What causes SMD LED beads to turn yellow or black?

LED is an acronym for "Light-Emitting Diode." Its basic structure consists of an electroluminescent semiconductor chip, which is secured to a lead frame using silver or white adhesive. Silver or gold wires are then used to connect the chip to the circuit board. Finally, the assembly is encapsulated in epoxy resin to protect the internal components, and an outer housing is installed; consequently, LED lights possess excellent shock resistance. Their applications span a wide range of fields, including consumer electronics—such as mobile phones, desk lamps, and home appliances—as well as industrial machinery manufacturing.

Causes of Yellowing in LED Chips:

The primary cause is "poisoning" of the phosphor within the LED chip. When the phosphor is infiltrated by sulfur or sulfur compounds, it undergoes a degradation process characterized mainly by darkening and deterioration. This renders the phosphor ineffective, resulting in the LED chip turning yellow. This contamination may originate from the solder paste used during the assembly process or may occur during spot welding. This issue is also directly related to the quality of the LED chip itself; if the chip possesses superior airtightness, it is far less susceptible to external sulfur contamination.

Following the soldering of surface-mount (SMD) LED chips, several defects may manifest: yellowing or blackening of the base, the phosphor encapsulant becoming transparent, a severe decline in luminous flux, and an increase in color temperature. The defect rate typically ranges from 1% to 10%. This phenomenon is commonly referred to within the industry as "LED vulcanization." To date, however, no definitive solution has been identified.

The underlying principle is as follows: The lead frame of an SMD LED chip consists of a copper substrate plated with silver (the silver layer serves to enhance brightness and reflect light). During high-temperature soldering, if the LED is exposed to sulfur or sulfur vapor—and if the chip's airtightness is compromised at elevated temperatures—a chemical reaction occurs between the silver layer on the lead frame and the sulfur: Ag + S → AgS↓. This reaction forms silver sulfide (AgS); depending on the extent of the reaction, the affected area may appear yellow or black. In severe cases where the entire silver layer has reacted, the gold bonding wire may fracture, resulting in an open circuit and total failure of the LED chip.