Pb-free solders for flip-chip interconnects

A variety of lead-free solder alloys were studied for use as flip-chip interconnects including Sn-3.5Ag, Sn-0.7Cu, Sn-3.8Ag-0.7Cu, and eutectic Sn-37Pb as a baseline. The reaction behavior and reliability of these solders were determined in a flip-chip configuration using a variety of under-bump metallurgies (TiW/Cu, electrolytic nickel, and electroless Ni-P/Au). The solder micro-structure and intermetallic reaction products and kinetics were determined. The Sn-0.7Cu solder has a large grain structure and the Sn-3.5Ag and Sn-3.8Ag-0.7Cu have a fine lamellar two-phase structure of tin and Ag₃Sn. The intermetallic compounds were similar for all the lead-free alloys. On Ni, Ni₃Sn₄ formed and on copper, Cu₆Sn₅Cu₃Sn formed. During reflow, the intermetallic growth rate was faster for the lead-free alloys, compared to eutectic tin-lead. In solidstate aging, however, the interfacial intermetallic compounds grew faster with the tinlead solder than for the lead-free alloys. The reliability tests performed included shear strength and thermomechanical fatigue. The lower strength Sn-0.7Cu alloy also had the best thermomechanical fatigue behavior. Failures occurred near the solder/intermetallic interface for all the alloys except Sn-0.7Cu, which deformed by grain sliding and failed in the center of the joint. Based on this study, the optimal solder alloy for flip-chip applications is identified as eutectic Sn-0.7Cu. Editor’s Note: A hypertext-enhanced version of this article can be found at www.tms.org/pubs/journals/JOM/0106/Frear-0106.html For more information, contact D.R. Frear, Interconnect Systems Laboratories, Motorola, Tempe, AZ 85284; (480) 413-6655; fax (480) 413-4511; e-mail darrel.frear@motorola.com.     

Electromigration in solder joints and solder lines

Electromigration may affect the reliability of flip-chip solder joints. Eutectic solder is a two-phase alloy, so its electromigration behavior is different from that in aluminum or copper interconnects. In addition, a flipchip solder joint has a built-in currentcrowding configuration to enhance electromigration failure. To better understand electromigration in SnPb and lead-free solder alloys, the authors prepared solder lines in v-grooves etched on Si (001). This article discusses the results of those tests and compares the electromigration failure modes of eutectic SnPb and SnAgCu flip-chip solder joints along with the mean-timeto-failure.     

A metallurgical assessment of SnPbAg solder for GaAs power devices

In this study, solder-based die-attach processes used to affix GaAs devices to heat-spreading carriers were investigated. The solder microstructures were assessed, focusing on void formation, the response of the solder, the backsurface metallization, and the carrier plating to die attachment and reflow thermal processes. Voided regions were found in all solder joints, with a dramatic sensitivity to temperature cycles. Gold-tin alloy phases were found to dominate the solder microstructure for all of the configurations. The total thermal budget was a critical issue in the formation and transformation of various phases as expected for low-melting-point alloys. The NiV-Au backmetal system was investigated to determine the suitability for die attachment. J.M. Parsey, Jr., earned his Ph.D. in materials science at the Massachusetts Institute of Technology in 1982. He is currently a section manager, advanced materials, at Motorola ESTL. Dr. Parsey is also a member of TMS. S. Valocchi earned her M.S. in mechanical engineering/thermal sciences at Arizona State University in 1989. She is currently a process engineer at Motorola. W. Cronin earned his B.Sc. in electrical engineering at Arizona State University in 1979. He is currently a section manager, metals and thinning, at Motorola. J. Mohr earned his M.S. in materials science at Arizona State University in 1990. He is currently a senior technologist at Motorola. B.L. Scrivner earned his B.S. in business administration at the University of Phoenix in 1994. He is currently a product manager at Motorola. K. Kyler earned his B.S. in chemistry at Brigham Young University in 1987. He is currently a process development engineer at Motorola PCRL.     

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.     

Flip-chip process using heat transfer from an induction-heating film

A new flip-chip technology to attach an IC chip directly to a substrate was studied using the heat transfer from an induction-heating film in an AC magnetic field. When applying a magnetic field of 230 Oe at 14 kHz, the temperature of a 600 μm-thick 5 mm × 5 mm Cu induction-heating film reached 250 °C within 60 s. The temperature of the glass substrate used in this process was kept below 118 °C at a distance of 1,350 μm from the Cu induction-heating film, which was maintained at 250 °C, implying that damage to a substrate can be minimized with the flip-chip process using heat transfer from an induction-heating film. Flip-chip bonding was successfully accomplished with the reflow of Sn-3.5Ag solder bumps by applying a magnetic field of 230 Oe at 14 kHz for 120 s to a Cu induction-heating film.     

