Astm stp 1530 2011

Journal of ASTM International Selected Technical Papers STP 1530 Lead-free Solders JAI Guest Editor: Narayan Prabhu Journal of ASTM International Selected Technical Papers STP1530 Lead-free Solders JAI Guest Editor: Narayan Prabu ASTM International 100 Barr Harbor Drive PO Box C700 West Conshohocken, PA 19428-2959 Printed in the U.S.A ASTM Stock #: STP1530 Library of Congress Cataloging-in-Publication Data Lead-free solders / JAI guest editor, Narayan Prabu p cm (Journal of ASTM International selected technical papers; STP1530) Includes bibliographical references and index ISBN: 978-0-8031-7516-7 (alk paper) Lead-free electronics manufacturing processes Solder and soldering Materials I Prabu, Narayan TK7836.L424 2011 2010053870 621.9’77 dc22 Copyright © 2011 ASTM INTERNATIONAL, West Conshohocken, PA All rights reserved This material may not be reproduced or copied, in whole or in part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of the publisher Journal of ASTM International „JAI… Scope The JAI is a multi-disciplinary forum to serve the international scientific and engineering community through the timely publication of the results of original research and critical review articles in the physical and life sciences and engineering technologies These peer-reviewed papers cover diverse topics relevant to the science and research that establish the foundation for standards development within ASTM International Photocopy Rights Authorization to photocopy items for internal, personal, or educational classroom use, or the internal, personal, or educational classroom use of specific clients, is granted by ASTM International provided that the appropriate fee is paid to ASTM International, 100 Barr Harbor Drive, P.O Box C700, West Conshohocken, PA 19428-2959, Tel: 610-832-9634; online: http://www.astm.org/copyright The Society is not responsible, as a body, for the statements and opinions expressed in this publication ASTM International does not endorse any products represented in this publication Peer Review Policy Each paper published in this volume was evaluated by two peer reviewers and at least one editor The authors addressed all of the reviewers’ comments to the satisfaction of both the technical editor(s) and the ASTM International Committee on Publications The quality of the papers in this publication reflects not only the obvious efforts of the authors and the technical editor(s), but also the work of the peer reviewers In keeping with long-standing publication practices, ASTM International maintains the anonymity of the peer reviewers The ASTM International Committee on Publications acknowledges with appreciation their dedication and contribution of time and effort on behalf of ASTM International Citation of Papers When citing papers from this publication, the appropriate citation includes the paper authors, ⬘⬘paper title’’, J ASTM Intl., volume and number, Paper doi, ASTM International, West Conshohocken, PA, Paper, year listed in the footnote of the paper A citation is provided as a footnote on page one of each paper Printed in Bridgeport, NJ February, 2011 Foreword THIS COMPILATION OF THE JOURNAL OF ASTM INTERNATIONAL (JAI), STP1530, on Lead-free Solders, contains papers published in JAI encompassing the environmental and health concerns of the exposure to lead during soldering and the success and failures of lead-free solders This STP is sponsored by ASTM Committee D02 on Petroleum Products and Lubricants The JAI Guest Editor is Professor K Narayan Prabhu, Department of Metallurgical & Materials Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore, India Contents Overview Wetting Behavior of Solders G Kumar and K N Prabhu vii A Review of Pb-Free High-Temperature Solders for Power-Semiconductor Devices: Bi-Base Composite Solder and Zn–Al Base Solder Y Takaku, I Ohnuma, Y Yamada, Y Yagi, I Nakagawa, T Atsumi, M Shirai, and K Ishida 27 Wetting Behaviour and Evolution of Microstructure of Sn–Ag–Zn Solders on Copper Substrates with Different Surface Textures Satyanarayan and K N Prabhu 50 Solder Joint Reliability of SnBi Finished TSOPs with Alloy 42 Lead-Frame under Temperature Cycling W Wang, M Osterman, D Das, and M Pecht 74 The Microstructural Aspect of the Ductile-to-Brittle Transition of Tin-Based Lead-Free Solders K Lambrinou and W Engelmaier 89 Ball Grid Array Lead-Free Solder Joint Strength