LED is the light emitting diode, which is a semiconductor solid light emitting device that converts electric energy into light energy. Its core is the PN junction. In addition to the forward conduction, reverse cut-off and breakdown characteristics of the general PN junction, it also has the light emitting characteristics under certain conditions. Its structure mainly includes the following parts: lead, bracket, encapsulation glue, bonding wire, LED chip, solid crystal glue and phosphor. The discoloration failure of LED lamp bead is closely related to its material, structure, packaging process and service conditions. The following will analyze the causes of its discoloration through specific cases.
Cause of sealant
1 Foreign matters remained in the sealant
The appearance of the failed lamp bead shows local discoloration and blackening, as shown in Figure 2. Uncover the packaging adhesive and find a black foreign matter mixed in the packaging adhesive. Use scanning electron microscope and energy dispersive spectrometer (SEM&EDS) to analyze the composition of the foreign matter and confirm that its main components are aluminum (Al), carbon (C), oxygen (O), and also contain a small amount of impurity elements. The test results are shown in Figure 3. According to the failure background fed back by users, the foreign object was introduced during the packaging process.
2. Colloid discoloration of packaging adhesive due to chemical corrosion
The defective product is the glass light tube lamp. The internal LED light strip is bonded and fixed on the glass tube with one component room temperature curing silicone rubber. The LED light beads on the light strip at the adhesive fixing position turn yellow and dark. The invalid lamp bead sealant is made of silicone rubber. SEM&EDS was used to test the element composition of the sealant, and it was found that sulfur (S) was more detected than the normal lamp bead sealant.
Generally, sulfur, organic disulfide, polysulfide and other sulfur containing substances can be used as vulcanizing agents to cause vulcanization crosslinking reaction of rubber, thus changing the structure of rubber, showing the phenomenon of yellowing and darkening in color and increasing in thermal decomposition temperature. According to the TGA test of the thermal decomposition temperature of the lamp bead packaging gel, the temperature of the failed lamp bead packaging rubber at the weight loss of 2%, 5%, 10%, 15% and 20% is more than 25 ℃ higher than the same weight loss temperature of the same batch of good product packaging rubber. The thermal decomposition curve of the packaging rubber is shown in Figure 5, which confirms the phenomenon that the thermal decomposition temperature of the packaging rubber increases due to vulcanization and crosslinking. ICPOES was used to further analyze the chemical composition of the fixed single component cured silicone rubber and found that it contained about 400ppm sulfur (S).
It can be seen that the reason why the LED lamp beads turn yellow and dark is that the sulfur containing (S) gas volatilized from the one component room temperature curing silicone rubber used for bonding and fixing in the glass lamp tube during the curing process invades into the LED packaging rubber, which causes further vulcanization and crosslinking reaction of the packaging rubber, and the secondary vulcanization and crosslinking lead to the yellow and dark of the packaging gel. Subsequent users switched to plastic lamp tubes without one component cured silicone rubber, and the lamp beads did not change color. Therefore, the LED manufacturer shall consider the compatibility of different materials used in various parts of the product during product design, material selection and manufacturing to avoid subsequent reliability problems caused by incompatible materials.
Phosphor sedimentation
When the lamp beads were assembled into LED lamps and stored in the warehouse, the color temperature drift failure occurred. The encapsulation glue of the failed LED lamp beads changed from orange to light yellow. Through I-V characteristic test, it was found that the lamp beads could light normally, and the I-V curve was normal, but the brightness changed. Take some invalid lamp beads and take out the sealing glue by mechanical opening. It is found that transparent particles remain on the surface of the bracket. SEM&EDS is used to test the particle composition, and the results show that it contains high content of strontium (Sr), as shown in Figure 6; High content of strontium (Sr) and barium (Ba) were also detected on the contact surface between the encapsulation adhesive and the bracket.
In contrast, after the high-quality lamp beads are unsealed, the surface of the bracket is relatively clean. The main components of the surface are silver (Ag) and a small amount of carbon (C). Strontium (Sr) is not detected, and strontium (Sr) and barium (Ba) are not detected on the contact surface between the encapsulation adhesive and the bracket. Through testing the cross section composition of the defective product and the good product lamp bead sealant, it is known that the composition of the fluorescent powder used for both is the same, which is a mixture of yttrium aluminum garnet (mainly composed of oxygen (O), aluminum (Al) and yttrium (Y)) and strontium barium silicate (mainly composed of carbon (C), oxygen (O), silicon (Si), strontium (Sr), barium (Ba) and calcium (Ca)).