Reliability of flip-chip bonded RFID die using anisotropic conductive paste hybrid material

A reliability of flip-chip bonded die as a function of anisotropic conductive paste (ACP) hybrid materials, bonding conditions, and antenna pattern materials was investigated during the assembly of radio frequency identification(RFID) inlay. The optimization condition for flip-chip bonding was determined from the behavior of bonding strength. Under the optimized condition, the shear strength for the antenna printed with paste-type Ag ink was larger than that for Cu antenna. Furthermore, an identification distance was varied from the antenna materials. Comparing with the Ag antenna pattern, the as-bonded die on Cu antenna showed a larger distance of identification. However, the long-term reliability of inlay using the Cu antenna was decreased significantly as a function of aging time at room temperature because of the bended shape of Cu antenna formed during the flip-chip bonding process.     

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.     

The effect of pre-aging on the electromigration of flip-chip SnAg solder joints

The effect of pre-aging on electromigration is investigated in this study using flip-chip SnAg solder joints. The solder joints were pre-aged at 170°C for 1 h, 3 h, 5 h, 10 h, 25 h, and 50 h, and then they were subjected to electromigration tests of 0.9 A at 150°C. It was found that the average failure time increased about three times when the joints were pre-aged for 3 h to 25 h. But it decreased when the joints were overaged. It is proposed that the major contributor to the prolonged failure time may be the densification of the nickel and copper under-bump metallization (UBM) and the solder due to the aging treatment. The pre-aging treatment at 170°C may stabilize the microstructure of the solder. The vacancies in the solder were annihilated during the heat treatment, causing a slower diffusion rate. In addition, the UBM structure became denser after the pre-aging process. Thus, the denser UBM structure may lead to slower consumption rates of the nickel and copper layers, resulting in the enhancement of electromigration resistance.     

Characterization of micro-crack propagation through analysis of edge effect in acoustic microimaging of microelectronic packages

The miniaturization and three dimensional die stacking in advanced microelectronic packages poses a big challenge to their non-destructive evaluation by acoustic microimaging. In particular, their complicated structures and multiple interfaces make the interpretation of acoustic data even more difficult. A common phenomenon observed in acoustic microimaging of microelectronic packages is the edge effect phenomena, which obscures the detection of defects such as cracks and voids.In this paper, two dimensional finite element modelling is firstly carried out to numerically simulate acoustic microimaging of modern microelectronic packages. A flip-chip with a 140 µm solder bump and a 230 MHz virtual transducer with a spot size of 16 µm are modelled. Crack propagation in the solder bump is further modelled, and B-scan images for different sizes of micro-cracks are obtained. C-line plots are then derived from the simulated B-scan images to quantitatively analyze the edge effect. Gradual progression of the crack is found to have a predictable influence on the edge effect profile. By exploiting this feature, a crack propagation characterization method is developed. Finally, an experiment based on the accelerated thermal cycling test is designed to verify the proposed method.     

Thermoelectric power generation materials: Technology and application opportunities

Thermoelectric power sources have consistently demonstrated their extraordinary reliability and longevity for deep space missions (67 missions to date, more than 30 years of life) as well as terrestrial applications where unattended operation in remote locations is required. The development of new, more efficient materials and devices is the key to improving existing space power technology and expanding the range of terrestrial applications. The NASA Jet Propulsion Laboratory is leading collaborative research and development on novel advanced bulk materials capable of long-term operation at temperatures up to 1,300 K at more than 20% conversion efficiency. The research areas include refractory rare earth compounds and bulk three-dimensional nanostructures that emulate results obtained on low dimensional superlattices through “force engineering” and “self-assembling” techniques. Recent experimental results will be highlighted, and progress in transitioning thermoelectric technology to a more flexible, lower-cost modular array configuration suitable for various application opportunities will be discussed.     