under Monotonic Flexural Load P Geng Tensile Properties of Sn-10Sb-5Cu High Temperature Lead Free Solder Q Zeng, J Guo, X Gu, Q Zhu, and X Liu Loading Mixity on the Interfacial Failure Mode in Lead-Free Solder Joint F Gao, J Jing, F Z Liang, R L Williams, and J Qu 121 139 158 Empirical Modeling of the Time-Dependent Structural Build-up of Lead-Free Solder Pastes Used in the Electronics Assembly Applications S Mallik, N N Ekere, and R Bhatti 168 Rheological Characterisation and Empirical Modelling of Lead-Free Solder Pastes and Isotropic Conductive Adhesive Pastes R Durairaj, L W Man, and S Ramesh 186 Overview Lead containing solders are used extensively in the electronic packaging industry The lead based solders have excellent wetting characteristics and provide good electrical, thermal, and mechanical continuities However the lead present in these solders poses significant environmental hazards, such as the problem of disposal of electronic assemblies, landfill contamination, and toxicity toward human and wild life To mitigate these problems, a large number of lead free solders have been developed and introduced Although lead free solders are environmentally friendly, there are several technical issues, such as-wetting, solder joint reliability, solder joint strength, and other mechanical properties, which are not fully resolved This special issue on lead free solders addresses some of these concerns The compendium consists of ten research papers In the first paper, the factors affecting the wetting behavior of solders and the evolution of interfacial microstructures are reviewed and discussed The development of Pbfree high temperature solders for power semiconductor devices is reviewed in the second paper The effect of surface roughness on the wetting behavior and the evolution of microstructures of two lead free solders on copper substrates is discussed in the third paper A paper by Wang et al on solder joint reliability compares the fatigue life of SnBi finished thin-small-outlinepackage (TSOP) parts under thermal cycling to that of Sn finished parts The paper on microstructural aspects of the ductile-to-brittle transition focuses on specific aspects of the DBTT in the fracture behavior of tin-based lead-free solders The loading mixity on the interfacial failure mode in a lead-free solder joint is discussed in the sixth paper The paper by Phil Geng compares the solder joint strengths of BGA (Ball Grid Array) lead-free to that of eutectic lead (Sn–Pb) solder joint strengths The effect of the morphology of Cu6Sn5 intermetallic compounds on tensile properties of bulk solder and solder joint is discussed in a paper on Tensile properties of Sn10Sb-5Cu high temperature lead free solder Empirical modeling and rheological characterization of solder pastes used in electronic assemblies are discussed in the last two papers I sincerely thank all the authors for their contributions and sharing their knowledge I am indebted to the reviewers who have played an important role in the preparation of this STP by their constructive comments and suggestions I deeply appreciate the timely assistance and the excellent coordination of the review work by ASTM and JAI staff members It was wonderful working and interacting with them I am grateful to Dr George Totten of GE Totten & Associates, LLC, USA who inspired, encouraged, and initiated this work As guest editor, I earnestly hope that this STP on Lead free Solders will encourage and facilitate further research in the wonderful area of vii environmentally friendly lead free solders This compendium of research papers should serve as a valuable resource for students, researchers, and material scientists in the electronics industry to understand the existing leadfree solders better and initiate the development of newer solders K Narayan Prabhu Department of Metallurgical & Materials Engineering National Institute of Technology Karnataka, Surathkal Mangalore, India viii Reprinted from JAI, Vol 7, No doi:10.1520/JAI103055 Available online at www.astm.org/JAI Girish Kumar1 and K Narayan Prabhu2 Wetting Behavior of Solders ABSTRACT: Lead bearing solders have been used extensively in the assembly of modern electronic circuits However, increasing environmental and health concerns about the toxicity of lead has led to the development of lead-free solders Wetting of solders on surfaces is a complex and