Rheological characterisation of solder pastes and isotropic conductive adhesives used for flip-chip assembly

Solder pastes and isotropic conductive adhesives (ICAs) are widely used as a principal bonding medium in the electronic industry. This study investigates the rheological behaviour of the pastes (solder paste and isotropic conductive adhesives) used for flip-chip assembly. Oscillatory stress sweep test are performed to evaluate solid characteristic and cohesiveness of the lead-free solder pastes and isotropic conductive adhesive paste materials. The results show that the G' (storage modulus) is higher than G '' (loss modulus) for the pastes material indicating a solid like behaviour. It result shows that the linear visco-elastic region for the pastes lies in a very small stress range, below 10 Pa. in addition, the stress at which the value of storage modulus is equal to that of loss modulus can be used as an indicator of the paste cohesiveness. The measured cross-over stress at G'=G '' shows that the solder paste has higher stress at G'=G '' compared to conductive adhesives. Creep-recovery test method is used to study the slump behaviour in the paste materials. The conductive adhesive paste shows a good recovery when compared to the solder pastes. (C) 2008 Elsevier B.V. All rights reserved.

Wall-slip effects in SnAgCu solder pastes used in electronics assembly applications

Solder paste is the most important strategic bonding material used in the assembly of surface mount components in electronics manufacturing. As the trend towards miniaturisation of electronic products continues, there is an increasing demand for better understanding of the flow and deformation that is, the rheological behaviour of solder paste formulations. Wall slip plays an important role in characterising the flow behaviour of solder paste materials. The problem of wall slip arises due to the various attractive and repulsive forces acting between the solder particles and the walls of the measuring geometry. These interactions could lead to the presence of a thin solvent layer adjacent to the wall, which gives rise to slippage. In rheological measurements, slip effects can generally be avoided by using roughened surfaces for measuring geometries. In this paper, a novel technique is developed to study the effect of wall slip in the rheological measurements of lead-free solder paste. The viscosity and oscillatory data obtained for three different solder paste samples (from measuring geometries of different surface roughness) havebeen analysed and compared. In viscosity measurements, slip effects were dominant at low shear rates and the use of serrated surfaces was found to be quite effective in minimizing slip effects. Oscillatory measurements were also affected by roughening the surfaces of measuring geometries.

Modeling the structural breakdown of solder paste using the structural kinetic model

Solder paste is the most important strategic bonding material used in the assembly of surface mount devices in electronic industries. It is known to exhibit a thixotropic behavior, which is recognized by the decrease in apparent viscosity of paste material with time when subjected to a constant shear rate. The proper characterization of this time-dependent rheological behavior of solder pastes is crucial for establishing the relationships between the pastes structure and flow behavior; and for correlating the physical parameters with paste printing performance. In this article, we present a novel method which has been developed for characterizing the time-dependent and non-Newtonian rheological behavior 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 modeling 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.

Modelling the time-dependent behaviour of solder pastes used in the electronics assembly applications

Solder paste is the most widely used bonding material in the assembly of surface mount devices in electronic industries. It generally has a flocculated structure (show aggregation of solder particles), and hence are known to exhibit a thixotropic behavior. This is recognized by the decrease in apparent viscosity of paste material with time when subjected to a constant shear rate. The proper characterisation of this timedependent 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 novel method which has been developed for characterising the timedependent and non-Newtonian rheological behaviour of solder pastes as a function of shear rates. The objective of the study reported in this paper is to investigate the thixotropic build-up behaviour of solder pastes. The stretched exponential model(SEM) has been used to model the structural changes during the build-up process and to correlate model parameters with the paste printing process.

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.

Viscoelastic properties of solder paste and isotropic conductive adhesives used for flip-chip assembly

The printing of pastes (solder pastes and isotropic conductive adhesives) through very small stencil apertures required for flip-chip pitch sizes is expected to result in increased stencil clogging and incomplete transfer of paste to the printed circuit board pads. There is wide agreement in industry that the paste printing process accounts for the majority of assembly defects, and most defects originate from poor understanding of the effect of printing process parameters on printing performance.

