What is the reason for the yellowing and blackening of LED chips?

LED is an abbreviation for light emitting diode in English. Its basic structure is an electroluminescent semiconductor material chip, which is cured onto a bracket with silver or white glue, and then connected to the chip and circuit board with silver or gold wires. The chip is then sealed with epoxy resin around it to protect the internal core wires. The installation of the housing ensures that the LED light has good seismic performance. The application areas involve daily household appliances and machinery production such as mobile phones, desk lamps, and home appliances.

Reasons for yellowing of LED beads:
It is the fluorescent powder in the LED beads that has been poisoned. When the fluorescent powder is invaded by sulfur or sulfides, poisoning occurs, mainly manifested as blackening and deterioration, losing the function of the fluorescent pool powder, causing the LED beads to turn yellow. This poisoning may have been caused by solder paste during the assembly process, or it could have been obtained during spot welding. This is also related to the quality of LED beads. If the beads have good airtightness, they are less susceptible to external sulfur damage.

After the welding of the surface mounted LED chips is completed, the bottom will turn yellow and black, the fluorescent adhesive will become transparent, the luminous flux will decrease significantly, and the color temperature will increase. The proportion of defects ranges from 1-10%. This is commonly known as LED vulcanization phenomenon in the industry. But there is no solution heard.
The principle is that the bracket of the surface mount LED light bead is silver plated on a copper substrate (the silver layer will shine and reflect light). When the LED is welded at high temperature, it encounters sulfur or sulfur vapor. If the airtightness of the surface mount LED light bead is not particularly good at high temperature, it will cause a chemical reaction between the silver layer on the bracket and sulfur, forming AGS. Depending on the amount of reaction, its color varies from yellow to black. The severe silver layer has reacted completely, causing the gold wire to break and resulting in an open circuit in the LED light bead.