important phenomenon that affects the interfacial microstructure and hence the reliability of a solder joint The solder material reacts with a small amount of the base metal and wets the metal by intermetallic compound 共IMC兲 formation The degree and rate of wetting are the two important parameters that characterize the wetting phenomenon Contact angle is a measure of the degree of wetting or wettability of a surface by a liquid Spreading kinetics in a given system is strongly affected by the experimental conditions In reactive systems like soldering, wetting and chemical interfacial reactions are interrelated, and hence for successful modeling, it is essential to assess the effect of interfacial reactions on kinetics of wetting Solder wetting necessarily involves the metallurgical reactions between the filler metal and the base metal This interaction at the solder/base metal interface results in the formation of IMCs During soldering an additional driving force besides the imbalance in interfacial energies originates from the interfacial reactions The formation of IMC has significant influence on contact angle The presence of IMCs 共thin, continuous, and uniform layer兲 between solders and substrate metals is an essential requirement for good bonding Optimum thickness of an IMC layer offers better wettability and an excellent solder joint reliability However, due to their inherent brittle nature and tendency to generate structural defects, a too thick IMC layer at the interface may degrade the joint In Manuscript received February 25, 2010; accepted for publication April 15, 2010; published online June 2010 Dept of Mechanical Engineering, St Joseph Engineering College, Mangalore 575028, India, e-mail: srigk71@gmail.com Dept of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore 575025, India, e-mail: prabhukn_2002@yahoo.co.in Cite as: Kumar, G and Prabhu, K N., ‘‘Wetting Behavior of Solders,’’ J ASTM Intl., Vol 7, No doi:10.1520/JAI103055 Copyright © 2010 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959 DURAIRAJ ET AL., doi:10.1520/JAI103009 191 fore a linear function of viscosity ␩ A generalised kinetic equation for structural change 关25兴 d␭ = a共1 − ␭兲 − b␭␥˙ m dt 共6兲 where the first term on the right describes the rate of structural build up being proportional to the extent of un-built-up structure The second term describes the rate of breakdown proportional to the degree to which structure is already built up and to the magnitude of the shear rate Equilibrium is achieved at d␭ / dt = 0, leading to a = ␭共a + b␥˙ m兲 and ␭= b m 1+ ␥˙ a 冉冊 共7兲 Hence ␩ = ␩⬁ + ␩0 − ␩⬁ 1+ 冉 冊 b ␥˙ a m 共8兲 which is the Cross steady-state equation 关23,25兴 The zero shear viscosity, ␩0, is a critical material property and can prove valuable in making assessments of suspension and emulsion stability The parameter m is known as the rate constant It is dimensionless and is a measure of the degree of dependence of viscosity on shear rate in the shear-thinning region When m = 0, this indicates Newtonian behavior While m ⬎ 0, this means the viscosity decreases with increasing shear rate, ␥˙ , which is the condition for shear-thinning behavior The value of b / a is known as the consistency coefficient The cross model describes well the shear dependence of fluids over a wide range of shear rates as shown in Fig 关20兴 Experimentation Apparatus All the flow curve test measurements were carried out with the Physica MCR 301 controlled stress rheometer In order to avoid the formation of wall slip at the interface between the plate and conductive paste, a parallel plate geometry was chosen with a diameter of 25 mm A gap height of 0.5 mm was used between the upper and lower plates, as shown in Fig Prior to loading the sample onto the rheometer, the conductive paste was stirred for about 1–2 to ensure that the paste structure is consistent with the particles being redistributed into the paste A sample was loaded on the Peltier plate, and the geometry plate was then lowered to the gap of 0.5 mm The excess paste at the plate edges was carefully trimmed using a plastic spatula Then the sample was allowed to rest for about in order to reach the equilibrium state before 192 JAI • STP 1530 ON LEAD-FREE SOLDERS FIG 2—Plot of viscosity against shear rate for shear-thinning fluids identifying three separate regions: A zero-shear viscosity at low shear rates, a power law