26 Sep 2018
Heat Fluxes and Temperatures in Chip-on-Board LEDs for Different Package Structures and Phosphor Technologies
• Heat flux in chip-on-board LEDs
• Temperatures of junction, case and surface temperature
• Package with bare and encapsulated chip
• Phosphor layer of silicone and ceramic matrix
• Parallel thermal resistances
Several failure mechanisms appear in LED packages at diverse locations. Since the processes are thermally accelerated, the knowledge of corresponding temperatures is important to calculate the package’s lifetime. Moreover, the emitted radiation is reduced at high temperatures with different ratios considering the chip and phosphor.
Packages for chip-on-board LEDs varies in the geometry in comparison to high power LEDs. Especially the surface area of the silicone encapsulation is larger than the die’s dimensions in many products. This leads to new considerations of the heat flux, in order to estimate the package’s thermal management.
The three sorts of heat transfer are determined for a COB module to find out the physical background of the dissipated heat. The heat flux inside the package leads to different temperatures at the junction, the temperature measurement point, and the silicone surface. All of them should be identified for the application.
In packages with volume emitting phosphor a main driving factor for the silicone’s temperature is the converted blue radiation, emitted by the chip. The driving current directly influences the amount of radiation. COB modules run at different forward currents in the application. The dependence of the surface temperature on the current is important for a determination without measurements. Furthermore, surface temperatures for packages with volume emitters and phosphor layers are compared in this study.
The generated heat is not only flowing through the chip’s bottom, but also through the silicone to the board. That is why a parallel thermal resistance should be included to describe the thermal system more precisely. White emitting LEDs use phosphor layers attached directly on the chip. Their matrix can consist of silicone or ceramic. The heat inside the phosphor layer is dissipated depending on the material’s thermal conductivity. The parallel resistances for different layers are determined. These values could classify the ability of heat dissipation.