Therefore, the failure reason of LED lamp beads is that the silicate phosphor used has settled on the bottom of the packaging adhesive and the surface of the bracket, resulting in dispersion due to inconsistent light refraction laws, leading to color temperature drift and color change of the lamp beads.
Support cause
1 Foreign matters contaminate the support
One side of the failed lamp bead is discolored. After uncovering the sealant, 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. Uncover the white plastic around the discolored part of the lamp bead, and tin (Sn) and lead (Pb) components were also detected on the surface of the bracket in contact with the white plastic. Since the bracket of the foreign matter covered part is connected with the pin on the side of the lamp bead, the pin is soldered with tin lead.
It is obvious that if the pin is stained with excess solder paste when the lamp bead is surface mounted, the melted solder will climb up to the surface of the bracket connected to it along the pin to form a covering layer during welding. 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 pin welding part enters the surface of the bracket, forming a covering, which leads to the color change of the lamp bead.
2 Support corrosion
The middle part of the failed LED lamp bead is discolored and blackened. After unpacking, it is observed under the optical microscope that the surface of the whole bracket is significantly blackened. SEM&EDS is used to test the components of the blackened bracket. The results show that, in addition to the normal material components, the blackened bracket also contains a high content of corrosive sulfur (S) elements, and the silver coating on the bracket surface also shows a loose corrosion morphology, as shown in Figure 9. In general, when corrosive elements such as sulfur (S) and chlorine (Cl) are introduced into LED lamp beads due to material impurity or process pollution during production, under certain conditions (such as high temperature, water vapor residue, etc.), the metal supports of LED lamp beads are prone to corrosion, resulting in color change, electric leakage and other failures.
3 Poor quality of bracket coating
After the LED lamp beads are turned on and aged, they appear discoloration and blackening, and the failure rate is up to 30%. After removing the encapsulation glue on the lamp bead surface, it was found that the silver coating on the bracket surface lost its original brightness and was gray. SEM was used to observe the surface micromorphology of the support, and it was found that the silver layer on the support surface of the LED failure lamp beads was loose and had more holes than that of the unassembled semi-finished support.
The semi-finished product support and the failed LED were sliced, and the coating quality of its section was observed. It was found that the coating structure of the support was copper nickel plating followed by silver plating. Compared with the semi-finished product, the nickel coating of the failed product support became thinner, the silver layer on the surface became loose, and the boundaries of the nickel silver coating became blurred. The cross-section morphology of the support of the sample was 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. It is obvious that the nickel coating diffused to the surface of the silver layer.
It can be concluded that the reason for LED lamp bead discoloration is that the used bracket coating is poor, after aging, the silver layer is loose and has holes, and the nickel layer diffuses to the silver layer surface through the silver layer holes, resulting in the silver layer blackening and the lamp bead discoloration.
In many LED discoloration failure cases, the failure rate caused by discoloration or corrosion of the bracket is the highest. Therefore, the LED or bracket manufacturer should take some measures to prevent product failure. For example, select the support base material with good quality and corrosion resistance; Appropriate electroplating process conditions shall be adopted to ensure the formation of a coating with fine grains and dense structure, and the coating thickness shall be uniform and meet the protection requirements; For the support whose surface layer is silver, select effective silver protection process to improve the anti tarnish ability of the silver support; In the process of LED production and assembly, it is necessary to prevent the introduction of foreign pollution or corrosive substances, and ensure that the LED package is tight, so as to reduce the possibility of various corrosion caused by the intrusion of water vapor and oxygen in the environment.
The above analyses the causes and mechanisms of LED lamp bead discoloration failure caused by abnormal encapsulation adhesive, phosphor and support components, hoping to provide reference and guidance for the industry, so that LED manufacturers can take effective measures to prevent these failures in the process of material selection and manufacturing, and further improve the reliability of LED finished products.