It is normal for a full-color LED display to have dead lights. What is abnormal is that the number of dead lights exceeds a certain range. The current industry standard for dead lights on LED displays is three out of ten thousand. This standard was the standard several years ago. Huarong Optoelectronics has controlled the dead-light rate of most LED display cases at one in 300,000, far exceeding many peers, and has also been recognized by customers.
        So what is the reason for the dead light on the led display? What are the factors that cause the dead light of the led display?

        As we all know, the LED display dead light situation restricts the development of various companies specializing in the development and production of LED products, and also restricts the development of the LED industry. What are the factors that cause LED dead lights? In view of the current status of this industry, Huarong Optoelectronics learned from a media interview with industry experts to analyze the specific factors in LED R&D, production, sales and application from a professional perspective that can cause LED dead lights.

According to big data, there may be more than one hundred causes of LED dead lights. Limited 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, solid crystal glue and packaging) Start with glue) and introduce some of the possible causes of dead lights.

Gold Line
1. 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. However, the price of gold wire is expensive, which leads to high packaging costs. In the periodic table, four metal elements of gold, silver, copper and aluminum among the transition metal elements have high electrical conductivity. Many LED manufacturers try to develop materials such as copper alloy, gold-coated silver alloy wire, and silver alloy wire to replace expensive gold wires. Although these alternatives are superior to gold wires in some properties, they are much worse in terms of chemical stability. For example, silver wires and gold-coated silver alloy wires are susceptible to sulfur/chlorine/bromination corrosion, and copper wires are prone to oxidation. In the case of encapsulated silica gel similar to water-absorbent and breathable sponge, these alternatives make the bonding wire vulnerable to chemical corrosion, reduce the reliability of the light source, and use it for a long time, and the LED lamp beads are easy to break and die.

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, the measured resistance of the gold wire is one-fourth of the normal.

For suppliers, the smaller the diameter of the gold wire, the lower the cost, and the higher the profit when the price remains unchanged. For LED customers who use gold wires, purchasing gold wires that cut corners in diameter will have the risk of increased gold wire resistance and lower fusing 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 1.2 mil gold wire.

3. Surface defects

        (1) The surface of the wire should be free of nicks, pits, scratches, cracks, bumps, discounts 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, the surface defects on the wire surface will increase the current density and make the damaged part easily burned. At the same time, the resistance to mechanical stress is reduced, causing the damage of the inner lead to break.　　

        (2) The surface of the gold wire should be free of oil, rust, dust and other adherents, 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 impact generated by 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. Bonding wires with high breaking rate and elongation are more conducive to bonding. Gold wire that is too soft can cause the following disadvantages:

(1) The arch wire sags;

(2) Spherical instability;

(3) The neck of the ball is easy to shrink;

(4) The gold wire is easy to break.

Gold wire that is too hard can cause the following disadvantages:

(1) Punch out the chip electrode or epitaxial hole;

(2) The neck of the golden ball is broken;

(3) Difficulties in forming alloys;

(4) It is difficult to control the arch wire arc.

chip

1. The chip has poor antistatic ability

        The antistatic index of LED lamp beads depends on the LED light-emitting chip itself, and is basically independent of the expected packaging process of the packaging material, or the influencing factors are very small and subtle; LED lights are more susceptible to electrostatic damage, which is related to the distance between the two pins. The relationship between the two electrodes of the LED chip die is very small, generally within 100 microns, while the LED pin is about two millimeters. When the electrostatic charge is to be transferred, the greater the distance, the easier it is to form a large potential difference. , That is, high voltage. Therefore, it is more prone to electrostatic damage accidents after being sealed into LED lights.

2. Chip epitaxial defects

        During the high-temperature crystal growth of LED epitaxial wafers, impurities will be introduced into the substrate, residual deposits in the MOCVD reaction chamber, peripheral gas, and Mo source. These impurities will penetrate into the epitaxial layer to prevent the nucleation of gallium nitride crystals and form 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 film material.

3. Chip chemical residues

        Electrode processing is a key process for making LED chips, including cleaning, evaporation, yellowing, chemical etching, fusion, and grinding. Many chemical cleaning agents will be exposed. If the chip is not cleaned enough, harmful chemicals will remain. These harmful chemicals will electrochemically react with the electrode when the LED is energized, leading to dead lights, light decay, dimming, and blackening. Therefore, the identification of chip chemical residues is very important for LED packaging factories.

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, a spring clip is sometimes needed to fix the chip, so clip marks are generated. If the yellow light work is incomplete and the mask has holes, there will be excess metal in the light-emitting area. In the front-end process of the die, various processes such as cleaning, evaporation, yellowing, chemical etching, fusion, grinding, etc. must use tweezers, flower baskets, and carriers, so scratches on the die electrode may occur.

