Today we take the LED dead light as an example to analyze how many reasons are there:
According to the big data of failure analysis, there may be more than one hundred kinds of LED dead lights. It is limited to time. Today we only use LED light source as an example. The five raw materials from LED light source (chip, bracket, phosphor, solid crystal glue, encapsulant) And the start of the gold line), the introduction of some of the reasons that may lead to the death of the light.
chip
01, the chip has poor antistatic ability
The anti-static index of the LED lamp bead depends on the LED light-emitting chip itself, which is basically independent of the encapsulation material's expected packaging process, or the influence factor is small and very subtle; the LED lamp is more susceptible to electrostatic damage, which is separated from the two pins. Relationship, the distance between the two electrodes of the bare chip of the LED chip is very small, generally less than one hundred micrometers, and the LED pin is about two millimeters. When the electrostatic charge is to be transferred, the larger the spacing, the easier it is to form a large potential difference. , that is, high voltage. Therefore, it is often more prone to electrostatic damage accidents after being sealed into LED lights.
02, chip epitaxial defects
In the high-temperature crystal growth process of the epitaxial wafer , the substrate, the residual deposits in the MOCVD reaction chamber, the peripheral gas and the Mo source all introduce impurities, which will penetrate into the epitaxial layer and prevent nucleation of the gallium nitride crystal to form various kinds. Various epitaxial defects eventually form tiny pits on the surface of the epitaxial layer, which also seriously affect the crystal quality and performance of the epitaxial film material.
03, chip chemical residue
Electrode processing is the key process for making LED chips, including cleaning, evaporation, yellowing, chemical etching, fusion, and grinding. It will come into contact with many chemical cleaning agents. If the chip cleaning is not clean enough, it will cause harmful chemicals to remain. These harmful chemicals react electrochemically with the electrodes when the LEDs are energized, resulting in dead lights, light decay, darkness, and blackening. Therefore, identifying chip chemical residues is critical to LED packaging plants.
04, chip damage
Damage to the LED chip directly leads to LED failure, so it is important to improve the reliability of the LED chip. In the evaporation process, it is sometimes necessary to fix the chip with a spring clip, so that a pinch is generated. If the development of the yellow light is incomplete and the mask has holes, there will be residual metal in the light-emitting area. In the pre-stage process, various processes such as cleaning, evaporation, yellow light, chemical etching, fusion, grinding, etc. must use tweezers and flower baskets, carriers, etc., so there will be scratches of the die electrodes.
The effect of the chip electrode on the solder joint: the evaporation of the chip electrode itself is not reliable, resulting in the electrode being peeled off or damaged after the wire; the poor solderability of the chip electrode itself will lead to solder ball soldering; improper storage of the chip will lead to oxidation of the electrode surface and surface contamination. Etc., slight contamination of the bonding surface can affect the diffusion of metal atoms between the two, causing failure or solder joints.
05, the chip of the new structure process is incompatible with the light source material
The new structure of the LED chip electrode has a layer of aluminum, which acts to form a mirror in the electrode to improve the light extraction efficiency of the chip, and secondly to reduce the amount of gold used in the evaporation of the electrode to reduce the cost. However, aluminum is a relatively active metal. Once the packaging plant is not well controlled, the aluminum reflective layer in the gold electrode reacts with the chlorine in the glue to cause corrosion.
LED bracket
06, the silver plating layer is too thin
The existing LED light source on the market selects copper as the base material of the lead frame. In order to prevent oxidation of copper, the surface of the stent is usually plated with a layer of silver. If the silver plating layer is too thin, the stent is liable to yellow under high temperature conditions. The yellowing of the silver-plated layer is not caused by the silver-plated layer itself, but by the copper layer under the silver layer. At high temperatures, copper atoms diffuse and penetrate the surface of the silver layer, causing the silver layer to yellow. The oxidizability of copper is the biggest drawback of copper itself. When copper is oxidized, thermal and thermal performance will be greatly reduced. Therefore, the thickness of the silver plating layer is crucial. At the same time, both copper and silver are susceptible to corrosion by various volatile sulfides and halides in the air, causing darkening of the surface. Studies have shown that discoloration increases the surface resistance by about 20-80%, and the power loss increases, so that the stability and reliability of the LED are greatly reduced, and even lead to serious accidents.
07, silver plating layer vulcanization
The LED light source is afraid of sulfur because the sulfur-containing gas undergoes a vulcanization reaction with the silver plating layer of the light source through the porous structure of the silica gel or the support gap. After the vulcanization reaction of the LED light source, the functional area of ​​the product will be blackened, the luminous flux will gradually decrease, and the color temperature will drift significantly. The vulcanized silver after vulcanization will increase in conductivity with increasing temperature, and it is prone to leakage during use; The condition is that the silver layer is completely corroded and the copper layer is exposed. Since the gold wire two solder joints are attached to the surface of the silver layer, when the silver layer of the bracket functional region is completely vulcanized and eroded, the golden ball appears to fall off, and a dead light appears.
08, silver plating oxidation
Jinjian detection found that the sulfur/chlorine/bromine element is harder to find in the business of initial blackening of exposed LEDs. However, the silver-plated LED light source shows signs of blackening, which may be related to silver oxidation. However, pure element analysis and detection methods such as EDS spectrum analysis are not easy to determine oxidation, because oxygen in organic substances such as air environment, sample surface adsorption and encapsulant can interfere with the determination of detection results. Therefore, the conclusion of determining oxidation blackening needs to be used. SEM, EDS, micro-infrared spectroscopy, XPS and other professional testing and a series of reliability comparison experiments such as light, electricity, chemistry, environmental aging, combined with professional testing knowledge and electroplating knowledge for comprehensive analysis.
09, poor plating quality
The quality of the coating is mainly determined by the crystal structure of the metal deposit. Generally, the finer the crystal structure, the denser, smoother and more protective the coating. This finely crystallized plating layer is referred to as a "microcrystalline deposited layer". Jin Jian pointed out that a good plating layer should be crystallized, smooth, uniform and continuous, and no pollutants, chemical residues, spots, black spots, charring, roughness, pinholes, pitting, cracks, delamination, Defects such as foaming, wrinkling, peeling, yellowing, crystal plating, and partial plating.
In the practice of electroplating production, the thickness of the metal coating and the uniformity and integrity of the coating are one of the important indicators for checking the quality of the coating, because the protective properties and porosity of the coating are directly related to the thickness of the coating. The special change is the cathode coating, and as the thickness increases, the protective properties of the coating also increase. If the thickness of the coating is not uniform, the thinnest part is often destroyed first, and the remaining part of the coating is thicker and loses its protective effect.
The porosity of the coating is high, and corrosive gases such as oxygen enter the corroded copper matrix through the pores.
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