region at intermediate shear rates, and an infinite-shear viscosity at high shear rates 关23兴 starting the test All tests were conducted at 25° C with the temperature controlled by the Peltier-plate system Each test was repeated for three times for stabilisation 共with fresh samples used for each test兲 Paste Materials There are two main types of the paste materials used in the assembly flip chip devices, namely, 共i兲 solder paste and 共ii兲 ICAs Solder Paste Solder paste is one of the most widely used interconnection materials in the surface mount technology assembly process Solder paste is a homogenous and stable suspension of solder alloy particles suspended in a flux/vehicle system, as can be seen in Fig 3共a兲 The flux/vehicle is a combination of solvents, thickeners, binders, and fluxing agents 关13兴, as shown in Fig 3共b兲 Solder pastes consists of three main constituents, namely, 共a兲 Solder alloy particles which forms the base for the metallic bond, 共b兲 The flux system which helps to promote the formation of the metallic DURAIRAJ ET AL., doi:10.1520/JAI103009 193 FIG 3—共a兲 Solder particles and 共b兲 constituent of the flux vehicle system 关13兴 194 JAI • STP 1530 ON LEAD-FREE SOLDERS FIG 4—ICAs microstructure 共c兲 bond by providing a good wetting condition and for cleaning the surfaces, and The vehicle carrier system which facilitates the binding together of the solder powder particles and the flux system together, and for providing the desired rheological properties for processing and depositing the paste onto the PCB Isotropic Conductive Adhesives The ICAs consist of 70–80 % metal fillers dispersed in an epoxy resin, as shown in Fig During curing, the epoxy resin shrinks, which enables the metal fillers to come into contact, hence conducting electricity There are various types of metal fillers, e.g., silver, nickel, gold, copper, carbon, and metal-coated particles The most commonly used metal filler is silver Silver is preferred to other metal fillers because of its unique characteristic to conduct electricity even after it oxidises When the adhesive is cured, the filler particles are uniformly distributed and form a network within the polymer structure From these networks, electricity can pass through making the mixture electrically conductive and due to nature of the network, the current can flow in any direction The ICAs generally consist of resin such as epoxy 共most commonly兲, polyamides, silicones, and acrylic adhesives DURAIRAJ ET AL., doi:10.1520/JAI103009 195 TABLE 1—Constituent of solder paste and ICAs investigated Particle size Distribution 共␮m兲 Particle Shape P1 20–45 Spherical P2 P3 20–45 20–45 Spherical Flakes Paste Samples Paste Medium Flux vehicle system Flux vehicle system Epoxy resin Solder Alloys/Materials 共Percentage by Weight兲 Tin 95.5 %/copper 0.7 %/silver 3.8 % Tin 96.5 %/silver 3.5 % Silver 88 % Material and Sample Preparations Three different types of pastes were used in the study, and the details of the pastes 共lead-free solder pastes are labeled as P1 and P2, while ICA sample is labeled as P3兲 are presented in Table The pastes 共P1 and P2兲 contain metal particles, and P3 contains silver flakes of 88 wt % In order to minimize separation and prolong shelf life, manufacturers prescribe specific storage conditions, for example, the ICA and pastes used in this study are stored in a fridge unit at ⫺20 and −4 ° C, respectively As it is important to carry out the experiments on the paste samples at room temperature, the procedure used in the study is to bring out the ICA and solder paste out of the fridge prior to the rheological tests and to allow these pastes to attain room temperature All the rheological measurements are carried out at 25± 0.1° C Rheology Test Viscosity Test—In the stencil printing process, the viscosity of the paste must be low enough for the squeegee to force the paste through the stencil apertures but high enough to recover to its required shape and not flow beyond its stenciled area The viscosity test can be used to provide a quick indication of the viscosity of a solder pastes changes over a wide range of shear rates The experimental parameters utilized for the viscosity test is outlined in Table Thixotropy Test—Thixotropy test aims to simulate the structural breakdown and recovery of the solder paste and flux system In the hysteresis loop test, the shear rate were increased from 0.001 to 100 s−1 