Solder paste characterisation: towards the development of quality control (QC) tool

Purpose – The purpose of this paper is to develop a quality control tool based on rheological test methods for solder paste and flux media. Design/methodology/approach – The rheological characterisation of solder pastes and flux media was carried out through the creep-recovery, thixotropy and viscosity test methods. A rheometer with a parallel plate measuring geometry of 40mm diameter and a gap height of 1mm was used to characterise the paste and associated flux media. Findings – The results from the study showed that the creep-recovery test can be used to study the deformation and recovery of the pastes, which can be used to understand the slump behaviour in solder pastes. In addition, the results from the thixotropic and viscosity test were unsuccessful in determining the differences in the rheological flow behaviour in the solder pastes and the flux medium samples. Research limitations/implications – More extensive rheological and printing testing is needed in order to correlate the findings from this study with the printing performance of the pastes. Practical implications – The rheological test method presented in the paper will provide important information for research and development, quality control and production staff to facilitate the manufacture of solder pastes and flux media. Originality/value – The paper explains how the rheological test can be used as a quality control tool to identify the suitability of a developmental solder paste and flux media used for the printing process.

A methodology for characterising new lead-free solder paste formulations used for flip-chip assembly applications

The paper reports on the investigation of the rheological behaviour new lead-free solder pastes formulations for use in flip-chip assembly applications. The study is made up of three parts; namely the evaluation of the effect of plate geometry, the effect of temperature and processing environment and the effect of torsional frequencies on the rheological measurements. Different plate geometries and rheological tests were used to evaluate new formulations in terms of wall slip characteristics, linear viscoelastic region and shear thinning behaviour. A technique which combines the use of the creep-recovery and dynamic frequency sweep tests was used to further characterise the paste structure, rheological behaviour and the processing performance of the new paste formulations. 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 paste manufacture and packaging and for qualifying new flip-chip assembly lines

Effect of long-term aging on the rheological characteristics and printing performance of lead-free solder pastes used for flip-chip assembly

Stencil printing of solder pastes is a critical stage in the SMT assembly process as a high proportion of the solder-related defects can be attributed to this stage. As the trend towards product miniaturization continues, there is a greater need for better understanding of the rheological behaviour and printing performance of new paste formulations. This fundamental understanding is crucial for achieving the repeatable solder paste deposits from board-to-board and pad-to-pad required for more reliable solder interconnections. The paper concerns a study on the effect of ageing on the rheological characteristics and printing performance of new lead-free solder pastes formulations used for flip-chip assembly applications. The objective is to correlate the rheological characteristics of aged paste samples to their printing performance. The methodology developed can be used for bench-marking new lead-free paste formulations in terms of shelf life, the potential deterioration in rheological characteristics and their printing performance.

An investigation into the rheological properties of different lead-free solder pastes for surface mount applications

Purpose – The purpose of this paper is to investigate the rheological behaviour of three different lead-free solder pastes used for surface mount applications in the electronic industry.Design/methodology/approach – This study concerns the rheological measurements of solder paste samples and is made up of three parts. The first part deals with the measurement of rhelogical properties with three different measuring geometries, the second part looks into the effect of frequencies on oscillatory stress sweep measurements and the final part reports on the characterisation and comparison of three different types of Pb-free solder pastes. Findings – Among the three geometries, the serrated parallel plate was found effective in minimising the wall-slip effect. From the oscillatory stresssweep data with different frequencies; it was observed that the linear visco-elastic region is independent of frequency for all the solder paste samples. To understand the shear thinning behaviour of solder paste, the well known Cross and Carreau models were fitted to the viscosity data. Moreover,creep-recovery and dynamic frequency-sweep tests were also carried out without destroying the sample’s structure and have yielded useful information on the pastes behaviour.Research limitations/implications – More extensive research is needed to fully characterise the wall-slip behaviour during the rheological measurements of solder pastes. Practical implications – The rheological test results presented in this paper will be of important value for research and development, quality control and facilitation of the manufacturing of solder pastes and flux mediums. Originality/value – This paper shows how wall-slip effects can be effectively avoided during rheological measurements of solder pastes. The paper also outlines how different rheological test methods can be used to characterise solder paste behaviours

