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Study of the time-dependent rheological behaviour of lead-free solder pastes and flux mediums used for flip-chip assembly applications

Study of the time-dependent rheological behaviour of lead-free solder pastes and flux mediums used for flip-chip assembly applications

Mallik, Sabuj (2009) Study of the time-dependent rheological behaviour of lead-free solder pastes and flux mediums used for flip-chip assembly applications. PhD thesis, University of Greenwich.

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The two most important trends in electronic industry are "miniaturisation" and "increased functionality". Over the last fifteen years, the electronic manufacturing industries have experienced tremendous pressure to meet the requirements for miniaturised products, particularly, hand-held consumer products. Functionality of these products has also evolved at the same pace through packing in more and more features. As these trends are set to continue, there is an increasing demand for better understanding of soldering technology, particularly in the area of solder pastes used in the reflow soldering of surface mount devices. Successful assembly of electronic devices for ultra-fine pitch and flip-chip applications requires the deposition of small and consistent paste deposits from pad to pad, and from board to board. The paste printing process at this chip-scale geometry depends on conditions such as good paste roll, complete aperture filling and paste release from the apertures onto the substrate pads. This means that the paste flow and deformation behaviour is very important in defining the printing performance of any solder paste.

In order to understand rheological phenomena associated with the flow of solder pastes, it is necessary to understand time dependent rheological behaviour. Such behaviour is common to many industrial fluids and consequently has been of interest to rheologists for many years. The time dependent behaviour observed in solder pastes is largely due to the breakdown of the flocculated structure formed during storage or idle period. In general, the breakdown of the structure with shear results in a decrease in apparent viscosity. Recovery after shear is generally a very slow process and depends on the intensity of the breakdown and previous shear history.

The work reported in this thesis on the characterisation and modelling of time-dependent rheological behaviour of solder paste and flux mediums used in surface mount applications is made up of four main parts. The first part concerns the characterisation of the time-dependent behaviour of solder pastes and flux mediums. Two types of tests were performed at this stage: hysteresis-loop test and step-shear-rate test. In the second part of the study the time-dependent rheology of solder pastes and flux mediums has been modelled to evaluate the mechanisms for the break-down of the internal structure of the paste materials. A novel technique has been developed which combines the experimental rheological data with a modified structural kinetic model (SKM) to investigate the rate and extent of structural change of solder paste and flux medium. The third part of the study deals with the experimental and modelling studies of the short term build-up of solder paste and flux medium structure using the stretched exponential model. In the final part of the study the printing trials of four different solder paste samples were carried out to investigate the effect of post-print rest period on slumping behaviour of solder paste.

From the experimental characterisation it was evident that both the solder paste and flux samples are strongly thixotropic and shear-thinning in nature. The thixotropic breakdown behaviour of solder paste and fluxes has been satisfactorily modelled using a second-order structural kinetic model. The results from this study can be of great help for the solder paste manufacturers and formulators in quantifying and predicting the effect of long term shearing on the solder paste samples. The technique developed can also be utilized for similar materials such as solar pastes and conductive adhesives. The short-term build-up of solder pastes and fluxes has been successfully modelled using the stretched exponential model. The paste manufacturers and formulators can use the technique developed to predict and quantify the slumping behaviour of solder paste. The end-users, for example the electronics assemblers may use the technique to optimize their assembly process by minimising/preventing slumping of solder paste.

Item Type: Thesis (PhD)
Additional Information:
Uncontrolled Keywords: rheological behaviours, solder pastes, flux, metallurgy, flip-chip assembly,
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
Pre-2014 Departments: School of Engineering
School of Engineering > Department of Engineering Systems
School of Engineering > Electronics Manufacturing Engineering Research Group
Last Modified: 14 Aug 2017 13:22

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