Effects of Creep and Fatigue on the Reliability of SnAgCu Solder Joints in Thermal Cycling
Type of DegreePhD Dissertation
Industrial and Systems Engineering
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The reliability of electronic systems is threatened by the failure of one solder interconnection among thousands connecting several components to the PCB. These joints are exposed to different sorts of harsh circumstances such as vibration, drop, and thermal cycling. Thermal cycling stresses are the most common stimulus that leads to joints failure caused by environmental conditions and the on-off switching cycles of the circuit. Recent studies have shown that the failure mechanism under thermal cycling is caused by both creep and fatigue. Fatigue is dominant during the ramps, while creep is dominant during the dwelling. Such creep and fatigue damages are affected by various parameters such as dwelling time, ramp rate, and temperature range. Therefore, it essential to fundamentally understand the damage mechanisms related to creep and fatigue. Moreover, quantifying the damage under different testing parameters is critical for prediction modeling and material improvement purposes. For decades, lead-free alloys replaced the SnPb alloys in the market share after the lead prohibition, where SAC based alloys are the most common ones. These alloys have exceptional mechanical and thermal properties, but there is a concern about their reliability, especially in harsh applications. Thus, it is vital to investigate the behavior of the SAC-based alloys under these circumstances. Evaluating the reliability of SAC305 solder interconnection under various fatigue and creep conditions at different temperature levels, several stress levels, and dwelling times, quantify them in addition to establish prediction models are the main objectives of this study. This study is divided into two main parts; the first one is to test individual solder joints (spheres) under fatigue and creep at room temperature, where the second part includes testing joints under various temperatures. In the first part, three stress amplitudes; 16, 20, and 24 MPa, and various dwelling times; 0s, 10s, 60s, and 180s are utilized for testing the individual joints. Four levels of temperatures; 25oC, 75oC, 125oC, and -40oC, three stress amplitudes; 16, 20, and 24 MPa, and various dwelling times; 0s, 10s, 60s, and 180s are included in the second part. For creep-fatigue tests, test vehicle includes individual SAC305 solder joints attached to FR-4 glass epoxy boards. The surface finish for all test vehicles is OSP, and SAC305 is the only alloy tested in this research. In the first study, the effect of creep and fatigue on individual solder joints with various stress levels and dwelling times at room temperature is investigated. The second study aims to study the effect of the same parameters of stress and dwelling but at several hot and cold temperature levels. In the previously mentioned studies, both damage mechanisms of creep and fatigue are quantified in addition to the other behavioral and microstructural analysis. There are some differences in the analysis for each part. For part one, the hysteresis loop for each condition is generated where damage parameters, inelastic work per cycle, and plastic strain for each cycle are extracted. Qualitative and quantitative information about the damage experienced by these joints is obtained. These parameters are also utilized in generating the life prediction models and would be helpful in FEA models. The evolution during the life (number of cycles) in term of work per cycle and plastic strain are also observed and provide information about the effect of testing parameters on evolution trends. A two-parameter Weibull plot for each combination is established to examine the reliability under various conditions based on individual life. The evolution in microstructure is also investigated for fatigue and creep tests using SEM and optical microscopes. This study will provide a fundamental understanding of both failure mechanisms of creep and fatigue reliability under the thermal cycling of the SAC305 solder alloy. Also, modified prediction models based on Coffin-Manson, Morrow Energy, Norris-Landzberg, and Engelmaier will be constructed from the results. The preliminary results show that both creep and fatigue influence reliability, but the creep effect is very significant and remarkably degrade the fatigue life compared to fatigue.