The data shows that there may be more than 100 reasons for LED dead lights, which are limited by time. Today, we only take LED light sources as an example, from the five major raw materials of LED light sources (gold wire, chip, bracket, phosphor, die-bonding glue and encapsulation glue) To get started, introduce some of the reasons that may lead to dead lights.
Copper wire, copper alloy, gold-clad silver alloy wire, silver alloy wire instead of gold wire.
Gold wire has the advantages of high electrical conductivity, good thermal conductivity, corrosion resistance, good toughness, and excellent chemical stability, but the price of gold wire is expensive, which leads to high packaging costs. In the periodic table of elements, gold, silver, copper and aluminum among the transition metal elements have high electrical conductivity. Many LED manufacturers try to develop such as copper alloy, gold clad silver alloy wire, silver alloy wire to replace the expensive gold wire. While these alternatives are superior to gold wire in some properties, they are much worse in chemical stability, such as silver wire and gold-clad silver alloy wire susceptible to sulfur/chlorine/bromide corrosion, and copper wire susceptible to oxidation. In the case of encapsulated silica gel similar to water-absorbing and breathable sponge, these alternatives make the bonding wire susceptible to chemical corrosion, the reliability of the light source is reduced, and the LED lamp bead is easy to break and die after a long time of use.
2. Diameter deviation.
1 gram of gold can draw a gold wire with a length of 26.37m and a diameter of 50μm (2 mil), or a gold wire with a length of 105.49m and a diameter of 25μm (1 mil). If the length of the gold wire is fixed, and if the diameter of the incoming gold wire is half of the original diameter, then the measured resistance of the gold wire to be beaten is a quarter of the normal value.
For suppliers, the thinner the diameter of the gold wire, the lower the cost, and the higher the profit when the selling price remains the same. For LED customers who use gold wires, purchasing gold wires that cut corners in diameter will increase the resistance of the gold wires and reduce the fuse current, which will greatly reduce the life of the LED light source. For example, the life of 1.0 mil gold wire must be shorter than that of 1.2 mil gold wire.
3. Surface defects.
(1) The surface of the wire should be free from nicks, pits, scratches, cracks, bumps, folds and other defects that reduce the service life of the device that exceed 5% of the wire diameter. During the drawing process of the gold wire, surface defects on the surface of the wire will lead to an increase in the current density, making the damaged part easy to be burned, and at the same time reducing the ability to resist mechanical stress, causing the inner lead to break at the damaged part.
(2) The surface of the gold wire should be free of oil, rust, dust and other adhesions, which will reduce the bonding strength between the gold wire and the LED chip, and between the gold wire and the bracket.
4. The breaking load and elongation are too low.
A good gold wire that can withstand the shock of resin encapsulation must have a specified breaking load and elongation. At the same time, the breaking force and elongation of the gold wire play a key role in the quality of wire bonding, and the bonding wire with high breaking rate and elongation is more conducive to bonding. Too soft gold wire can cause the following problems:
(1) The arch wire sags;
(2) Spherical instability;
(3) The ball neck is easy to shrink;
(4) The gold wire is easy to break.
Gold wire that is too hard can cause the following problems:
(1) Punch holes in the chip electrodes or epitaxy;
(2) The neck of the gold ball is broken;
(3) Difficulty forming alloys;
(4) It is difficult to control the arc of the arch wire.
chip
The chip has poor antistatic ability.
The antistatic index of LED lamp beads depends on the LED light-emitting chip itself, which is basically independent of the expected packaging process of the packaging material, or the influencing factors are very small and subtle; LED lamps are more prone to electrostatic damage, which is related to the distance between the two pins. The distance between the two electrodes of the bare die of the LED chip is very small, generally within 100 microns, while the LED pin is about two millimeters. When the electrostatic charge is to be transferred, the larger the distance, 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 sealing into LED lights.
2. Chip epitaxy defects.
During the high-temperature crystallization process of LED epitaxial wafers, impurities are introduced into the substrate, residual deposits in the MOCVD reaction chamber, peripheral gas and Mo source. These impurities will penetrate into the epitaxial layer, preventing the nucleation of gallium nitride crystals and forming various Various epitaxial defects eventually form tiny pits on the surface of the epitaxial layer, which will also seriously affect the crystal quality and performance of the epitaxial wafer thin film material.
