High Strain Rate Material Characterization of Lead-Free SAC Solder Alloy and Solder Joint Reliability under Vibration and Thermal Loads
Type of DegreePhD Dissertation
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Electronics will experience high and low working temperatures during operations, handling, and storage in severe environments applications such as automotive, oil and gas, aerospace, medical technologies, and defense applications in addition to mechanical shock and vibration. Electronic devices like smartphones, tablets, and personal computers have grown recently; while being used on a daily basis, these gadgets might accidentally fall to the ground. It's crucial to comprehend the reliability characteristic of those gadgets under impact. One way to assess a given electronic device's drop and shock durability is through drop testing. Drop testing is described in a unified standard created by the Joint Electron Device Engineering Council (JEDEC). The appropriate acceleration output during an impact event was specified in the JESD22-B111 standard [JEDEC]. Electronics System like engine control units, variable valve sensor, crankshaft-camshaft sensors are located under-hood in automotive systems, are subjected simultaneously to mechanical vibrations and thermal loads. These conditions require the expansion of material performance prediction practices for such extreme environments. Furthermore, electronic assemblies may be subjected to extended periods of non-climate-controlled exposure prior to their service life. Temperatures in very harsh conditions can range from -65 to +200 °C with vibration G-level up to 10g in these applications [Eddy 1998, Hattori 1999, Johnson 2004 & Watson 2012]. Combined effect of elevated temperature and vibration can cause faster failure in electronics components. Most of the previous research of solder joints is focused on either single stress of vibration or thermal cycling. Very few researchers have studied the solder joint reliability under simultaneously high temperature and vibration. Previous research demonstrate multiple failure modes of solder fatigue, copper trace or lead fracture due to high cycle fatigue under vibration loads. Previously the literature on reliability of lead-free alloys under combined thermal loads, drop & shock and vibration is sparse. Solder interconnects may undergo large non-linear deformation under such loads. Nonetheless, lead-free solders are robust and brittle, which means that solder interconnections are more susceptible to high strains, which can lead to electronic device failure. We need a better understand of solder alloy in order to develop and improve the efficiency of electrical devices. Lead-free solder materials continue to evolve under varied thermal loads, which can lead to deterioration in mechanical parameters such as Ultimate Tensile Strength and elastic modulus. SAC lead-free solders and doped SAC solders are becoming popular choices for extreme temperature applications. Recently, many doped solder alloys are being introduced in the electronic industries such that SAC-Q (CYCLOMAX), SAC-R, Innolot, etc. These alloys have been produced with the addition of Ni, Co, Au, P, Ga, Cu and Sb to SAC solder alloy to boost the mechanical properties, thermal properties, wettability, melting temperature, shock and drop performance and solder alloy microstructure [Cai 2010, and Matahir 2011]. SAC-Q is a newer general purpose, thermal fatigue resistant doped solder alloy, used for surface mount technology. SAC-Q are formulated with Sn-Ag-Cu with addition of Bi (SAC+Bi). The material characteristic for non-linear modeling and reliability prediction are required for risk minimization with the use of alloys in high-reliability applications. Finite element modeling has been widely employed in electronic packaging for solder connections design and drop & shock simulation and vibration events. One of the most extensively used models for capturing non-linear solder behavior in thermo-mechanical stresses is the Anand constitutive model. The author investigated the influence of thermal aging and adding dopants on the mechanical response of SAC solder at high strain rates and low- and high-test temperatures with the inclusion of Anand constitutive model parameters The current work fills this gap in the state of the art by measuring mechanical characteristic of undoped SAC105 and doped SAC-Q solder alloys at low operation temperatures (-65°C to 200°C) at high strain rate (10-75 sec-1) after varied thermal aging periods up to one year. The fabricated specimens were isothermally aged up to 12 months at 50 °C before testing. Stress- Strain measurements have been done by conducting tensile tests using impact-hammer based tensile tests set up with environmental chambers. The Anand Viscoplastic model was employed to obtain 9 Anand parameters from the recorded tensile data in order to characterize the material constitutive behavior. Also, the evolution of Anand parameters for SAC solder alloys at high strain rates has been investigated induced under sustained periods of thermal aging. The Anand model's reliability has been assessed by comparing experimentally measured data with predicted data using determined model constants for both solder alloys. There was a strong correlation established between experimental data and Anand predicted data. The Anand parameters have been implemented in an FE-framework to simulate the drop events for a ball-grid array package on printed circuit board assembly to determine hysteresis loop and plastic work density. In this research, drop and shock test event has been simulated using shock pulse with 1500 g and 0.5 ms using input-g method in ANSYSTM with implicit solver. The plastic work per shock event is a measure of the damage progression in the solder interconnects. Evolution in hysteresis loops and PWD have been studied and compared for various thermal aging durations and storage temperatures in addition to extreme operating temperatures. This research also presents reliability for SAC105 alloy compositions at elevated test temperature and vibration. Pristine test board with lead-free SAC daisy chain CABGA packages have been subjected to harmonic vibration at their 1st natural frequency at three test temperatures (25°C, 55°C and 155°C) and vibration with amplitude of 5g, and 10g. Resistance data were measured at each test conditions using high speed data acquisition. Material properties of the printed circuit board at elevated temperatures have been measured using tensile tests. High speed camera is also used to capture the vibration event during testing. The experimental system characteristic such as mode shapes and natural frequencies and displacement amplitudes for each test condition compared with FE models. A comparison of strain data from the center of test board has been done DIC. Stresses in solder interconnects have been extracted from model of packages. The plastic work density of critical solder joint extracted using FEA based model for test vehicle. Failure mode analysis has been done for test board. Anand Viscoplasticity material data from the prior studies by the authors have been used to capture the high-strain rate temperature dependent aging behavior of the solder joints. In this research, an energy-based model has been proposed to predict the high frequency fatigue life under simultaneous temperature-vibration.