The effect of joint size on the creep properties of microscale lead-free solder joints at elevated temperatures

Solder joints in electronic packaging systems are becoming smaller and smaller to meet the miniaturization requirements of electronic products and high density interconnect technology. Furthermore, many properties of the real solder joints at the microscale level are obviously different from that of bulk solder materials. Creep, as one of the key mechanical properties at elevated temperatures, can impair the reliability of miniature solder joints in electronic devices. However, there is a lack of knowledge about the comparative creep properties of microscale solder joints of different sizes. Most previous studies have focused on the creep properties of bulk solder materials or solder joints of the same size. In this research, to determine whether a size effect exists for creep properties of solder joints or not, we characterized the creep behaviors of Sn–3.0Ag–0.5Cu lead-free solder joints under tensile loading modes using microscale butt-joint specimens with a copper-wire/solder/copper-wire sandwich structure with two different sizes. Also, the creep failure mechanisms were investigated. Experimental results show that the creep activation energy and creep stress exponent are very similar for both sizes of solder joint. However, under the same testing conditions, the joints with a larger size exhibit a much higher steady-state creep rate and a shorter creep lifetime than the smaller joints.     

Critical studies to improve service reliability of Sn–Ag–Cu solder joints by thermal treatments

Lead-free electronic packages intended for use in applications such as aerospace, military, and other highly demanding service conditions, necessitate exceptional mechanical reliability of lead-free electronic solder joints under realistic service conditions. Most current design strategies employed for improving the reliability of lead-free electronic solder joints are aimed at developing suitable alloying additions and reinforcements to the solder itself. At present there exists no suitable methodology to minimize the effects of service conditions while the solder joint is in service. Since thermomechanical fatigue reliability of electronic solder joints is closely related to the crack nucleation that occurs during very early stages of repeated thermal excursions, this study is based on subjecting solder joints to a limited number of thermal shock (TS) cycles in a chosen temperature regime to nucleate cracks, then evaluating their effectiveness in improving reliability when the solder joints are subjected to additional TS cycles in a different temperature regime. This study is a preliminary investigation, aimed at developing suitable methodology to minimize the effects of damage to lead-free solder joint specimens subjected to repeated thermal excursions during service, by imposing appropriate thermal treatments. These thermal treatments can be automatically implemented at programmed intervals during the service life of the electronic packages. Methods employed in these studies may also be useful to enhance long-term service reliability and to obtain a conservative estimate of long-term service reliability.     

温度对Cu颗粒增强复合钎料蠕变性能的影响

蠕变性能是影响钎焊接头可靠性的重要指标之一.采用搭接面积为1 mm2的单搭接钎焊接头,在恒定载荷下,测定了Cu颗粒增强锡铅基复合钎料钎焊接头的蠕变寿命,分析并讨论了温度对该复合钎料蠕变寿命的影响.结果表明:Cu颗粒增强的锡铅基复合钎料的蠕变抗力优于传统63Sn37Pb共晶钎料;钎焊接头蠕变寿命随温度的升高而降低,并且温度对复合钎料钎焊接头蠕变寿命的影响较传统63Sn37Pb钎料明显.     

Reliability behavior of lead-free solder joints in electronic components

With more consumer products moving towards environmentally friendly packaging, making solder Pb-free has become an urgent task for electronics assemblies. Solder joints are responsible for both electrical and mechanical connections. Solder joint does not have adequate ductility to ensure the repeated relative displacements due to the mismatch between expansion coefficients of the chip carrier and the circuit board. Materials behavior of solder joints involves a creep–fatigue interaction, making it a poor material for mechanical connections. The reliability of solder joints of electronics components has been found playing a more important role in service for microelectronics components and micro-electro-mechanical systems. So many researchers in the world investigated reliability of solder joints based on finite element simulation and experiments about the electronics devices, such as CR, QFP, QFN, PLCC, BGA, CSP, FCBGA and CCGA, which were reviewed systematically and extensively. Synchronously the investigation on reliability of solder joints was improved further with the high-speed development of lead-free electronic packaging, especially the constitutive equations and the fatigue life prediction equations. In this paper, the application and research status of constitutive equations and fatigue life prediction equations were reviewed, which provide theoretic guide for the reliability of lead-free solder joints.     

