| dc.description.abstract | In the rapidly advancing fields of automotive, oil exploration, and military electronics, components are increasingly required to operate under extreme conditions, characterized by high and low temperatures and transient loads with high strain rates. Such demanding environments subject electronic products to strain rates ranging from 1 to 100 per second, and temperatures from -65 °C to 200 °C. These harsh conditions often result in significant mechanical stresses on solder joints, which are among the most vulnerable points in electronic assemblies during operations involving shock and vibration. This research work aims to investigate the mechanical behavior of solder alloys under these extreme conditions, focusing particularly on the impact of high strain rates, thermal aging, and elevated temperatures on the reliability of solder joints.
The research focuses on three types of solder alloys: a conventional un-doped solder SAC305, and two doped solders, SAC-Q and M758. These doped solders have been recently developed to enhance resistance to aging and creep, without compromising strength or melting point. Experimental investigations were conducted to characterize the mechanical behavior of these solders across a wide temperature range (-65 °C to +200 °C) and strain rates (10–75 s⁻¹). The specimens were tested in both pristine and thermally aged conditions, with aging durations extending up to one year at 100 °C. Key mechanical properties, including Ultimate Tensile Strength (UTS) and Elastic Modulus (E), are measured and compared across the different solder alloys. The results revealed significant variations in mechanical properties due to temperature, strain rate, and aging, with doped solders such as SAC-Q and M758 exhibiting better resistance to degradation compared to SAC305. The addition of dopants like Bismuth (Bi) and Nickel (Ni) improved the performance of SAC-Q and M758 by reducing property deterioration over time, particularly under prolonged thermal storage.
The study also focuses on the extraction of Anand Model constants for both pristine and aged SAC solders across various aging durations. The Anand Viscoplasticity Model equations are fitted to the experimental stress-strain data, allowing for accurate predictions of solder behavior under different conditions. To validate the predictive capability of the Anand Model, the stress-strain curves predicted by the model are compared with the experimental data, for understanding the accuracy of the model in simulating the real-world behavior of solder alloys. The research further explores the evolution of Anand Parameters with respect to thermal aging duration for SAC305, SAC-Q, and M758 solder alloys. In addition to experimental and modeling efforts, this thesis explores the predictive potential of Bismuth content in solder alloys. A novel prediction framework was developed to estimate mechanical properties, such as UTS and E, for Bi-enhanced solders based on trends observed in experimental data. Higher Bi content was shown to reduce property degradation at high strain rates, making such solders more suitable for applications with prolonged high-temperature exposure.
The study also includes a finite element analysis (FEA) for drop/shock events using the extracted Anand Model constants for a PCB-PBGA324 package assembly. The board-level drop tests are performed according to JEDEC standards, employing the Input-G method. The analysis aims to compare hysteresis loops and accumulated plastic work densities for the various SAC solders, evaluating the extent of damage to solder joints per impact under various test conditions. Additionally, for vibration study a test vehicle comprising a PCB-CABGA208 Package assembly with SAC-Q solder joints is prepared and tested to failure. Test conditions include temperatures of up to 150 °C, and harmonic vibrations up to 10g acceleration levels. The experimental data obtained from the tests are correlated with FEA-based modal analysis results. The research investigates the effect of operating temperatures on the first natural frequency of test boards and captures vibration events using high-speed cameras and Digital Image Correlation (DIC) method. Finally, the dissertation addresses the characteristic life and failure modes of solder joints in Package-PCB assemblies subjected to high-temperature vibration. Both experimental and FEA methods are used to establish damage relationships for life prediction. By combining these experimental and computational approaches, this research contributes to a thorough understanding of the mechanical behavior of solder joints under harsh environmental conditions.
The comprehensive datasets, validated constitutive models, and predictive frameworks developed in this research advance the state of the art in solder joint reliability. The findings provide critical insights into the material selection and design of electronic assemblies for harsh environments, contributing to enhanced performance and reliability. This work has significant implications for industries such as automotive, aerospace, and defense, where the failure of electronic components can have catastrophic consequences. In summary, this thesis delivers a holistic approach to understanding and predicting solder joint behavior under extreme conditions. Through experimental investigations, advanced material modeling, and FEA simulations, it addresses key challenges in solder joint reliability. The contributions of this research are expected to guide future developments in solder materials and reliability analysis, ensuring the safe and efficient operation of electronic assemblies in demanding environments. | en_US |
