With the current widespread application of LED displays, SMD LED displays have firmly established their position in the market. After being used for a period of time, SMD LED displays may experience dead lights and soldering failures. This period can range from a few months to several years, or it could be that such problems occurred during the production process by the LED display manufacturer. It can be said that dead lights and soldering failures have always been inevitable issues for LED display manufacturers in the packaging of SMD components. Analyzing from the perspective of the light bulbs, there are the following several reasons:
1.The failure of LED electronic screens to light up is related to the LED light source. The five major materials of the LED light source are money, chips, supports, phosphors, bonding, and encapsulation glue. If any of these components has a problem, it may cause the LED display to have dead lights.
2. Gold wire has the characteristics of high conductivity, good thermal conductivity, corrosion resistance, excellent chemical stability and good toughness. However, the price of gold wire is relatively high, which leads to the packaging cost being several times or even dozens of times higher than that of other materials.
3. During the drawing process of gold wire, defects will appear on the surface of the wire material. If these defects exceed 5% of the wire diameter, such as scratches, abrasions, protrusions, cracks, pits and folds, they will cause an increase in current density, making the damaged part prone to being burned, and at the same time reducing the ability to resist mechanical stress, resulting in the fracture of the internal lead connection point. In addition, if the surface of the gold wire is contaminated with rust, oil stains and dust, it will reduce the bonding strength between the gold wire and the LED chip, as well as between the gold wire and the support, causing the failure of the light-emitting diode.
4. The anti-static ability of LED light beads is related to the LED light-emitting chips, and has little or no significant impact on the packaging process and materials. If the LED electronic screen is affected by static electricity, it may cause dead lights. That is to say, the static influence on LED light beads is related to the distance between the two pins of the LED light bead. If the distance between the two electrodes of the bare crystal of the LED chip is very small, generally less than 100 micrometers, while the LED pins are 2mm. When the static charge needs to be transferred, the larger the distance, the easier it is to form a large potential difference, that is, a high voltage. Therefore, the packaged LED light beads are often more susceptible to static electricity influence.
5.During the high-temperature crystal growth process of LED epitaxial chips, if the residues of deposition materials, gases outside the MOCVD reactor chamber, and the Mo source introduce impurities, these impurities will penetrate the epitaxial layer and prevent the formation of gallium nitride crystals, resulting in various epitaxial defects. This leads to the formation of tiny pits on the epitaxial layer surface, which will seriously affect the crystal quality and performance of the epitaxial film material.
6. In the process of manufacturing LED chips, there is a crucial step called residual electrode processing, which involves cleaning, yellow light exposure, fusion, evaporation, chemical etching, and grinding. These processes come into contact with many chemical cleaning agents. If the chip is not cleaned thoroughly, harmful chemicals will remain. These residual harmful chemicals will react electrochemically with the electrodes when the LED is powered on, resulting in phenomena such as light attenuation, blackening, dimming, and dead lights.
7.Damage to the chips of the LED electronic screen will directly lead to the failure of the LEDs. Therefore, it is necessary to enhance the reliability of the LED chips. If the developing process of yellow light operation is incomplete and the mask has holes, there will be residual excess metal in the light-emitting area. During the front-end manufacturing process, various processes such as cleaning, evaporation, yellow light exposure, chemical etching, fusion, grinding, etc., must use tweezers, baskets, carriers, etc. Therefore, there may be electrode scratches on the crystal grains. During the evaporation process, sometimes spring clips are needed to fix the chips, thus causing indentation marks. The influence of the chip electrodes on the solder joints includes the unreliable evaporation of the chip electrodes themselves, resulting in the detachment or damage of the electrodes after soldering. Improper chips can cause oxidation and surface contamination on the electrode surfaces. Minor contamination on the bonding surface may cause the diffusion of metal atoms between the two, resulting in failure or false soldering. And the poor weldability of the chip electrodes themselves can lead to false soldering of the solder balls.
8.In the general market, the main material for the lead frame of LED light sources is copper. To prevent copper from oxidizing, a layer of silver is usually electroplated on the surface of the frame. If the silver layer is too thin, it will easily turn yellow under high temperature conditions. Therefore, the yellowing of the silver layer is not caused by the silver layer itself, but is affected by the copper layer beneath it. At high temperatures, copper atoms will diffuse and penetrate into the surface of the silver layer, causing the silver layer to turn yellow. The oxidizability of copper is the biggest drawback of copper. Once copper is in an oxidized state, the thermal conductivity and heat dissipation performance will significantly decrease. Therefore, the thickness of the silver layer is of vital importance. At the same time, both copper and silver are susceptible to corrosion by various volatile sulfides and halides and other pollutants in the air, causing their surfaces to darken and change color. Studies have shown that the color change increases the surface resistance by approximately 20-80%, increases electrical energy loss, and significantly reduces the stability and reliability of LEDs, even leading to serious accidents.