3. Chip Chemical Residues.
Electrode processing is a key process for making LED chips, including cleaning, evaporation, yellow light, chemical etching, fusion, and grinding. It will come into contact with many chemical cleaning agents. If the chip is not cleaned sufficiently, harmful chemicals will remain. These harmful chemicals will electrochemically react with the electrodes when the LED is energized, resulting in dead lights, light decay, dim light, and blackening. Therefore, identification of chip chemical residues is crucial for LED packaging companies.
Electrode processing is a key process for making LED chips, including cleaning, evaporation, yellow light, chemical etching, fusion, and grinding. It will come into contact with many chemical cleaning agents. If the chip is not cleaned sufficiently, harmful chemicals will remain. These harmful chemicals will electrochemically react with the electrodes when the LED is energized, resulting in dead lights, light decay, dim light, and blackening. Therefore, identification of chip chemical residues is crucial for LED packaging companies.
4. Damage to the chip.
The damage of the LED display chip will directly lead to the failure of the LED, so it is very important to improve the reliability of the LED chip. During the evaporation process, the chip is sometimes fixed with spring clips, so clip marks will be generated. If the yellow light work is not fully developed and the mask has holes, there will be residual metal in the light-emitting area. In the front-end process of the die, various processes such as cleaning, evaporation, yellow light, chemical etching, fusion, grinding and other operations must use tweezers, flower baskets, carriers, etc., so there will be scratches on the die electrodes.
The influence of the chip electrode on the solder joints: the chip electrode itself is not reliably evaporated, resulting in the electrode falling off or damage after the bonding wire; the poor solderability of the chip electrode itself will lead to solder balls soldering; improper storage of the chip will lead to oxidation of the electrode surface and contamination of the surface. Etc., slight contamination of the bonding surface can affect the diffusion of metal atoms between the two, resulting in failure or false soldering.
5. The incompatibility between the chip of the new structure process and the light source material
There is a layer of aluminum in the electrode of the LED chip of the new structure, and its function is to form a layer of reflector in the electrode to improve the light extraction efficiency of the chip, and secondly, it can reduce the amount of gold used when the electrode is evaporated to a certain extent, thereby reducing the cost. However, aluminum is a relatively active metal. Once the packaging factory does not strictly control the incoming materials and uses glue containing excessive chlorine, the aluminum reflective layer in the gold electrode will react with the chlorine in the glue, resulting in corrosion.
LED bracket
The silver-plated layer is too thin.
Existing LED light sources on the market choose copper as the base material of the lead frame. In order to prevent oxidation of copper, a layer of silver is generally plated on the surface of the bracket. If the silver-plated layer is too thin, the stent is prone to yellowing under high temperature conditions. The yellowing of the silver-plated layer is not caused by the silver-plated layer itself, but is affected by the copper layer under the silver layer. At high temperature, 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 itself. Once the copper is in an oxidized state, the thermal conductivity and heat dissipation performance will be greatly reduced. So the thickness of the silver layer is very important. At the same time, both copper and silver are easily corroded by pollutants such as various volatile sulfides and halides in the air, making their surfaces dark and discolored. Studies have shown that discoloration increases the surface resistance by about 20-80%, and the power loss increases, which greatly reduces the stability and reliability of the LED, and even leads to serious accidents.
2. Silver plating layer vulcanization.
The LED light source is afraid of sulfur, because the sulfur-containing gas will pass through the porous structure of the silica gel or the gap of the bracket, and undergo a vulcanization reaction with the silver-plated layer of the light source. After the vulcanization reaction of the LED light source occurs, the functional area of the product will be blackened, the luminous flux will gradually decrease, and the color temperature will drift significantly. The condition is that the silver layer is completely corroded and the copper layer is exposed. Since the second solder joint of the gold wire is attached to the surface of the silver layer, when the silver layer in the functional area of the bracket is completely vulcanized and corroded, the gold ball will fall off, resulting in a dead lamp.
3. Silver coating oxidation.
In the case of the initial diagnosis of LED blackening, it was found that the elements of sulfur/chlorine/bromine were more difficult to find. However, the signs of blackening of the silver-plated layer of the LED light source were obvious, which may be related to silver oxidation. However, pure element analysis and detection methods such as EDS energy spectrum analysis are not easy to determine oxidation, because oxygen elements existing in the air environment, sample surface adsorption, and organic substances such as encapsulation will interfere with the determination of the detection results, so the conclusion of judging 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, etc., combined with professional testing knowledge and electroplating knowledge for comprehensive analysis.
