R. Durairaj

Reflow profile study of the Sn‐Ag‐Cu solder

A study of the Sn‐Ag‐Cu lead‐free solder reflow profile has been conducted. The purpose of the work was to determine the Sn‐Ag‐Cu reflow profile that produced solder bumps with a thin intermetallic compound (IMC) layer and fine microstructure. Two types of reflow profiles were studied. The results of the experiment indicated that the most significant factor in achieving a joint with a thin IMC layer and fine microstructure was the peak temperature. The results suggest that the peak temperature for the Sn‐Ag‐Cu lead‐free solder should be 230°C. The recommended time above liquidus is 40 s for the RSS reflow profile and 50‐70 s for the RTS reflow profile.

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.

Sub process challenges in ultra fine pitch stencil printing of type-6 and type-7 Pb-free solder pastes for flip chip assembly applications

Purpose – The study investigates the sub process behaviour in stencil printing of type‐6 and type‐7 particle size distribution (PSD) Pb‐free solder pastes to assess their printing limits.Design/methodology/approach – Two solder pastes were used in a design of experiments approach to find optimal printing parametersFindings – Solder paste printing has been achieved to ultimately produce 30 μm deposits at 60 μm pitch for full area array patterns using a type‐7 Pb‐free solder paste. For a type‐6 PSD solder paste, full area array printing was limited to 50 μm deposits at 110 μm pitch. However, for peripheral printing patterns, 50 μm deposits at 90 μm pitch were obtained. The disparities in the behaviour of the two paste types at different geometries can be attributed to differences in the sub‐processes of the stencil printing. The paste release of the type‐6 paste from the stencil apertures at fine pitch was superior to the type‐7 paste, which may be attributed to the finer particle paste producing an increas...

Understanding the process window for printing lead-free solder pastes

Solder paste is primarily used as a bonding medium for surface mount assemblies (SMA) in the electronics industry, and is typically deposited using the stencil printing process. Stencil printing is a very important and critical stage in the reflow soldering of surface mount devices, and a high proportion of all SMA defects are related to this process. This is likely to continue with the drive toward the introduction of lead-free solder pastes. Work is continuing on the metallurgical properties of these lead-free solders, including solder joint strength and material compatibility. However, the initial challenge for the new Pb-free formulations is in achieving repeatable solder deposit from print to print and from pad to pad. To meet this challenge, new flux formulations are being developed. For a smooth transition to Pb-free soldering formulations, a proper understanding of the solder paste printing performance is necessary. The key parameters that affect solder paste printing have been identified and are the subject of numerous studies. In lead-free solder paste, the replacement of lead with other elements (including Bi, Cu) changes the density of this dense suspension. In this paper, we investigate the effects of printer parameters, i.e. squeegee speed and pressure (defined as the process window) on the printing performance of a variety of lead-free solder pastes. A three-level design of experiment on these factors was used. Comparisons are presented with lead-rich solder pastes. The metal content of the lead-free solders had a significant effect on the process window.

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 40 mm diameter and a gap height of 1 mm 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 s...

A Study of Inter-Metallic Compounds (IMC) Formation and Growth in Ultra-Fine Pitch Sn-Ag-Cu Lead-Free Solder Joints

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. As the trend towards further miniaturisation of pocket and handheld consumer electronics products continue apace, the requirements for even smaller solder joints continue. With further reductions in the size of solder joints, the reliability of solder joints becomes more and more critical to the long-term performance of these electronic products. One of the most important factors which are known to influence solder joint reliability is the intermetallic compound (IMC) layer formed between the solder and the substrate. Although the formation of an IMC layer signifies good bonding between the solder and substrate, its main disadvantage is that it is also known to be the most brittle part of the solder joint. Thus as the miniaturisation trend continues and solder joints become even smaller in size, the nature and impact of IMCs on solder joint reliability becomes even more of a concern with the introduction of new lead-free solder alloy systems. Although the IMC formation in Sn-Pb alloys have been studied extensively and a number of empirical relationships have been developed, there are very few reports on the Sn-Ag-Cu alloy systems and there is wide agreement that the existing empirical kinetic relationships developed for Sn-Pb alloy can not be extrapolated to fit the new Sn-Ag-Cu alloy systems. The objectives of the study were therefore to study the effect of solder volume on IMC layer formation and growth rate, the effect of reflow soldering profile on Pb-free solder joint microstructure and IMC formation, and the effect of the rework process on the Pb-free solder joint microstructure and IMC formation. This study on the formation and growth of IMC in the new Sn-Ag-Cu lead-free solder alloy system is divided into three main parts. The first part concerns the study of the effect of solder volume on IMC formation and growth rate. The second part of the study concerns the experimental investigation of the effect of reflow soldering profile on solder joint microstructures and IMC formation. The focus of the final part of the study is the investigation of the effect of the rework process on the Pb-free solder joint microstructure and IMC formation. The key research questions explored in the study include the following: what is the effect of solder volume (size) on the IMC layer? What is the effect of primary and secondary rework reflow profile parameters on IMC formation? The results of the experimental work conducted show that the solder volume has very little influence on the growth pattern of the IMC, suggesting that the IMC layer thickness is not very volume sensitive. Further work is however needed, given continuing trend towards further miniaturisation of hand held and pocket consumer electronic products - the size of the solder joint might in future be limited by the IMC layer thickness. With further reductions in joint size, with IMC layer thickness remaining the same, the ratio of the IMC layer thickness to solder joint size increased and adversely impact the joint reliability

Rheological Studies of PMMA-PVC Based Polymer Blend Electrolytes with LiTFSI as Doping Salt

In this research, two systems are studied. In the first system, the ratio of poly (methyl methacrylate) (PMMA) and poly (vinyl chloride) (PVC) is varied, whereas in the second system, the composition of PMMA–PVC polymer blends is varied with dopant salt, lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) with a fixed ratio of 70 wt% of PMMA to 30 wt% of PVC. Oscillation tests such as amplitude sweep and frequency sweep are discussed in order to study the viscoelastic properties of samples. Elastic properties are much higher than viscous properties within the range in the amplitude sweep and oscillatory shear sweep studies. The crossover of and is absent. Linear viscoelastic (LVE) range was further determined in order to perform the frequency sweep. However, the absence of viscous behavior in the frequency sweep indicates the solid-like characteristic within the frequency regime. The viscosity of all samples is found to decrease as shear rate increases.

Utilisation of corn starch in production of 'green' polymer electrolytes

Abstract ‘Green’ polymer electrolytes were fabricated from corn starch (CS), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 1-allyl-3-methylimidazolium chloride, [Amim] Cl by solution casting method. This communication covers the influence of different proportions of [Amim] Cl concentration in plasticising the CS∶LiTFSI matrix in term of the degree of co-ordination, ionic conductivity and relative viscosity. The possible dipole–dipole interaction between the chemical constituents was confirmed based on the Fourier transform infrared analysis. The highest ionic conductivity of 4·18×10−4 S m−1 was achieved at room temperature for the composition of CS∶LiTFSI∶[Amim] Cl (14 wt-%∶6 wt-%∶80 wt-%). The incorporation of environmental-friendly [Amim] Cl in the polymer electrolyte is significantly important in improving the ionic conductivity by increasing the amorphous morphology that consequently improves the ion transport mechanism. The highest conducting sample possesses the lowest relative viscosity with the value of 3·98 attributed to the greater structural disorderliness.

Study of the rheological behaviours 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 lead-free 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.

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.