A sandwich structure of multi-chip COB LED with double flat glass boards coated with phosphors film by screen printing technique

Recently, high power multi-chip COB (chip on board) LED has become industrial trends. The substrate materials of the boards are mainly divided into two types: ceramics and aluminum. However, both of the materials are opaque. Moreover, encapsulation process of the traditional LEDs with mixtures of silicone or epoxy resin encapsulant and phosphors are indispensable procedure for LEDs packaging. The emitting lights still loss in the reflection layers, board, chips, encapsulant, resin and etc. In general, the light extraction efficiency of COB LEDs is low. In this study, transparent flat glasses were utilized as substrate materials. And phosphor film was coated onto the surface of glass by screen printing technique (SPT) to replace the encapsulation process of traditional LED production. Finally, double-sides light emitting structure of white LEDs with high light extraction efficiency was fabricated and characterized. The resulting warm white LEDs with correlated color temperature (CCT) of 3000K exhibited a high luminous efficiency of 150lm/W. The irritation angle was about 250 degree. Color tolerance is less than 7 SCDM. The light extraction of sandwich structure increased by about 25%, compared with traditional COB structures. These results are very valuable and may be used for the application of the high power LED package.