Aging Effects in Sac Solder Joints

The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work on aging effects, we have demonstrated that the observed material behavior variations of Sn-Ag-Cu (SAC) lead free solders during room temperature aging (25 C) and elevated temperature aging (125 C) were unexpectedly large and universally detrimental to reliability. Such effects for lead free solder materials are especially important for the harsh applications environments present in high performance computing and in automotive, aerospace, and defense applications. However, there has been little work in the literature, and the work that has been done has concentrated on the degradation of solder ball shear strength (e.g. Dage Shear Tester). Current finite element models for solder joint reliability during thermal cycling accelerated life testing are based on traditional solder constitutive and failure models that do not evolve with material aging. Thus, there will be significant errors in the calculations with the new lead free SAC alloys that illustrate dramatic aging phenomena. In the current work, we have extended our previous studies to include a full test matrix of aging temperatures and solder alloys. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on four different SAC alloys (SAC105, SAC205, SAC305, SAC405) that were aged for various durations (0-6 months) at room temperature (25 C), and several elevated temperatures (50, 75, 100, and 125 C). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. The chosen selection of SAC alloys has allowed us to explore the effects of silver content on aging behavior (we have examined SACN05 with N = 1%, 2%, 3%, and 4% silver; with all alloys containing 0.5% copper). Variations of the mechanical and creep properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) have been observed and modeled as a function of aging time and aging temperature. In this paper, we report on the results of the creep experiments. INTRODUCTION Eutectic or near eutectic tin/lead (Sn/Pb) solder (melting temperature TM = 183 °C) has been the predominant choice of the electronics industry for decades due to its outstanding solderability and reliability. However, legislation that mandates the banning of lead in electronics has been actively pursued worldwide during the last 15 years due to the environmental and health concerns. Although the implementation deadlines and products covered by such legislation continue to evolve, it is clear that laws requiring conversion to lead-free electronics are becoming a reality. Other factors that are affecting the push towards the elimination of lead in electronics are the market differentiation and advantage being realized by companies producing so-called “green” products that are lead-free. A large number of research studies are currently underway in the lead-free solder area. Although no “drop in” replacement has been identified for all applications; Sn-Ag, Sn-Ag-Cu (SAC), and other alloys involving elements such as Sn, Ag, Cu, Bi, In, and Zn have been identified as potential replacements for standard 63Sn-37Pb eutectic solder. Several SAC alloys have been the proposed by various user groups and industry experts. These include 96.5Sn-3.0Ag-0.5Cu (SAC305), 95.5Sn-3.8Ag-0.7Cu (SAC387), 95.5Sn-3.9Ag-0.6Cu (SAC396) and Proceedings of the SEM Annual Conference June 1-4, 2009 Albuquerque New Mexico USA ©2009 Society for Experimental Mechanics Inc. 95.5Sn-4.0Ag-0.5Cu (SAC405). For enhanced reliability during high strain rate exposures (e.g. shock and drop), several alloys with lower silver content have been recommended including 98.5Sn-1.0Ag-0.5Cu (SAC105) and 99Sn-0.3Ag-0.7Cu (SAC0307). The main benefits of the various SAC alloy systems are their relatively low melting temperatures compared with the 96.5Sn-3.5Ag binary eutectic alloy, as well as their superior mechanical and solderability properties when compared to other lead free solders. Solder joint fatigue is one of the predominant failure mechanisms in electronic assemblies exposed to thermal cycling. Reliable, consistent, and comprehensive solder constitutive equations and material properties are needed for use in mechanical design, reliability assessment, and process optimization. Mechanical characterization of solder materials has always been hampered by the difficulties in preparing test specimens that reflect the same true material making up the as actual solder joints (e.g. match the solder microstructure). Solder uniaxial samples haven been fabricated by machining of bulk solder material [1-8], or by melting of solder paste in a mold [9-18]. Use of a bulk solder bars is undesirable, because they will have significantly different microstructures than those present in the small solder joints used in microelectronics assembly. In addition, machining can develop internal/residual stresses in the specimen, and heat generated during turning operations can cause localized microstructural changes on the exterior of the specimens. Reflow of solder paste in a mold causes challenges with flux removal, minimization of voids, microstructure control, and extraction of the sample from the mold. In addition, many of the developed specimens have shapes that significantly deviate from being long slender rods. Thus, undesired non-uniaxial stress states will be produced during loading. Other investigators have attempted to extract constitutive properties of solders by direct shear or tensile loading [6, 19-28], or indenting [29-32], of actual solder joints (e.g. flip chip solder bumps or BGA solder balls). While such approaches are attractive because the true solder microstructure is involved, the unavoidable non-uniform stress and strain states in the joint make the extraction of the correct mechanical properties or stress-strain curves from the recorded load-displacement data very challenging. Also it can be difficult