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Objectives: For improved inter-study reproducibility and ultimately improved patient care, researchers and dentists need to know what electromagnetic radiation (light) is emitted from the light-curing unit (LCU) they are using and what is received by the resin. This information cannot be obtained from a dental radiometer, even though many studies have used a dental radiometer.Methods: The light outputs from six LCUs (two QTH and four broad-spectrum LED units) were collected in real-time using an integrating sphere connected to a fiberoptic spectrometer during different light exposures.Results: It was found that the spectral emissions were unique to each LCU, and there was no standardization in what was emitted on the various ramp (soft-start) settings. Relative to the normal use setting, using the ramp setting reduced the radiant energy (J) delivered from each LCU. For one of the four broad-spectrum LED LCUs, the spectral emissions in the violet range did not increase when the overall radiant power output was increased. In addition, this broad-spectrum LED LCU emitted no light from the violet LED chip for the first 5s and only emitted violet light when the ramp phase finished.Conclusions: A single irradiance value derived from a dental radiometer or from a laboratory grade power meter cannot adequately describe the output from the LCU. Manufacturers should provide more information about the light output from their LCUs. Ideally, future assessments and research publications that include resin photopolymerization should report the spectral radiant power delivered from the LCU throughout the entire exposure cycle. [ABSTRACT FROM AUTHOR]

Tin-Lead (Sn-37wt%Pb) eutectic solder joints on a chip scale package (CSP) attached to a printed circuit board have been tested under power cycling from 0 to 100 °C each cycle. Fatigue failure by near interfacial crack growth was discovered to progress from the outside edge of the joints towards the center of the package, for all joints. Creep/fatigue and shear overload microscopic failure modes were found on the solder joint fracture surfaces. The eutectic microstructure of the joints was found to coarsen as the thermal cycling took place, changing the inherent material properties besides accumulating damage. Voids and secondary cracks were shown to have an insignificant effect on the rate and direction of fatigue crack growth compared to the location of the joint on the package, which determines the stress on the joint. The fracture morphology due to fatigue crack growth was nearly identical close to and far from the crack initiation site, revealing similar damage mechanisms during all stages of life. Nonlinear finite element analyses were carried out to estimate the state of stress and inelastic strain in the solder joints under the imposed thermal cycling conditions. Transient heat transfer analysis followed by a structural analysis including plasticity and creep effects was performed using three-dimensional models. The maximum inelastic strains, obtained using the finite element simulation were compared against the crack propagation area at equivalent cycle numbers for several solder joints along the diagonal cross-section of the package. The crack propagation area and the inelastic strain correlated well irrespective of the presence of solder defects, such as voids. A damage-based model is introduced to analyze crack trajectory in the solder joints. The proposed model enables a computationally efficient algorithm for fatigue lifetime and crack front progression estimation in solder joints. [Copyright &y& Elsevier]

Empirical fatigue life models such as the Coffin–Manson rule and its variants are commonly used at the present time to predict the reliability of microelectronic packages. While there have been reports of substantial error in empirical correlations relative to the experiments, this has not been accompanied by a rigorous understanding of the sources of the error. In this paper we systematically explore the various modeling errors in the fatigue life prediction. These errors include those in geometry representation, material behavior, load history and boundary condition application, and in the numerical solution procedure. As part of the study, experimentally validated correlations between temperature cycling and power cycling are developed for a TI 144 chip-scale package. The accuracy of the predicted life under power cycling conditions compared to the experimentally determined life is used as the basis for judging the model accuracy. The criticality of spatial refinement, temporal refinement, and accurate boundary conditions, including the often ignored natural convection boundary conditions, and their effect on predicted life is described in detail. It is shown that model errors can be a significant part of both the constitutive life models and the application models that use the constitutive life models to predict the fatigue life under a given environmental condition. It is also shown that with careful model building, solutions accurate to within 3% can be obtained. [Copyright &y& Elsevier]

Abstract: Ce/Cr co-doped Y3Al5O12 (Ce/Cr:YAG) exhibits enriched emission spectrum in the red region due to a non-radiative energy transfer from the Ce3+(2E) level to Cr3+(4T) level. Excitation spectra collected at Cr3+ red emission show an intense excitation broad band at 458nm confirming the energy transfer, which is responsible for the two orders of magnitude increase in the intensity of the red emission compared to YAG doped with Cr alone. The huge increase in the Cr red emission in Ce/Cr:YAG compared to Cr:YAG suggests that the former is an efficient red and yellow phosphor and has potential in white LEDs and laser applications. [Copyright &y& Elsevier]