Understanding the process window for printing lead-free solderpastes

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     

<Emphasis Type="Italic">Aloe vera</Emphasis> (L.) Burm.f. extract reduced graphene oxide for supercapacitor application

An eco-friendly and effective reducing agent to convert Graphene Oxide (GO) to reduced Graphene Oxide (rGO) is reported. The oxygen scavenging property of Aloe vera (L.) Burm.f. (AV) extract is successfully utilized to remove oxygen functionalities on GO. The synthesized reduced Graphene oxide (ARGO) is analyzed using UV–Visible spectroscopy, Raman spectroscopy and FT-IR analysis. Complete GO reduction is achieved within 3 h with 30 mM AV extract and is confirmed by the XRD results. The high resolution transmission electron microscopy images provide clear evidence for the formation of single layer of graphene (rGO). The mechanism of reduction of GO by the AV extract is proposed. The rGO shows good charge storage properties with stable cycling up to 1000 cycles, demonstrated by the electrochemical method. The findings suggest that the Aloe vera (L.) Burm.f. (AV) extract reduced graphene oxide was found to be suitable for supercapacitor applications.     

Investigation of wall-slip effect on lead-free solder paste 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 flow geometries, 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.     

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.     

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.     

Double Cluster Head Heterogeneous Clustering for Optimization in Hybrid Wireless Sensor Network

Wireless Personal Communications - The growth of ubiquitous and pervasive computing is largely derived from the contribution of Wireless Sensor Network (WSN) in several fields such as medicine,...     

Electrochemical machining process parameter optimization using particle swarm optimization

Electrochemical machining (ECM) is a nontraditional process used for the machining of hard materials and metal‐matrix composites. Composites are used in several applications such as aerospace, automobile industries, and medical field. The determination of optimal process parameters is difficult in the ECM process for obtaining maximum material removal rate (MRR) and good surface roughness (SR). In this paper, a multiple regression model is used to obtain the relationship between process parameters and output parameters. Particle swarm optimization (PSO) is one of the optimization techniques for solving the multiobjective problem; it is proposed to optimize the ECM process parameters. Current (C), voltage (V), electrolyte concentration (E), and feed rate (F) are considered as process parameters, and MRR and SR are the output parameters used in the proposed work. The developed multiple regression is statistically analyzed by regression plot and analysis of variance. The optimized value of MRR and SR obtained in PSO is 0.0116 g/min and 2.0106 μm, respectively. Furthermore, PSO is compared with the genetic algorithm. The PSO technique outperforms the genetic algorithm in computation time and statistical analysis. The obtained values are validated to test the significance of the model, and it is noticed that the error value for MRR and SR is within 0.15%.     

Thixotropy flow behaviour of solder and conductive adhesive pastes

This paper presents results on the thixotropic behavior of two suspensions; solder paste and isotropic conductive adhesives (ICAs). These materials are widely used as bonding 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 behavior was investigated through two rheological test; (i) hysteresis-loop test and (ii) steady shear rate test. In the hysteresis-loop test, the shear rate was increased from 0.01 to 10 s^–1 and then decreased from 10 to 0.01 s^–1. Meanwhile, in the steady shear rate test, the materials were subjected to a constant shear rate of 0.1, 1 and 10 s^–1 for a period of 1800 s. The solder paste exhibited a higher degree of structural breakdown compared to the isotropic conductive adhesives (ICAs). Both the suspensions showed a high degree of shear thinning behavior with time. Existing thixotropy model such as Weltman and Hahn were applied to understand the rate of structural change for solder paste and ICAs. The Weltman model (r0.95) showed a strong correlation with the experimental data compared to Hahn model (r0.95). The rate of structural breakdown increases with the value of B ₁, from Weltman model. The change in the microstructure of solder paste increases with the B ₁ but these trends were only observed for isotropic conductive adhesives at lower shear rates.     

Numerical Simulations on Efficient Microwave Processing of Thermoplastics with Ceramic Composites

A theoretical analysis has been carried out to study efficient heating of one-dimensional thermoplastic (Nylon 66 and PET) slabs via polymer-ceramic-polymer composite. Detailed spatial distributions of power and temperature are illustrated for specific cases. Uniform heating with an enhanced processing rate may be obtained for a thick Nylon sample with specific thickness of the Al₂O₃ composite, whereas the SiC composite leads to an enhanced processing rate with higher thermal runaway. For the thin Nylon sample, the SiC composite is effective due to an enhanced processing rate whereas Al₂O₃ is not effective due to a reduced processing rate. It is also found that thin thermoplastics with less dielectric loss (PET) may be efficiently processed using microwave with SiC as an intermediate layer, whereas Al₂O₃ leads to uniform heating with high processing time. On the other hand, for thick PET samples both Al₂O₃ and SiC composites are not effective due to very high processing times as well as high thermal runaway.     

Performance and Emission characteristics of cotton seed and neem oil biodiesel with CeO2 additives in a single-cylinder diesel engine

Energy utilisation from renewable sources plays a vital role in meeting the demands of a clean environment. Commercialisation of biodiesel is comparatively less than that of other alternative sources due to its suitability and yield. This paper is focused on performance and emission characteristics of neem oil biodiesel and cotton seed oil biodiesel blended with cerium oxide as an additive. The blending proportion was B10, B20, B30, B40 and 100% diesel. The testing was performed in a single-cylinder diesel engine coupled with an exhaust gas analyser. The performance characteristics were obtained in between the brake power with specific fuel consumption and emission characteristics such as carbon monoxide, carbon dioxide and other gases. It was observed that the combination of B20 proportion with CeO₂ blend produces effect results with other blends in specific fuel consumption and reduced emission behaviour.