9.The LED light source is afraid of sulfur because sulfur-containing gases can pass through the porous structure of the silica gel or the gaps of the support, and react with the silver coating of the light source through a sulfidation process. After the sulfidation reaction occurs in the LED light source, the functional area of the product will become blackened, the luminous flux will gradually decrease, and the color temperature will show a significant drift; the sulfide after sulfidation will increase its conductivity with the rise in temperature, and during use, it is very likely to cause leakage; a more serious situation is that the silver layer is completely corroded, and the copper layer is exposed. Since the gold wire solder points are attached to the surface of the silver layer, when the silver layer of the support functional area is completely sulfided and corroded, the gold ball will fall off, resulting in a dead light.
10.In the practice of electroplating production, the thickness of the metal coating, as well as the uniformity and integrity of the coating, are among the important indicators for evaluating the quality of the coating. This is because the protective performance, porosity, etc. of the coating are all directly related to its thickness. The coating type is a cathodic coating. As the thickness increases, the protective performance of the coating also improves. If the coating thickness is not uniform, the weakest part of the coating is usually the first to be damaged, and even if the remaining parts have a thicker coating, they will lose their protective effect. The coating has a high porosity, and corrosive gases such as oxygen can enter and corrode the copper substrate through the pores.
11. The material of the water-jet material is crucial for the heat dissipation of the LED packaging frame. If the PPA frame is made of water-jet material, it will reduce the plastic properties of PPA, resulting in the following problems: poor heat resistance, easy deformation, yellowing, and decreased reflectivity; high water absorption rate, causing size changes and mechanical strength reduction due to water absorption of the frame; poor adhesion to metals and silicon gel, making it difficult to bond, and not matching with many types of silicon gel. These potential issues make it difficult for the light bulbs to be used at slightly larger power levels. Once the usage power exceeds the range, the initial brightness is very high, but it decays rapidly, and the light becomes dim within a few months.
12. The fluorescent powder containing hydrolyzable nitrides is prone to hydrolysis and thus becomes ineffective.
13.The self-heating mechanism of the phosphor causes the temperature of the phosphor layer to often be higher than that of the p-n junction of the LED chip. The reason is that the conversion efficiency of the phosphor cannot reach 100%. Therefore, a part of the blue light absorbed by the phosphor is converted into yellow light. In high light energy density LED packages, the other part of the light energy absorbed by the phosphor becomes heat. Since the phosphor is usually mixed with silicone, and the thermal conductivity of silicone is very low, only 0.16 W/mK, the heat generated by the phosphor accumulates in a smaller local area, causing local high temperature. The higher the light density of the LED, the greater the heat generated by the phosphor. When the temperature of the phosphor reaches above 450 degrees Celsius, carbonization will occur near the phosphor particles. Once there is carbonization and blackening in a certain area of the silicone, its light conversion efficiency will be lower. This area will absorb more light energy emitted by the LED and convert more heat. The temperature continues to increase, causing the area of carbonization to expand larger and larger.
14. The base material of the silver adhesive is an epoxy resin type material. Its thermal expansion coefficient is much larger than that of the chip and the support. In the cold and hot shock usage environment of the light bulb, due to the heat issue, stress will be generated. The effect will be more intensified in a highly temperature-volatile environment. The adhesive itself has tensile fracture strength and elongation rate. When the tensile force exceeds this limit, the adhesive will break. The bonding adhesive peels off at the interface, the heat dissipation becomes extremely poor, the heat generated by the chip cannot be conducted out, the junction temperature rises rapidly, and this greatly accelerates the process of light attenuation.
15.In the cold and hot shock usage environment of the light bulb, stress will be generated due to the heat issue. The effect will be more severe in an environment with drastic temperature changes. The gel itself has tensile fracture strength and elongation rate. When the tensile force exceeds this limit, the gel will break.
16.The reasons for the failure of the LED electronic screen include five factors: the gold wire, the chip, the LED support, the phosphor, and the encapsulation glue. If any of these factors is affected, it will cause the LED display to have a failure of lights.
