PCB Technical - PCB circuit board flying probe test false open circuit

With the rapid development of the PCB industry, testing equipment continues to be technically updated, and the market has increasingly higher requirements for the stability and high efficiency of testing equipment. In order to make the test equipment as stable as possible, people play a pivotal role in ensuring that the test equipment is stable, reliable and efficient. During the testing process, false open circuits will inevitably occur, especially when there are many false open circuits (≥10 places), operators and process personnel should pay attention to them, and analyze from the aspects of equipment, process data and printed circuit board products. And resolved. In the actual work of equipment maintenance and related equipment process feedback analysis, this article takes the Italian Seica series PCB flying probe test as an example, and summarizes the reasons and solutions for the frequent false open circuit of PCB flying probe test equipment. For your reference and discussion.

1. Determine whether it is caused by unstable equipment

The easiest way to judge that the equipment is working properly: use the old file data that has passed the test and the corresponding qualified PCB board (removal of surface oxides, etc.) for testing. If there is still an open circuit, it should be equipment failure, otherwise it should be process data Or the problem of the printed circuit board to be tested. When the device has problems, use the following methods to check and analyze.

1. Software maintenance

First, use the equipment maintenance (also called self-check) software to run according to the prompts to see if any damaged parts or errors are found: If there are damaged parts or errors, you should replace the corresponding parts and correct them according to the error prompts.

Secondly, use the DMC software to check whether the feedback system of each axis is working properly. The correct operation steps are: minimize the test system - open DMC under the (Start\\Program\\DMC) program or on the desktop (the TEST of DMC appears) Interface) - press the emergency stop switch - move each axis by hand, observe the digital change and sensitivity of the grating feedback system of each axis, and whether the number changes within the specified range; then activate TESTXY.DMC and TESTZ.DMC-RUN to observe each Whether the axis can return to the zero position (the position of each corresponding axis is read as '0').

2. Hardware failure

Hardware failure is more likely to occur in all failures, because the quality of the test system is closely related to the working environment (temperature, humidity, etc.) of the test equipment, the length of the working time, maintenance and other factors. According to my summary, the hardware failure There are Z-axis linear motors, grating rulers, grating feedback data line adapters and L-shaped probes.

(1) Z-axis linear motor: Due to the long-term and high-frequency operation of the linear motor, it is easy to blacken the moving parts of the motor and produce black scale, resulting in inflexible up and down and increased motor load. Therefore, the linear motor should be removed regularly (about 6 months) and cleaned with anhydrous alcohol. Be careful when removing and cleaning the guide rail ball.

(2) Grating ruler: Grating is the core component of all high-precision equipment positioning. The quality of the grating is directly related to the accuracy and stability of the equipment. However, most of the gratings are caused by bad environment or poor air source. The workshop environment may make the surface of the grating ruler have too much dust. The dust directly affects the feedback signal, resulting in many open circuits. In order to remove the dust on the grating ruler, it should be cleaned with absolute ethanol. Note that when cleaning, use fine gauze gloves (dipped in a little absolute alcohol) to gently clean in one direction. Do not scrub back and forth and use excessive force (to prevent scratching the grating) ; At the same time, the air source is required to enter the equipment after drying, filtering oil, and filtering water, otherwise it will affect the service life and measurement accuracy of the equipment.

(3) Raster data feedback line adapter: PCB flying probe test equipment moves faster, once the equipment is in working condition, there will be strong jitter. Therefore, the grating data cable connector may generally have poor contact with the socket after a period of use due to inertia. In order to avoid this situation, check the plugs before turning on the device every day before turning on the device.

(4) L-shaped probe: The quality of the stylus is also one of the important factors that cause the open circuit. The stylus is mainly manifested by the dullness of the needle tip, the poor contact between the needle and the needle plug, and the regular needle calibration (at least once per week). When the stylus is passivated, the stylus must be replaced. After changing the needle, remember to reset the number of uses to zero. Check whether the needle and the needle are loose at any time, and ensure that the stylus is automatically calibrated at least once a week.

2. Process data conversion

The process data of the new file is wrong when it is first generated, which is also the cause of the open circuit. Many process personnel have errors in the generated network diagram file when converting the CAM data. Most of the cases belong to the hole or pad attributes of each layer and surface. Inconsistent. Therefore, the process personnel are required to review the data file repeatedly when it appears.

Third, the printed circuit board product problem

If the test equipment and process data are excluded, the other situation should be a problem with the PCB product itself, which is mainly manifested in warpage, solder mask, and irregular characters.

(1) Warpage: In order to hurry, some production planners often dispense with the hot air leveling process and send them directly to the final inspection. If the product is not subjected to heat leveling, the product warpage is greater than the allowable range of the test equipment. Therefore, the process of heat leveling cannot be omitted, and at the same time, inspection and test personnel are required to add warpage measurement before the test.

(2) Solder mask: often products with relatively severe open circuits will have unsatisfactory results because part of the through holes are blocked by the solder mask. During the test, try to avoid the transfer holes (or ensure that the holes are conductive). Pass without error) test.

(3) Characters: Many PCB manufacturers will print characters first and then electronically measure them. As long as the position of the characters is slightly offset or the character negative is not accurate enough, the thin surface stickers and small holes may be partially covered by the characters. Therefore, in order to avoid open circuits caused by characters, it is more reasonable that a printed circuit board with fine surface mounts, small holes (Φ<0.5), and high density of fine lines should be electrically tested before the characters.

There are many reasons for the false open circuit of PCB electrical test, but the general situation is not different from the above three situations. In order to eliminate the problem as soon as possible, a specific and comprehensive analysis should be carried out according to the specific situation to improve efficiency.