1 Introduction

LED display has the advantages of high brightness, flexible installation, high efficiency and low consumption, which makes it widely used in fields such as relatively flexible display area and long viewing distance. At present, there are nearly a thousand LED display manufacturers in China, but less than a hundred have a certain production scale. In today's fierce market competition, users have higher and higher requirements for product quality.

The cost performance of products has become one of the most important factors in determining whether high-tech products can occupy a large market share. At present, consumers have paid more and more attention to the cost performance of products when choosing products, that is, whether the use value of the product is equivalent to the sales value of the product itself. Of course, the use value of the product depends on many factors, such as the technical indicators of the product, the functions provided by the product, the leading position in the market, and of course the life of the product.

For products, their life can be divided into economic life, technical life, service life and average life. The economic life of a product (also known as the value life) is the life determined based on the product's usage fee (including maintenance fees and depreciation fees), and usually refers to the product's service life on the basis of the lowest annual average cost of use; economic life It is used to determine the best depreciation period and the best renewal time of the product; the technical life is the period during which the product has technical value, that is, the entire experience period from the beginning of use of the product to its replacement by new technologically more advanced equipment , The length of technical life depends on the speed of invisible wear and tear of equipment.

In fact, the current technical life of LED display screens is generally only 3 years; the service life refers to the time from when the product is put into use to its complete failure. As an integrated system integrating LED display technology, visual principles, computer technology, video technology, and electronic technology, LED display can be divided into independent parts, and each part is replaceable, so in actual use , LED Mesh display life is quite long, and some even up to decades.

The average life (also known as the mean time between failures or MTBF) refers to the time elapsed between two failures, which is a statistical average value. The determination of the MTBF value usually adopts theoretical statistics and empirical statistics. Theoretical statistics It is the data obtained by accumulating and averaging according to the actual situation of devices, components and constraints. The empirical statistical method is based on the destructive records of the factory or laboratory, and the data obtained by accumulating and averaging. Regardless of the method used, the reliability of the product changes regularly. Taking the failure rate as an example, its change curve with time is like a bathtub, which is commonly referred to as the "bathtub curve", as shown in Figure 1. From the bathtub curve, it can be seen that the life cycle of a product can be divided into three stages: early life cycle; useful life cycle; wear-out life cycle.



Figure 1 The "bathtub curve" of product failure

The figure above shows the famous "bathtub" curve, the left sloping part is the early failure rate, its failure rate is generally high and declines quickly as time goes by. The middle part of the curve is the service life, and its failure rate is generally very low and basically fixed. The far right part is the attrition period, and the failure rate increases rapidly. Electronic product manufacturers generally eliminate early failures as much as possible through testing, aging, screening, etc., and then provide them to customers for use. When the service life is about to end, the product is about to enter a period of high incidence of failure, and needs to be scrapped or replaced.

2. The impact of environmental factors on product life

2.1 Effect of temperature on product life

As we all know, any product has a low failure rate within its service life and only under suitable working conditions. As an integrated electronic product, the LED display is mainly composed of a control board with electronic components, a switching power supply, and a light emitting diode. components, etc., and the life of all of them is closely related to the operating temperature. If the actual working temperature exceeds the specified range of use of the product, not only the service life will be shortened, but the product itself will also be seriously damaged.

Taking a capacitor as an example, without considering factors such as ripple, frequency, ESR, etc., the estimated life of a capacitor can be expressed by the following formula:



Among them, L0 represents the lifetime at the maximum operating temperature, Tmax represents the maximum operating temperature, and Ta represents the actual ambient temperature. It can be seen that if the ambient temperature increases by 10°C, the life of the capacitor will be doubled.



Figure 2 Relationship between capacitor life and ambient temperature

It can be clearly seen from the curve on the right side of the above figure that as the working environment temperature of the capacitor rises, its effective life shortens sharply. Among them, the effective life refers to the time when the capacitor reaches a given failure rate. For electronic components, its failure rate is more affected by temperature. According to calculations, when the temperature increases by 10°C, its failure rate increases by 100 times, that is, the average life expectancy decreases by 10 times. Therefore, how to make the product work at a suitable temperature is crucial to the product life. At present, the most commonly used method is to adopt various heat dissipation methods and maintain ventilation inside and outside the display screen.

2.2 Influence of dust on product life

In order to maximize the average life of the LED display, the threat of dust cannot be ignored. Working in a dusty environment, because the printed board absorbs dust, and the deposition of dust will affect the heat dissipation of electronic components, which will cause the temperature of the components to rise, which will lead to a decrease in thermal stability and even leakage, and in severe cases, it will cause burnout . In addition, dust can also absorb moisture, corrode electronic circuits, and cause short-circuit failures. Although dust is small in size, its harm to products cannot be underestimated. Therefore, it is necessary to clean it regularly to reduce the probability of failure. When cleaning the dust inside the display, remember to disconnect the power and operate with care.