Review on microstructure evolution in Sn–Ag–Cu solders and its effect on mechanical integrity of solder joints

The use of Pb-bearing solders in electronic assemblies is avoided in many countries due to the inherent toxicity and environmental risks associated with lead. Although a number of “Pb-free” alloys have been invented, none of them meet all the standards generally satisfied by a conventional Pb–Sn alloy. A large number of reliability problems still exist with lead free solder joints. Solder joint reliability depends on mechanical strength, fatigue resistance, hardness, coefficient of thermal expansion which are influenced by the microstructure, type and morphology of inter metallic compounds (IMC). In recent years, Sn rich solders have been considered as suitable replacement for Pb bearing solders. The objective of this review is to study the evolution of microstructural phases in commonly used lead free xSn–yAg–zCu solders and the various factors such as substrate, minor alloying, mechanical and thermo-mechanical strains which affect the microstructure. A complete understanding of the mechanisms that determine the formation and growth of interfacial IMCs is essential for developing solder joints with high reliability. The data available in the open literature have been reviewed and discussed.     

Intermetallic compounds in 3D integrated circuits technology: a brief review

Abstract

The high performance and downsizing technology of three-dimensional integrated circuits (3D-ICs) for mobile consumer electronic products have gained much attention in the microelectronics industry. This has been driven by the utilization of chip stacking by through-Si-via and solder microbumps. Pb-free solder microbumps are intended to replace conventional Pb-containing solder joints due to the rising awareness of environmental preservation. The use of low-volume solder microbumps has led to crucial constraints that cause several reliability issues, including excessive intermetallic compounds (IMCs) formation and solder microbump embrittlement due to IMCs growth. This article reviews technologies related to 3D-ICs, IMCs formation mechanisms and reliability issues concerning IMCs with Pb-free solder microbumps. Finally, future outlook on the potential growth of research in this area is discussed.     

Design of solder joints for fundamental studies on the effects of electromigration

The continuous miniaturization of high performance electronic devices has reached a level at which current densities are large enough to make electromigration (EM) a significant issue affecting the electrical and mechanical reliability of solder joints. A new design of solder joints that controls the extent of regions experiencing relatively uniform current density, as well as regions with large current density gradient was developed. Current density distribution of this newly designed solder joint was calculated using finite element analysis (FEA), which was used to guide the characterization of EM of real solder joints. As a part of the effort in evaluating the suitability of the new joint configuration for evaluating the fundamental issues in EM, eutectic PbSn solder joints were fabricated using this design. EM effects due to applied current, current density distribution, and joint thickness of eutectic PbSn solder joints present in this joint configuration were investigated. Findings based on this new design can facilitate fundamental studies of EM issues that affect the reliability of solder joints.     

Degradation of Sn37Pb and Sn3.5Ag0.5Cu solder joints between Au/Ni (P)/Cu pads stressed with moderate current density

Sn37Pb (SP) and Sn3.5Ag0.5Cu (SAC) ball grid array (BGA) solder joints between Au/Ni (P)/Cu pads were stressed with a moderate current density of 6.0 × 10² A/cm² at an ambient temperature of 125°C up to 600 h. The solder joint reliability was evaluated in terms of temperature measurement, microstructural analysis and mechanical strength test. It was confirmed that no obvious electromigration occurred with this moderate current density. However, the local temperature of solder joints rose considerably due to massive Joule heating, which degraded the solder joint reliability seriously. Phase coarsening was observed for both solders and it was particularly apparent in the SP solder joints. Compared to the SP, the SAC was found to be more reactive and hence a thicker intermetallic compound (IMC) was developed during the current stressing. Nevertheless, the IMC thickening was not as remarkable as expected with current stressing at high temperature. It exhibited a sub-parabolic growth manner that was mainly controlled by grain boundary diffusion. However, a sufficiently thick IMC layer initially formed during reflow soldering and the low diffusivity of the Ni atoms retarded the growth. The shear strength of the solder joints was found to decrease severely with the current stressing time. This degradation was attributed to the large stresses arising from localized thermal mismatch, phase coarsening, volume shrinkage of IMC evolution, Ni–P layer crystallization and the pad cracking during current stressing.     

