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

Abstract

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 rheological 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.

Quantitative Evaluation of Voids in Lead Free Solder Joints

“…As stated earlier, all solder pastes share the same solder alloy composition and metal content. The paste samples were mainly different in terms of flux medium and the solder particle size distribution.Flux in a solder paste formulation plays a key role in determining the soldering defect rate and the ultimate solder joint reliability [6]. The composition of a typical vehicle/flux has some 5 -20 constituents.…”

Rheology of F620 solder paste and flux

“…The flux, presented in Figure 1, is a combination of solvents, thickeners, fluxing agents and blinders (Hwang, 1989). It is known that the formulation of the flux directly influences the deformation, flow characteristic and printing performance of the solder paste (Mallik, 2009). However, the exact chemical composition of the flux is unknown as the producers of flux media do not disclose this information, which makes it difficult to interpret the results obtained from the rheological tests.…”

Újraömlesztéses forrasztás stencilnyomtatási folyamatának modellezése

“…A forraszpaszták viszkozitása a legelső nyomtatástól kezdve nyomtatásról nyomtatásra csökken, és legalább 5-6 nyomtatás szükséges az állandósult állapot eléréséhez a viszkozitás szempontjából. A viszkozitás időfüggésének mérése eddig csak konstans nyírási ráták mellett valósult meg [8,9]. Ez egyrészt nem ad pontos képet a nyomtatás közbeni viszkozitás csökkenésről az eltérő időtartamok miatt; a mérés időtartama több perc [10,11], míg a stencilnyomtatás időtartama 30-90 s. Másrészt elhanyagolták azt, hogy a stencilnyomtatási folyamat során a nyomtatások között állásidő van (hossza 15-60 s), mely során a forraszpaszta viszkozitása növekszik.…”