2.3 Effect of humidity on product life

Although almost all LED displays can work normally in an environment with a humidity of 95%, humidity is still an important factor affecting product life. Moisture gas will enter the interior of the IC device through the bonding surface of the packaging material and components, causing the internal circuit to oxidize and corrode the circuit breaker, and the high temperature during the assembly and welding process will cause the moisture gas entering the IC to expand and generate pressure, causing the plastic from Internal separation (delamination) on the chip or lead frame, wire bond damage, chip damage, internal cracks and cracks extending to the surface of the component, or even component bulging and popping, aka "popcorn", which will lead to assembly Parts are repaired or even scrapped. What's more important is that those invisible and potential defects will be integrated into the product, causing problems in the reliability of the product. Reliability improvements in wet environments include the use of moisture barrier materials, dehumidifiers, protective coatings/covers, etc.

2.4 Influence of corrosive gases on product life

Humidity and saline air environments can cause degradation of system performance, as they can exacerbate the corrosion effects of metal parts, and also favor the generation of primary cells, especially when dissimilar metals are in contact. Another detrimental effect of moisture and salty air is the formation of films on the surface of non-metallic components that degrade the insulating and dielectric properties of these materials, thereby creating leakage paths. Moisture absorption by insulating materials can also cause an increase in the material's bulk conductivity and dissipation coefficient. Reliability improvements in humid and salt air environments include the use of hermetic seals, moisture barrier materials, dehumidifiers, protective coatings/covers, and the reduction of dissimilar metals.

2.5 Effect of electromagnetic radiation on product life

The interference of radio frequency radiation to electronic systems generally comes from two ways. One avenue is the direct infiltration of electrical noise disturbances from the radiated field into the system. Experiments have shown that when the field strength reaches 5V/m, the system will definitely go wrong, and the electromagnetic interference is enough to change the value of the CPU program counter PC, making the microcomputer mistakenly "jump out" of the program being executed, especially for small signal circuits. However, the memory cannot work normally when the field strength is 15V/m. Another way for radio frequency radiation interference is introduced through the power supply. The transmission line on the outer frame is equivalent to the receiving antenna, which introduces radiation interference into the system. When the interference is severe, it can burn the power supply of the system itself.

Electromagnetic radiation also interferes with video and communication signals. Television stations with a frequency close to radar radiation suffer the most serious image and audio interference. Radiation interference signals enter from the TV antenna and are processed together with the TV signal to form interference. On other channels, when the radiation field strength is high, direct detection or interference can be formed. Another reason why the radiation field interferes with the TV set and affects its reliability, so that it burns out is that the radiation interference enters through the power line. Due to the re-radiation effect of the power line, the radiation field strength of the external space is as high as 40-50V/m instantaneously, so that the interference pulse entering the power line and the induction effect of the space field will cause the circuit with weak anti-interference ability in the TV to malfunction. . Often, reducing the effects of interference requires filtering and shielding and the selection of less sensitive components and circuits.

2.6 Impact of vibration on product life
Electronic equipment is often subjected to environmental shock and vibration during normal use and testing. This environment can cause physical damage to components and structural parts when the mechanical stress caused by the resulting deflection exceeds the allowable operating stress of the component.

The natural frequency of electronic products is an important parameter that must be considered in the design process, because if the natural frequency is within the vibration frequency range, resonance may occur, which will greatly increase the deflection of the subsystem and cause the stress to exceed the safety line. Vibration environments can be particularly harsh on electrical connectors because it can induce relative motion between connector components. This movement combined with other environmental stresses can produce fretting corrosion. This can generate wear debris, causing large changes in contact resistance. Reliability improvement methods for vibration stress include using stiff materials, controlling resonance and reducing free motion.

2.7 Effect of load on product life

Regardless of whether it is an integrated chip or an LED tube or a switching power supply, whether it works under the rated load or not, the load is also an important factor affecting the life. Because any component has a fatigue damage period, take the power supply as an example, the brand power supply can output 105%~135% power, but if the power supply is operated under such a high load for a long time, it will inevitably accelerate the aging of the switching power supply. Of course, the switching power supply does not necessarily fail immediately, but it will reduce its life rapidly.

Summary:

It can be seen from the influence of the above-mentioned various environmental factors on the product reliability index that each environmental factor experienced by the product during the life cycle requires consideration in the design process. This ensures that sufficient environmental strength is incorporated into the reliability design. Of course, improving the use environment of the product and regular maintenance of the product can not only eliminate hidden troubles in time, but also help improve the reliability of the product and prolong the average life of the product.