Study on the behavior characteristics of solder balls for FCBGA package

Warpage issues are of concern for flip chip Ball Grid Array (FCBGA) package reliability on thin substrates. Despite advantages in electrical performance of thin substrates, FCBGA warpage hampers reliability of the solder balls making up the ball grid array. Warpage and plastic strain of both the FCBGA package and solder balls are calculated under assembly conditions. Temperature dependent material properties were investigated using finite element analysis. FEM calculations of thermal deformation and equivalent plastic strain of solder balls considered to determine overall reliability of the solder balls. Solder ball plastic strain decreased with substrate core thickness. This can explain why solder ball reliability decreases with substrate thickness. Substrate surface warpage fluctuated significantly as core thickness decreased due to reduced stiffness. In addition, it is possible that solder balls and surface mounted components can be affected by surface fluctuations. Although thinner substrates have advantages in terms of electrical performance, high warpage is a critical root cause of package failure. Despite these tradeoffs, developing thinner substrates while avoiding warpage is a forefront issue.     

Reliability of flip-chip bonded RFID die using anisotropic conductive paste hybrid material

A reliability of flip-chip bonded die as a function of anisotropic conductive paste (ACP) hybrid materials, bonding conditions, and antenna pattern materials was investigated during the assembly of radio frequency identification(RFID) inlay. The optimization condition for flip-chip bonding was determined from the behavior of bonding strength. Under the optimized condition, the shear strength for the antenna printed with paste-type Ag ink was larger than that for Cu antenna. Furthermore, an identification distance was varied from the antenna materials. Comparing with the Ag antenna pattern, the as-bonded die on Cu antenna showed a larger distance of identification. However, the long-term reliability of inlay using the Cu antenna was decreased significantly as a function of aging time at room temperature because of the bended shape of Cu antenna formed during the flip-chip bonding process.     

Structural materials issues for the next generation fission reactors

Generation-IV reactor design concepts envisioned thus far cater to a common goal of providing safer, longer lasting, proliferation-resistant, and economically viable nuclear power plants. The foremost consideration in the successful development and deployment of Gen-W reactor systems is the performance and reliability issues involving structural materials for both in-core and out-of-core applications. The structural materials need to endure much higher temperatures, higher neutron doses, and extremely corrosive environments, which are beyond the experience of the current nuclear power plants. Materials under active consideration for use in different reactor components include various ferritic/martensitic steels, austenitic stainless steels, nickel-base superalloys, ceramics, composites, etc. This article addresses the material requirements for these advanced fission reactor types, specifically addressing structural materials issues depending on the specific application areas.     

电子互连无铅钎料及焊点蠕变行为研究进展

针对近年来无铅钎料及焊点的蠕变失效问题,综合评述了蠕变变形行为及其在焊点可靠性评估中的应用。首先系统地介绍无铅钎料的蠕变行为,探讨含合金元素/颗粒无铅钎料蠕变性能改性机制。其次评述焊点蠕变行为,探讨焊点成分以及不同基板材料对焊点蠕变特性影响的研究进展。再次结合具体电子器件,采用有限元模拟,分析基于有限元的焊点蠕变响应及疲劳寿命预测,评估焊点可靠性。最后针对无铅钎料及焊点蠕变行为的未来发展进行展望,分析其研究中存在的问题及解决办法,为焊点可靠性进一步研究提供理论支撑。     

Low-thermal-expansion copper composites via negative CTE metallic elements

Thermal management is an important issue in electronic packaging due to the increasing complexity, miniaturization, and high density of components in modern devices. In high-power-dissipation packages, heat sinks are essential for preventing thermal damage to heat-sensitive components on the silicon chip. However, commonly used heat-sink materials (e.g., copper and aluminum alloys) have a much higher coefficient of thermal expansion (CTE) than silicon. CTE mismatch between the various materials in an electronic package can lead to stresses that can trigger complex failure mechanisms like component distortion, stress rupture, thermomechanical fatigue, and creep, thereby seriously degrading device reliability and lifetime. Therefore, it is highly desirable to minimize the CTE mismatch by developing new heat-sink materials having CTEs that are close to the CTE of silicon. In this work, low-thermal-expansion copper composites with CTEs as low as 4 ppm/°C have been fabricated by employing a negative thermal-expansion alloy—equiatomic Ni-Ti, which has a CTE of approximately −21 ppm/°C. The use of negative CTE elements, especially those with very large negative CTE values, offers an attractive route for controlling the thermal-expansion behavior of various metallic and nonmetallic materials. H. Mavoori earned his Ph.D. in materials science and engineering from Northwestern University in 1996. He is a post-doctoral member of the technical staff of the Applied Materials and Metallurgy Research Group at Bell Laboratories, Lucent Technologies. Dr. Mavoori is also a member of TMS. S. Jin earned his Ph.D. in materials science at the University of California at Berkeley in 1974. He is currently technical manager at Bell Laboratories, Lucent Technologies. Dr. Jin is also a member of TMS. Author’s Note: Unless otherwise indicated, compositions are in weight percent.     