4. Poor electroplating quality.
The quality of the coating is mainly determined by the crystalline structure of the metal deposition layer. Generally speaking, the finer the crystalline structure, the denser and smoother the coating, and the higher the protective performance. Such finely crystalline coatings are called "microcrystalline deposits". A good electroplating layer should have a fine, smooth, uniform and continuous coating without contaminants, chemical residues, spots, black spots, charring, roughness, pinholes, pitting, cracks, delamination, blistering, and blisters. Skin wrinkling, coating peeling, yellowing, crystalline coating, local no coating and other defects.
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 to check the quality of the coating, because the protective performance and porosity of the coating are directly related to the thickness of the coating. The peculiarity is the cathodic coating, and as the thickness increases, the protective performance of the coating also increases. If the thickness of the coating is not uniform, the thinnest part is often destroyed first, and the rest of the coating will lose its protective effect no matter how thick it is.
The porosity of the coating is large, and corrosive gases such as oxygen will enter through the pores to corrode the copper substrate
5. Organic pollution
Because a variety of potions containing organic substances will be used in the electroplating process, if the silver-plated layer is not cleaned properly or the potions with poor quality and deterioration are used, these residual organic substances will be exposed to light, heat and electricity in the environment where the light source is lit. Under the action of organic matter, chemical reactions such as redox may occur, resulting in discoloration of the surface of the silver-plated layer.
6. Nozzle material
The material of the plastic is the key to the heat conduction of the LED package bracket. If the PPA bracket is a nozzle material, the plastic performance of the PPA will be reduced, resulting in the following problems: poor high temperature tolerance, easy deformation, yellowing, low reflectivity; high water absorption , the stent will cause dimensional changes and decrease in mechanical strength due to water absorption; poor bonding with metal and silica gel, it is relatively picky and does not match many silica gels. These potential problems make it difficult to use the lamp beads at a slightly higher power. Once the power range is exceeded, the initial brightness is very high, but the attenuation is very fast, and the lamp will be dim after a few months.
phosphor
1. Hydrolysis of phosphor powder
Nitride phosphors are easily hydrolyzed and fail.
2. The mechanism of self-heating of phosphors
The self-heating mechanism of the phosphor makes the temperature of the phosphor layer often 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%, so part of the blue light absorbed by the phosphor is converted into yellow light, and another part of the light energy absorbed by the phosphor in the high light energy density LED package becomes heat. Since the phosphor is usually mixed with silica gel, and the thermal conductivity of silica gel is very low, only 0.16 W/mK, the heat generated by the phosphor will accumulate in a small local area, resulting in local high temperature, and the greater the optical density of the LED. The greater the calorific value of the phosphor. When the temperature of the phosphor reaches above 450 degrees Celsius, the silica gel near the phosphor particles will be carbonized. Once the silica gel is carbonized and blackened somewhere, its light conversion efficiency is lower, that area will absorb more light energy from the LED and convert more heat, and the temperature will continue to increase, making the carbonized area larger and larger.
die glue
1. Stripping of silver glue
The matrix of the conductive silver glue is epoxy resin material, and the thermal expansion coefficient is much larger than that of the chip and the bracket. In the environment where the lamp bead is used in the cold and thermal shock, stress will be generated due to thermal problems, and the effect will be more severe in the environment with severe temperature changes. To exacerbate, the colloid itself has tensile breaking strength and elongation, and when the tensile force is exceeded, then the colloid will crack. The die-bonding adhesive is peeled off at the interface, the heat dissipation deteriorates sharply, the heat generated by the chip cannot be exported, and the junction temperature rises rapidly, which greatly accelerates the process of light decay.
2. Silver glue layering
The silver powder particles are dispersed in the slurry system in a suspended state. Due to the influence of many factors such as density difference, charge, cohesion, force and the structure of the dispersion system between the silver powder and the matrix, the phenomenon of silver powder sedimentation and stratification often occurs. It will cause the product to sag when the paste is hung, and the thickness of the coating will be uneven, which will even affect the physical and chemical properties of the coating film. Layering will also affect the heat dissipation, bonding strength and electrical conductivity of the device.