        The impact of chip electrodes on solder joints: the chip electrode itself is not vapor-deposited reliably, causing the electrode to fall off or damage after the wire bonding; the chip electrode itself is poorly solderable, which will cause the solder ball to solder; improper chip storage will cause the electrode surface to be oxidized and the surface is stained And so on, the slight contamination of the bonding surface may affect the diffusion of metal atoms between the two, resulting in failure or false welding.

5. 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 functions to form a mirror in the electrode to improve the light output efficiency of the chip, and secondly, it can reduce the amount of gold used when evaporating the electrode to a certain extent, thereby reducing the cost. However, aluminum is a relatively active metal. Once the packaging factory does not control the incoming materials strictly and uses glue with excessive chlorine content, the aluminum reflective layer in the gold electrode will react with the chlorine in the glue, resulting in corrosion.

LED bracket

1. The silver plating layer is too thin

        The existing LED light sources on the market choose copper as the base material of the lead frame. In order to prevent the oxidation of copper, a layer of silver is generally plated on the surface of the support. If the silver plating layer is too thin, the stent is easy to yellow 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 temperatures, copper atoms will diffuse and penetrate to the surface of the silver layer, making the silver layer yellow. The oxidizability of copper is the biggest drawback of copper itself. Once the copper is oxidized, the thermal conductivity and heat dissipation performance will be greatly reduced. Therefore, the thickness of the silver plating layer is very important. At the same time, both copper and silver are susceptible to the corrosion of 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 to 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 vulcanization

        LED light sources are afraid of sulfur. This is because sulfur-containing gas will pass through the porous structure of the silica gel or the gap between the brackets and cause 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 conductivity of the vulcanized silver sulfide increases with the increase in temperature, and leakage is very likely to occur during use; more serious The situation 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 stent is completely vulcanized and corroded, the gold ball appears to fall off, resulting in a dead light.

3. Silver plating layer oxidation

        In the preliminary diagnosis of LED blackening, it is found that the more difficult the sulfur/chlorine/bromine elements are, the more difficult it is to find. However, there are obvious signs of blackening of the silver plating layer of the LED light source, 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 glue will interfere with the determination of the detection results, so 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, etc., combined with professional testing knowledge and electroplating knowledge for comprehensive analysis.

4. Poor plating 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, smoother, and higher protective properties of the coating. This kind of fine crystalline coating is called "microcrystalline deposition." A good electroplating layer should be fine, smooth, uniform, and continuous, and no pollutants, chemical residues, spots, black spots, burnt, rough, pinholes, pits, cracks, delamination, blisters, and bubbles are allowed. Skin wrinkles, coating peeling, yellowing, crystalline coating, partial 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 for checking the quality of the coating, because the protective performance and porosity of the coating are directly related to the thickness of the coating. The special change is the cathode coating. As the thickness increases, the protective performance of the coating also improves. If the thickness of the coating is not uniform, the thinnest part is often destroyed first, and the remaining parts of the coating will lose its protective effect no matter how thick the coating is.

        The coating has more porosity, and corrosive gases such as oxygen will enter and corrode the copper matrix through the pores

5. Organic pollution

        Because a variety of organic substances are used in the electroplating process, if the silver plating layer is not cleaned or the poor quality or deteriorating syrup is used, these residual organic substances will be affected by light, heat and electricity once the light source is lit. Under the action of the organic matter, chemical reactions such as oxidation-reduction may occur to cause discoloration of the surface of the silver plating layer.

6. Nozzle material

        The plastic material 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, and low reflectivity; high water absorption , The stent will change its size and decrease its mechanical strength due to water absorption; it has 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 higher power. Once the power range is exceeded, the initial brightness is very high, but it decays quickly, and the lamp will be dimmed after a few months.

Phosphor

1. Phosphor hydrolysis

        Nitride phosphors are easily hydrolyzed and fail.

2. The self-heating mechanism of phosphor

        The self-heating mechanism of the phosphor makes the temperature of the phosphor layer often higher than the p-n junction of the LED chip. The reason is that the conversion efficiency of phosphors cannot reach 100%, so part of the blue light absorbed by the phosphors is converted into yellow light, and the other part of the light energy absorbed by the phosphors in the high light energy density LED package becomes heat. Since 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, causing local high temperature, and the higher the optical density of the LED The greater the calorific value of the phosphor. When the temperature of the phosphor exceeds 450 degrees Celsius, the silica gel near the phosphor particles will be carbonized. Once the silica gel in a certain place is carbonized and blacked, its light conversion efficiency is lower. This area will absorb more light energy emitted by the LED and convert more heat. The temperature continues to increase, making the carbonized area larger and larger.

