In recent years, a series of reliability problems such as color temperature drift, reduced lumen, and poor light output caused by the discoloration of LED lamp beads have increased. Many LED product manufacturers and users have suffered serious economic losses. The reliability of LED products is increasingly concerned by manufacturers and users. The quality of the LED lamp beads as the basic unit of the product directly affects the reliability of the finished LED. This article analyzes most of the root causes of discoloration and failure of LED lamp beads.
(1) Residual foreign objects in the encapsulant
Inadvertent introduction of foreign substances in the lamp bead during the packaging process causes the lamp bead to become discolored and black. This part of the reason is basically not very common, and it will be screened out during the factory inspection of the lamp bead.
(2) Discoloration of the colloid caused by the chemical attack of the encapsulant
During the LED lamp assembly process, gas containing sulfur (S) volatilized during the curing process was used to invade the LED encapsulant, which caused further vulcanization and crosslinking reaction of the encapsulant, and then vulcanization and crosslinking again. It causes yellowing and darkening of the encapsulating colloid. Subsequent users switched to plastic tubes that did not use single-component cured silicone rubber, and the lamp beads did not change color. Therefore, LED manufacturers should consider the compatibility of different materials used in each part of the product when selecting materials and manufacturing products to avoid subsequent reliability problems due to incompatible materials.
(3) Phosphor settling
When the lamp beads were assembled into LED lamps and stored in a warehouse, the color temperature drift failed, and the encapsulant of the failed LED lamp beads changed from orange to light yellow. The IV characteristics of the lamp beads were tested and found that the lamp beads can be normally lit, and the IV curve Normal, only the brightness of the light changes. Take some failed lamp beads and take out the encapsulant by mechanical unsealing. It is found that there are transparent particles remaining on the surface of the bracket. Using SEM & EDS to test the composition of the particles, the results show that it contains a high content of strontium (Sr) element.
(4) Foreign body contaminated stent
Because the bracket covered by the foreign object is connected to the pin on the side of the lamp bead, and the pin is soldered with tin and lead. Obviously, if the pins are attached with excess solder paste during the surface mounting of the lamp beads, when soldering, the molten solder will climb along the pins to the surface of the bracket connected thereto, 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 soldered, the solder of the pin welding part enters the surface of the bracket, forming a covering, which causes the lamp bead to discolor.
(5) Corrosion of bracket
Generally, when the LED lamp beads introduce corrosive elements such as sulfur (S) and chlorine (Cl) due to impure materials or process pollution during the production process, under certain conditions (such as high temperature and water vapor residue), their The metal bracket is prone to corrosion, resulting in discoloration and leakage of the lamp beads.
(6) The plating quality of the bracket is poor
The LED lamp bead turns black after discoloration, and the failure rate is as high as 30%. After removing the encapsulant on the surface of the lamp beads, it was found that the silver coating on the surface of the bracket lost its original brightness and appeared gray. Using SEM to observe the micro-morphology of the surface of the bracket, it was found that compared with the unassembled semi-finished bracket, the silver surface of the bracket of the LED failure lamp bead was loose and there were more holes. The semi-finished stent and the failed LED were made into slices, and the quality of the cross-section plating was observed. It was found that the plating structure of the stent was copper nickel plating and silver plating. Compared with the semi-finished product, the nickel plating of the failed stent became thinner and the surface silver layer became loose, The boundaries of the nickel-silver coating become blurred.
The proportion of failures caused by discoloration or corrosion of the stent is the highest. Therefore, LED or bracket manufacturers should take some measures to prevent product failure.
For example: choose a good quality and corrosion-resistant stent substrate; adopt appropriate electroplating process conditions to ensure the formation of a fine-grained, densely structured coating, the thickness of the coating is uniform and meets the protection requirements; for the stent with silver surface coating, choose effective The 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 to ensure that the LED package is tight, to reduce the moisture and oxygen in the environment. The possibility of intrusion and various corrosion.