LED lamp bead dead lamp analysis

Jin Jian company's difference with traditional detection analysis: the traditional detection company will tell death lamp is gold wire broken, solid crystal glue out, burning, chip simple phenomenon such as corrosion, Jin Jian will further analysis the glue is good or not lineup or artificial pressure, glue poisoning caused by glue or glue curing or plating to protect water, is too much electricity or extension defects or electrode ohmic contact undesirable, is fab covertly exchange structure craft or operators hand sweat contamination or chemical residues. This is why jinjineng, born of materials, is called the holmes of LED.

Today we will take LED dead lights as an example to analyze how many causes there are:

According to the big data of failure analysis of jinjin contact, there may be more than 100 kinds of causes of LED dead lamp. Due to time limitation, today we only take LED light source as an example, starting from five major raw materials of LED light source (chip, bracket, phosphor, solid crystal glue, packaging glue and gold wire), and introduce some possible causes of dead lamp.

chip

Chip antistatic ability is poor

The anti-static index of LED lamp beads depends on the LED LED chip itself, which has nothing to do with the expected packaging technology of the packaging material, or the influencing factors are small and subtle. LED lamp is more prone to electrostatic damage, which is related to the distance between two pins. The distance between the two electrodes of the bare crystal of LED chip is very small, generally less than 100 microns, while the lead pin is about two millimeters. When the electrostatic charge is transferred, the larger the distance is, the more likely to form a large potential difference, that is, a high voltage. Therefore, sealed LED lights are more prone to static damage accidents.

Ii. Chip epitaxial defects

LED epitaxial wafer in the process of high temperature and long jing, substrate, MOCVD reaction cavity residual sediments, Mo and peripheral gas source will introduce impurities, these impurities will infiltration of epitaxial layer, prevent the gallium nitride crystal nucleation, form the extension of various defects, eventually forming a small hole in the surface of epitaxial layer, which will seriously affect the epitaxial wafer thin film material of crystal quality and performance.

Chip chemical residues

Electrode processing is the key process of making LED chips, including cleaning, evaporation, yellow light, chemical etching, fusion, grinding, will come into contact with a lot of chemical cleaning agents, if the chip is not clean enough, will cause the residue of harmful chemicals. These harmful chemicals will occur electrochemical reactions with the electrodes when LED is energized, resulting in dead lights, light decay, dimming, blackening and other phenomena. Therefore, the identification of chemical residues on the chip is very important for the LED packaging plant.

Damage to chips

LED chip damage will directly lead to LED failure, so it is very important to improve the reliability of LED chip. In the process of evaporation, a spring clip is sometimes needed to fix the chip, so there will be a clip mark. Incomplete development and holes in the mask will result in residual excess metal in the luminous area. Tweezers, flower baskets and carriers must be used for the preparation of grains in the previous stage, such as cleaning, evaporation, yellow light, chemical etching, fusion, grinding, etc., so the scratch of grain electrode may occur.

Influence of the chip electrode on the welding spot: the evaporation of the chip electrode itself is not reliable, resulting in the electrode falling off or damage after the welding line; The poor weldability of chip electrode itself will lead to virtual welding of welding ball. Improper storage of the chip will lead to oxidation of the electrode surface, surface contamination, etc., and slight contamination of the bonding surface may affect the diffusion of metal atoms between the two, resulting in failure or virtual welding.

The chip of the new structure process is incompatible with the material of the light source

The new structure includes an aluminum layer in the electrode of the LED chip, which forms a reflective mirror in the electrode to improve the light output efficiency of the chip, and can reduce the amount of gold used in the evaporation of the electrode to some extent, thus reducing the cost. However, aluminum is a relatively active metal. Once the packaging factory does not strictly control the incoming materials, and USES the glue containing excessive chlorine, the aluminum reflective layer in the gold electrode will react with the chlorine in the glue, resulting in corrosion.