随机振动载荷下塑封球栅阵列含铅焊点疲劳寿命模型

对塑封球栅阵列(PBGA)封装器件Sn37Pb焊点进行了正弦振动、随机振动实验,得到各个载荷下焊点的疲劳寿命结果。建立了三维有限元模型,进行与实验条件一致的有限元分析,计算焊点的应力;将实验结果与有限元计算相结合,并基于Steinberg寿命预测模型,发展了随机振动载荷下焊点疲劳寿命预测方法。结果表明,疲劳寿命模型预测结果与实验结果吻合较好,该方法可应用于PBGA封装焊点在随机振动载荷下的疲劳寿命评估,为PBGA封装器件的设计与使用提供指导。     

Study on creep characterization of nano-sized Ag particle-reinforced Sn–Pb composte solder joints

In the present work, the creep strain of solder joints is measured using a stepped load creep test on a single specimen. Based on the experimental results, the constitutive model on the steady-state creep strain is established by applying a linear curve fitting for the nano-sized Ag particle-reinforced Sn37Pb based composite solder joint and the Sn37Pb solder joint, respectively. It is indicated that the activation energy of the Ag particle-reinforced Sn37Pb based composite solder joints is higher than that of Sn37Pb solder joints. It is expected that the creep resistance of the Ag particle-reinforced Sn37Pb based composite solder joints is superior to that of Sn37Pb solder.     

Life expectancies of Pb-free SAC solder interconnects in electronic hardware

The transition from lead (Pb) bearing solder to Pb-free solder has arisen in response to government restrictions on the use of lead (Pb) by the European Union. As a result, electronic manufacturers have sought a material comparable to the conventional 63Sn37Pb solder that has been traditionally used to assemble electronic hardware. Based on extensive review of various solder combination, the majority of electronic manufacturers appear to be adopting a tin–silver–copper (SAC) solder as a popular Pb-free solder replacement. Significant investments have been made by many researchers to characterize the material behavior and durability of this solder system. While the exact composition of the SAC solder is still in question, it now appears that the 96.5Sn3.0Ag0.5Cu (SAC305) solder is gaining wider acceptance as the favored Pb-free replacement, for surface mount assemblies that are going to be subjected predominantly to cyclic thermal environments. This paper presents a review of our current understanding of the life expectancy of Pb-free SAC solder interconnects for electronic hardware. To this end, the paper focuses on material characterization of SAC solder, as well as its temperature cycling and vibration fatigue reliability. From this review, SAC solder interconnects are shown to be suitable for providing adequate life expectancies for temperature cycling in electronic hardware. However, it is clear that there are differences between SAC and the conventional Sn37Pb solder, that need to be understood in order to design reliable electronic hardware.     

Fracture mechanics analysis of the effect of substrate flexibility on solder joint reliability

Solder joint fatigue failure is a serious reliability concern in area array technologies, such as flip chip and ball grid array packages of integrated-circuit chips. The selection of different substrate materials could affect solder joint thermal fatigue life significantly. The reliability of solder joints in real flip chip assembly with both rigid and compliant substrates was evaluated by the accelerated temperature cycling test and thermal mechanical analysis. The mechanism of substrate flexibility on improving solder joint thermal fatigue lifetime was investigated by fracture mechanics methods. Two different methods (crack tip opening displacement, CTOD and virtual crack closure technique, VCCT) are used to determine the crack tip parameters which are considered as the indices of reliability of solder joints, including the strain energy release rate and phase angle for the different crack lengths and temperatures. It was found that the thermal fatigue lifetime of solder joints in flip chip on flex assembly (FCOF) was much longer than that of flip chip on rigid board assembly (FCOB). The flex substrates could dissipate energy that otherwise would be absorbed by solder joints, that is, substrate flexibility has a great effect on solder joint reliability and the reliability improvement was attributed to flex buckling or bending during thermal cycling.     

