Formation of Ag3Sn plates in SnAgCu solder bumps

Special experiments are designed to obtain the solid reactants directly from a liquid solder during phase transformation. Series of such tests performed throughout reflow, which enables to investigate the entire formation process of intermetallic Ag3Sn plates out of liquid SnAgCu solder bumps. The results show that Ag3Sn plates are formed first in the middle of the cooling stage. In the plane, they have two preferable growth directions. By varying thermal conditions during reflow, the formation mechanism of these plates is discussed

Crystallographic structure and mechanical behaviour of SnAgCu solder interconnects under a constant loading rate

With the continuing increase of the integration density in electronics, dimensions of interconnections for electronic components have been miniaturized to a scale that is comparable to those of their crystallographic structure. For instance, a SnAgCu solder joint in the flip chip package can contain only one or a few grains. In this case, the mechanical behaviour of the micro-joint is expected to shift from a polycrystalline-based to single-crystal one. Considering the further miniaturization, both the crystallographic structure and mechanics of each component (Ag3Sn, Cu6Sn5 and beta-Sn matrix) within a grain and the adjacent SnCu interface will play an important role in the reliability of the micro-joint due to their size comparable with that of a grain, irregular geometry, their heterogeneous distribution and considerably different properties. In addition, at such a small scale, the non-local effect on deformation of beta-Sn should be taken into account to interpret mechanical interactions between components. In this paper, a shearing test, in which it is possible to apply a constant loading to a SnAgCu joint is deigned to investigate mechanics of substructure within a SnAgCu grain and near the SnCu interface

Grain features of SnAgCu solder and their effect on mechanical behaviour of micro-joints

SnAgCu alloy, which promises compatible properties with Sn-Pb solder, has been identified as one of the most potential Lead-free solders for electronic interconnections. However, due to the miniaturization of solder joints, a micro-joint of this material contains only few grains. In this case, the mechanical behaviour of solder alloys shifts from the polycrystal-based to single-crystal based. Since P-Sn, the matrix of SnAgCu solder, has a contracted body-centred tetragonal structure, its grains are expected to have anisotropic properties, which are important, the reliability of a micro-joint. The present paper studies the inelastic anisotropic behaviour of this material. In order to analyse the effect of grain features, solder joints at different size are formed under the different cooling rate. An in-situ shear test is then performed to correlate the mechanical behavior of a joint to its microstructural features. The results show that the decrease in the joint's dimension results in the diminishment of the number of grains, and that the inelastic behaviour of SnAgCu grains is orientation-dependen

Mesomechanical modelling of SnAgCu solder joints in flip chip

In modern microelectronic packages (considered here as a mesoscale), the size of microstructural features of an alloy is compatible with the scale of an entire element that can contain only one or a few grains. In this case, the mechanical behaviour of the element deviates from isotropic/homogenous character at the macroscopic scale of a bulk specimen, comprising a large number of randomly oriented grains. Generally, a crystal-plasticity model, which is based on dislocation sliding in certain slip systems, is applied to describe a local lattice-induced anisotropic behaviour. However, even at a room temperature, the movement of dislocations is not a single mechanism of the inelastic behaviour of eutectic SnAgCu solder due to its low melting point. Under a low-magnitude loading condition, creep also has an effect due to a movement of vacancies. At high temperatures, this creep can become a dominant mechanism for the inelastic behaviour, diminishing the role of the crystal-plasticity model. This paper accounts for the creep component of deformation and unites it with the traditional crystal-plasticity model. In addition, deformation due to thermal expansion is introduced into the constitutive equation to capture the major mechanisms of the mechanical behaviour of a SnAgCu solder micro-joint used in electronics

Formation of Sn dendrites and SnAg eutectics in a SnAgCu solder

The formation behaviour of grains and their components, including Sn dendrites, Cu6Sn5 and Ag3Sn intermetallic compounds (IMCs), in a SnAgCu alloy is investigated in an experiment, capable to obtain the solid reactants directly from the liquid solder during the liquid-to-solid phase transformation. The results show that Cu6Sn5 IMCs are formed first in a grain; then large Sn dendrites; fibre-like Ag3Sn IMCs are formed ahead of the β-Sn matrix in the coupling process generating eutectics

Mechanical Behaviour of Grains in SnAgCu Solder Joints

A collection of slides from the authors conference presentation is given

Sexual practices among unmarried adolescents in Tanzania

\ud Sexual activities are increasingly changing from the cultural point of view what they used to be. Knowledge of these practices among adolescents may be a basis to create awareness among adolescents on practices that involve risks. This study aims to assess sexual practices among unmarried adolescents in Tanzania. A cross-sectional survey was conducted among in-school and out-of-school but unmarried adolescents aged 10 to 19 in five locations in Tanzania. A questionnaire was used to collect information and to characterize sexual practices among these adolescents. About 32% of adolescents reported being sexually active; a higher proportion being males than females. The only inquired and reported sexual practices include vaginal sex, masturbation, oral and anal sex. About 15% of sexually active adolescents reported having multiple sexual partners. Significantly more males reported having multiple partners than females. Nearly 42% of sexually active adolescents reported having used a condom during most recent sexual act. Females reported older partners at first sexual act. Adolescents experience several sexual practices that include penetrative and non-penetrative. More males reported being sexually active than females. Despite adolescents reporting having multiple sexual partners, reported condom use during the most recent sexual act was low. We advocate for a more enhanced approach of reproductive health education that includes safer sex to adolescents without forgetting those in-schools.\u

Mechanical behaviour of grains in SnAgCu solder joints

A collection of slides from the authors conference presentation is given