LED is a light-emitting diode, which is a semiconductor solid-state light-emitting device that converts electrical energy into light energy. Next, it also has luminous properties. Its structure mainly includes the following parts: leads, brackets, packaging glue, bonding wires, LED chips, die-bonding glue and phosphors. The discoloration failure of LED lamp beads is closely related to its material, structure, packaging process and service conditions. The following will analyze the cause of discoloration through specific cases.

     The reason for encapsulation

  1 Foreign matter remaining in the encapsulant

The appearance of the failed lamp bead is partially discolored and blackened, as shown in Figure 2. The packaging glue was uncovered, and a black foreign matter was found to be mixed in the packaging glue. The composition of the foreign matter was analyzed with a scanning electron microscope and an energy dispersive spectrometer (SEM&EDS), and it was confirmed that its main components were aluminum (Al), carbon (C), oxygen (O ) elements and a small amount of impurity elements, the test results are shown in Figure 3. Combined with the failure background of user feedback, it can be seen that the foreign matter was introduced during the packaging process.

  2 The encapsulation glue is corroded by chemical substances and the color of the colloid changes

The failed product is a glass light tube lamp, and the internal LED light strip is bonded and fixed on the glass tube with a one-component room temperature curing silicone rubber. The LED light beads on the light strip at the glued part appear yellow and dark. The material of the failed lamp bead encapsulant is silicone rubber. SEM&EDS was used to test the elemental composition of the encapsulant, and it was found that more sulfur (S) elements were detected in it than in the normal lamp bead encapsulant.

Usually, sulfur-containing substances such as sulfur, organic disulfides and polysulfides can be used as vulcanizing agents to cause vulcanization and cross-linking reactions in rubber, thereby changing the structure of rubber, showing the phenomenon of yellowing and darkening in color and rising thermal decomposition temperature. . Through the TGA test of the thermal decomposition temperature of the lamp bead packaging gel, it can be known that the temperature of the failed lamp bead packaging gel at the weight loss of 2%, 5%, 10%, 15% and 20% is higher than that of the same batch of good product packaging gel. If the temperature is higher than 25 ℃, the thermal decomposition curve of the encapsulation adhesive is shown in Figure 5, which confirms that the thermal decomposition temperature of the encapsulation adhesive increases due to vulcanization and crosslinking. ICPOES was used to further analyze the chemical composition of the one-component curing silicone rubber used for fixing, and it was detected that it contained about 400ppm of sulfur (S) element.

It can be seen that the reason for the yellowing and darkening of the LED lamp beads is that the one-component room temperature curing silicone rubber used for bonding and fixing in the glass lamp tube intrudes into the LED encapsulant during the curing process. , causing further vulcanization and cross-linking reactions to occur in the encapsulant, and re-vulcanization and cross-linking cause the encapsulation colloid to turn yellow and dark. Subsequent users switched to plastic lamps that did not use one-component curing silicone rubber, and there was no discoloration of the lamp beads. Therefore, LED manufacturers should consider the compatibility of different materials used in various parts of the product when selecting materials for product design and manufacturing, so as to avoid subsequent reliability problems caused by material incompatibility.

     Phosphor settling

When the lamp beads were assembled into LED lamps and stored in the warehouse, the color temperature drift failure occurred, and the packaging glue of the failed LED lamp beads changed from orange to light yellow. The I-V characteristic test was carried out on it, and it was found that the lamp beads could be lit normally, and the I-V curve Normal, only the brightness of the light changes. Take some failed lamp beads, take out the packaging glue by mechanical unsealing, and find that transparent particles remain on the surface of the bracket, use SEM&EDS to test the composition of the particles, and the results show that it contains a high content of strontium (Sr) elements, as shown in Figure 6; High content of strontium (Sr) and barium (Ba) elements were also detected on the interface between the glue and the stent.

In contrast, after unsealing good-quality lamp beads, the surface of the bracket is relatively clean, the main components of the surface are silver (Ag) and a small amount of carbon (C) elements, and no strontium (Sr) element is detected, and the packaging glue and the bracket Strontium (Sr) and barium (Ba) elements were also not detected on the contact surface. By testing the cross-sectional composition of the failed product and the good product lamp bead packaging glue, it is known that the phosphor powder used in the two has the same composition, both of which are yttrium aluminum garnet (the main components are oxygen (O), aluminum (Al) and yttrium (Y) ) and strontium barium silicate (main components are carbon (C), oxygen (O), silicon (Si), strontium (Sr), barium (Ba) and calcium (Ca)) mixed phosphor.