Solid crystal glue

1. Stripping of silver glue

        The matrix of conductive silver glue is epoxy resin material, and the coefficient of thermal expansion is much larger than that of chips and brackets. In the environment where the lamp bead is used under the impact of cold and heat, stress will be generated due to thermal problems, and the effect will be greater in an environment with severe temperature changes. To exacerbate, the colloid itself has tensile breaking strength and elongation. When the tensile force exceeds, the colloid will crack. The die-bonding glue peels off at the interface, the heat dissipation deteriorates sharply, the heat generated by the chip cannot be discharged, and the junction temperature rises rapidly, which greatly accelerates the light decay process.

2. Silver glue layering

        Silver powder particles are dispersed in the slurry system in a suspended state. Due to the influence of density difference, charge, cohesion, force, and structure of the dispersion system between the silver powder and the matrix, the phenomenon of sedimentation and stratification of silver powder often occurs. Quickly will cause the product to sag during slurrying, and the coating thickness will be uneven, and even affect the physical and chemical properties of the coating film. Delamination will also affect the heat dissipation, bonding strength and conductivity of the device.

3. Silver ion migration

        A customer used silica gel encapsulation and a vertical flip-chip light source bonded with conductive silver glue to leak electricity. 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 spread from the bottom positive silver glue area to the side of the PN junction on the upper side of the chip. Therefore, it is determined that the leakage failure of the bad lamp is most likely to come from The silver ions of the solid crystal silver glue are caused by ion migration on the side of the chip. The silver ion migration phenomenon is gradually formed during the use of the product. With the increase of the migration phenomenon, the silver ion will eventually conduct the chip PN junction, causing a low resistance path on the side of the chip, resulting in abnormal leakage current of the chip. It even causes a short circuit of the chip. 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 use silica gel encapsulation and silver glue to bond the lamp beads of the vertical flip chip carefully, choose the gold-tin eutectic welding method to fix the chip on the bracket, and strengthen the waterproof characteristic test of the lamp.

4. Solid crystal glue does not dry

        The curing agent of silicone for LED encapsulation contains platinum (platinum) complex, and this platinum complex is very susceptible to poisoning. The poisoning agent is any one containing nitrogen (N), phosphorus (P), and sulfur (S). If the compound is poisoned by the curing agent, the silicone will not be cured completely, which will cause the linear expansion coefficient to be higher and the stress to increase.

Packaging adhesive
1. Poor heat resistance of glue
       

According to our test, pure silica gel does not begin to crack until 400 degrees Celsius, but the heat resistance of modified silica gel with epoxy resin is reduced to the level of epoxy resin. When this modified silica gel is applied to high-power LEDs Or in a high-temperature environment, the colloid will turn yellow and black and crack the lamp.

2. The glue does not dry

        The curing agent of silicone for LED encapsulation contains platinum (platinum) complex, and this platinum complex is very susceptible to poisoning. The poisoning agent is any one containing nitrogen (N), phosphorus (P), and sulfur (S). If the compound is poisoned by the curing agent, the silicone will not be cured completely, which will cause the linear expansion coefficient to be higher and the stress to increase.

        The substances that are prone to silica gel "poisoning" include: organic compounds containing N, P, S, etc.; heavy metal ion compounds such as Sn, Pb, Hg, Sb, Bi, and As; and organic compounds containing unsaturated groups such as ethynyl groups. 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 using Sn catalyst

        Soft polyvinyl cyanide: plasticizer, stabilizer

Flux

        Engineering plastics: flame retardants, enhanced heat-resistant agents, ultraviolet absorbers, etc.

        Silver-plated, gold-plated surface (the electroplating solution during manufacturing is the main reason)

        Degassing generated by the solder register (caused by heating and curing of silicone)

3. The coefficient of expansion of the packaging glue is too large

        In the hot and cold shock environment of the lamp beads, stress will be generated due to the heat problem, and the effect will be aggravated in an environment with drastic temperature changes. The colloid itself has tensile breaking strength and elongation. When the tensile force exceeds, the colloid will be Cracked.

4. The glue contains chlorine

        However, at present, domestic epoxy resin manufacturers generally have small production scales, backward management models and production processes, and low 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 urgently needs to be upgraded. The chlorine in the epoxy resin not only causes chlorination corrosion on the silver plating layer of the stent, alloy wires or other active metals and chip electrodes (aluminum reflective layer), but also can complex with amine curing agents and affect the curing of the resin. Chlorine content is an important physical property index of epoxy resin. It refers to the mass fraction of chlorine contained in epoxy resin, including organic chlorine and inorganic chlorine. Inorganic chlorine will affect the electrical properties of the cured resin. The content of organochlorine indicates the content of the part of the chlorohydrin group that has not undergone ring-closing reaction in the molecule. Its content should be reduced as much as possible, otherwise it will also affect the curing of the resin and the curing

性能。