Reliability of Pb-free solders for harsh environment electronic assemblies

Abstract

Since 2006 and the implementation of environmental regulations, the electronic industry has moved to Pb-free solders. Harsh environment industries that were exempted from the regulations will soon have to follow suit. However, a suitable replacement solder for use in harsh environments still has to be validated and reliability models are yet to be established. In this review, research that led to the selection of currently used Pb-free alloys and the continuing search for high reliability alloys are described. Sn pest and Sn whiskers, potential major threats for electronics operating in harsh environments, are highlighted. This review also focuses on the microstructure, mechanical properties and deformation mechanisms of Pb-free alloys. Emphasis is placed on Sn–Ag–Cu alloys, now considered to be the alloys of choice for replacement of Sn–Pb solders. The reliability of Pb-free electronic assemblies is studied, focusing on thermal fatigue, believed to be the main source of failure through creep–fatigue mechanisms. The validity of models for Pb-free solder joints life time prediction is assessed and the lack of cohesiveness among the available reliability data is examined.     

The ‘IMC rings’ growth mechanism in Sn–Ag–Cu solder ball and NiAu-plated BGA ball pad

Due to environmental pollution concerns, the law says the lead (Pb) inside electronics devices must be eliminated. Lots of lead-free materials have been introduced and been used for electronic products and Sn–Ag–Cu (SAC) is one of most popular lead-free representatives and has been used in high-volume production. The most popular IC packages, BGA packages which have higher I/O counts, and better thermal and electrical performance than lead-frame type packages, use solder balls of SAC for lead-free applications to connect with printed circuit boards. A particular phenomenon, so-called ‘IMC rings’, is only observed on BGA solder ball pad surfaces after the SAC solder balls are mounted on BGA ball pads which are plated with NiAu. It has not been found in either eutectic solder or Sn–Ag solder welding on plated NiAu pads. No significant evidence exists to show that ‘IMC rings’ degrade the strength of solder joints or cause earlier failures in mechanical tests. ‘IMC rings’ appear to be an inevitable outcome after the SAC is soldered onto a plated NiAu ball pad. This study is to find the growth mechanism of ‘IMC rings’ on the ball pad which is created between SAC ball and plated NiAu pad during solder ball temperature reflow. The design of the experiment and data have been discussed.     

Microstructures formed from mixed solders on copper substrate

Abstract

With the development and use of a variety of Pb free solders, it is probable that some solder joints in electronic assemblies may be made with solders of two different compositions. To investigate possible microstructures resulting from such procedure, samples were prepared using small balls of four different Sn–Ag–Cu (SAC) Pb free solders, as well as Sn–Zn–Al solder, melted together with eutectic Pb–Sn solder paste and also various SAC solder pastes, on a copper substrate. It was observed that using eutectic Pb–Sn solder paste with an SAC solder ball introduced some Pb–Sn eutectic microstructure and changed the ternary eutectic present from Ag₃Sn–Cu₆Sn₅–Sn to Ag₃Sn–Pb–Sn. Use of an SAC solder paste with Sn–Zn–Al solder introduced an apparent Ag–Cu–Zn ternary compound, replacing Zn lamellae of the Sn–Zn eutectic. With eutectic Pb–Sn solder paste, the Pb–Sn–Zn ternary eutectic was formed. It was noted that use of a high Sn solder results in rapid dissolution of the copper substrate.     

Reliability of SnAgCu/SnAgCuCe solder joints with different heights for electronic packaging

Finite element simulation and experiments were used to analyze the reliability of SnAgCu/SnAgCuCe solder joints with different heights. With the increase of height of solder joints, the tensile strength of SnAgCu and SnAgCuCe solder joints drops significantly. With the same height, the tensile strength of SnAgCuCe solder joints show enhanced reliability comparing with that of SnAgCu solder joints. The heights of lead-free solder joints can markedly influence the failure modes of SnAgCu/SnAgCuCe solder joints, three models [intermetallic compound (IMC), solder/IMC, solder] can be demonstrated with the variation of height. Based on finite element methods, it is found the maximum stress concentrated location show a relationship with heights of solder joints, smaller height can keep the high reliability of solder joints.     

