2023-12-29
The three major reasons for the discoloration of LED beads are 6 cases
The three major reasons for the discoloration of LED beads are 6 cases
The reliability of LED products has been increasingly concerned by manufacturers and users. As a basic component of the product, the quality of LED beads directly affects the reliability of LED products, and often occurs in practical applications due to the failure of LED beads leading to functional abnormalities or even complete failure of the finished product. In recent years, a series of reliability problems such as color temperature drift, lumen reduction and poor light effect have been increasing due to the discoloration of lamp beads, which has caused many LED product manufacturers and users to suffer serious economic losses. This paper analyzes the root cause of discoloration failure of LED lamp beads through several failure cases.
LED beads discoloration reasons, packaging glue reasons
(1) Residual foreign bodies in the packaging glue
The appearance of the failure lamp bead presents local discoloration and blackness. When the encapsulation was uncovered, a black foreign body was found mixed in the encapsulation. The composition analysis of the foreign body was carried out by SEM and energy dispersive spectrometer (SEM&EDS) [5-6], and it was confirmed that the main components of the foreign body were aluminum (Al), carbon (C), oxygen (O), and a small amount of impurity elements were also contained. The test results are shown in the figure below. Combined with the failure background of user feedback, it can be seen that the foreign body was introduced in the packaging process.
(2) The packaging adhesive is eroded by chemicals and colloidal discoloration occurs
The failed product is the glass light tube lamp, and the internal LED lamp strip is fixed on the glass tube using single-component room temperature curing silicone rubber. The LED lamp bead on the lamp belt of the solid adhesive part appears yellow and dark phenomenon. SEM&EDS was used to test the element composition of the failed lamp bead encapsulation adhesive, and it was found that more sulfur (S) was detected than that of the normal lamp bead encapsulation adhesive.
Usually sulfur, organic disulfide and polysulfide and other sulfur-containing substances can be used as vulcanizing agents, so that rubber vulcanization cross-linking reaction, so that the structure of rubber changes, showing yellow and dark color, thermal decomposition temperature rise phenomenon.
According to the TGA test thermal decomposition temperature of the lamp bead encapsulation gel, the temperature of the failed lamp bead encapsulation adhesive at the weight loss of 2%, 5%, 10%, 15% and 20% is more than 25 ℃ higher than the temperature of the same batch of good product encapsulation adhesive at the same weight loss. The thermal decomposition curve of the packaging adhesive is shown as the following figure. It was confirmed that the thermal decomposition temperature of the packaging adhesives increased due to vulcanization crosslinking. ICPOES was used to further analyze the chemical composition of the fixed one-component cured silicone rubber, which was detected to contain about 400 ppm of sulfur (S).
It can be seen that the reason for the yellow and dark LED lamp beads is that the sulfur containing (S) gas volatilized by the single-component room temperature curing silicone rubber used for bonding and fixing in the glass lamp tube intrudes into the LED packaging glue during the curing process, making the packaging glue undergo further vulcanization cross-linking reaction, and the revulcanization cross-linking causes the packaging colloids to turn yellow and dark. The subsequent user changed to the plastic lamp tube without the use of single-component cured silicone rubber, and there was no phenomenon of lamp beads discoloration.
Therefore, LED manufacturers should consider the matching between different materials used in product design and manufacturing, to avoid the subsequent reliability problems caused by material incompatibility
Usually sulfur, organic disulfide and polysulfide and other sulfur-containing substances can be used as vulcanizing agents, so that rubber vulcanization cross-linking reaction, so that the structure of rubber changes, showing yellow and dark color, thermal decomposition temperature rise phenomenon.
According to the TGA test thermal decomposition temperature of the lamp bead encapsulation gel, the temperature of the failed lamp bead encapsulation adhesive at the weight loss of 2%, 5%, 10%, 15% and 20% is more than 25 ℃ higher than the temperature of the same batch of good product encapsulation adhesive at the same weight loss. The thermal decomposition curve of the packaging adhesive is shown as the following figure. It was confirmed that the thermal decomposition temperature of the packaging adhesives increased due to vulcanization crosslinking. ICPOES was used to further analyze the chemical composition of the fixed one-component cured silicone rubber, which was detected to contain about 400 ppm of sulfur (S).
It can be seen that the reason for the yellow and dark LED lamp beads is that the sulfur containing (S) gas volatilized by the single-component room temperature curing silicone rubber used for bonding and fixing in the glass lamp tube intrudes into the LED packaging glue during the curing process, making the packaging glue undergo further vulcanization cross-linking reaction, and the revulcanization cross-linking causes the packaging colloids to turn yellow and dark. The subsequent user changed to the plastic lamp tube without the use of single-component cured silicone rubber, and there was no phenomenon of lamp beads discoloration.
Therefore, LED manufacturers should consider the matching between different materials used in product design and manufacturing, to avoid the subsequent reliability problems caused by material incompatibility