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

Influence of solder paste components on rheological behaviour

Rheological properties of solder pastes are very important for a high quality surface mount technology process. The stencil/screen printing process of solder pastes is one of the most critical steps in the SMT assembly process, as most of the assembly defects can often be shown to originate from paste rheology and associated poor printing performance. This paper concerns an investigation of the effect of solder paste composition on the rheological properties and behaviour of four different solder pastes. We report on the evaluation of three different paste formulations based on the no-clean flux composition, with different alloy composition, metal content and particle size using a range of rheological characterisation techniques - including viscosity measurements, yield stress, oscillatory and creep-recovery tests. Our results show that in the viscosity test, all solder pastes exhibited a shear thinning behaviour in nature with different highest maximum viscosity. In the region of shear thinning behaviour the paste 3 delivered the best results. Viscosity test helps to understand the solid and cohesive behaviour of solder pastes. Good solid and cohesive behaviour indicates a good paste roll and helps to avoid paste bleeding. The yield stress test has been used to study the effect of temperature on the flow behaviour of solder pastes. Yield stress was measured for a range of temperature from 15deg C to 35deg C with an increment of 5degC. The result indicated a decreasing of the yield stress point if the temperature was increased. Paste 4 has shown the minimum dependence on temperature. The oscillatory test has been used to find out the linear visco-elastic range and to study the solid and liquid like behaviours of solder pastes. Paste 1 indicated the biggest linear visco-elastic region (LVR) and the highest value of G' and G" which means solder paste 1 will be needed a higher squeegee pressure in the printing process. In the creep recovery test paste 4 showed the best- - recovery and the lowest values of creep and recovery compliance which indicated a good printing behaviour. The test also has showed the solder paste with smaller particle size exhibit less recovery

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

Development of an in-situ, non-destructive ultrasonic monitoring technique for solder pastes

This paper concerns the use of a non-destructive ultrasonic technique for characterising the rheological properties of solder paste and specifically, the use of through-mode microsecond ultrasonic pulses for evaluation of viscoelastic properties of paste materials at the molecular level. Ultrasonic techniques are a widely used and a reliable form of non-destructive testing of materials. This is because techniques such as ultrasounds while used for testing or monitoring material properties, has offered immense benefits in applications where access to the sample is restricted or when handling the sample for testing could interfere with the monitoring or analysis process. Very often, this would mean that the measurements taken are not a true representation of the behaviour of the material (due to externally incorporated changes into the material's physical state during the removal or testing process). Ultrasonic based techniques are being increasingly used for quality control and production monitoring functions which requires evaluation of the changes in material properties over wide range of industrial applications such as cement paste quality, plastic/polymer extrusion process, dough, and even sugar content in beverage drinks. In addition, ultrasound techniques are of great interest for their capacity to take rapid measurements in systems which are optically opaque. The viscometer and rheometer are two of the most widely used rheological instruments used in industry for monitoring the quality of solder pastes, during the production and packaging stage. One of the potential limitations of viscometer and rheometer based measurements is that the collection and preparation of the solder paste samples can irreversibly alter the structure and flow behaviour of the sample. Hence the measurement may not represent the actual quality of the whole production batch. Secondly, rheological measurements and the interpretation of rheological data is a very technical and time consuming process, which requires professionally trained R&D personnel. It is for these reasons that materials suppliers (who formulate and produce solder pastes) and solder paste consumers (especially, contract electronics manufacturers) are keen to see the development of simple, easy to use and accurate techniques for the theological characterisation of solder pastes. The results from the work show that the technique can be used by R&D personnel involved in paste formulation and manufacture to monitor the batch-to-batch quality and consistency.

In-situ non-destructive ultrasonic rheology technique for monitoring different lead-free solder pastes for surface mount applications

This paper investigates the application of a non-destructive ultrasonic technique for characterising the rheological properties of solder paste through the use of through-mode microsecond ultrasonic pulses for evaluation of viscoelastic properties of lead-free solder paste containing different types of flux. Ultrasonic techniques offer a robust and reliable form of non-destructive testing of materials where access to the sample is restricted or when sample handling can interfere with the monitoring or analysis process due to externally incorporated changes to the material’s physical state or accidental contamination during the removal or testing process. Ultrasonic based techniques are increasingly used for quality control and production monitoring functions which requires evaluation of changes in material properties for a wide range of industrial applications such as cement paste quality, plastic/polymer extrusion process, dough and even sugar content in beverage drinks. In addition, ultrasound techniques are of great interest for their capability to take rapid measurements in systems which are optically opaque. The conventional industry approach for characterising the rheological properties of suspensions during processing/packaging stage is mainly through the use of viscometer and some through the use of rheometer. One of the potential limitations of viscometer and rheometer based measurements is that the collection and preparation of the solder paste samples can irreversibly alter the structure and flow behaviour of the sample. Hence the measurement may not represent the actual quality of the whole production batch. Secondly, rheological measurements and the interpretation of rheological data is a very technical and time consuming process, which requires professionally trained R&D personnel. The ultrasound technique being proposed provides simple, yet accurate and easy to use solution for the in-situ rheological characterisation of solder pastes which will benefit the materials suppliers (who formulate and produce solder pastes) and solder paste consumers (especially, contract electronics manufacturers). The results from the work show that the technique can be used by R&D personnel involved in paste formulation and manufacture to monitor the batch-to-batch quality and consistency.