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.

Effect of reflow profile and thermal cycle ageing on the intermetallic formation and growth in lead-free soldering

The formation and growth of intermetallic compound layer thickness is one of the important issues in search for reliable electronic and electrical connections. Intermetallic compounds (IMCs) are an essential part of solder joints. At low levels, they have a strengthening effect on the joint; but at higher levels, they tend to make solder joints more brittle. If the solder joint is subjected to long-standing exposure of high temperature, this could result in continuous growth of intermetallic compound layer. The brittle intermetallic compound layer formed in this way is very much prone to fracture and cold therefore lead to mechanical and electrical failure of the joint. Therefore, the primary aim of this study is to investigate the growth of intermetallic compound layer thickness subjected to five different reflow profiles. The study also looks at the effect of three different temperature cycles (with maximum cycle temperature of 25 0C, 40 0C and 60 0C) on intermetallic compound formation and their growth behaviour.. Two different Sn-Ag-Cu solder pastes (namely paste P1 and paste P2) which were different in flux medium, were used for the study. The result showed that the growth of intermetallic compound layer thickness was a function of ageing temperature. It was found that the rate of growth of intermetallic compound layer thickness of paste P1 was higher than paste P2 at the same temperature condition. This behaviour could be related to the differences in flux mediums of solder paste samples used.

Investigation of wall-slip effect on lead-free solder paste and isotropic conductive adhesives

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 ensure successfulpaste release after the printing process. Wall-slip plays animportant role in characterising the flow behaviour of solderpastes and isotropic conductive adhesives. The study investigates the wall-slip formation in solder paste andisotropic conductive adhesives using flow visualisation technique. The slip distance was measured for parallel plate with different surface roughness in order to quantify the wallslip formations in these paste materials. An ink marker line was drawn between the parallel plate and the free surface of the sample. The parallel was rotated slowly at a constant shear rate of 0.05 sec-1 and the displacement of the ink marker was observed using a video microscope and image capturing software was utilised to capture the displacement of ink marker. From this study, it was found that the wall-slip effect was evident in all the paste materials. In addition, the different surface roughness of the parallel plates did not prevent the formation of wall-slip. This study has revealed that the wallslip effect could used to understand the flow behaviour of the paste in the stencil printing process.

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.

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

Using computer models to identify optimal conditions for flip-chip assembly and reliability

Flip-chip assembly, developed in the early 1960s, is now being positioned as a key joining technology to achieve high-density mounting of electronic components on to printed circuit boards for high-volume, low-cost products. Computer models are now being used early within the product design stage to ensure that optimal process conditions are used. These models capture the governing physics taking place during the assembly process and they can also predict relevant defects that may occur. Describes the application of computational modelling techniques that have the ability to predict a range of interacting physical phenomena associated with the manufacturing process. For example, in the flip-chip assembly process we have solder paste deposition, solder joint shape formation, heat transfer, solidification and thermal stress. Illustrates the application of modelling technology being used as part of a larger UK study aiming to establish a process route for high-volume, low-cost, sub-100-micron pitch flip-chip assembly.

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

Investigation of wall-slip behaviour in lead-free solder pastes and isotropic conductive adhesives

Slippage due to wall depletion effect is well-known in rheological investigation. 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 different flowgeometries, gap heights and surface roughness on the paste viscosity was investigated. The utilisation of different measuring geometries has not clearly showed the presence of wall-slip in the paste samples. The existence of wall-slip was found to be pronounced when gap heights were varied using the parallel plate geometry. It was also found that altering the surface roughness of the parallel plate measuring geometry did not significantly eliminate wall-slip as expected. But results indicate that the use of a relatively rough surface helps to increase paste adhesion to the plates and to a certain extent inducing structural breakdown in 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.

Thixotropic studies of lead-based solder, lead-free solder and conductive adhesive pastes

The stencil printing process is an important process in the assembly of Surface Mount Technology (SMT)devices. There is a wide agreement in the industry that the paste printing process accounts for the majority of assembly defects. Experience with this process has shown that typically over 60% of all soldering defects are due to problems associated with the flow properties of solder pastes. Therefore, the rheological measurements can be used as a tool to study the deformation or flow experienced by the pastes during the stencil printing process. This paper presents results on the thixotropic behaviour of three pastes; lead-based solder paste, lead-free solder paste and isotropic conductive adhesive (ICA). These materials are widely used as interconnect medium in the electronics industry. Solder paste are metal alloys suspended in a flux medium while the ICAs consist of silver flakes dispersed in an epoxy resin. The thixotropy behaviour was investigated through two rheological test; (i) hysteresis loop test and (ii) steady shear rate test. In the hysteresis loop test, the shear rate were increased from 0.001 to 100s-1 and then decreased from 100 to 0.001s-1. Meanwhile, in the steady shear rate test, the materials were subjected to a constant shear rate of 0.100, 100 and 0.001s-1 for a period of 240 seconds. All the pastes showed a high degree of shear thinning behaviour with time. This might be due to the agglomeration of particles in the flux or epoxy resin that prohibits pastes flow under low shear rate. The action of high shear rate would break the agglomerates into smaller pieces which facilitates the flow of pastes, thus viscosity is reduced at high shear rate. The solder pastes exhibited a higher degree of structural breakdown compared to the ICAs. The area between the up curve and down curve in the hysteresis curve is an indication of the thixotropic behavior of the pastes. Among the three pastes, lead-free solder paste showed the largest area between the down curve and up curve, which indicating a larger structural breakdown in the pastes, followed by lead-based solder paste and ICA. In a steady shear rate test, viscosity of ICA showed the best recovery with the steeper curve to its original viscosity after the removal of shear, which indicating that the dispersion quality in ICA is good because the high shear has little effect on the microstructure of ICA. In contrast, lead-based paste showed the poorest recovery which means this paste undergo larger structural breakdown and dispersion quality in this paste is poor because the microstructure of the paste is easily disrupted by high shear. The structural breakdown during the application of shear and the recovery after removal of shear is an important characteristic in the paste printing process. If the paste’s viscosity can drop low enough, it may contribute to the aperture filling and quick recovery may prevent slumping.

Rheological characterisation and printing performance of Sn/Ag/Cu solder pastes

Lead-free solder paste printing process accounts for majority of the assembly defects in the electronic manufacturing industry. The study investigates rheological behaviour and stencil printing performance of the lead-free solder pastes (Sn/Ag/Cu). Oscillatory stress sweep test was carried out to study the visco-elastic behaviour of the lead-free solder pastes. The visco-elastic behaviour of the paste encompasses solid and liquid characteristic of the paste, which could be used to study the flow behaviour experienced by the pastes during the stencil printing process. From this study, it was found that the solid characteristics (G0) is higher than the liquid characteristic (G0 0) for the pastes material. In addition, the results from the study showed that the solder paste with a large G0 = G0 0 has a higher cohesiveness resulting in poor withdrawal of the paste during the stencil printing process. The phase angles (d) was used to correlate the quality of the dense suspensions to the formulation of solder paste materials. This study has revealed the value of having a rheological measurement for explaining and characterising solder pastes for stencil printing. As the demand for lead free pastes increases rheological measurements can assist with the formulation or development of new pastes.

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.

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.

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.

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

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.