and then decreased from 100 to 0.001 s−1 This property maybe essential for understanding the rolling motion of the paste during the printing process as the squeegee pushes the paste TABLE 2—Experimental parameters for flow curve test Experimental Values Initial Shear Rate 共s−1兲 0.001 Final Shear Rate 共s−1兲 100 Number of Measuring Point 31 Interval Between Measuring Point 共s兲 Overall Duration 共s兲 100 196 JAI • STP 1530 ON LEAD-FREE SOLDERS back and forth In this test, the pastes are subjected to low and high shear rates over a period of time, and the recovery of the paste as a function of the viscosity is noted after the removal of high shear rate Results and Discussion Viscosity Figure shows the plots for viscosity against the shear rate for all three paste samples As expected, as the shear rate is increased, all the pastes showed a decrease in the viscosity The drop in viscosity clearly indicates that the pastes are shear thinning in nature and the structure of the pastes was undergoing changes due to destruction of flocculation in the suspensions 关17兴 As mentioned previously, the viscosity measured at low shear rates will be useful in assessing the suspension stability With respect to this, at a low shear rate of 0.001 s−1, the highest viscosity was recorded for sample P2, followed by P1 and P3, as shown in Fig The high viscosity measured for sample P1 could be due to the strong interaction between tin/silver particles with the flux medium as opposed to the tin/copper/silver system formulation in sample P2 From the result, sample P1 showed a good stability at low shear rates, which could indicate the particles and flux medium will not separate In addition, the high viscosity will give the paste a good cohesive behavior and prevent slumping during and after the printing process In contrast to P1 and P2, the viscosity measured for sample P3 was the lowest These results are in line to that reported by Durairaj et al 关1兴 The low viscosity attributed o sample P3 could be merely due the poor interface wetting of the epoxy resin and the silver flake In addition, the irregular shapes of the silver flakes could have decreased the flocculation in the systems, hence reducing the overall viscosity of the conductive adhesives 关4,5,7,11兴 The experimental viscosity data was fitted to the power law and Cross model, as shown in Fig For all samples, the Cross model showed a better fit compared to the power law model Despite the fact that both the model was designed to evaluate the shear-thinning behavior of suspensions, the results indicate otherwise There could be two possible explanations: First, the shear rate investigated may to wide and fall beyond the range of the power law model The second reason could be attributed to the presence of three regions: First, Newtonian region; second, shear-thinning region; and third, Newtonian region, which is easily captured by the Cross model The results from the experiment seem to correlate well with previous studies 关4兴 and also prove that the pastes 共solder paste and ICAs兲 show the three regions when the samples are sheared from the low to high shear rates Hence for a wider shear rates, the Cross model provides a better experimental fit compared to the power law model A further analysis was carried on the fitted data, as shown in Tables and The power law and the Cross model used to quantify the viscosity/shear rate profile for the shear-thinning solder pastes and fit to the experimental data In a power law model, as the power index, n, lies between zero and one, ⬍ n DURAIRAJ ET AL., doi:10.1520/JAI103009 197 FIG 5—Flow curve of pastes: 共a兲 lead-free solder paste, P1; 共b兲 lead-free solder paste, P2; and 共c兲 ICA paste, P3 198 JAI • STP 1530 ON LEAD-FREE SOLDERS TABLE 3—Variables used in the power law model Samples P1 P2 P3 Consistency Coefficient, K 共Pa s兲 454.55 2913.3 72.068 Power Law Indexes, n 0.409 23 0.161 09 0.353 70 Correlation Ratio, R2 0.871 66 0.671 38 0.726 83 ⬍ 1, this indicates that the viscosity of the sample being tested was exhibiting shear-thinning behavior 关5,21,22兴 Based on Table 3, the n values of P1, P2, and P3 samples were 0.409 23, 0.161 09, and 0.353 70, respectively Since the n values lie between zero and one, the viscosities of all three samples were experiencing shear-thinning behavior In a Cross model equation, the rate constant, m, is a measure of the degree of dependence