Antony Marks

A Study of the Rheological Properties of Lead free Solder Paste formulations used for Flip-Chip Interconnection

The market for solder paste materials in the electronics sector is very large and consists of material and equipment suppliers and end users. These materials are used to bond electronic components (e.g. flip-chip, BGA) to printed circuit boards (PCBs) across a range of dimensions where the solder interconnects can be 50 microns to 1 mm in size. For materials suppliers, the trends in the market are towards environmentally friendly materials (e.g. lead-free solders) that can be used at ever-smaller dimension where the properties of the materials must ensure reliable product performance. Equipment suppliers, for example printing machine manufacturers, are continually updating their equipment characteristics to ensure better print yield of solder paste onto a PCB. Whilst the End Users must ensure that the combination of materials and equipment used will provide the required product quality in terms of reliable interconnection performance. This study concerns the rheological characterisation of different lead-free solder paste formulations used for flip-chip interconnections, and is made up of three parts. The first part deals with the measurement of rheological 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 lead-free solder paste formulations. The objective of the study is to investigate the rheological behaviour of the three lead-free solder paste formulations used for flip-chip interconnection. Our study shows that of the three plate geometries evaluated, the serrated parallel plate geometry was more effective in minimizing the wall-slip. Our results also show that for the oscillatory stress-sweep measurement, the linear visco-elastic region (LVR) is independent of frequency for the three solder paste formulations. The results also show how wall-slip effects can be minimized in rheological measurements of solder pastes. The paper also outlines how different rheological test methods can be used to characterise solder paste behaviours and useful guide for both paste manufacturers and process engineers implementing flip-chip assembly.

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 stress‐sweep 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 ...

Effect of Abandon Time on Print Quality and Rheological Characteristics for Lead-Free Solder Pastes used for Flip-Chip Assembly

The lack of understanding relating to the correlation between paste characteristics and print performance presents a challenge with regards to accurately predicting the outcome of the printing process. One such paste characteristic, the abandon time, is critical in reducing defects on a modern assembly line, due to the requirement of achieving an acceptable print after a delay in production. This study concerns the investigation of the abandon time for different lead-free solder paste formulations used for flip- chip assembly. The objective of the study is to determine the length of time a paste could be plausibly left on the stencil before the quality of print degrades. In the study, a series of viscosity and oscillation shear tests were carried out to aid with understanding the nature of the pastes. Results show that for all the pastes, there is a degradation of the pastes elastic and viscous behaviour after a period of 50 hours, thus demonstrating a breakdown in paste structure, which can be correlated to the deterioration of pastes printing quality. The results also show that increasing the length of the abandon time leads to an increase in the incidence of paste bridging, which is correlated to the decrease in storage and loss modulus from the oscillatory tests. The utility of the results of the study is in assisting paste manufacturers and process engineers in the identification of the process window with respect to the paste property changes with various abandon time scenarios, such as those experienced within a typical industrial production line.

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 (PCBpsilas) across a range of dimensions where the solder interconnects can be in the order of 0.05 mm to 5 mm 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 pastespsila 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 pastespsila 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.

Investigation of Wall-slip Effect on Paste Release Characteristic in Flip chip Stencil Printing Process

As the trend toward further miniaturisation of pocket and handheld consumer electronic products continues apace, the requirements for even smaller solder joints will continue. With further reductions in the size of solder joints, the reliability of solder joints will become more and more critical to the long-term performance of electronic products. Solder joints play an important role in electronics packaging, serving both as electrical interconnections between the components and the board, and as mechanical support for components. With world-wide legislation for the removal/reduction of lead and other hazardous materials from electrical and electronic products, the electronics manufacturing industry has been faced with an urgent search for new lead-free solder alloy systems and other solder alternatives. In order to achieve high volume, low cost production, the stencil printing process and subsequent wafer bumping of solder paste has become indispensable. 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. The printing of ICAs and lead-free solder pastes through the very small stencil apertures required for flip chip applications was expected to result in increased stencil clogging and incomplete transfer of paste to the printed circuit pads. Paste release from the stencil apertures is dependent on the interaction between the solder paste, surface pad and aperture wall; including its shape. At these very narrow aperture sizes the paste rheology becomes crucial for consistent paste withdrawal because for smaller paste volumes surface tension effects become dominant over viscous flow. Successful aperture filling and release will greatly depend on the rheology of the paste material. Wall-slip plays an important role in characterising the flow behaviour of solder paste materials. The 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. The wall slip effect can play an important role in ensuring successful paste release after the printing process. The aim of this study was to investigate the influence of the paste microstructure on slip formation for the paste materials (lead-free solder paste and isotropic conductive adhesives). The effect of surface roughness on the paste viscosity was investigated. It was also found that altering the surface roughness of the parallel plate measuring geometry did not significantly eliminate wall slip as was expected. But results indicate that the use of a relatively rough surface helps to increase paste adhesion to the plates, inducing structural breakdown of the paste. Most importantly, the study also demonstrated on how the wall slip formation in the paste material could be utilised for understanding of the paste microstructure and its flow behaviour.

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.

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 materials 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.