Mechanical and Microstructural Behavior Evolution of Lead Free Solder Materials Under Different Thermal Exposures
Type of DegreePhD Dissertation
Restriction TypeAuburn University Users
MetadataShow full item record
With the growth of electronic packaging industries, ranging from automobile to hand-held products, reliability is a major concern. Also, to keep pace with the increasing demand of end users for integration, miniaturization, light weight, high speed, and multi-functionality of portable electronic devices such as mobile phones, digital cameras, as well as personal digital assistant (PDA), electronic packaging industries are intended to manufacture packages of high density and smaller dimension. These packages consist of smaller solder joint interconnects with solder balls in each area and short distance between the solder balls. The reliability of smaller electronic packages is greatly affected by environment and application where it is being used, compared to the traditional one. Solder joints provide mechanical support, electrical and thermal interconnection between packaging levels in microelectronics assembly systems. Proper functioning of these interconnections and the reliability of the electronic packages depend largely on the mechanical properties of the solder joints. Lead free solders are common as interconnects in electronic packaging due to their relatively high melting point, attractive mechanical properties, thermal cycling reliability, and environment friendly chemical properties. However, Lead free electronic assemblies are often subjected to thermal cycling during qualification testing or during actual use. During the dwells at the constant high temperature extreme, the lead free solders joints will experience thermal aging phenomena, resulting in microstructural evolution and material property degradation. Additional aging effects can also occur in the ramp periods from low to high temperature. In real scenario, CTE mismatch occurs between die and PCB causes shear fatigue on solder joints and affects the reliability of the entire package. Many studies were conducted to investigate the effect of aging on lead free solder alloy properties and the reliability of assemblies in thermal cycling. In addition, the effect of thermal cycling on mechanical properties evolution of lead free solder joints was also observed. However, no study has focused on how thermal cycling with different profiles (ramp and dwell times) affects the mechanical property evolution in lead free solders. This research involves several projects to create a database of mechanical properties of bulk and real solder joints, and the associated mechanical and microstructural evolution under different thermal exposures. In the first project, mechanical behavior evolution of SAC305 lead free solder material was investigated under different thermal exposures. Uniaxial test specimens were prepared by reflowing solder in rectangular cross-section glass tubes with a controlled temperature profile. After reflow solidification, the samples were placed into the environmental chamber and thermally cycled from -40 C to +125 C under a stress-free condition (no load). Several thermal cycling profiles were examined including: (1) 150 minute cycles with 45 minutes ramps and 30 minutes dwells, (2) air-to-air thermal shock exposures with 30 minutes dwells and near instantaneous ramps, and (3) 90 minute cycles with 45 minutes ramps and 0 minutes dwells (thermal ramp only), (4) no cycling (simple aging at high temperature extreme). After the preconditioning, mechanical properties including stress-strain and creep response were explored at room temperature. Stress-strain behavior under different thermal exposures has been compared in terms of exposure time. Creep tests were performed at three different stress levels (σ = 10, 12, and 15 MPa). Creep response recorded measuring secondary creep strain rate. The creep response of SAC305 solder material under different thermal exposures has been compared in terms of stress levels. Mechanical behavior under different thermal exposures has been compared to explore the most detrimental thermal exposures. In the second project, the mechanical behavior evolution of SAC+3% (SAC_Q) lead free solder material under different thermal exposures has been investigated. At the first phase, tensile testing, and creep testing were conducted to extract the mechanical properties including effective elastic modulus, ultimate tensile strength (UTS), yield strength (YS), and secondary creep strain rate. For tensile testing, 8-10 samples were tested at room temperature at a strain rate of 0.001(sec-1) to extract the stress-strain properties. Creep testing was also conducted at room temperature at three different stress levels (10, 12, and 15 MPa). Creep properties were evaluated in terms of secondary creep strain rate. At the second phase, tensile properties evolution of both SAC305 and SAC+3%Bi were compared for different thermal exposures in terms of exposure time to understand the reliability performance of both lead free solder alloy under harsh environment. Also, secondary creep strain rate of both SAC305 and SAC+3%Bi under different thermal exposures has been compared with stress levels. In the third project, the evolution of mechanical properties of both SAC305 and SAC+3%Bi solder joints under different thermal exposures have been explored. Mechanical properties were recorded as modulus, hardness, and yield strength. The size of the solder joints were 30 mils. Mechanical properties were extracted using nanoindentation technique. For nanoindentation, samples were prepared by attaching the package on epoxy mold using glue followed by grinding, polishing, and finally optical microscopy (OM) to find out single grain joint for avoiding grain orientation effect on mechanical properties. After the sample preparation, all samples were preconditioned and then tested at room temperature to measure the mechanical properties. For each exposure time, 10 indents were made in a row and average was taken to extract the mechanical properties. Mechanical properties were compared under different thermal exposures with exposure time for each solder alloy. Also, mechanical properties evolution under different thermal exposures for both bulk solder and solder joint of SAC305 and SAC+3%Bi were compared to observe the effect of thermal exposures on small scale and large scale specimen. In the final project, the microstructural evolution of SAC305 (96.5Sn-3.0Ag-0.5Cu) and SAC+3%Bi (Sn-4.0Ag-0.5Cu-3.0Bi-0.05Ni-0.007Ge) bulk solder have been investigated for different thermal exposures including isothermal aging, slow thermal ramping, and slow thermal cycling utilizing Scanning Electron Microscopy (SEM). Particularly, microstructural changes occurring under different thermal exposures within fixed regions have been monitored in selected lead free bulk solder material to create time-lapse imagery of the microstructure evolution. For the microstructural study, a slot was made on an epoxy mold and the sample was inserted into the slot followed by grinding, polishing, and exposed in a thermal chamber for 0, 1, 5, and 20 days. Finally, the topography of the microstructure of a fixed region was captured using the SEM system. This process generated several images of the microstructure as the thermal exposures progressed containing the visual and quantitative information of Bi diffusion in β-Sn matrix and the evolution of Ag3Sn IMC particles as a function of exposure time during different thermal exposures.