Scale
The grating scale is a core component of high-precision equipment, and its accuracy directly impacts the stability of the equipment. However, it can be contaminated due to environmental factors or poor-quality air supply, leading to unstable feedback signals and false open issues.
To maintain the proper function of the grating scale, it should be regularly cleaned with anhydrous ethanol. Use a light, single-directional wiping motion to clean the grating scale, avoiding excessive force to prevent scratches. Additionally, ensure that the air supply is adequately dried, filtered for oil, and filtered for water to maintain the long-term stability of the equipment.
- Grating Data Feedback Connector
Since flying probe tests are often performed at high speeds, this can cause strong vibrations in the equipment, which may lead to poor contact between the grating data feedback connector and its socket.
As this is an inherent issue, there is no particularly effective solution. It is recommended to check all connectors before powering on the equipment to ensure stable contact.
- L-Type Probes
The quality of L-type probes also directly affects false open issues, mainly due to probe tip dulling or poor contact with the connector.
It is recommended to perform automatic calibration at least once a week. If the probe tip becomes dull, it should be replaced promptly, and the usage count should be reset. Regularly check the tightness of the connection between the probe and the connector to ensure no loosening occurs.
Process Data Errors
Sometimes, errors occur during the initial generation of process data. For example, when production personnel convert CAM data, issues may arise in the generated netlist file, typically due to inconsistencies between holes and pads on different layers or sides. This inconsistency prevents the testing equipment from correctly identifying the connections, resulting in false open judgments.
Therefore, when encountering false open issues, production personnel need to carefully and repeatedly review the relevant data files to ensure consistency and accuracy in all process data.
PCB Issues
If the testing equipment and process data have been ruled out as causes, the problem is likely related to the PCB itself. Common PCB issues include warping, solder mask problems, and incorrect character printing.
Warping Issues
During PCB manufacturing process, some production staff may skip the hot air leveling process to save time, sending the boards directly to the final inspection. This can result in PCB warping that exceeds the testing equipment’s tolerance range, thus affecting the test’s accuracy.
To avoid this, the hot air leveling process should never be skipped. Additionally, testing personnel should check the PCB’s warpage before testing to ensure it falls within the equipment’s allowable limits.
Solder Mask Issues
In some cases, particularly with severe open issues, the conductive holes on the PCB may be partially or completely covered by the solder mask, leading to false open judgments during flying probe testing.
To prevent this, it is best to avoid testing through via holes, or to perform appropriate treatment on the solder mask, such as localized removal and thickness control, to ensure that the hole’s conductivity is unaffected.
Character Printing Issues
Many PCB manufacturers print characters before testing. However, if the character print is misaligned or the film accuracy is insufficient, small surface-mount components or small holes (especially those with a diameter of Φ<0.5) may be covered by the characters, affecting the test results.
To address this, it is recommended to adopt a “test first, then print characters” process flow for high-density surface-mount PCB, small holes, and fine lines to avoid interference from the character printing and improve test accuracy.
Ending
The causes of false open issues in flying probe testing are varied, but they generally fall into the three categories discussed above. However, different situations may manifest different forms of false open problems, so troubleshooting must be approached with flexibility and a comprehensive analysis based on specific circumstances. By carefully identifying the root causes and taking appropriate measures, these issues can be effectively mitigated, leading to improved accuracy and efficiency in flying probe testing.
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