The reliability of LED products has been increasingly concerned by manufacturers and users. As the basic unit of the product, the quality of the LED lamp bead directly affects the reliability of the finished LED product. In practical applications, the failure of the LED lamp bead often leads to abnormal function or even complete failure of the finished product. In recent years, cases of a series of reliability problems such as color temperature drift, lumen reduction and poor light output effect have been increasing due to discoloration of lamp beads, causing many LED product manufacturers and users to suffer serious economic losses.

1. Reasons for encapsulation

(1) Residual foreign matter in the encapsulant

    The appearance of the failed lamp beads shows local discoloration and blackening. When the encapsulant was uncovered, it was found that there was a black foreign matter mixed in the encapsulant. The components of the foreign matter were analyzed by scanning electron microscope and energy dispersive spectrometer (SEM&EDS) [5-6], and it was confirmed that its main components were aluminum (Al), carbon (C) ), oxygen (O) elements, and also contains a small amount of impurity elements. Combining with the failure background reported by the user, it can be known that the foreign matter was introduced during the packaging process.

(2) Colloidal discoloration occurs when the encapsulant is eroded by chemical substances

    The failed product is a glass light tube lamp. The LED light strip inside is bonded and fixed on the glass tube with one-component room temperature curing silicone rubber. The LED lamp beads on the light strip at the solid glue part appear yellow and dark. The material of the failed lamp bead encapsulant is silicone rubber. SEM&EDS was used to test the element composition of the encapsulant, and it was found that more sulfur (S) elements were detected than the normal lamp bead encapsulant.

2. Phosphor sedimentation

    After the lamp beads were assembled into LED lamps and stored in the warehouse, the color temperature drift failure occurred, and the encapsulation glue of the failed LED lamp beads changed from orange to light yellow. The IV characteristic test was carried out on it, and it was found that the lamp beads could light up normally, and the IV curve Normal, only the brightness of the light output changes. Take some failed lamp beads and take out the encapsulant by mechanical unpacking. It is found that transparent particles remain on the surface of the bracket. The particle composition is tested by SEM&EDS, and the results show that it contains a high content of strontium (Sr) element.

    In contrast, after the good-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) elements, and no strontium (Sr) element is detected. Strontium (Sr) and barium (Ba) elements were also not detected on the contact surface. By testing the cross-sectional composition of the failed and good lamp bead encapsulant, it is known that the phosphors used in both have the same composition, both of which are yttrium aluminum garnet (the main components are oxygen (O), aluminum (Al) and yttrium (Y) . ) mixed with barium strontium silicate (the main components are carbon (C), oxygen (O), silicon (Si), strontium (Sr), barium (Ba) and calcium (Ca)).