First-principles study on elastic properties of Cu, (Cu1−x,Nix)3Sn and interfacial mechanical properties of (Cu1−x,Nix)3Sn/Cu in the lead-free solder joint

With human's increasing attention to the environment protection, the transition of electronic assembly technology from tin-lead to lead-free is a matter of necessity. Sn-Ag-Cu solder alloy systems have been considered as the most potential substitute for traditional Sn-Pb solder alloys. It has been accepted that, in protective atmosphere, the peak reflow temperature can be reduced and the wetting performance of lead-free solder joints can be improved. Ar is the most plentiful rare inert gas. To date, however, few researches on using Ar as reflow atmosphere have been reported. The focus of this study is to investigate the influence of Ar reflow atmosphere on the microstructure and properties of lead-free solder joints. Reflow process in Ar atmosphere was conducted for OSP-pad PCBs. Board level reflow in different atmospheres and microstructural characterization of the solder joints after both reflow process and environmental tests were carried out. The experimental results indicated that Ar atmosphere could obviously improve the wetting performance and reliability of solder joints in comparison with the solder joints fabricated in air atmosphere.

The reliability of an electronic assembly is often limited by the fatigue failure of one of the interconnected solder joints due to the mismatch of Coefficient of Thermal Expansion (CTE) between the components and Printed Circuit Board (PCB). Fatigue properties of lead-free solder joints are normally studied on the large bulk samples. However, the microstructure and failure mechanism of actual size solder joints are different than bulk samples. The surface finish also affects the fatigue behavior of solder joints. The objective of this study is to investigate the effect of surface finish on the fatigue properties of the high-Bi lead-free solder joints. Lead-free alloys for harsh applications are commonly micro-alloyed with small percentages of metal elements, such as Bi, to enhance the fatigue life. In this study, individual solder joins were cycled using Instron micromechanical testers in stress-controlled experiments. The tested solder materials were Sn-3Ag-0.5Cu (SAC305), Sn-3.5Ag-0.7Cu-3Bi-l.5Sb-0.125Ni (Innolot), Sn-3.41Ag-0.52Cu-3.3Bi (CycloMax), and Sn-0.92Cu-2.46Bi (Ecolloy). The surface finishes were OSP, ImAg, and ENIG. Characteristic fatigue life and work dissipation per cycle were measured and compared. The results showed that characteristic fatigue life decreased with the increase of stress amplitude. Solder joints with high Ag and Bi contents demonstrated outstanding fatigue properties. OSP and ImAg surface finishes demonstrated more stable fatigue properties, while ENIG surface finish tended to show an embrittlement fracture between the Ni layer and solder joints during high-stress-amplitudes fatigue test.

There is an increasing demand for replacing tin-lead (Sn/Pb) solders with lead-free solders in the electronics industry due to health and environmental concerns. The European Union recently passed a law to ban the use of lead in electronic products. The ban will go into effect in July of 2006. The Japanese electronics industry has worked to eliminate lead from consumer electronic products for several years. Although currently there are no specific regulations banning lead in electronics devices in the United States, many companies and consortiums are working on lead-free solder initiatives including Intel, Motorola, Agilent Technologies, General Electric, Boeing, NEMI and many others to avoid a commercial disadvantage. The solder joints reliability not only depends on the solder joint alloys, but also on the component and PCB metallizations. Reflow profile also has significant impact on lead-free solder joint performance because it influences wetting and microstructure of the solder joint. A majority of researchers use temperature cycling for accelerated reliability testing since the solder joint failure mainly comes from thermal stress due to CTE mismatch. A solder joint failure could be caused by crack initiation and growth or by macroscopic solder facture. There are conflicting views of the reliability comparison between lead-free solders and tin-lead solders. This paper first reviews lead-free solder alloys, lead-free component lead finishes, and lead-free PCB surface finishes. The issue of tin whiskers is also discussed. Next, lead-free solder joint testing methods are presented; finite element modeling of lead-free solder joint reliability is reviewed; and experimental data comparing lead-free and tin-lead solder joint reliability are summarized. Finally the paper gives perspectives of transitions to totally lead-free manufacturing.

Electronic products are evolving towards miniaturization, high integration, and multi-function, which undoubtedly puts forward higher requirements for the reliability of solder joints in electronic packaging. Approximately 70% of failure in electronic devices originates during the packaging process, mostly due to the failure of solder joints. With the improvement of environmental protection awareness, lead-free solder joints have become a hot issue in recent years. This paper reviews the research progress on the reliability of lead-free solder joints and discusses the influence of temperature, vibration, tin whisker and electromigration on the reliability of solder joints. In addition, the measures to improve the reliability of solder joints are analyzed according to the problems of solder joints themselves, which provides a further theoretical basis for the study of the reliability of solder joints of electronic products in service.

