MICROSTRUCTURAL EFFECTS ON CREEP AND FRACTURE MECHANICS OF SN-AG-CU SOLDERSMICROSTRUCTURAL EFFECTS ON CREEP AND FRACTURE OF Sn-Ag-Cu SOLDERS

During service and/or storage, Sn-Ag-Cu (SAC) solder alloys are subjected to temperatures ranging from 0.4 to 0.8 Tm (where Tm is the melting temperature), while also experiencing cyclic strains. These conditions lead to significant microstructural changes, and thereby influence the creep response of the solder, hence the long-term reliability of microelectronic packages. Accurate prediction of long term reliability of SAC solders necessitates quantitative study of microstructure coarsening as well as development of creep models that adapt to ongoing microstructural changes. In this work, microstructure dependence of the creep behavior of two solder alloys (Sn-3.5Ag-0.5Cu and Sn-1.0Ag-0.5Cu) has been studied both experimentally and analytically; a previously developed microstructurally-adpative creep model has been modified to account for the significant contribution of the proeutectic phase to the overall creep behavior of solders with a small amount of eutectic.Additonally, handheld electronic devices are also subjected to drops, which makes the solder joints susceptible to fracture at high strain rates. This is particularly important for solder micro-bumps which connect devices in three-dimensional (3D) electronics, since these micro-bumps typically contain a large proportion of brittle intermetallic compounds (IMC), making these micro-joints highly susceptible to fracture. In this work, the fracture mechanics of thin micro-joints has been studied and a fracture mechanism map has been plotted.The main objectives of the proposed work are to (i) characterize microstructural evolution and model the key microstructural parameters that evolve during different thermal and thermo-mechanical excursions, (ii) study the creep behavior of solders with different thermal/thermo-mechanical histories, (iii) develop a microstructurally adaptive composite model capable of predicting the creep behavior of the solders in situ during microstructural evolution, (iv) study the mixed-mode fracture behavior (fracture toughness Gc and fracture mechanisms) of the thin solder joints under dynamic loading condition (v) develop a fracture mechanism map which may be used for material selection and joint design to enhance the reliability of 3D microelectronic packages. As such, this work provides fundamental insight into the relationship between the solder / joint microstructure and the steady state creep behavior as well as high strain rate fracture mechanics of Sn-Ag-Cu lead-free solders

Head Impact and Brain Injury Analysis for Evaluation and Improvement of Novel Bicycle Helmets

Brain injury is one of the major health concerns in sport. Studying brain injury needs insight into the mechanical response of the brain to traumatic loadings. Impact tests on anthropomorphic test devices and finite element human models are used to gain insight into the mechanism of brain injury. In this research, the predicted head kinematics and brain responses of different anthropomorphic test devices and biofidelic human models in various head impacts were studied and compared to understand the mechanism of brain injury for evaluation and improvement of new bicycle helmets. The effect of brain material models, the geometry of the head, brain-skull connection, and loading condition was investigated for a mild deceleration of the head by comparing the strain response of brain models with the strains measured using magnetic resonance imaging. The simulated strain responses from all the models agreed with the experimental peak strain locations but were lower in magnitude. The neck contributed measurably to the rotational acceleration of the head and, in turn, the brain strain. Assessment of two methods of head impact reconstruction was performed by simulating the unhelmeted and helmeted head impacts using an anthropomorphic test device and a human model. Favorable agreement (Less than 20% difference) in the brain injury measures of the two methods was observed in all impacts while higher difference (up to 50%) in head accelerations was observed for some impacts. The effect of the head surrogate on the performance of bicycle helmets was investigated by comparing the kinematic response and brain injury measures of two headforms at three oblique impact orientations with three helmet models. The results showed that differences in the headforms` center of gravity led to differences in headforms` rotational accelerations (up to 60%) which caused noticeable differences (an average 61%) in their brain injury measures. A finite element model of an aluminum honeycomb helmet liner was developed and applied to oblique headform drop tests. The importance of the honeycomb liner shear strength on the performance of the helmet in mitigating head rotational acceleration was shown where a 30% decrease in the liner shear strength mitigated the head rotational acceleration by 35%

Decision-making for flexible manufacturing systems using DEMATEL and SAW

Flexible manufacturing system (FMS) is an important component of competitive strategy, which could be used for improving organizational performance, productivity, and profitability. The goal of this research is to use DEMATEL approach for finding the intensity of influence of selected criteria. Then, in order to evaluate flexible manufacturing systems, the results of DEMATEL are used in SAW method. A questionnaire was developed and ten professional experts working in various departments of Aluminum Composite Panel Industry are asked to answer its questions. The obtained results reveal that in this case, it is a better choice not to implement and develop FMS

Protective Effects of Hydro-alcoholic Extract of Apium Graveolens on Carbon Tetrachloride Induced Hepatotoxicity in Sprague-Dawley male Rats

Background & Objective: Apium graveolens has antioxidant property because of its flavonoid compounds. In the present study, we investigated the hepatoprotective effects of hydroalcoholic extract of Apium graveolens in Sprague Dawley male rats treated with carbon tetrachloride (CCl4).Materials & Methods: In this study, 48 rats (weighted 180-200 gram) were divided into six groups each having eight rats. The six groups were control, sham, CCl4, and three experimental groups. The experimental groups (4-6) received 1ml/kg CCl4 twice a week and olive oil with the ratio of 1:1 by intraperitoneal injection for 40 days. In addition, they received 2 cc hydroalcoholic extract of Apium graveolens by daily gavage with the concentrations of 200, 400, and 600 mg/kg/cc, respectively. The sham group received only olive oil with the mentioned ratio and the other group received CCl4 and olive oil. At the end, Alkaline Phosphatase (ALP), Aspartate amino Transferase (AST), Alanine amino Transferase (ALT), and the serum concentrations of albumin, total and direct bilirubin were measured and analyzed by using statistical ANOVA Test. Results: Injection of CCl4 increased the serum levels of liver enzymes and total and direct bilirubin but it decreased the concentration of serum albumin. On the other hand, hydroalcoholic extract of Apium graveolens decreased the liver enzymes and increased albumin level (p<0.001). Conclusion: The results of this study reveals that the consumption of hydro-alcoholic extract of Apium graveolens maintain the integrity of the liver and protects it against damage

Recrystallization and Ag3Sn Particle Redistribution During Thermomechanical Treatment of Bulk Sn-Ag-Cu Solder Alloys

Sn-Ag-Cu (SAC) solders are susceptible to appreciable microstructural coarsening during storage or service. This results in evolution of joint properties over time and thereby influences the long-term reliability of microelectronic packages. Accurate reliability prediction of SAC solders requires prediction of microstructural evolution during service. Microstructure evolution in two SAC solder alloys, such as, Sn-3.0Ag-0.5Cu (SAC 305) and Sn-1.0Ag-0.5 Cu (SAC 105), under different thermomechanical excursions, including isothermal aging at 150 degrees C and thermomechanical cycling (TMC) was studied. In general, between 200 and 600 cycles during TMC, recrystallization of the Sn matrix was observed, along with redistribution of Ag3Sn particles because of dissolution and reprecipitation. These latter effects have not been reported before. It was also observed that the Sn grains recrystallized near precipitate clusters in eutectic channels during extended isothermal aging. The relative orientation of Sn grains in proeutectic colonies did not change during isothermal aging