to separate the various contributions to the observed behavior from the solder material and other materials in the assembly (bond pads, silicon die, PCB/substrate, etc.). The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments [8, 13, 15-16, 18, 23-24, 28, 31, 33-51]. The observed material behavior variation during thermal aging/cycling is universally detrimental to reliability and includes reductions in stiffness, yield stress, ultimate strength, and strain to failure, as well as highly accelerated creep. Such aging effects are greatly exacerbated at higher temperatures typical of thermal cycling qualification tests. However, significant changes occur even with aging at room temperature [13, 15-16, 23-24, 33-41, 48]. As early as 1956, Medvedev [33] observed a 30% loss of tensile strength for bulk solder Sn/Pb solder stored for 450 days at room temperature. In addition, he reported 4-23% loss of tensile strength for solder joints subjected to room temperature storage for 280-435 days. In 1976, Lampe [34] found losses in shear strength and hardness of up to 20% in Sn-Pb and Sn-Pb-Sb solder alloys stored for 30 days at room temperature. Miyazawa and Ariga [35-36] measured significant hardness losses and microstructural coarsening for Sn-Pb, Sn-Ag, and Sn-Zn eutectic solders stored at 25 C for 1000 hours, while Chilton and co-workers [37] observed a 10-15% decrease in fatigue life of single SMD joints after room temperature aging. Several studies [38-41] have also documented the degradation of Sn-Pb and SAC solder ball shear strength (10-35%) in area array packages subjected to room temperature aging. The effects of room temperature isothermal aging on constitutive behavior have also been investigated [13, 15-16, 48]. Chuang, et al. [13] characterized the reductions in yield stress and increases in elongations obtained in Sn-Zn eutectic solder during aging at room temperature. In addition, Xiao and Armstrong [15-16] recorded stress-strain curves for SAC 396 specimens subjected to various durations of room temperature aging, and found losses of ultimate tensile strength of up to 25%. The effects of room temperature aging on the mechanical properties and creep behavior of SAC alloys have been extensively discussed by the authors (Ma, et. al. [48]). The measured stress-strain data demonstrated large reductions in stiffness, yield stress, ultimate strength, and strain to failure (up to 40%) during the first 6 months after reflow solidification. In addition, even more dramatic evolution was observed in the creep response of aged solders, where up to 100X increases were found in the steady state (secondary) creep strain rate (creep compliance) of lead free solders that were simply allowed to sit in a room temperature environment. The SAC solder materials in room temperature aged joints were also found to enter the tertiary creep range (imminent failure) at much lower strain levels than virgin joints (non aged, immediately after reflow solidification). We also demonstrated that there are corresponding changes in the solder joint microstructure occurring during room temperature aging. The magnitudes of the material behavior evolution occurring in lead free SAC solder joints were found to be much larger (e.g. 25X) than the corresponding changes occurring in traditional Sn-Pb assemblies. The effects of aging at elevated temperature are the most widely studied due to the dramatic changes in the microstructure and mechanical properties that result. Aging effects (reduced effective stiffness and ultimate strength) have been observed for solder subjected to elevated temperature aging (e.g. 125 C) [8, 15-16, 18, 2324]. Pang, et al. [20] measured microstructure changes, intermetallic layer growth, and shear strength degradation in SAC single ball joints subjected to elevated temperature aging. Darveaux [21] performed an extensive experimental study on the stress-strain and creep behavior of area array solder balls subjected to shear. He found that aging for 1 day at 125 C caused significant effects on the observed stress-strain and creep behavior. The aged specimens were also found to creep much faster than un-aged ones by a factor of up to 20 times for both SAC305 and SAC405 solder alloys. Xiao and Armstrong [13-14] measured stress-strain curves for SAC396 specimens subjected to elevated temperature aging at 180 C. At this highly elevated temperature, they observed a quick softening of the material during the first 24 hours followed by a gradual hardening with time. Dutta, et al. [31] used impression techniques to measure the creep behavior of SAC405 solder joints and observed large increases in the secondary creep rates with aging at 180 C. Wiese and Kolter [28] demonstrated analogous large increases in the creep rates for SAC387 joints by directly loading flip chip assemblies that were aged at 125 C. Several studies have been performed on the degradation of BGA ball shear strength with elevated temperature aging at 125 C or 150 C [42-46]. All of these investigations documented both microstructure coarsening and intermetallic layer growth. In addition, Hasegawa, et al. [42] measured elastic modulus reductions with aging by testing thin solder wires, while Chiu and co-workers [46] found significant reductions in drop reliability during elevated temperature aging. Finally, Ding, et al. [47] explored the evolution of fracture behavior of SnPb tensile samples with elevated temperature aging. In our prior work on elevated temperature aging effects [49], we demonstrated that the observed material behavior variations of SAC305 and SAC405 lead free solders during isothermal aging at 125 C were unexpectedly large and universally detrimental to reliability. The measured stress-strain data demonstrated large reductions in stiffness, yield stress, ultimate strength, and strain to failure (up to 50%) during the first 6 months