As environmental issues are raising more interest and are becoming crucial factors in all parts of the world, more and more environmental-friendly electronics products are emerging. Usually this means the introduction of products with lead-free solders. However, the reliability of lead-free solders is still a serious concern despite the vast research done in this field. This paper will describe the interconnect reliability of three kinds of solder joints respectively prepared with lead-free solder paste and lead-free PBGA components, lead-free solder paste and tin-lead-silver PBGA components, and tin-lead solder paste and tin-lead-silver PBGA components. Lead-free and tin-lead solders were composed of eutectic tin-silver-copper and tin-lead, respectively. In addition, the study also presents the effect of multiple reflow times. The study focuses on the microstructures of different assemblies. The particular interest is on the assemblies soldered with lead-free solder paste and tin-lead-silver PBGA components, since the SnPbAg solder on the bumps of the PBGA components were exposed to the reflow profile meant for the lead-free SnAgCu solder. Thus, these SnPbAg solder bumps were in the molten state almost twice as long as the rest of the solders. This had a notable effect on the reliability of these solder joints as we will be showing later in this paper. The test boards were temperature-cycled for 2500 cycles between −40 and +125°C (a 30-minute cycle). PBGA solder joint failures were monitored with a real time monitoring system. Optical and scanning electron microscopy was used to inspect the broken solder joints and their microstructure. The results of tests indicate that the number of reflow times can significantly affect the lifetime of PBGA solder joints. The most notable changes can be seen in the solder joints made with tin-lead-silver PBGA components and tin-silver-copper solder paste soldered with a lead-free reflow profile. The general trend was that the reliability of the solder joints increased in proportion to the number of reflow times. Mainly two factors are believed to have the major effect on the reliability of PBGA solder joints, voids, and microstructural changes in solder.

The creep properties of Lead-free solder joints were evaluated by Shear Punch (SP) method. Lead-free solder joints with Sn-4Ag, Sn-4Ag-0.5Cu and Sn-4Ag-1.0Cu solder tested and compared to the lead solder joint with Sn-37Pb. SP creep tests were performed at temperature of 30, 50 and 80°C. For the SP method, constant shear stresses from 3 to 22 MPa were applied into the SP specimen(10 × 10 × 0.5 mm). From the SEM, IMC layer of Cu6Sn5 were observed on the interface between solder and Cu. From the SEM and EDX, the Lead-free solder joints have much thicker interfacial IMC layer than the Sn-37Pb solder joint. From the power law relationships, the Lead-free solder joints show superior creep resistance than the Sn-37Pb solder joint. And among Lead-free solder joints, at 30 and 50°C, the ternary SnAgCu solder joints have high creep resistance than the binary SnAg solder joint. Sn-4Ag-1.0Cu solder joint shows the best creep resistance at 30°C, and Sn-4Ag-0.5Cu solder joint shows the best creep resistance at 50°C. However, at high temperature 80°C, all of the Lead-free solder joints have similar creep properties. The activation energy were expressed with creep strain rate and temperature. The ternary SnAgCu solder joints have the higher activation energy than the binary SnAg solder joint at 17.68 and 18.72 MPa. Average activation energy of Lead-free solder joints were lower than that of Sn-37Pb 75.11 kJ/mol. The average activation energy of Sn-4Ag-1.0Cu, Sn-4Ag-0.5Cu and Sn-4Ag solder joint at high stress is 102.68, 62.52 and 58.61 kJ/mol, respectively.

A major problem faced by electronic packaging industries is the poor reliability of lead free solder joints. One of the most common methods utilized to tackle this problem is by doping the alloy with other elements, especially bismuth. Researches have shown Bismuth doped solder joints to mostly fail near the Intermetallic (IMC) layer rather than the bulk of the solder joint as commonly observed in traditional SAC305 solder joints. An understanding of the properties of this IMC layer would thus provide better solutions on improving the reliability of bismuth doped solder joints. In this study, the authors have used three different lead free solders doped with 1%, 2% and 3% bismuth. Joints of these alloys were created on copper substrates. The joints were then polished to clearly expose the IMC layers. These joints were then aged at 125 °C for 0, 1, 2, 5 and 10 days. For each aging condition, the elastic modulus and the hardness of the IMC layers were evaluated using a nanoindenter. The IMC layer thickness and the chemical composition of the IMC layers were also determined for each alloy at every aging condition using Scanning Electron Microscopy (SEM) and EDS. The results from this study will give a better idea on how the percentage of bismuth content in lead free solder affects the IMC layer properties and the overall reliability of the solder joints.