3. Silver ion migration
A customer used silica gel package, and the vertical flip-chip light source bonded with conductive silver glue had leakage phenomenon. Through the analysis of bad lamp beads. Abnormal silver elements are detected on the side of the chip, and it can be observed that the silver particles gradually diffuse from the bottom anode silver glue area to the side of the upper P-N junction near the top of the chip in a dendritic shape. Therefore, it is determined that the leakage failure of the bad lamp bead is very likely to come from It is caused by the ion migration of the silver ions of the die-bonding silver paste on the side of the chip. The migration phenomenon of silver ions is gradually formed during the use of the product. With the aggravation of the migration phenomenon, the silver ions will eventually conduct the P-N junction of the chip, resulting in a low-resistance path on the side of the chip, resulting in abnormal leakage current of the chip. In severe cases Even cause the chip to short circuit. There are many reasons for silver migration, but the main reason is that the silver-based material is damp. After the silver glue is damp, the invading water molecules ionize the silver and migrate along the side of the chip under the action of the vertical electric field from bottom to top. Therefore, it is recommended that customers carefully use silica gel packaging, silver glue to bond vertical flip-chip lamp beads, use gold-tin eutectic welding to fix the chip on the bracket, and strengthen the detection of waterproof characteristics of lamps.
4. Solid crystal glue does not dry
The curing agent of silicone for LED encapsulation contains platinum (platinum) complex, which is very easy to be poisoned. The poisoning agent is any one containing nitrogen (N), phosphorus (P) and sulfur (S). Compounds, once the curing agent is poisoned, the silicone curing is incomplete, which will cause the linear expansion coefficient to be high and the stress to increase.
encapsulant
1. Poor heat resistance of glue
According to our test, pure silica gel does not start to crack until 400 degrees, but the heat resistance of modified silica gel with epoxy resin is lowered to the level of epoxy resin. When this modified silica gel is applied to high-power LEDs Or in a high temperature environment, there will be phenomena such as colloid yellowing, blackening, cracking, and dead lights.
2. The glue does not dry
The curing agent of silicone for LED encapsulation contains platinum (platinum) complex, which is very easy to be poisoned. The poisoning agent is any one containing nitrogen (N), phosphorus (P) and sulfur (S). Compounds, once the curing agent is poisoned, the silicone curing is incomplete, which will cause the linear expansion coefficient to be high and the stress to increase.
The substances that are prone to "poisoning" of silica gel include: organic compounds containing N, P, S; heavy metal ion compounds such as Sn, Pb, Hg, Sb, Bi, As; organic compounds containing unsaturated groups such as ethynyl. Pay attention to the following materials:
? Organic rubber: sulfur vulcanized rubber such as gloves
? Epoxy resin, polyurethane resin: amine, isocyanate curing agent
? Comprehensive silicone RTV rubber: especially the use of Sn-type catalysts
? Soft polyvinyl cyanide: plasticizer, stabilizer
? Flux
? Engineering plastics: flame retardants, reinforced heat-resistant agents, ultraviolet absorbers, etc.
? Silver-plated, gold-plated surface (the plating solution at the time of manufacture is the main reason)
? Outgassing from Solder register (caused by heat curing of silicone)
3. The linear expansion coefficient of the encapsulant is too large
In the environment where the lamp bead is subjected to thermal shock, stress will be generated due to thermal problems, and the effect will be intensified in environments with drastic temperature changes. The colloid itself has tensile breaking strength and elongation. When the tensile force exceeds, the colloid will cracked. 4. Glue contains chlorine
However, at present, domestic epoxy resin production enterprises generally have small production scale, backward management mode and production process, and low degree of automation of operating machinery, which makes it difficult to guarantee various parameters of epoxy resin. The production of low-quality epoxy resin is related to the status quo of my country's current industry, and the industry is in urgent need of upgrading.
Chlorine in epoxy resin not only causes chlorination corrosion to silver-plated layer, alloy wire or other active metal and chip electrode (aluminum reflective layer) of bracket, but also can complex with amine curing agent to affect the curing of resin. Chlorine content is an important physical property index of epoxy resin, which refers to the mass fraction of chlorine contained in epoxy resin, including organic chlorine and inorganic chlorine. Inorganic chlorine can affect the electrical properties of the cured resin. The content of organochlorine indicates the content of the part of the chlorohydrin group in the molecule that has not undergone a ring-closure reaction.
