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Sn-Ag-Cu (SAC) solder alloys, such as Sn-3.0Ag-0.5 Cu (SAC305) are the popular choices of lead-free solders replacing SnPb solders. However, SAC solders are more brittle in nature due to stiffness and excessive intermetallic compounds growth at the solder joint to pad interface. This leads to higher risk of solder joints failures. Memory module-type smaller lead-free ball grid array (BGA) packages are constantly under dynamic stresses during handling and thermal stresses during operations. It is important to understand the dynamic performance and long-term reliability of memory module lead-free BGAs. It is believed that the printed circuit board (PCB) design variations cause dynamic and long-term failure discrepancies in the fields. In this paper, different pad and trace designs are introduced to evaluate the effects of PCB design variations on the bend and accelerated thermal cycling (ATC) performance of lead-free solder joints. Pad designs with nonsolder mask defined, solder mask defined (SMD), and a unique web design are assembled and tested. Different solder alloys, including SAC305, Sn-1.0Ag-0.5Cu (SAC105) SAC105, and SnPb solders, have been evaluated in this paper. Different PCB materials have also been evaluated in the test. Four-point monotonic bend tests are performed to characterize the bending performance variations with different PCB designs and compared with conventional Sn-Pb solder. The SMD pad is shown to have the best bend performance among all other types of designs in this paper. In addition, this design also shows improvement in mitigation of PCB pad cratering with lead-free solders. Wide trace width seems to degrade the strength and is not preferred. Just as it shows superior shock resistance when compared with SAC305, the SAC105 solder alloy also shows better bend performance. There is no significant improvement in bend performance with web design. After aging treatment, bend performance of both SAC305 and SAC105 degraded by up to 34% and 29%, respectively. However, the bend performance of eutectic SnPb is actually improved after aging. ATC tests are performed to investigate the effects of design variations on the long-term reliability of lead-free solder joints; SMD design shows less reliability life than others. The implications of these results for the reliability of lead-free solder joints are discussed in this paper. [ABSTRACT FROM PUBLISHER]

In this paper, we report a comprehensive set of accelerated thermal cycling (ATC) tests that were performed on test vehicles with different package types, sizes, pitches, and solder joint alloy metallurgies using four different thermal cycling profiles: 0 to 100, -40 to 125, -55 to 125, and -60 to 150^\circC. Samples from the tests were analyzed for their failure modes, and failure rates were calculated by using Weibull statistics. The characterized life for each test condition was determined and analyzed. The impact of solder alloy metallurgies, package types, sizes, and pitches on acceleration factors of the ATC tests to fatigue life was also analyzed and discussed. The quantified discrepancies among several acceleration factors from different studies compared to the experimental data presented in this paper are illustrated. The results provide valuable guidance on the effects of package types, size, pitches, and solder joints alloy metallurgies on various ATC test conditions. In addition, failure analysis was performed at different stages of the tests for each thermal cycling condition. Dramatic failure mode shifts at extreme ATC conditions were observed. The significance and the long-term impact of the failure modes and failure mechanism shift between various ATC test conditions to the life prediction of lead-free solders are extensively discussed. [ABSTRACT FROM PUBLISHER]

The interaction between isothermal aging and the long-term reliability of fine-pitch ball grid array (BGA) packages with Sn-3.0Ag-0.5Cu (wt.%) solder ball interconnects was investigated. In this study, 0.4-mm fine-pitch packages with 300- μm-diameter Sn-Ag-Cu solder balls were used. Two different package substrate surface finishes were selected to compare their effects on the final solder composition, especially the effect of Ni, during thermal cycling. To study the impact on thermal performance and long-term reliability, samples were isothermally aged and thermally cycled from 0°C to 100°C with 10 min dwell time. Based on Weibull plots for each aging condition, package lifetime was reduced by approximately 44% by aging at 150°C. Aging at 100°C showed a smaller impact but similar trend. The microstructure evolution was observed during thermal aging and thermal cycling with different phase microstructure transformations between electrolytic Ni/Au and organic solderability preservative (OSP) surface finishes, focusing on the microstructure evolution near the package-side interface. Different mechanisms after aging at various conditions were observed, and their impacts on the fatigue lifetime of solder joints are discussed. [ABSTRACT FROM AUTHOR]

Recently studies of mechanical properties of lead-free solders show that large reductions in stiffness, yield stress, ultimate strength, and strain to failure (up to 35%) were found after 2 months of aging at room temperature. It also shows that the tensile properties of both lead-free solders and Sn–Pb solders tend to become relatively stable after 10 days of aging at room temperature. In this study, in order to minimize any room temperature aging contribution in the investigation of the dependence of temperature and strain rates, all specimens tested were preconditioned after 10 days of aging at room temperature. All tests were conducted under the same conditions. Testing has been performed at three strain rates, 10−3, 10−4, and 10−5 s−1, in temperature range from −40 to 150 °C. A linear relationship was found between the temperature and the tensile properties (elastic modulus, yield stress, and ultimate stress), while a power law relationship was found between strain rate and tensile properties. Constitutive models have also been developed based on the experimental data with multiple variables of strain rate and temperature for both lead-free and lead content solders. With the obtained constitutive models, tensile properties of lead-free solders can be predicted at any testing strain rate and temperature. [ABSTRACT FROM AUTHOR]

The constitutive modeling of creep has been extensively studied due to the important of the creep failure mode in solder joints. However, there are very few studies that considered room temperature aging contributions in their creep modeling studies. This study investigated constitutive modeling of creep of solders by taking into account the possible contribution room temperature aging. Lead-free solder (Sn–4.0Ag–0.5Cu) was found to have a higher creep resistance than Sn–Pb solder at the same stress level and testing temperature. The higher creep resistance was contributed by the second phase intermetallic compounds, Ag3Sn and Cu6Sn5. The precipitation of these intermetallic compounds can significantly block the movement of dislocations and increase the creep resistance of the material. Constitutive models of creep for both lead-free and Sn–Pb eutectic solders were constructed based on the experimental data. The activation energy for SAC405 is much higher than that of Sn–Pb, which also indicates that SAC405 possesses higher creep resistance. The constitutive models can be used in finite element analysis of actual electronic packages to predict solder joint failure. The creep mechanisms of both lead-free and Sn–Pb eutectic solders were also extensively discussed in this dissertation. Dislocation gliding and climb is believed to be the major failure mode at high stresses, while lattice diffusion and grain boundary diffusion is believed to be the major failure mode at low stress levels. Grain boundary sliding is believed to contribute to creep deformation at both high-stresses and low-stresses. For eutectic Sn–Pb, superplastic deformation is a major the creep mechanism at low-stresses and high-temperatures. [ABSTRACT FROM AUTHOR]

The characterization of lead-free solders, especially after isothermal aging, is very important in order to accurately predict the reliability of solder joints. However, due to lack of experimental testing standards and the high homologous temperature of solder alloys ( Th > 0.5 Tm even at room temperature), there are very large discrepancies in both the tensile and creep properties provided in current databases for both lead-free and Sn–Pb solder alloys. Some recent researches show that the room temperature aging has significant effects on mechanical properties of solders. This paper is intended to review all available data in the field and give rise to the possible factors including room temperature effects which causes the large discrepancies of data. This review of the research literatures has documented the dramatic changes that occur 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 previous studies involving solder material characterization or finite element predictions of solder joint reliability during thermal cycling. It is widely acknowledged that the large discrepancies in measured solder mechanical properties from one study to another arise 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 change 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 well-defined relationship, it is doubtful that finite element reliability predictions can ever reach their full potential. [ABSTRACT FROM AUTHOR]