The LED bracket

Today we will take LED dead lights as an example to analyze how many causes there are:

According to the big data of failure analysis of jinjin contact, there may be more than 100 kinds of causes of LED dead lamp. Due to time limitation, today we only take LED light source as an example, starting from five major raw materials of LED light source (chip, bracket, phosphor, solid crystal glue, packaging glue and gold wire), and introduce some possible causes of dead lamp.

chip

Chip antistatic ability is poor

The anti-static index of LED lamp beads depends on the LED LED chip itself, which has nothing to do with the expected packaging technology of the packaging material, or the influencing factors are small and subtle. LED lamp is more prone to electrostatic damage, which is related to the distance between two pins. The distance between the two electrodes of the bare crystal of LED chip is very small, generally less than 100 microns, while the lead pin is about two millimeters. When the electrostatic charge is transferred, the larger the distance is, the more likely to form a large potential difference, that is, a high voltage. Therefore, sealed LED lights are more prone to static damage accidents.

Ii. Chip epitaxial defects

LED epitaxial wafer in the process of high temperature and long jing, substrate, MOCVD reaction cavity residual sediments, Mo and peripheral gas source will introduce impurities, these impurities will infiltration of epitaxial layer, prevent the gallium nitride crystal nucleation, form the extension of various defects, eventually forming a small hole in the surface of epitaxial layer, which will seriously affect the epitaxial wafer thin film material of crystal quality and performance.

Chip chemical residues

Electrode processing is the key process of making LED chips, including cleaning, evaporation, yellow light, chemical etching, fusion, grinding, will come into contact with a lot of chemical cleaning agents, if the chip is not clean enough, will cause the residue of harmful chemicals. These harmful chemicals will occur electrochemical reactions with the electrodes when LED is energized, resulting in dead lights, light decay, dimming, blackening and other phenomena. Therefore, the identification of chemical residues on the chip is very important for the LED packaging plant.

Damage to chips

LED chip damage will directly lead to LED failure, so it is very important to improve the reliability of LED chip. In the process of evaporation, a spring clip is sometimes needed to fix the chip, so there will be a clip mark. Incomplete development and holes in the mask will result in residual excess metal in the luminous area. Tweezers, flower baskets and carriers must be used for the preparation of grains in the previous stage, such as cleaning, evaporation, yellow light, chemical etching, fusion, grinding, etc., so the scratch of grain electrode may occur.

Influence of the chip electrode on the welding spot: the evaporation of the chip electrode itself is not reliable, resulting in the electrode falling off or damage after the welding line; The poor weldability of chip electrode itself will lead to virtual welding of welding ball. Improper storage of the chip will lead to oxidation of the electrode surface, surface contamination, etc., and slight contamination of the bonding surface may affect the diffusion of metal atoms between the two, resulting in failure or virtual welding.

The chip of the new structure process is incompatible with the material of the light source

The new structure includes an aluminum layer in the electrode of the LED chip, which forms a reflective mirror in the electrode to improve the light output efficiency of the chip, and can reduce the amount of gold used in the evaporation of the electrode to some extent, thus reducing the cost. However, aluminum is a relatively active metal. Once the packaging factory does not strictly control the incoming materials, and USES the glue containing excessive chlorine, the aluminum reflective layer in the gold electrode will react with the chlorine in the glue, resulting in corrosion.

The LED bracket

Silver coating is too thin

The existing LED light sources on the market choose copper as the matrix material of lead frame. In order to prevent the oxidation of copper, the surface of the support should be plated with a layer of silver. If the silver coating is too thin, under high temperature conditions, easy to yellow support. The yellowing of the silver coating is not caused by the silver coating itself, but by the copper layer below the silver layer. At high temperatures, copper atoms diffuse and penetrate the surface of the silver layer, making it yellow. Copper's oxidizability is its biggest drawback. Once the oxidation state of copper occurs, the thermal conductivity and heat dissipation performance will be greatly reduced. So the thickness of the silver coating is very important. At the same time, copper and silver are susceptible to various volatile pollutants in the air, such as sulfur compounds and halides corrosion, make their surface dark color. Studies have shown that the discoloration increases the surface resistance by about 20 ~ 80%, and increases the power loss, which greatly reduces the stability and reliability of LED, and even leads to serious accidents.

Silver plating layer vulcanization

LED light source is afraid of sulfur, this is because the sulfur containing gas through its porous structure of silica gel or bracket gap, and the light source silver coating sulfide reaction. After curing reaction of LED light source, the functional area of the product will darken, the luminous flux will gradually decline, and the color temperature will drift obviously. The conductivity of silver sulfide increases with the increase of temperature. A more serious condition is that the silver layer is completely corroded and the copper layer is exposed. As the gold wire ii solder joint is attached to the surface of the silver layer, when the silver layer in the functional area of the support is completely vulcanized, the gold ball falls off, resulting in dead light.

