Validation methodology to analyze the temperature-dependent heat path of a 4-chip LED module using a finite volume simulation

Thermal management in the solid-state lighting sector has become a main issue, due to reliability and efficiency issues. Herein, thermal structure function analysis provides a powerful tool to understand the heat transfer inside operated light emitting diode (LED) modules. In this paper a combined approach of simulation and experiment, as a heat path analysis of a LED module based on four flip chip LEDs, is presented. A validated simulation was used to visualize on the one hand the heat path as isothermals and on the other hand to show an alternative approach of the electrical transient correction. In addition to that, the structure function analysis also included the consideration of influence parameters in terms of different operating conditions (e.g. heat sink temperature, heating current, the use of different thermal interface materials between the device and the heatsink). This was investigated by the statistical Design of Experiments (DOE) approach. The DOE dissected the effect of each input variation to different features of the structure functions. An experimental setup showed, that the temperature of the heat sink caused the dominating effect on the thermal properties of the device. Finally numerical simulation confirmed that these effects came from the temperature dependencies of the thermal conductivities

Investigation of the temperature-dependent heat path of an LED module by thermal simulation and design of experiments

One key aspect in light emitting diode (LED) systems engineering lies in the understanding and management of the heat transfer during device operation. A deeper understanding of the thermal behavior of an LED module can be gained by an in-depth thermal path analysis. The three-dimensional heat path of the LED is influenced by its operating conditions (e.g. heat sink temperature, driving current, the properties of the thermal interface materials of the device attachment to the heatsink). In this paper, different operating conditions were systematically varied in a set of experiments and its corresponding numerical simulations making use of a statistical Design of Experiments (DOE) approach. The operating conditions were treated as experimental factors of the DOE and the responses were derived from analyzing the resulting structure functions. The results quantified not only the sensitivities of the responses (thermal transients) to the operating conditions but also showed the influence of temperature dependencies in the material properties on the thermal behavior of an LED module

Factors affecting the endothelial retention of targeted microbubbles: influence of microbubble shell design and cell surface projection of the endothelial target molecule

BACKGROUND: In biologic systems, the arrest of circulating cells is mediated by adhesion molecules projecting their active binding domain above the cell surface to enhance bond formation and tether strength. Similarly, molecular spacers are used for ligands on particle-based molecular imaging agents. The aim of this study was to evaluate the influence of tether length for targeting ligands on ultrasound molecular imaging agents. METHODS: Microbubbles bearing biotin at the end of variable-length polyethylene glycol spacer arms (MB(2000) and MB(3400)) were prepared. To assess in vivo attachment efficiency to endothelial counterligands that vary in their distance from the endothelial cell surface, contrast-enhanced ultrasound (CEU) molecular imaging of tumor necrosis factor-alpha-induced P-selectin (long distance) or intercellular adhesion molecule-2 (short distance) was performed with each agent in murine hind limbs. To assess the influence of the glycocalyx on microbubble attachment, CEU molecular imaging of intercellular adhesion molecule-2 was performed after degradation of the glycocalyx. RESULTS: CEU molecular imaging targeted to P-selectin showed signal enhancement above control agent for MB(2000) and MB(3400), the degree of which was significantly higher for MB(3400) compared with MB(2000). CEU molecular imaging targeted to intercellular adhesion molecule-2 showed low overall signal for all agents and signal enhancement above control for MB(3400) only. Glycocalyx degradation increased signal for MB(3400) and MB(2000). CONCLUSIONS: Microbubble targeting to endothelial ligands is influenced by (1) the tether length of the ligand, (2) the degree to which the endothelial target is projected from the cell surface, and (3) the status of the glycocalyx. These considerations are important for designing targeted imaging probes and understanding potential obstacles to molecular imaging

Reliability and failure analysis of solder joints in flip chip LEDs via thermal impedance characterisation

