Correlation of solder paste rheology with computational simulations of the stencil printing process

Soldering technologies continue to evolve to meet the demands of the continuous miniaturisation of electronic products, particularly in the area of solder paste formulations used in the reflow soldering of surface mount devices. Stencil printing continues to be a leading process used for the deposition of solder paste onto printed circuit boards (PCBs) in the volume production of electronic assemblies, despite problems in achieving a consistent print quality at an ultra-fine pitch. In order to eliminate these defects a good understanding of the processes involved in printing is important. Computational simulations may complement experimental print trials and paste characterisation studies, and provide an extra dimension to the understanding of the process. The characteristics and flow properties of solder pastes depend primarily on their chemical and physical composition and good material property data is essential for meaningful results to be obtained by computational simulation.This paper describes paste characterisation and computational simulation studies that have been undertaken through the collaboration of the School of Aeronautical, Mechanical and Manufacturing Engineering at Salford University and the Centre for Numerical Modelling and Process Analysis at the University of Greenwich. The rheological profile of two different paste formulations (lead and lead-free) for sub 100 micron flip-chip devices are tested and applied to computational simulations of their flow behaviour during the printing process.

Ultra-fine pitch flip-chip assembly using isotropic conductive adhesives

This paper discusses results from a highly interdisciplinary research project which investigated different packaging options for ultra-fine pitch, low temperature and low cost flip-chip assembly. Isotropic Conductive Adhesives (ICAs) are stencil printed to form the interconnects for the package. ICAs are utilized to ensure a low temperature assembly process of flip-chip copper column bumped packages. Results are presented on the structural integrity of novel electroformed stencils. ICA deposits at sub-100 micron pitch and the subsequent thermo-mechanical behaviour of the flip-chip ICA joints are analysed using numerical modelling techniques. Optimal design rules for enhanced performance and thermomechanical reliability of ICA assembled flip-chip packages are formulated.

Ultra-fine pitch stencil printing for a low cost and low temperature flip-chip assembly process

This paper presents the results of a packaging process based on the stencil printing of isotropic conductive adhesives (ICAs) that form the interconnections of flip-chip bonded electronic packages. Ultra-fine pitch (sub-100-mum), low temperature (100degC), and low cost flip-chip assembly is demonstrated. The article details recent advances in electroformed stencil manufacturing that use microengineering techniques to enable stencil fabrication at apertures sizes down to 20mum and pitches as small as 30mum. The current state of the art for stencil printing of ICAs and solder paste is limited between 150-mum and 200-mum pitch. The ICAs-based interconnects considered in this article have been stencil printed successfully down to 50-mum pitch with consistent printing demonstrated at 90-mum pitch size. The structural integrity or the stencil after framing and printing is also investigated through experimentation and computational modeling. The assembly of a flip-chip package based on copper column bumped die and ICA deposits stencil printed at sub-100-mum pitch is described. Computational fluid dynamics modeling of the print performance provides an indicator on the optimum print parameters. Finally, an organic light emitting diode display chip is packaged using this assembly process

Historical highlights in organoarsenic chemistry

Organoarsenic compounds have given insight into important theoretical topics in chemistry and proved to have beneficial pharmacological effects

Use Of Micro-Stereology And Quantitative Cyto-Chemistry To Determine The Effects Of Crude Oil-Derived Aromatic-Hydrocarbons On Lysosomal Structure And Function In A Marine Bivalve Mollusk Mytilus-Edulis

The marine bivalve mollusc,Mytilus edulis (blue mussel), is a noted accumulator of many environmental pollutants and is increasingly used for the chemical and biological assessment of environmental impact. The toxic effects of crude oil-derived aromatic hydrocarbons (30 μg/l total hydrocarbons) on the lysosomal-vacuolar system of the digestive cells have been investigated in cryostat sections of hexane-frozen digestive glands. Exposure to aromatic hydrocarbons reduced the cytochemically determined latency of lysosomal β-N-acetylhexosaminidase; lysosomal volume density and surface density increased while the numerical density decreased. Experimental exposure resulted in the formation of very large lysosomes which are believed to be largely autophagic in function and these results indicate a significant structural and functional disturbance of digestive cell lysosomes in response to hydrocarbons.

Determination Of Fine-Scale Vertical-Distribution Of Microbes And Meiofauna In An Intertidal Sediment

A simple sampling device is described which produces thin (1 mm) sections of sediment cores. The sampler has been tested on fine sand of an intertidal sandflat and used to study the vertical distribution, over part of a tidal cycle in August, 1981, of migrating algae in the surface 20 mm of sand. Two species of Diplonies and one of Navicula showed marked changes in vertical distribution as the sandflat was flooded, but the distribution of bacteria in the sime samples did not show any change with tidal state. Spatial separation of different species of harpacticoid oppepods within the surface 20 mm of sand has also been demonstrated using this sampler, and the results suggest that different species may occupy particular fine-scale spatial niches within the sand column. The depth separation of nematode species was less well defined, except for two species with apparently the same feeding mode which were isolated from one another vertically.