Study of the rheological behaviour of Sn-Ag-Cu solder pastes and their correlation with printing performance

Solder paste plays an important role in the electronic assembly process by providing electrical, mechanical and thermal bonding between the components and the substrate. The rheological characterisation of pastes is an important step in the design and development of new paste formulations. With the ever increasing trend of miniaturisation of electronic products, the study of the rheological properties of solder pastes is becoming an integral part in the R&D of new paste formulations and in the quality monitoring and control during paste manufacture and electronic assembly process. This research work outlines some of the novel techniques which can be successfully used to investigate the rheology of leadfree solder pastes. The report also presents the results of the correlation of rheological properties with solder paste printing performance. Four different solder paste samples (namely paste P1, P2, P3 and P4) with different flux vehicle systems and particle size distributions were investigated in the study. As expected, all the paste samples showed shear thinning behaviour. Although the samples displayed similar flow behaviour at high shear rates, differences were observed at low shear rates. In the stencil printing trials, round deposits showed better results than rectangular deposits in terms of paste heights and aperture filling. Our results demonstrate a good correlation between higher paste viscosity and good printing performance. The results of the oscillatory and thixotropy tests were also successfully correlated to the printing behaviour of solder paste.

Modelling of the time-dependent flow behaviour of lead-free solder pastes used for flip-chip assembly applications

The market for solder paste materials in the electronic manufacturing and assembly sector is very large and consists of material and equipment suppliers and end users. These materials are used to bond electronic components (such as flip-chip, CSP and BGA) to printed circuit boards (PCB's) across a range of dimensions where the solder interconnects can be in the order of 0.05mm to 5mm in size. The non-Newtonian flow properties exhibited by solder pastes during its manufacture and printing/deposition phases have been of practical concern to surface mount engineers and researchers for many years. The printing of paste materials through very small-sized stencil apertures is known to lead to increased stencil clogging and incomplete transfer of paste to the substrate pads. At these very narrow aperture sizes the paste rheology and particle-wall interactions become crucial for consistent paste withdrawal. These non-Newtonian effects must be understood so that the new paste formulations can be optimised for consistent printing. The focus of the study reported in this paper is the characterisation of the rheological properties of solder pastes and flux mediums, and the evaluation of the effect of these properties on the pastes' printing performance at the flip-chip assembly application level. Solder pastes are known to exhibit a thixotropic behaviour, which is recognised by the decrease in apparent viscosity of paste material with time when subjected to a constant shear rate. The proper characterisation of this time-dependent theological behaviour of solder pastes is crucial for establishing the relationships between the pastes' structure and flow behaviour; and for correlating the physical parameters with paste printing performance. In this paper, we present a number of methods which have been developed for characterising the time-dependent and non-Newtonian rheological behaviour of solder pastes and flux mediums as a function of shear rates. We also present results of the study of the rheology of the solder pastes and flux mediums using the structural kinetic modelling approach, which postulates that the network structure of solder pastes breaks down irreversibly under shear, leading to time and shear dependent changes in the flow properties. Our results show that for the solder pastes used in the study, the rate and extent of thixotropy was generally found to increase with increasing shear rate. The technique demonstrated in this study has wide utility for R&D personnel involved in new paste formulation, for implementing quality control procedures used in solder paste manufacture and packaging; and for qualifying new flip-chip assembly lines

Effect of temperature on slumping behaviour of lead-free solder paste and its rheological simulation

Variation in temperature can have a significant impact on the rheological characterisation of solder pastes used in the electronic assembly of surface mount devices. This paper concerns the study of the effect of temperature on slumping characteristics of lead-free solder pastes. The identification of the slumping characteristics can help in the correlation of the pastes characteristics to its printing performance. Further issues, which aid in justifying the undertaking of such a study, include the temperature differences identified both at the squeegee during the print, and during reflow. Due to these temperature variations, it is imperative to understand how slump differs with a temperature gradient

Flexible diel vertical migration behaviour of zooplankton in the Irish Sea

The diel vertical migration (DVM) of the whole plankton community was investigated in the central and coastal Irish Sea. Generally, more than 60% of the plankton did not perform significant DVM. A correlation analysis of the weighted mean depths of different organisms and their potential predators suggested relationships between two groups, Oithona spp., copepod nauplii and fish larvae, and between Pseudocalanus elongatus, Calanus spp. and chaetognaths. The organisms showing significant DVM were chaetognaths (Sagitta spp.), Calanus spp. and P. elongatus. Calanus spp. showed clear ontogenic variations in DVM, and along with P. elongatus demonstrated great flexibility both in the amplitude and direction of migration. P. elongatus did not migrate in the coastal area and Calanus spp. showed a clear reverse migration. The direction of migration appeared to be related to the vertical position of the chaetognaths in the water column during the day.

Seasonal movements and behaviour of basking sharks from archival tagging: No evidence of winter hibernation

Habitat selection processes in highly migratory animals such as sharks and whales are important to understand because they influence patterns of distribution, availability and therefore catch rates. However, spatial strategies remain poorly understood over seasonal scales in most species, including, most notably, the plankton-feeding basking shark Cetorhinus maximus. It was proposed nearly 50 yr ago that this globally distributed species migrates from coastal summer-feeding areas of the northeast Atlantic to hibernate during winter in deep water on the bottom of continental-shelf slopes. This view has perpetuated in the literature even though the 'hibernation theory' has not been tested directly. We have now tracked basking sharks for the first time over seasonal scales (1.7 to 6.5 mo) using 'pop-up' satellite archival transmitters. We show that they do not hibernate during winter but instead undertake extensive horizontal (up to 3400 km) and vertical (> 750 m depth) movements to utilise productive continental-shelf and shelf-edge habitats during summer, autumn and winter. They travel long distances (390 to 460 km) to locate temporally discrete productivity 'hotspots' at shelf-break fronts, but at no time were prolonged movements into open-ocean regions away from shelf waters observed. Basking sharks have a very broad vertical diving range and can dive beyond the known range of planktivorous whales. Our study suggests this species can exploit shelf and slope-associated zooplankton communities in mesopelagic (200 to 1000 m) as well as epipelagic habitat (0 to 200 m).