after reflow solidification. After approximately 1000 hours of aging, the lead free solder joint material properties were observed to degrade at a slow but constant rate. In addition, even more dramatic evolution was observed in the creep response of aged solders, where up to 500X increases in the secondary creep rates were observed for aging up to 6 months. The solder materials in aged joints were also found to enter the tertiary creep range (imminent failure) at much lower strain levels than virgin joints (non aged, tested immediately after reflow solidification). We also correlated the changes in mechanical behavior during aging with changes that occur in the solder joint microstructure, and showed that the magnitudes of the material behavior evolution occurring in lead free SAC solder joints are much larger (e.g. 100X) than the corresponding changes occurring in traditional Sn-Pb assemblies. One of the most important observations from our prior work on creep behavior was the demonstration that a “cross-over point” occurs during the elevated temperature aging (125 C) of lead free and tin-lead solders. This cross-over point occurred after approximately 50 hours of aging at 125 C, and marked the point where the two lead free solders began to creep at higher rates than standard 63Sn-37Pb solder for the same stress level. Such an effect was not observed for solder joints aged at room temperature (25 C). The presence of the cross-over point with elevated temperature aging can possibly explain existing reliability data for area array packages where lead free packaging becomes less reliable than the analogous Sn-Pb case when the upper limit of the thermal cycling test is increased. As demonstrated above, the literature has documented the dramatic changes occurring in the constitutive and failure behavior of solder materials and solder joint interfaces during isothermal aging. However, these effects have been largely ignored in most other studies involving solder material characterization or finite element predictions of solder joint reliability during thermal cycling. It is also widely acknowledged that the large discrepancies in measured solder mechanical properties from one study to another are due to differences in the microstructures of the tested samples. This problem is exacerbated by the aging issue, as it is clear that the microstructure and material behavior of the samples used in even a single investigation are moving targets that are evolving rapidly even at room temperature. Furthermore, the effects of aging on solder behavior must be better understood so that more accurate viscoplastic constitutive equations can be developed for SnPb and SAC solders. Without such relations, it is doubtful that finite element reliability predictions can ever reach their full potential. In the current work, we have extended our previous studies [48-51] to include a full test matrix of aging temperatures and solder alloys. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on four different SAC alloys (SAC105, SAC205, SAC305, SAC405) that were aged for various durations (0-6 months) at room temperature (25 C), and several elevated temperatures (50, 75, 100, and 125 C). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. The chosen selection of SAC alloys has allowed us to explore the effects of silver content on aging behavior (we have examined SACN05 with N = 1%, 2%, 3%, and 4% silver; with all alloys containing 0.5% copper). Variations of the mechanical and creep properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) have been observed and modeled as a function of aging time and aging temperature. In this paper, we report on the results of the creep experiments. EXPERIMENTAL PROCEDURE Uniaxial Test Sample Preparation In the current study, mechanical measurements of aging effects and material behavior evolution of lead free solders have been performed. We have avoided the specimen preparation pitfalls present in many previous studies by a using a novel procedure where solder uniaxial test specimens are formed in high precision rectangular cross-section glass tubes using a vacuum suction process. The tubes were then cooled by water quenching and sent through a SMT reflow to re-melt the solder in the tubes and subject them to any desired temperature profile (i.e. same as actual solder joints). The solder is first melted in a quartz crucible using a pair of circular heating elements (see Figure 1). A thermocouple attached on the crucible and a temperature control module is used to direct the melting process. One end of the glass tube is inserted into the molten solder, and suction is applied to the other end via a rubber tube connected to the house vacuum system. The suction forces are controlled through a regulator on the vacuum line so that only a desired amount of solder is drawn into the tube. The specimens are then cooled to room temperature using a user-selected cooling profile. In order to see the extreme variations possible in the mechanical behavior and microstructure, we are exploring a large spectrum of cooling rates including water quenching of the tubes (fast cooling rate), air cooling with natural and forced convection (slow cooling rates), and controlled cooling using a surface mount technology solder reflow oven. For the reflow oven controlled cooling, the tubes are first cooled by water quenching, and they are then sent through a reflow oven (9 zone Heller 1800EXL) to re-melt the solder and subject it to the desired temperature profile. Thermocouples are attached to the glass tubes and monitored continuously using a radio-frequency KIC temperature profiling system to ensure that the samples are formed using the desired temperature profile (same as actual solder joints). Figure 2 illustrates the reflow temperature profiles used in this work for SAC and SnPb solder specimens. (a) SAC (105/205/305/405)