Therefore, the reason for the failure of the LED lamp bead is that the silicate phosphor used has settled to the bottom of the encapsulant and the surface of the bracket, resulting in dispersion due to inconsistent light refraction laws, resulting in color temperature drift and discoloration of the lamp bead.

     Cause of stent

  1 Foreign matter contamination of the bracket

One side of the failed lamp bead changes color. After removing the encapsulant, it can be seen that the surface of the bracket at the discolored part is covered with a layer of foreign matter. The element composition test of the foreign matter shows that its main components are tin (Sn) and lead (Pb) elements. The measured results are shown in Figure 8. The white plastic around the discolored part of the lamp bead was uncovered, and tin (Sn) and lead (Pb) components were also detected on the surface of the bracket in contact with the white plastic. Because the bracket at the part covered by foreign matter is connected to the pin on one side of the lamp bead, and the pin is soldered with tin-lead.

Obviously, if the lamp beads are surface-mounted, the pins are covered with excess solder paste, then during soldering, the molten solder will climb along the pins to the surface of the bracket connected to it, forming a covering layer. Therefore, the reason for the failure of the LED lamp bead in this case is that when the LED lamp bead is assembled and welded, the solder at the soldering part of the pin enters the surface of the bracket and forms a covering, which causes the lamp bead to change color.

  2 bracket corrosion

The middle part of the failed LED lamp bead became discolored and blackened. After unpacking, it was observed under an optical microscope, and it was found that the surface of the entire bracket was obviously blackened. SEM&EDS was used to test the components of the blackened bracket. The results showed that, in addition to the normal material composition , the blackened stent also has a relatively high content of corrosive sulfur (S) elements, and the silver-plated layer on the surface of the stent also presents a loose corrosion morphology locally, as shown in Figure 9. Usually, in the production process of LED lamp beads, when corrosive elements such as sulfur (S) and chlorine (Cl) are introduced due to the impurity of the material itself or the pollution of the process, under certain conditions (such as high temperature, water vapor residue, etc.), its The metal bracket is extremely prone to corrosion, resulting in discoloration, leakage and other failures of the lamp beads.

  3 Poor quality of stent plating

After the LED lamp beads are lit and aged, they will change color and blacken, and the failure rate is as high as 30%. After removing the encapsulation glue on the surface of the lamp bead, it was found that the silver plating on the surface of the bracket lost its original brightness and appeared gray. Using SEM to observe the microscopic morphology of the surface of the bracket, it is found that compared with the unassembled semi-finished bracket, the silver layer on the surface of the LED failure lamp bead is loose and has more holes.

The semi-finished bracket and the failed LED were made into slices, and the coating quality of the section was observed. It was found that the coating structure of the bracket was nickel-plated copper and then silver-plated. Compared with the semi-finished product, the nickel coating of the bracket became thinner, and the silver layer on the surface became loose. The boundary of nickel-silver plating becomes blurred, and the cross-sectional morphology of the sample bracket is shown in Figure 10. AES was used to test the composition of the superficial layer of the failed LED bracket, and it was found that there would be nickel (Ni) element in it. The test results are shown in Figure 11b. Obviously, the nickel plating diffused to the surface of the silver layer.

It can be concluded that the reason for the discoloration of the LED lamp bead is that the coating of the bracket used is poor. After aging, the silver layer is loose and produces holes, and the nickel layer diffuses to the surface of the silver layer through the holes of the silver layer, resulting in blackening of the silver layer and discoloration of the lamp bead.

Among the numerous LED discoloration failure cases, the proportion of failures caused by bracket discoloration or corrosion is the highest. Therefore, LED or bracket manufacturers should take some measures to prevent product failure. For example: choose a good-quality, corrosion-resistant bracket base material; adopt suitable electroplating process conditions to ensure the formation of a coating with fine grains and a dense structure, and the thickness of the coating is uniform and meets the protection requirements; The advanced silver protection process improves the anti-discoloration ability of the silver bracket; in the process of LED production and assembly, it should prevent the introduction of external pollution or corrosive substances, and ensure that the LED is tightly packaged to reduce the impact of water vapor and oxygen in the environment. The possibility of various corrosions caused by intrusion.

The reasons and mechanism of discoloration and failure of LED lamp beads caused by abnormal encapsulation glue, phosphor powder and bracket components have been analyzed above, hoping to provide reference and guidance for the industry, so that LED manufacturers can take effective measures in the process of material selection and manufacturing to prevent these problems. The occurrence of failure phenomenon further improves the reliability of LED finished products.