Eight, silver layer oxidation

Gold detection in contact with the initial diagnosis of LED blackening business found that sulfur/chlorine/bromine elements are more difficult to find, however, LED light source silver coating blackening signs are obvious, which may be related to silver oxidation. But pure element EDS EDS analysis testing means are not easy to determine, because present in the air environment, the surface adsorption and encapsulation adhesive organic matter in the determination of oxygen will interfere with the test results and therefore determining black oxide conclusion need using SEM, EDS, micro ir, XPS and other professional testing and optical, electrical, chemical, environment aging and a series of reliability contrast experiment, combining with the professional knowledge of testing knowledge and plating for comprehensive analysis.

Ix. Poor electroplating quality

The quality of the coating is mainly determined by the crystal structure of the metal deposit layer, in general, the smaller the crystal structure, the more compact, smooth coating, the higher the protection performance. This thin coating of crystals is called a microcrystalline deposit. Jinjian pointed out that a good electroplating coating should be fine, smooth, uniform, continuous coating crystallization, do not allow pollutants, chemical residues, spots, black spots, charred, rough, pinhole, pitting, crack, layering, foaming, peeling, coating peeling, yellowing, crystal coating, local no coating defects.

In the practice of electroplating, the thickness of metal coating, the uniformity and integrity of coating are one of the important indexes to check the quality of coating, because the protection performance and porosity of coating are directly related to the thickness of coating. The special change is the cathode coating, with the increase of the thickness, the protection performance of the coating also improves. If the thickness of the coating is not uniform, often its thinnest place is destroyed first, the rest of the coating thickness will lose the protective effect.

The porosity of the coating is more, oxygen and other corrosive gases through the pores into the corrosion of copper matrix

Organic pollution

Jin Jian also pointed out that because of the plating process will use a variety of organic liquid medicine, silver coating if cleaning is not clean, or use of poor quality and bad potions, once these residual organic matter in the source of light environment, under the action of light, heat and electricity, organic matter will probably be the chemical reactions such as oxidation reduction led to the silver coating surface discoloration.

11. Spout material

The material of plastic is the key to heat conduction of LED packaging support. According to the golden mirror test, if the PPA support is nozzle material, the plastic performance of PPA will be reduced, resulting in the following problems: poor high-temperature bearing capacity, easy deformation, yellowing, low reflectivity; High water absorption rate, the bracket will cause size change and mechanical strength decline due to water absorption; With metal and silica gel poor combination, compared with the rubber, and many silica gel are not matched. These potential problems make it difficult to use the beads at slightly higher power levels. Once the beads are out of the power range, the initial brightness is very high, but the attenuation is rapid, and the lamp will dim in a few months.

phosphors

Xii. Phosphor hydrolysis

Nitride phosphors are easily hydrolyzed and fail.

Mechanism of spontaneous heat of phosphors

Due to the mechanism of spontaneous heating of phosphors, the temperature of phosphors layer is often higher than the p-n junction of LED chips. The reason is that the conversion efficiency of phosphors cannot reach 100%, so part of the blue light absorbed by phosphors is converted to yellow light, while the other part of the light energy absorbed by phosphors in the high-light energy density LED package is turned into heat. As phosphors are usually mixed with silica gel, and the thermal conductivity of silica gel is very low, only 0.16w /mK, the heat generated by phosphors will accumulate in small local areas, resulting in local high temperature. The greater the optical density of LED, the greater the calorific value of phosphors will be. When the temperature of phosphor reaches more than 450 degrees Celsius, it will carbonize the silica gel near the phosphor particles. Once a certain area of silica gel carbonized black, its light conversion efficiency is lower, the area will absorb more LED light energy and convert more heat, the temperature continues to increase, making the carbonized area more and more large.

Solid crystal glue

14. Silver glue stripping

Is epoxy resin conductive silver glue matrix materials, thermal expansion coefficient is much larger than chips and stents, used in lamp bead cold and hot shock environment, can produce stress because of the hot issues, severe environment temperature change effect will be more intensified, colloid itself have tensile fracture strength and the extension rate, when the pull over, colloid is cracked. When the solid crystal glue is stripped at the interface, the heat dissipation becomes worse sharply. The heat generated by the chip cannot be derived, and the junction temperature rises rapidly, which greatly accelerates the process of light decay.