In the past few years, many studies have been reported on reliability of lead-free solder joint. Almost all of them have described evaluation of solder alloy composition of printed wiring board surface-finish. What seems to be lacking, however, is the influence of component placing condition to reliability and reliability under several stress conditions. The purpose of this work is to investigate the influence of the factor on reliability of the solder joint. We used chip size package as a test piece, which is a more popular semiconductor package, and thermal fatigue test and mechanical fatigue test were carried out. We choose experimental conditions as follow, solder materials (Sn-Ag-Cu, Sn-Ag-Cu-Bi, Sn-Zn-Bi, Sn-Pb), chip size package placing condition (on one side, on both sides), printed wiring board surface finish (organic solderability protector on Cu, Au plating). We got the result that chip size package placing condition, printed wiring board surface finish had more effect on reliability of the lead-free solder joint than solder materials. It is clear that placing condition and printed wiring board surface-finish greatly affects reliability of thermal fatigue and mechanical fatigue, respectively. In conclusion, the influence of the factor on reliability of lead-free solder joint was investigated from several points of view. The work helps us to clarify the influence of the factor on reliability of lead-free solder joint and lead to the guidelines for introduction of lead-free solder.

This article deals with the growth of intermetallic compounds (IMCs) in solder joints and influence of these IMCs on properties of soldered joints. The test specimens of printed circuit boards (PCBs) with different surface finishes were made for this experiment. Tin_G (galvanic tin), Tin_C (immersion tin), Cu (pure copper), OSP (organic solderability preservative) and ENIG (electroless nickel immersion gold) surface finishes were chosen. The soldered joints were created on the test specimens by reflow soldering process concretely vapour-phase soldering. The surface mount solder joints of LCCC resistor of size 1206 were studied. Two lead-free solder paste and one tin-lead solder paste were chosen for research of IMCs and their influence on properties of soldered joints. The test specimens were exposed to elevated aging for research of IMCs growth in solder joint. The specimens were thermal stressed in hot air oven at the temperature of 150°C for a periods of 0, 1, 2, 4, 8, and 16 days. The shear test was used for research of IMCs growth influence on strength of soldered joints. The results of analysis of IMCs in solder joints using confocal and metallographic microscopes and the results of shear strength measurement will be presented in this article.

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.

Accelerated R&D efforts in industry, universities, national laboratories, and government agencies have recently identified several promising lead-free solders to replace lead-containing solders in microelectronic applications. The leading candidates are Sn-3.5Ag, Sn3.5Ag-0.7Cu, Sn-3.5Ag-4.8Bi, and Sn-0.7Cu (in weight percent). These lead-free solders are all tin-rich with a melting temperature between 210°C and 227°C. They are recommended for such soldering applications as surfacemount technology, plated-through-hole, ball grid arrays, flip chip, and others. Despite the R&D progress and the proliferation of published technical findings on lead-free solders, our knowledge and understanding of these new materials are still at an infancy stage compared to lead-containing solders. Thus, unanswered questions and unresolved issues still persist: Can we make and properly test reliable leadfree solder joints? What are the implications of higher reflow temperatures required for the new solders? Are the new surface finishes needed? What is the lead-free solder candidate for flipchip applications? Can we maintain the solder hierarchy between the first and second level interconnection? What are the reliability issues of new solder alloys? What are the solidification mechanisms in solder joints? How well are microstructure-property relations understood in lead-free solder alloys and joints? Have interfacial reactions between tin-rich solders and new surface finishes been adequately characterized? What are the influences of microstructural evolution during thermomechanical processes? There is much to be learned regarding thermal fatigue mechanisms in solder joints, creep and fatigue interactions, corrosion behavior, etc. To Developments in Lead-Free Solders and Soldering Technology

With the high operating temperature in power electronics devices, the reliability of lead-free solder joints in electronic packages at high temperatures is important. Solder joint reliability depends on a large extent on the mechanical properties of the thin intermetallic compound layer, especially hardness and Young's modulus. In this paper, the mechanical properties of Cu6Sn5, the main component of IMC, at high temperature are measured by nanoindentation. After being processed and analyzed, data are compared with the room temperature one. The results show that temperature changes have effect on the mechanical properties of lead-free solder joints.

The purpose of this paper is to study effect of thermal cycle condition on the reliability of lead-free flip chip solder joint. Flip chip assembly was subjected to thermal cycle (MIT-STD-883) condition. Two dimensional finite element analysis (FEA) has been carried out using ANSYS commercial software. There are two types of lead-free solder (Sn96.5-Ag3.5 and Sn95.5-Ag3.8-Cu0.7). They used flip chip solder joints have been evaluated. For each lead-free flip chip solder joint, thermal stress and strain have been studied. The plastic strain is mainly effect on thermal fatigue of solder joint, so two types of lead-free solder joint compare with Sn63-Pb37 solder joint in equivalent plastic strain, and thermal fatigue life of these three types of solder joint has been analyzed and assessed.

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.

Crystal plasticity finite element study of deformation behavior in commonly observed microstructures in lead free solder joints