Effect of temperature gradient on moisture diffusion in high power devices and the applications in LED packages

Abstract

High power electronic devices create hot spots, which give rise to the junction temperature significantly higher than the ambient temperature. This induces a situation that the moisture diffusion from environment to device is in the direction against temperature gradient direction. In addition, far field relative humidity is different from the environment surrounding the packaged devices. In this paper, two mechanisms of moisture transport are studied. First, the localized relative humidity related to the far field ambient environment is investigated. Second, moisture diffusion in the presence of temperature gradient inside package is studied. Several scenarios are investigated with the use of an LED package as example. IntroductionDriven by the demand in compact and lightweight products, there has been a trend for ongoing miniaturization of packages. Similar trends can be observed in the development of LED packages [1,2]. One of the critical concerns is the reliability of such packages under humidity condition [3].High power electronic devices create hot spots, which give rise to the junction temperature significantly higher than the ambient temperature. This creates the temperature gradient, not only inside electronic packages, but also the air environment that surrounds the packaged devices. According to the reliability method developed by Intel [4], it has been conceived that the hot spots in high power electronic devices, such as in CPU devices, will have localized "drying" or a reduction in relative humidity (RH) in a micro-environment. Consequently, it is predicted that the devices at on condition will have longer life than that at off condition under the same environmental humidity conditions.In this paper, we divide the problem into two parts: moisture transport in air and solids, respectively. In air, assuming that the partial moisture pressure is in equilibrium, the relationship between the local relative humidity and the far field relative humidity can be derived. The local relative humidity and temperature can then be used as boundary conditions for solving moisture diffusion in solids. Next, the moisture diffusion in the presence of temperature gradient is investigated in package. Since thermal diffusion in solids is much greater than the moisture diffusivity, the temperature field can be solved with a steady state solution, while the moisture diffusion is treated as a subsequent transient stage analysis. As an example, the above methodology is applied to a mid-high power LED package.

Optical degradation mechanisms of mid-power white-light LEDs in LM-80-08 tests

“…14. Same observation had also been observed by Fan et al [32]. The effect of moisture ingress into the electronic packages, though not LED packages, had been reported as considerable even at a low relative humidity of 10%, as in the case of the aging chamber [33].…”

LED's luminous flux lifetime prediction using a hybrid numerical approach

“…Heat generated inside LEDs must be effectively released out of the system to the environment; otherwise the LED's operating temperature will become higher and higher. Many studies have shown that high operating temperature can damage the LED package in several ways, such as lumen depreciation, die-attach delamination, epoxy degradation, high mechanical stresses due to mismatch of thermal expansion coefficients of materials, decrease of electron-hole recombination efficiency in the active layer, and color shift [5,6,[10][11][12]. Any of these effects can reduce the useful LED lifetime significantly.…”