15. Silver glue layering

Silver powder particles in a state of suspension dispersion in slurry system, due to the density difference between silver and matrix, charge, cohesive force, force and decentralized system structure, and many other factors, often appear settlement stratification, silver if settlement too fast will make the product in a plasma generation sag, coating thickness is not uniform, and even affect the material properties of the coating, stratification can also affect device of heat dissipation, bonding strength and electric conductivity.

16. Silver ion migration

A customer USES silica gel package, conductive silver adhesive bonding vertical flip light source leakage phenomenon, commissioned the gold mirror to find the cause. Jin Jian through analyzing bad lamp bead, in detect abnormal silver elements on the side of the chip, and can be observed that silver particles from the positive silver glue at the bottom of the regional extension on dendrite morphology gradually spread to the upper chip nearby on the side of the p-n junction, so Jin Jian determine bad lamp bead leakage failure is likely to be for silver ions from the solid crystal silver ion migration caused by the chip on the side. The phenomenon of silver ion migration is gradually formed in the process of using the product. With the aggravation of the phenomenon of migration, the silver ion will eventually conduct the p-n junction of the chip, resulting in the low resistance path on the side of the chip, resulting in abnormal leakage current of the chip, and even short circuit of the chip in serious cases. There are many reasons for the silver migration, but the main reason is that the silver-based material is affected by moisture. When the silver glue is affected by moisture, the intruding water molecules ionize the silver and migrate along the side of the chip under the action of the electric field from bottom to top. Therefore, it is recommended that customers carefully use silica gel package and silver adhesive to bond the lamp beads of vertical flip chip, fix the chip on the support by means of gold and tin eutectic welding, and strengthen the detection of waterproof characteristics of lamps and lanterns.

Xvii, solid crystal glue dry

The curing agent of organic silicon used in LED packaging contains platinum (platinum) complex, which is very easy to be poisoned. The toxicant is any compound containing nitrogen (N), phosphorus (P), sulfur (S).

Encapsulation adhesive

Poor heat resistance of glue

According to our test, pure silica gel does not begin to crack until 400 degrees, but the heat resistance of modified silica gel with epoxy resin is pulled down to the level of epoxy resin. When this modified silica gel is applied to high-power LED or high-temperature environment, the phenomenon of colloid yellow, black, cracking and dead lamp will occur.

Nineteen, glue dry

The curing agent of organic silicon used in LED packaging contains platinum (platinum) complex, which is very easy to be poisoned. The toxicant is any compound containing nitrogen (N), phosphorus (P), sulfur (S).

Silicone "toxic" substances are: containing N, P, S and other organic compounds; Sn, Pb, Hg, Sb, Bi, As and other heavy metal ion compounds; An organic compound containing an unsaturated group such as acetylene. Pay attention to the following materials:

▪ organic rubber: sulfuric rubber such as gloves

▪ epoxy resin, polyurethane resin: amine, isocyanate curing agent

▪ comprehensive silicone RTV rubber: especially using Sn catalyst

▪ soft polyvinyl chloride: plasticizer, stabilizer

Forced the flux

▪ engineering plastics: flame retardants, heat enhancers, uv absorbers, etc

▪ silver-plated, gold-plated surface (electroplating bath during manufacturing is the main reason)

The degasification generated by Solder register (caused by heating and curing of organic silicon)

Twenty, packaging glue line expansion coefficient is too large

In the cold and thermal impact of the lamp beads in the use of the environment, because of thermal stress, temperature changes in the environment will be more severe effect, the gel itself has tensile fracture strength and elongation rate, when the tension exceeds, then the gel is split.

Glue contains chlorine

However, at present, epoxy resin production enterprises in China generally have small production scale, backward management mode and production technology, and low automation degree of operation machinery, which makes it difficult to guarantee the parameters of epoxy resin. The production of low quality epoxy resin is related to the current industrial situation in China, which is in urgent need of upgrading.