of viscosity on shear rate in the shear-thinning region 关20兴 When m = 0, this indicates Newtonian behavior While m ⬎ 0, this means that the viscosity decreases with increasing shear rate, ␥˙ Based on Table 4, the rate constant, m, values of all three type of pastes were more than zero, which indicated shear-thinning behavior 共P1 = 0.699 67, P2 = 0.873 12, P3 = 0.583 89兲 The correlation coefficient, R2, showed the relationship between the viscosity and the shear rate R2 is measure of how well the data correlated The closer it is to one, the closely correlated the data is 关26兴 Based on Tables and 4, the correlation ratios, R2, of the Cross model for all three pastes were higher than that of the power law model While P3 showed the highest R2 value, 0.993 29, while P1 and P3 showed 0.981 87 and 0.975 03, respectively The R2 values of all three pastes were said to be almost perfect linear relationship between viscosities and shear rates because the R2 value is ⬃1 Often, the magnitudes of the consistency and the flow behavior indexes of a solder pastes depend on the specific shear rates range being used so that when comparing the properties of different solder pastes, an attempt should be made to determine them over a specific range of shear rates From the point of view of approximation of the obtained results, the power law model is good; however the Cross model can describe the results more precisely This follows from the facts that the Cross model provides more information on rheological properties of a suspension in a wide range of shear rates As mentioned earlier, the power law model does not describe the low shear and TABLE 4—Variables used in the Cross model Samples P1 P2 P3 Zero Shear Viscosity, ␩0 共Pa s兲 90 163 830 000 9180 Infinite Shear Viscosity, ␩⬁ 共Pa s兲 38.359 97.084 8.61 Rate Constant, m 0.699 67 0.873 12 0.583 89 Correlation Ratio, R2 0.981 87 0.975 03 0.993 29 DURAIRAJ ET AL., doi:10.1520/JAI103009 199 high shear rates constant-viscosity data of shear-thinning fluids Of these reasons, the power law model does not fit well to the experimental data as the Cross model did Thixotropic Behavior of Pastes Figure shows the hysteresis loop for all three paste samples The samples were constantly subjected to high shear rate, 100 s−1, with time and recover to their initial shear rate, 0.001 s−1 The overall time interval was 240 s The effect of increasing shear rate on the viscosity for the paste samples was being investigated The drop in viscosities for all three samples clearly indicates that the pastes are shear thinning in nature and the structure of the pastes was undergoing changes due to the destruction of flocculations in the suspensions 关14兴 All three samples show a hysteresis area for which an area between the up and down curves is observed The region between the up curve and down curves in the hysteresis curve is an indication of the thixotropic behavior of the pastes Therefore, all three samples studied are thixotropic suspensions The enclosed area within the curves indicates the extent of the structural breakdown in the sample for the applied shear The plot of the effect of the shear rate on the viscosity is presented in Fig 6共a兲 for P1 pastes As expected, the viscosity of the pastes drops with increasing shear rate, which indicates shear-thinning behavior of the pastes The area between the down curve and up curve indicates that the P1 paste is thixotropic in nature, which have been confirmed in previous studies on solder pastes 关15,16兴 P1 paste shows the highest degree of thixotropy because of the high hysteresis area among all three pastes The large area within the hysteresis loop in the P1 paste indicates that the sample undergone a large structural breakdown P1 paste is said to have the weakest structural bonding, which easily is being broken down by increasing shear rate The stronger attraction between the particles in P2 paste leads to a good recovery after the shear rate is removed and P2 is said to have a strong thixotropic behavior While for P3 paste, the particle size ranges from to 10 ␮m, which is smaller than P1 and P2 pastes, where the particle size is around 20– 45 ␮m These smaller particles of P3 paste tend to fill up the spaces between the flocs and form stronger bond Therefore, P3 paste, which consisted of smallest particle size, is said to be strongly thixotropic Based on Fig 6共b兲 and 6共c兲, the