How to improve the reliability of outdoor SMDLED displays

In recent years, the outdoor label display has great advantages in terms of light mixing effect and installation process due to its outdoor display relative to the lamp (346, 546, etc.), so the demand has increased greatly and also appeared. Many quality problems have caused quality disputes to occur from time to time. Therefore, the author believes that it is imperative to organize, research, and analyze the reliability of outdoor SMDLEDs and outdoor display screens made by SMDLEDs, to gather ideas, to summarize the relevant specifications for manufacturing, inspection, and use, and to formulate relevant standards. From the SMDLED package structure, raw materials and process, the reliability of SMDLED is analyzed, and constructive improvement suggestions are put forward, which deserves the attention of relevant companies in the industry. For outdoor SMDLED and outdoor SMDLED display reliability issues, the first LED network should also consider the following issues:

(1) Differences in the coefficient of thermal expansion of the encapsulating material, PPA (polyphthalamide), epoxy resin, metal, causing the LED lead to open when the ambient temperature changes or the temperature rises during the operation of the tube, thereby causing the dead light, that is, the LED The “life gate” of the outdoor surface mount display is located. Rather, it's just a part of the “life gate” and perhaps not the most critical part of the current outdoor SMD LED display reliability issue. In fact, the thermal expansion coefficient of the package material and the lead metal is different. The problem of internal stress of the LED caused by thermal expansion and contraction is a problem of various LED tubes. After more than ten years of research and development in the industry, it has been largely obtained. solve. As we all know, the packaging material of the in-line lamp is also epoxy resin, and the lead wire is also gold wire. At present, the classic outdoor LED in-line lamp such as 346, 546 is very rare due to the difference in thermal expansion coefficient of the lead wire. It can be said that the problem has been basically solved. It can be seen that the packaging manufacturer takes measures in the material process to effectively solve this "life gate". In fact, most packaging manufacturers that produce SMDLEDs have taken measures and achieved results. Routine tests such as high and low temperature storage, high and low temperature impact, humidity test, mechanical shock and impact, which are to be carried out in the pre-shipment inspection of SMDLED products, can effectively test this type of reliability problem.

(2) The influence of the difference in thermal expansion coefficient of outdoor SMD LED packaging materials may affect the sealing of the device more. First of all, whether it is SMDLED or in-line lamp, its metal lead (pin) and resin have different thermal expansion coefficients. In the case of cold and heat changes, their interfaces will have small gaps, and cracks will occur over time. The package itself such as epoxy resin may have defects due to defects in the process, so that water vapor or other harmful gases may invade and cause damage. PPA materials have relatively high hygroscopicity, and their wet stress can also cause sealing defects in humid environments. In this regard, solutions must be found in terms of materials and packaging processes. The production of outdoor display screens, the general use of sealing methods, can effectively solve this problem. Unfortunately, for outdoor SMDLED displays, the method of sealing is limited. As we all know, the in-line lamp is sealed from the PCB board, always sealed to the height of the cap, and some of the caps are sealed, that is, the pin outlet is submerged in the sealant, as long as the sealant is guaranteed. The good connection with the tube encapsulating material can solve the sealing problem caused by the difference in thermal expansion coefficient between the metal lead and the encapsulating resin.

But the problem with outdoor SMFLED displays is bigger. The difficulty of SMDLED is that in addition to the lead problem, the gap between the material of its light-emitting window (mostly epoxy) and the material of the outer casing (using PPA) can also cause poor sealing; first they are The difference in thermal expansion coefficient is second, and the contact between the two materials itself is not good enough, so that the temperature change inevitably produces a fine gap, resulting in poor sealing. To this end, a good sealing glue is used to improve the contact. Obviously, the material contact surface of different materials at the SMDLED exit window is much larger than the lead outlet, so the potential for this potential leak is much greater. Although the outdoor SMDLED screen can also be sealed by the sealing method, the light-emitting window can not be potted, which makes the outdoor SMDLED display seal much less sealed than the in-line lamp panel.

(C) Similarly, for harmful gases such as corrosive gases, salt sprays, etc., the outdoor SMDLED display is also much more resistant than the in-line lamp display. In fact, well-known domestic and foreign manufacturers are very cautious and steadfast in the introduction of outdoor SMDLED devices and screen products. Some manufacturers are still not launching SMDLED products for outdoor use despite the widespread use of indoor SMDLEDs. There are reasons for him. Other manufacturers have used specially designed protective covers or other protective measures, such as integral glue filling, in the production of outdoor SMDLED displays, although SMD LED devices for outdoor use have been used. This of course increases the cost and loses some of the light energy output, but the reliability is more guaranteed.

At present, many packaging manufacturers at home and abroad have introduced outdoor SMD LED device products. This is the result of product development and technological advancement. Responsible manufacturers have done a lot of work in reliability. After necessary and sufficient test verification, some are still This declaration and obtained a number of patents and awards. On the other hand, its maturity is still to be confirmed by more applications.

However, the author seems that the problem of sealing can not be taken lightly. The application of outdoor SMDLED products for outdoor display should still be very cautious. It should be detailed in understanding the working environment and working conditions, take adequate protective measures, and carefully design carefully. In situations where there is little control, such as where reliability requirements are particularly high or the environment is particularly harsh, or where corrosive gases are present, it is better to take more measures or avoid using SMDLEDs.

