(1) Packaging materials, PPA (polyphthalamide), epoxy resin, and the difference in thermal expansion coefficients of metals will cause the LED leads to open when the ambient temperature changes or the temperature rises when the tube is working, resulting in a dead light, which is LED The "life gate" of the outdoor surface mount display. To be precise, this is only a part of the "fate gate", perhaps not the most critical part of the current outdoor SMD LED display reliability problem. In fact, the thermal expansion coefficient of the packaging material and the lead metal are different. The internal stress of the LED caused by thermal expansion and contraction is a problem that all kinds of LED tubes have. After more than ten years of research and development in the industry, it has been obtained to a large extent. solve. As we all know, the packaging material of the direct plug-in lamp is also epoxy resin, and the lead wire is also gold wire. At present, such as 346, 546 and other classic outdoor LED direct plug-in lamps, the phenomenon of dead light caused by the difference in the thermal expansion coefficient of the lead is very rare. , It can be said that the problem has been basically solved. It can be seen that packaging manufacturers can effectively solve this "fate gate" by taking measures in material technology. In fact, most packaging manufacturers producing SMDLEDs have already taken measures and achieved results. Routine tests such as high and low temperature storage, high and low temperature impact, humidity test, mechanical vibration and impact that must be carried out in the pre-factory inspection of SMDLED products can effectively test this kind of reliability problems.
(2) The influence of the difference in thermal expansion coefficient of outdoor SMDLED packaging materials may be more affecting the sealing performance of the device. First of all, whether it is a SMDLED or a direct plug-in lamp, the thermal expansion coefficient of the metal lead (pin) is different from that of the resin. In the case of cold and heat changes, small gaps will appear at their interface, which will cause cracks over time. The packaging itself such as epoxy resin may also have defects due to defects in the process and its compactness. In this way, water vapor or other harmful gases may invade and cause harm. PPA material is relatively hygroscopic, and its wet stress can also cause sealing defects in a humid environment. In this regard, we must find a solution in terms of materials and packaging technology. The production of outdoor display screens generally adopts the method of sealing glue, which can effectively solve this problem. Unfortunately, for outdoor SMD LED displays, the effect of sealing glue is limited. As we all know, the in-line lamp is sealed from the PCB board up to the height of the cap, and part of the cap is sealed, that is to say, the pin outlet is submerged in the sealant, as long as the sealant is ensured A good connection with the tube packaging material can solve the sealing problem caused by the difference in thermal expansion coefficient between the metal lead and the packaging resin.
(3) Similarly, for the hazards of harmful gases such as corrosive gases, salt spray, etc., the resistance of outdoor SMDLED displays is much worse than that of in-line lamp displays. In fact, well-known domestic and foreign manufacturers are very cautious and stable in launching outdoor SMDLED devices and screen products. Although indoor SMDLEDs have been widely used, some manufacturers still have not launched outdoor SMDLED products. Of course There is his reason. Some manufacturers still use specially designed protective covers or other protective measures, such as integral glue filling, even though they have adopted SMDLED devices suitable for outdoor use when making outdoor SMDLED displays. This of course increases the cost and loses some light energy output, but the reliability is more guaranteed.
(4) The biggest difference between the production of SMDLED screens and the direct plug-in led tube display is that the direct plug-in lamp is assembled and soldered by wave soldering or manual soldering, while SMDLED must be reflow soldered. Wave soldering is when the components are inserted and the PCB board is passed through the molten solder pool. At this time, it is the PCB board and the pins of the components that are heated. For the LED tube, the hot spot is the contact solder point between the lead and the PCB. The distance from the PN junction of the LED is generally about 10mm. Reflow soldering is completely different. It heats the entire device. In the preheating stage, the entire device is heated to 150~200oC, and the temperature during soldering reaches 240~260oC. In this way, the heating of the LED is more than that of a direct plug-in lamp. Much more severe.
(5) The humidity sensitivity level of components is extremely important for the preparation before reflow soldering. The SMDLED and plastic packaged components used in the display screen, such as chip resistors and chip capacitors, are all non-sealed solid surface mount components. In the process before they are stored or reflow soldering, if they absorb moisture, when such packages are exposed to the high temperature of reflow soldering, the vapor pressure inside the package will increase significantly. Under certain conditions, the pressure will increase. It causes internal delamination of the packaging material to separate from the die and/or lead frame/base, or internal cracks that do not extend to the outside of the package, or bonding damage, narrowing of metal lines, bonding warping, and thin film Cracks, or notches under the bond, etc., in the most serious case, this stress will cause cracks on the outside of the package, a phenomenon called "popcorn".
Not only SMDLED, all SMD devices must be attached great importance to reflow soldering. If the components do not meet the humidity-sensitive conditions, (plastic components are of this type) must ensure that there is no moisture intrusion, or baking before soldering Otherwise, delamination will occur during reflow soldering. Damaged components.
(6) Whether it is an indoor LED or an outdoor LED, regardless of the package, in-line or surface mount, electrostatic discharge will cause damage. Moreover, electrostatic damage is concealed, latent, random, and cumulative, which has a great impact on the reliability and life of the display. Only by taking adequate electrostatic protection measures in every link of production, transportation, storage, and processing, can the long-term safe and effective use of the device be ensured.
(7) For outdoor LED displays, whether they are in-line lamps or SMD LED displays, how to resist ultraviolet rays is an important issue. Ultraviolet rays can cause changes in the performance of semiconductor materials and reduce the luminous efficiency of LEDs. At the same time, the packaging materials are aged, the air tightness and transparency are reduced, the luminous ability of the LED is reduced, the brightness of the screen is reduced, and the transparency of the epoxy resin is reduced, not only the light output efficiency is reduced, the brightness is reduced, and more light is absorbed. It also causes the junction temperature to rise, which ultimately affects the life of the tube. The solution is to use packaging materials and glue with good UV resistance.
(8) Realizing and improving the reliability of LED displays is a system engineering issue that involves all aspects of the enterprise. The reliability of LED tubes and SMDled chips is the most important element. In addition, good design, production, installation, use and maintenance are also required. In the final analysis, reliability engineering is a concentrated expression of the company's comprehensive strength. As for outdoor SMD LED displays, as a new variety of LED displays, its superiority and development prospects are beyond doubt. But after all, the history is relatively short, the experience is limited, and the accumulated data is not much. In order to continuously improve the performance and reliability of outdoor SMD LED displays, it is necessary for the whole industry, upstream and downstream manufacturers to work together to form a good interaction, complement each other, and seek common development. . The most urgent task is to formulate relevant standards. This has become the consensus of entrepreneurs and experts in the industry. The industry association has also begun to entrust influential manufacturers in the industry to draft standards. It will soon enter the review and approval stage, and hope to get more attention and support from enterprises and experts.