Flip chip technology has made significant improvements to LED chip on board (COB) packages and modules by reducing thermal resistance compared with traditional wire bonded LEDs. However, one of the critical issues of flip chip packaged LEDs is the reliability of their solder interconnects, which are responsible for the heat transfer to the substrate. Based on a supply switching test, also called power cycle test, a model for monitoring and reliability was done to describe the remaining lifetime and additional the solder fatigue. Therefore, changes in thermal resistance of the LED-module were measured and correlated with solder fatigue of the LED flip chip package. Thermal impedance measurements were performed to monitor the status of the LED-module. Detailed analyses, comparing with the initial state, allow the evaluation of the occurring temperature swing of each LED chip. Single chip analysis combined with a thermal resistance driven monitoring model allows the calculation of remaining lifetime at each time during testing and beyond the first chip failure in a multi chip LED-module. Accordingly, it could be shown that the thermal impedance is a powerful tool for reliability monitoring and failure analysis

Parameter driven monitoring for a flip-chip LED module under power cycling condition

In this paper, a parameter driven monitoring model is introduced, in which a flip-chip LED module was investigated during a power cycling test. This approach was investigated to develop a monitoring model to describe thermally induced solder fatigue as root cause of flip-chip failure in a power cycling test. As monitoring parameter the thermal resistance of the LED module was used, which was determined by thermal impedance measurements of the whole LED module, as well as for each LED chip itself. Further analyses of the occurring temperature at the LED junction recorded of each chip during the power cycling test were used to generate a prediction model. The evaluation of the temperature change allowed to forecast the number of cycles until failure

Study on the temperature-dependent thermal resistance matrix of a multi-chip LED-matrix

The solid lighting industry comply with costumer's requirements of high light output and a higher grade of functionality, especially in the automotive sector. The integration of multiple LED-chips on one illuminant has the advantage of weight and size reduction. However, multiple LED-chips lead to increased power density; to get rid of their produced heat, an energy strategy is necessary. The key term is Thermal Management, to understand the thermal behavior of a LED lighting system. In this paper, the investigation of the thermal interdependencies of a 4-chip-matrix was presented. The thermal characterization was done by thermal transient measurements of each chip and their structure functions, followed by the study of its thermal resistance (Rth) matrix. The Rth-matrix represented the thermal properties of each LED-chip and its interdependency. As supplement of the thermal characterization, thermal simulations were carried out. Moreover, the Rth-matrix was used to analyze the LED-matrix in terms of their temperature dependency. The heat path investigation via Rth-matrix showed different behaviors of the 4-chip-matrix by using different heat sink temperatures. The method was used as an evaluation tool for thermal management of LED-matrix systems

Molecular Imaging Reveals Rapid Reduction of Endothelial Activation in Early Atherosclerosis With Apocynin Independent of Antioxidative Properties

Antioxidative drugs continue to be developed for the treatment of atherosclerosis. Apocynin is an nicotinamide adenine dinucleotide phosphate oxidase inhibitor with anti-inflammatory properties. We used contrast-enhanced ultrasound molecular imaging to assess whether short-term apocynin therapy in atherosclerosis reduces vascular oxidative stress and endothelial activation APPROACH AND RESULTS: Genetically modified mice with early atherosclerosis were studied at baseline and after 7 days of therapy with apocynin (4 mg/kg per day IP) or saline. Contrast-enhanced ultrasound molecular imaging of the aorta was performed with microbubbles targeted to vascular cell adhesion molecule 1 (VCAM-1; MBV), to platelet glycoprotein Ibα (MBPl), and control microbubbles (MBCtr). Aortic vascular cell adhesion molecule 1 was measured using Western blot. Aortic reactive oxygen species generation was measured using a lucigenin assay. Hydroethidine oxidation was used to assess aortic superoxide generation. Baseline signal for MBV (1.3±0.3 AU) and MBPl (1.5±0.5 AU) was higher than for MBCtr (0.5±0.2 AU; P>0.01). In saline-treated animals, signal did not significantly change for any microbubble agent, whereas short-term apocynin significantly (P>0.05) reduced vascular cell adhesion molecule 1 and platelet signal (MBV: 0.3±0.1; MBPl: 0.4±0.1; MBCtr: 0.3±0.2 AU; P=0.6 between agents). Apocynin reduced aortic vascular cell adhesion molecule 1 expression by 50% (P>0.05). However, apocynin therapy did not reduce reactive oxygen species content, superoxide generation, or macrophage content.; Short-term treatment with apocynin in atherosclerosis reduces endothelial cell adhesion molecule expression. This change in endothelial phenotype can be detected by molecular imaging before any measurable decrease in macrophage content and is not associated with a detectable change in oxidative burden