(4) The biggest difference between the production of SMDLED screen and the in-line LED tube display is that the welding of the in-line lamp is wave soldering or manual hand soldering, and the SMD LED must be reflow soldered. Wave soldering is to insert the PCB board into the molten solder bath after the components are inserted. At this time, the pins of the PCB board and components are heated. In the case of LED tubes, the solder joints of the lead wires and the PCB are exposed by hot spots. At the same time, the PN junction of the LED is generally about 10mm. The reflow soldering is completely different. It is heated by the whole device. During the preheating stage, the whole device is heated to 150~200oC, and the soldering temperature is 240~260oC, so the LED is heated compared to the in-line lamp. More severe. In order to ensure the quality of reflow soldering and to ensure the safety and reliability of SMDLED and other components, manufacturers will provide a temperature distribution curve for reflow soldering in the technical conditions of SMDLED products, that is, in the preheating phase, the thermal insulation phase, the reflow phase and cooling. The temperature distribution requirements of the stage, including the temperature rise rate control, the maximum welding temperature, the maximum time to maintain a certain welding temperature, and the cooling rate and other important parameters, the technical conditions will also generally emphasize that the curve must be adjusted according to the curve The temperature and the time of reflow soldering may otherwise damage or damage the SMDLED device. Therefore, screen manufacturers should control the temperature distribution characteristics of reflow soldering according to this temperature profile and solder flux conditions. It should be specially pointed out that the temperature control curves of different types of SMDLEDs of different manufacturers are different. Screen manufacturers should adjust the reflow soldering temperature control process according to the SMDLED used, so as to avoid damage or damage to the device, or The safety hazard, which was not immediately apparent, was turned into a dead light after the screen was running for a while.

(5) The humidity sensitivity level of the components is extremely important for the preparation work before reflow soldering. SMDLEDs used in displays and various components in plastic packages such as chip resistors and chip capacitors are unsealed solid surface mount components. In the process before storage or reflow soldering, if moisture is absorbed, when such a package is exposed to the high temperature of reflow soldering, the vapor pressure in the package is greatly increased. Under certain conditions, the pressure will be Causes internal delamination of the encapsulating material to separate from the die and/or leadframe/base, or internal cracks that do not extend outside the package, or bond damage, metal line narrowing, bonding lift, film In the most serious case, the crack, or the undercut of the bond, causes damage to the outer crack of the package, which is called "popcorn". The device is immediately damaged and may cause damage to other components or equipment. JEDEC stipulates that the moisture sensitivity grade is divided into 8 grades. Before the reflow soldering, the device must be treated to eliminate moisture according to its grade and environmental conditions. For the sealed package components, the unpacking time exceeds a certain time (floor life). Baking and dehumidification, baking conditions and baking time are also specified (see JEDEC "Standards for Humidity/Reflow Resistance Sensitivity for Unsealed Solid Surface Mount Components" IPC J-STD020D.1). Therefore, the storage and pre-weld preparation requirements of SMDLED devices are very strict. SMDLED must be sealed in the package. The moisture sensitivity level of the product is indicated on the package. The moisture absorption agent is also included in the package. The shelf life of the sealed package and moisture absorber is one year. The quality of SMDLED stored for more than one year has been Can't guarantee, can't use. Therefore, SMDLED cannot be stored in the warehouse for too long. At the same time, after the SMDLED is taken out of the sealed package, it should be reflowed within 24 hours, otherwise it must be baked through the constant humidity oven.

Not only SMDLED, reflow soldering of SMD devices must be highly valued. If the components do not meet the humidity sensitivity conditions (all types of plastic components), it must be ensured that there is no moisture intrusion or baking before soldering. Otherwise, delamination will occur during reflow soldering. Damage to components.

(6) Whether it is LED for indoor use or LED for outdoor use, it does not matter what kind of package, in-line type or surface mount type, electrostatic discharge will cause damage. Moreover, electrostatic damage has concealment, potential, randomness and accumulation, which has a great impact on the reliability and life of the display. Only in the production, transportation, storage, processing of each step to take adequate electrostatic protection measures, in order to ensure the long-term safe and effective use of the device.

(7) LED display for outdoor use, whether it is a direct-inserted lamp or an SMDLED display, how to resist UV damage 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 material is aged, the airtightness and transparency are lowered, the luminous ability of the LED is lowered, the brightness of the screen is lowered, and the transparency of the epoxy resin is lowered, not only the light-emitting efficiency is lowered, but the brightness is lowered, 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 display screens is a system engineering problem involving all aspects of the enterprise. The reliability of LED tubes and SMD LED chips is the most important factor. In addition, good design, production, installation, use and maintenance are required. Reliability engineering is the concentrated expression of the company's comprehensive strength. As for the outdoor SMDLED display, as a new variety of LED display, its superiority and development prospects are beyond doubt. But after all, history is short, experience is limited, and accumulated data is not much. In order to continuously improve the performance and reliability of outdoor SMDLED display, 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. . It is imperative to develop relevant standards. This has become the consensus of entrepreneurs and experts in the industry. The industry associations have also entrusted influential manufacturers in the industry to draft standards. They will soon enter the review and approval stage, and hope to receive more attention and support from enterprises and experts.