viscosities of P2 and P3 pastes drop with increasing shear rates, which is consistent with that of the P1 paste and shows that the pastes exhibit shear-thinning behavior For the P2 paste, the downward curve crosses the ascending curve, the cross over point being situated at 0.01 s−1 On this downward path, the viscosity increase at low shear rate indicates that a network structure is able to be rebuilt when the shear rate goes under a critical value 关17兴 The smallest hysteresis loop area 共P2 paste兲 corresponds to a thixotropic state for which the inter-particle bonds would be strong enough to favour a quick rearrangement of the structure This could be attributed to the strong interaction of the small Sn particles Figure 6共c兲 shows superposed upward and downward curves at high shear rates for P3 paste This could just as well correspond to an extremely thixotro- 200 JAI • STP 1530 ON LEAD-FREE SOLDERS FIG 6—Hysteresis loop of pastes: 共a兲 Lead-free solder paste, P1; 共b兲 lead-free solder paste, P2; and 共c兲 ICA paste, P3 DURAIRAJ ET AL., doi:10.1520/JAI103009 201 pic material, capable of rebuilding its structure almost instantaneously 关17兴 To support this justification, a steady shear rate test was carried out From the steady shear rate test, sample P2 recorded the highest viscosity 共94000 Pa.s兲 at zero shear rate followed by P1 共14 400 Pa.s兲 and P3 共13 100 Pa.s兲 In the middle interval, which high shear rate is applied, P3 is observed to undergo the largest structural breakdown followed by P2 and P1 As stated earlier, P3 has the lowest viscosity at zero shear rate; this suggests that P3 has less resistance to flow compared to sample P1 and P2 Therefore, P3 paste has the largest structural breakdown when high shear rate is subjected Although P3 showed the largest structural breakdown, its rapid structural build up interpreted that P3 is a strongly thixotropic paste material Conclusion In this study, the viscosities of several commercial solder pastes 共lead-free and ICA paste兲 are examined to find the effect of shear rate on the viscosity and to establish the correlation between paste viscosity and stencil printing process Furthermore, the power law and the Cross model used to quantify the viscosity/ shear rate profile for the shear-thinning solder pastes and fit to the experimental data From the experimental results, as the shear rates increased, the viscosities of the three pastes 共solder pastes and ICA兲 decreased 共shear thinning兲 In addition, the lead-free solder pastes exhibited the highest viscosity at low shear rates, which indicates that the dispersion of the paste is the more stable ICA paste In a stencil printing process, a paste of too high viscosity needs more energy to force the paste through the aperture and leads to poor surface wetting The statistical data show that the Cross model fits well to the experimental data than the power law model because it provides information in a wider range of shear rates The presence of an area between the down curve and up curve shows that the paste materials are thixotropic in nature The findings from the study show that a smaller particle size leads to a large surface area and better inter-particle attraction The structural breakdown and recovery of the pastes are important parameters that can be used in the development of new formulation of solder pastes and ICAs References 关1兴 关2兴 关3兴 关4兴 Durairaj, R., Ekere, N N., and Salam, B., “Thixotropy Flow Behaviour of Solder and Conductive Adhesives Paste,” J Mater Sci.: Mater Electron., Vol 15, 2004, pp 677–683 Nguty, T A., Ekere, N N., and Adebayo, A., “Correlating Solder Paste Composition with Stencil Printing Performance,” IEEE/CPMT International Electronics Manufacturing Technology Symposium, September 1999, pp 305–312 Lapasin, R., “Rheological Characterisation of Solder Pastes,” J Electron Mater., Vol 23共6兲, 1994, pp 525–532 Durairaj, R., Jackson, G J., Ekere, N N., Glinski, G., and Bailey, C., “Correlation of Solder Paste Rheology with Computational Simulations of the Stencil Printing Process,” Soldering Surf Mount Technol., Vol 14共1兲, 2002, pp 11–17 202 JAI • STP 1530 ON LEAD-FREE SOLDERS 关5兴 关6兴 关7兴 关8兴 关9兴 关10兴 关11兴 关12兴 关13兴 关14兴 关15兴 关16兴 关17兴 关18兴 关19兴 关20兴 关21兴 关22兴 关23兴 Evans, J and Beddow, J., “Characterisation of Particle Morphology and Rheological Behaviour in Solder Paste,” 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