QFN封装焊点可靠性的有限元分析

建立了QFN(quad flat non-leaded)的1/4模型,用Anand模型构建了Sn3.0Ag0.5Cu的本构方程,并比较了不同焊点形态时QFN的可靠性.结果表明,在温度循环载荷下,QFN器件应力的最大值位于拐角处的焊点的上表面处,且其应力值变化具有周期性和叠加性;无缩回设计的引脚比有缩回设计的引脚具有更好的可靠性,但对于无缩回设计的引脚,其焊脚高度和焊点长度对其塑性功的累积无明显影响;钎料厚度对焊点塑性功的累积有明显影响,焊点的单位体积塑性功与钎料厚度成反比;中央散热焊盘的焊接面积对焊点塑性功的累积有一定影响,在QFN的焊接过程中可以适当增加其焊接面积.     

Microstructural evolution and tensile properties of Sn-5Sb solder alloy containing small amount of Ag and Cu

The near peritectic Sn-5Sb Pb-free solder alloy has received considerable attention for high temperature electronic applications, especially on step soldering technology, flip-chip connection. In the present study, a separate addition of the same amount of Ag and Cu are added with the near-peritectic Sn-5Sb solder alloy to investigate the effect of a third element addition on the microstructural, thermal and mechanical properties of the newly developed ternary solder alloys. The results indicate that the melting point of Sn-5Sb solder is enhanced by Ag and Cu additions. Besides, the Ag and Cu content refine the microstructure and form new intermetallic compounds (IMCs) with the near-peritectic Sn-5Sb solder alloy. The tensile tests revealed that all alloys exhibit higher mechanical strength with increasing strain rate and/or decreasing testing temperature, suggesting that the tensile behavior of the three alloys is strain rate and temperature dependence. The yield and ultimate tensile strength are higher for Sn-5Sb-0.7Cu alloy compared with Sn-5Sb and Sn-5Sb-0.7Ag alloys. Good mechanical performance of Sn-5Sb-0.7Cu solder is often correlated to a fine β-Sn grain size and more dispersed Cu-Sn IMC particles, which makes the solder exhibit high strength and yield stress.     

The properties of tin-bismuth alloy solders

Tin-bismuth alloys may be an alternative to lead-based solders for low-temperature applications, but very little is known about their manufacturability and reliability. This article presents an overview of these issues. First, experiments to determine the wetting properties of the Sn-Bi solder are presented. The results show that Sn-Bi solders do not wet bare copper well, but that they do wet copper having a hot-dipped Sn-Bi coating. Next, the effects of aging on the microstructure of Sn-Bi solders are described. The results show that during aging, tin is de-pleted from the solder/base metal interface. The two-phase Sn-Bi microstructure coarsens during aging; the rate of coarsening can be slowed by adding 1.0 wt. % Cu to the solder. The aging also affects the shear strength of the solder joints, where aged joints show an increase in maximum shear stress and ductility at failure.     

THE DESIGN OF AN INTEGRATED SYSTEM FOR PREDICTING AND ANALYZING SOLDER JOINT SHAPE AND RELIABILITY IN SMT

1.IntroductionOnecriticalaspectinelectronicpackagingisthefatigUe/creep--inducedfailureinsolder(tin--leadbasedalloy)interconnections.previouswork,[l--ZJhaveprovedthatthesolderjointreliabilitydependsonthesolderjointshape(amongotherparameters).Manyissueshavefocusedontheresearchaboutsolderjointshapesince90.However,asystematicmethodofpredictingandanalyzingsolderjointshapeandreliability,whichcanheusedindirectingpracticalengineering,isstillnotgiven.Basedontheobject--orientedaPPmach,anintendedsystem…     

微电子封装无铅钎焊的可靠性研究

微电子封装材料的无铅化就是目前微电子封装技术的发展方向之一,微电子封装材料的无铅化对产品的质量可靠性带来的影响与有铅钎料封装时代不同,所以对此影响进行研究,并据此选择产品生产过程中合适的无铅钎料,制定适当的生产工艺评估标准来提高产品的可靠性有着较大的现实意义。阐述了微电子封装采用无铅钎料的必要性,概述了无铅钎料的研究现状,并着重分析讨论了微电子封装无铅钎焊的可靠性问题。