Numerical study of thermomechanical fatigue influence of intermetallic compounds in a lead free solder joint

As electronics is increasingly present the reliability of automotive and aircraft equipment is linked to the reliability of electronic boards. Solder bumps are subjected to multiple stresses (e.g. mechanical, thermal, thermo-mechanical, coupled electro-thermal) due to usage conditions. In the scope of RoHS directive, solder joints are made of lead-free alloys. Very promising candidates to replacing standard SnPb solders in electronic assemblies are based on Tin-Silver-Copper alloys, commonly referred to as SAC. The intermetallic compounds (IMC) located at the interfaces of a lead free solder joint form a layer with usually different mechanical properties from the rest of the volume. They depend on the type of finishes and the solder alloy reacting together. The resulting compound is stiffer and more fragile than the solder itself. The IMC are of great concern when it comes to the thermomechanical fatigue characterization of a solder. Indeed, the thickness of the IMC layer increases in function of the exposure duration to high temperature due to atomic diffusion. The whole rigidity of the assembly increases and can lead to some changes in the solder behavior in the vicinity of the interfaces. Fatigue characterization requires to correlate the relevant failure mechanisms of a model in order to be predictive in different conditions. For that purpose, finite element analyses (FEA) must be accurate and representative. Therefore, it has been decided to evaluate the influence of IMC prone to be implemented in simulations. The drifting stiffness, the concentration of stress or the accumulation of viscoplastic strain near the interfaces are the main aspects investigated in this study using two different FE models made in ABAQUS. First, the simulation of a shear test performed on “grooved” single lap shear specimens 1 is used to quantify the error made on the shear modulus with different thicknesses of IMC. This model is also necessary to determine the creep behavior of the lead free solder in another study by fitting the response of such a stack to the experimental curves. A second model composed of a solder ball with its interfacial IMC has been made (cf. Figure 1). It is used to calculate the increasing in global stiffness due to IMC. The final purpose is to highlight the weaknesses to take into account in fatigue life design for which IMC could be responsible. In a final study this model is to be implemented with a proper nonlinear solder material behavior to correlate with thermomechanical fatigue tests of BGA components

Witness Recantation Study: Preliminary Findings

In September 2012, the National Registry of Exonerations began a research study of all the cases in our database that involve post-conviction recantations by witnesses or victims. This is the first systematic study of recantations ever conducted. Its purpose is to identify patterns and trends among these cases, with a particular focus on the circumstances that first elicit the false testimony, and on the official reactions to the recantations by judges and other authorities. Our data set includes all the cases in the Registry as of February 28, 2013 – a total of 1,068 cases, 250 of which involve recantations. We developed a coding system to track various features of these cases, from the crime (or alleged crime) and initial investigation through the conviction, post-conviction proceedings, re-investigation, and exoneration of the defendant. In early April, we finished coding the cases, and we presented initial findings at the Innocence Network Conference on April 20th. This memo provides background on the project and a brief summary of our findings thus far

Witness Recantation Study: Preliminary Findings

In September 2012, the National Registry of Exonerations began a research study of all the cases in our database that involve post-conviction recantations by witnesses or victims. This is the first systematic study of recantations ever conducted. Its purpose is to identify patterns and trends among these cases, with a particular focus on the circumstances that first elicit the false testimony, and on the official reactions to the recantations by judges and other authorities. Our data set includes all the cases in the Registry as of February 28, 2013 – a total of 1,068 cases, 250 of which involve recantations. We developed a coding system to track various features of these cases, from the crime (or alleged crime) and initial investigation through the conviction, post-conviction proceedings, re-investigation, and exoneration of the defendant. In early April, we finished coding the cases, and we presented initial findings at the Innocence Network Conference on April 20th. This memo provides background on the project and a brief summary of our findings thus far

Beyond Adherence Thresholds: A Simulation Study of the Optimal Classification of Longitudinal Adherence Trajectories From Medication Refill Histories

International audienceBackground: The description of adherence based on medication refill histories relies on the estimation of continuous medication availability (CMA) during an observation period. Thresholds to distinguish adherence from non-adherence typically refer to an aggregated value across the entire observation period, disregarding differences in adherence over time. Sliding windows to divide the observation period into smaller portions, estimating adherence for these increments, and classify individuals with similar trajectories into clusters can retain this temporal information. Optimal methods to estimate adherence trajectories to identify underlying patterns have not yet been established. This simulation study aimed to provide guidance for future studies by analyzing the effect of different longitudinal adherence estimates, sliding window parameters, and sample characteristics on the performance of a longitudinal clustering algorithm

IMPLEMENTING E-LEARNING IN THE ROMANIAN EDUCATIONAL SYSTEM - A PRIORITY IN THE CONTEXT OF EU INTEGRATION

This paper intends to examine the development of e-Learning in Romania and to evaluate the gap between Romania and other members of the European Union (EU). Considering that Romania is part of the EU since 2007, it is imperative to achieve, in the shortest possible time, a real convergence with other member states. This requires finding the most effective ways to accelerate the development and increase the competitiveness. Using extensive IT&C technologies represent such a way, and public services – education, too – are among the development priorities on the agendas of all policies, both nationally and European. Thus, the subject treated in the paper is not only present but also of strategic importance for the immediate future of Romania.e-learning, e-education, IT&C

A Deleting Derivations Algorithm for Quantum Nilpotent Algebras at Roots of Unity

This paper extends an algorithm and canonical embedding by Cauchon to a large class of quantum algebras. It applies to iterated Ore extensions over a field satisfying some suitable assumptions which cover those of Cauchon's original setting but also allows for roots of unity. The extended algorithm constructs a quantum affine space $A'$ from the original quantum algebra $A$ via a series of change of variables within the division ring of fractions $\mathrm{Frac}(A)$. The canonical embedding takes a completely prime ideal $P\lhd A$ to a completely prime ideal $Q\lhd A'$ such that when $A$ is a PI algebra, ${\rm PI}\text{-}{\rm deg}(A/P) = {\rm PI}\text{-}{\rm deg}(A'/Q)$. When the quantum parameter is a root of unity we can state an explicit formula for the PI degree of completely prime quotient algebras. This paper ends with a method to construct a maximum dimensional irreducible representation of $A/P$ given a suitable irreducible representation of $A'/Q$ when $A$ is PI.Comment: 27 page

The acute effect of maximal voluntary isometric contraction pull on start gate performance of snowboard and ski cross athletes

This study investigated whether adding a maximal voluntary isometric contraction to developing snowboard and ski cross athletes’ warm-up could reduce start time. A secondary aim was to assess the appropriateness of start performance as a talent identification tool for junior athletes by determining whether differences in time could be explained by participant age and anthropometry. Twenty sub-elite athletes (male: n = 11, female: n = 9, age: 15.0 ± 1.4 years) participated. No differences were found for start time (7.5 m) between maximal voluntary isometric contraction and standardised (no-maximal voluntary isometric contraction) warm-up or gender (maximal voluntary isometric contraction; males: 1.36 ± 0.07 s, females: 1.41 ± 0.03 s, no-maximal voluntary isometric contraction; males: 1.35 ± 0.01 s, females: 1.38 ± 0.10 s, P > 0.05). A strong relationship between body mass and start time to 7.5 m (r = −0.78, r2 = 0.61, P < 0.05) was observed. Use of maximal voluntary isometric contraction-based warm-ups with developing snowboard cross and ski cross athletes may not be beneficial to improving performance