Analysis and research on the reliability of central air conditioner wire controller display

Central air conditioners are simpler, more beautiful and more elegant than traditional air conditioners, and they are perfectly integrated with home decoration. They are accepted by more and more people, and the installation volume increases year by year. With the popularization of central air conditioners, after-sales complaints caused by wire controller failures have increased significantly. Research on the reliability of wire controllers is of great significance to improving product quality and reducing maintenance costs.

1 Background

The air conditioner wire controller has multiple garbled display faults in the application (Figure 1). After power on, the entire screen is displayed, and the wire controller cannot be observed, but the function is normal and can be turned on and off and the mode is switched. Through in-depth analysis of the faulty parts, it was found that some of the wire controllers with garbled display faults can be eliminated after re-plugging the FPC and cleaning the connector with board cleaning water, while the garbled display fault of other parts cannot be eliminated.

Figure 1: Line control fault phenomenon

2 Analysis and solution of the cause of garbled display on the wire controller

From the perspective of returned faulty parts, there are mainly two situations where garbled characters are displayed: recoverable and unrecoverable. The following is an analysis of the causes of the two situations.

2.1 The garbled code failure is stable and cannot be recovered

① Chip abnormality: Some faulty products are normal after replacing the chips. The faulty chips are assembled on the normal wire controller to reproduce the fault. The chip 46 pin is short-circuited to the ground. After the chip is opened, it is found that there are obvious burn marks at the 46 pin, which is judged to be caused by EOS. The IC antistatic test results show that the antistatic ability of manufacturer A is up to 5000 V in human body mode and 2000 V in mechanical mode. After failure, they all show short circuit. One order starts from 2000 V and increases by 1000 V each time. When tested to 5000 V, a short circuit failure occurs. Another order is directly tested at 5000 V, and the pins are damaged. The test shows that the antistatic ability of manufacturer B IC is up to 8000V in human body mode (maximum output of the device is 8000 V) and up to 2000 V in mechanical mode, which is better than that of manufacturer A. For display abnormality caused by chip damage, the antistatic ability of the chip needs to be improved. At the same time, the production line strengthens anti-static control, checks the electrostatic protection during the installation of the wire controller, the use of ion fans, the operation of the production site, and isolates non-anti-static materials to reduce electrostatic damage to the chip.

Figure 2 QT2 characteristic curve of chip pin EOS damage and unsealed object

② LCD abnormality

The LCD of some wire controllers was replaced and it was normal. Analysis of the LCD found that the ITO (indium tin oxide) leads of the LCD were scratched.

Figure 3 LCD ITO lead scratches

ITO lead scratch improvement measures: a. The effect must be fully inspected at the glue coating post in the previous process. If there are glue scratches and impurities, they must be picked out and reworked immediately to ensure that each piece of glass is OK. b. Separate areas for good glued products and bad products. Good glued products to be exposed must be used up within 4 hours. Regular training is given to employees on the formal basket pulling and inserting techniques, glass picking and placing gestures and techniques. c. When pulling and inserting the basket, work in a grid order. It is not allowed to insert the basket from the middle of the glass, and only baskets can be used for transportation. d. Perform a secondary inspection of the glue coating effect before exposure. e. Confirm the pattern effect under the CCD lens every 2 batches during the development process. f. Improve the AOI electrical measurement business skills of employees. g. In the black film high-frequency test link, the synchronous delay is 2 seconds, and the naked eye can also identify and select the display defects to prevent missed inspections.

Figure 4 LCD ITO scratch improvement

The ITO leads of some LCDs were normal, but the short circuit alarm was detected during the electrical test, which was determined to be caused by the short circuit of conductive impurities between the LCD boxes. During the LCD production process, tiny conductive impurities fell into the box, causing poor conduction between the upper and lower parts. The conductive impurities are close to the thickness of the LCD box, and cannot be completely and effectively screened out under normal electrical test conditions.

Figure 5 Short circuit failure between LCD cells

Countermeasures to improve short circuit between boxes: a. Clean the turnover steel basket regularly, maintain and inspect the running parts of the equipment, and deal with and clean any wear and metal chips in time. b. Optimize test parameters, increase the electrical test, pin test and module test from 15 V to 30 V high voltage test.