The chlorine in epoxy resin not only causes chlorination corrosion on the silver coating layer of the support, alloy wire or other active metal and chip electrode (aluminum reflectance layer), but also can interact 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 content of chlorine in epoxy resin, including organic chlorine and inorganic chlorine. Inorganic chlorine will affect the electrical properties of cured resin. The content of organochlorine indicates the content of the group of chlorol in the part of the molecule that has no closed-loop reaction, and its content should be reduced as much as possible, otherwise it will affect the curing of resin and the properties of cured products.

In order to prevent chlorine corrosion of electrode more effectively and reduce the quality risk of the whole industry, jinjian has launched the "jinjian chlorine-free certification" testing service exclusively to LED manufacturers all over the world. "Jinjian chlorine-free certification" aims to confirm whether LED glue and chip contain excessive chlorine elements, and the detection accuracy reaches PPM level. The content of the certification report can be found at www.gmatg.com. With the service of "gold mirror chlorine free certification", LED buyers can rest assured to purchase raw materials with "gold mirror chlorine free certification", greatly reducing the risk of raw material procurement.

Gold wire

22. Copper alloy, gold-coated silver alloy wire, and silver alloy wire replace expensive gold wire

Gold wire has the advantages of high conductivity, good thermal conductivity, corrosion resistance, toughness and chemical stability, but the high price of gold wire leads to high packaging cost. In the periodic table of elements, gold, silver, copper and aluminum have high conductivity. Many LED manufacturers are trying to develop copper alloys, gold-coated silver wires, and silver-alloy wires to replace expensive gold wires. Although these alternatives are superior to gold wire in some properties, they are far less chemically stable, such as the vulnerability of silver wire and gold-coated silver wire to sulfur/chlorine/bromide corrosion and the oxidation of copper wire. In the case of encapsulated silica gel, which is similar to an absorbent and breathable sponge, these alternatives make the bonding wire vulnerable to chemical corrosion, reduce the reliability of the light source, and lead beads are prone to break and die after long use.

23. Diameter deviation

Gold wire with a length of 26.37m and a diameter of 50 microns (2 mil) can be drawn from 1 gram of gold, or gold wire with a length of 105.49m and a diameter of 25 microns (1 mil). If the length of the gold wire is fixed, if the diameter of the incoming gold wire is half of the original, then the resistance measured on the gold wire is a quarter of the normal.

Jinjian test points out that for suppliers, the thinner the diameter of the gold wire, the lower the cost, and the higher the profit if the price remains unchanged. However, for LED customers who use gold wire, the purchase of gold wire that is cut corners on the diameter will lead to the risk of increased resistance of gold wire and reduced fusing current, which will greatly reduce the life of LED light source. The gold wire life of 1.0 mil is necessarily shorter than that of 1.2 mil, but it cannot be tested simply by the packaging factory. In this case, we can provide incoming inspection of the diameter of the gold wire.

24. Surface defects

(1) the surface of the wire shall be free of scratches, pits, scratches, cracks, bulges, discounting and other defects that reduce the service life of the device over 5% of the wire diameter. During the drawing process of gold wire, the surface defects on the surface of the wire will lead to the increase of current density, making the damaged part easy to be burnt out, and reducing the ability to resist mechanical stress, resulting in the fracture of the damaged part of the inner lead.

(2) the surface of the gold wire shall be free of grease, rust, dust and other adhesion, which will reduce the bonding strength between the gold wire and LED chip and between the gold wire and the support.

Too low tensile load and elongation

A good gold wire that can withstand the impact of resin encapsulation must have a specified tensile load and elongation. At the same time, the breaking force and elongation of the gold wire play a key role in the bonding quality of the lead. Too soft gold wire will lead to the following defects :(1) sagging arch wire; (2) spherical instability; (3) the ball neck is easy to contract; (4) gold wire is easy to break. Too hard wire can lead to the following defects :(1) holes in the chip electrode or epitaxial; (2) broken neck of golden ball; (3) difficulty in forming alloys; (4) arc control of arch wire is difficult.

conclusion

After listed so many reasons, we can conclude the conclusion, the death of an LED light, simple representation may be dozens of law leads to different road, problems encountered in the past LED industry after the most and guess, can't see the question essence, fundamentally solve the problem, Jin Jian that focus on LED material testing company after coming out, put a lot of experience in theoretical, through collaboration from different professional team, high precision testing equipment, collection industry failure cases, on the basis of large data, draw more accurate conclusions.