Annealing effect to constitutive behavior of Sn–3.0Ag–0.5Cu solder

Sn–Ag–Cu based solder alloys are replacing Sn–Pb solders in electronic packaging structures of commercial electric devices. In order to evaluate the structural reliability, the mechanical property of solder material is critical to the numerical simulations. Annealing process has been found to stabilize material properties of Sn–37Pb solder material. In the current study, the annealing effect on tensile behaviour of Sn–3.0Ag–0.5Cu (SAC305) solder material is investigated and compared with Sn–37Pb solder. It is found that the tensile strength for both materials are more stabilized and consistent after the annealing process, nevertheless, the annealing process will improve the plasticity of SAC305 solder dominated by dislocation motion, and impede the occurrence of hardening deformation in Sn–37Pb solder dominated by grain-boundary sliding mechanism. Furthermore, the annealing effect is quantified in the proposed constitutive model based on unified creep–plasticity theory. The parameters are calibrated against the measured stress–strain relationships at the tensile strain rates ranging from 1?×?10^?4 to 1?×?10^?3 s^?1. The numerical regressions for dominant parameters in the proposed model reveal the intrinsic differences between SAC305 and Sn–37Pb solders under annealing treatment.     

Effect of solder filler thickness on the mechanical stability of fiber-solder-ferrule joint under temperature cyclic loading

The base materials of package and ferrule are often gold-coated Kovar and Invar, they both have relatively low coefficient of thermal expansion (CTE). Solder 63Sn37Pb dissolves Au substantially and forms brittle AuSn₄, which may cause catastrophic failure in the fiber-solder-ferrule (FSF) joint in the long-term application. It is well known that thermal fatigue creep is one of the crucial factors affecting the life and reliability of a solder joint in electronic and optoelectronic assemblies. Therefore, it is important to understand the behavior of the FSF joint under thermal cyclic loading. In this study, four different thicknesses of solder filler in a FSF joint were examined. By using the finite element method (FEM), the equivalent creep strains of eutectic lead-tin solder were compared. The joints were subjected to 5 cycles of temperature cycling test, i.e., −65 to 150^∘C. It was found that the thicker solder filler is subjected to a larger equivalent creep strain than the thinner solder filler. It is discussed the vertical shift of the optical fiber, which is sensitive to temperature and has effects on the power loss coupling. Modeling and experimental results show that 0.5 mm is the best inner diameter of ferrule that provides the lowest displacement and, thus, the lowest power loss under temperature cycle.     

A review of the influencing factors on anisotropic conductive adhesives joining technology in electrical applications

New interconnect materials are always necessary as a result of evolving packaging technologies and increasing performance and environmental demands on electronic systems. Polymer-based conductive-adhesive materials have become widely used in many electronic packaging interconnect applications. Among all the conductive-adhesive materials, the anisotropic conductive adhesives (ACA) (or anisotropic conductive adhesive films, ACF) have gained popularity as a potential replacement for solder interconnects. The interest in using ACA instead of solder comes partly from the fact that the use of ACA for the direct interconnection of flipped silicon chips to printed circuits (flip chip packaging) offers numerous advantages such as reduced thickness, improved environmental compatibility, lowered assembly process temperature, increased metallization options, reduced cost, and decreased equipment needs. In this review, a summary of our understanding of the electrical, physical, thermal, chemical, environmental, and cost behaviors of ACA in conjunction with various packaging applications is elaborated. First, the formulation and curing kinetics of ACA materials, as well as the conduction mechanisms of ACA joints, are introduced; second, the influencing factors, including the boding process (boding temperature, boding pressure, curing conditions, reflow and misalignment processes, etc), the environmental factors (temperature, humidity, impact load, etc), and the properties of the components (the properties of the ACA, substrates, conductive particles, the bump height, etc), on the reliability of ACA joining technology are presented. Finally, future research areas and remaining issues are pointed out. The purpose is simply to pinpoint the most important papers that have played significant role for the advancement of the ACA bonding technology.