Figure 6 Improvement of short circuit between LCD cells

2.2 The garbled display of the remote control can be restored to normal after simple processing

① After cleaning the LCD FPC (flexible printed circuit) connector, it returned to normal: This type of faulty product displays dark scratches when powered on, and the test function is normal. First, it is judged that the LCD fault is caused. The phenomenon is not eliminated after replacing the LCD, and the fault still exists after replacing the main chip, so the chip and LCD abnormalities are excluded. After cleaning the FPC connection pin seat, it is normal. The wire controller FPC connection pin seat uses a 0.5X40P (patch) type connector with small size, narrow spacing, and dense pins on the pin seat. The multimeter measures the short spacing between the two pins and determines that it is caused by foreign matter. With the widespread use of high-density and miniaturized terminals, the effective wall thickness of the insulator is getting thinner and thinner. For example, there is excess metal on the surface or inside of the insulator, tin beads and tin slag are generated on the board surface during the production process, and pollutants such as flux are damp, which will affect the connection equipment and cause short circuits, leakage and poor insulation. For connectors with reduced insulation performance, it is not recommended to apply glue to the connector to prevent conductive substances from being brought in during the application of glue. If glue is really needed, the cleanliness of the auxiliary materials and the method of glue application must be controlled, the table must be cleaned regularly and the brushes must be replaced. Automatic glue sprayers should be used to spray glue on PCBs.

Figure 7 Needle holder tin slag short circuit and PCB glue brushing

② The FPC was not assembled properly and was inserted crookedly, resulting in a short circuit between the two pins. The faulty FPC was obviously inserted crookedly, and the reinforcement plate was not parallel to the lower end of the needle seat. After re-inserting the FPC, it returned to normal. Feedback from the production process showed that the LCD FPC was soft and difficult to insert. At the same time, due to the high speed of the production line and the close color of the FPC and the needle seat, it was difficult to observe whether it was inserted in place. For this type of assembly abnormality, the LCD manufacturer was urged to extend the reinforcement plate at the FPC interface, and the extended part was not attached to the FPC, which was convenient for picking up during insertion. At the same time, a white silk screen line was added at the lower end of the FPC to facilitate alignment during insertion to prevent crooked insertion.

Figure 8 Extended FPC reinforcement plate

③ Reinstall the LCD FPC and it is normal: This type of problem accounts for the largest proportion of after-sales failures. The faulty parts returned to the power-on display with multiple strokes, the test function is normal, and no abnormalities are found in the measurement of the chip and LCD. The fault was not eliminated after cleaning the pin seat. Check that the FPC is assembled in place without skew. Remove the LCD and reinstall it. It is normal. X-ray observation of the matching of the wire controller FPC and the pin seat, no short circuit was found, the margin and offset are good, and there is no misalignment. However, the use of a multimeter to measure the main chip and the pin seat of the faulty wire controller found that the two pins were short-circuited. Preliminary judgment is that the reliability of the FPC and the connector is poor.

Figure 9 FPC connector terminal dimensions

1) Check the FPC connector terminal material used by the wire controller

Manufacturer A: Phosphor bronze, gold-plated on nickel base (with nickel strip); Reinforcement sheet: Phosphor bronze, tin-plated on copper base; Plastic shell: heat-resistant resin (LCP); Cover: heat-resistant resin (PA9T)

Manufacturer B: Phosphor bronze, gold-plated on nickel base (with nickel strip); Reinforcement sheet: Phosphor bronze, tin-plated on copper base; Plastic shell: heat-resistant resin (LCP); Cover: heat-resistant resin (LCP)

2) Check the dimensions of the FPC connector terminals

The comparison found that there was no significant difference in the connector materials and dimensions of manufacturers A and B, and the wired controllers using the connection terminals of the two manufacturers all displayed garbled characters, which was determined to be unrelated to the manufacturer's materials. After removing the glue on the bottom of the needle seat, there was still a short circuit between the two pins of the needle seat during the actual production test, which determined that the glue was not the main cause of the garbled characters on the wired controller.