Study of the Mechanical Behavior of Lead-free Solder Joints and Its Individual Phases under Various Harsh Environments
Type of DegreePhD Dissertation
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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, environmental conditions, such as, operating temperature, aging temperature, and aging time significantly affect these properties due to the microstructural evolution of the solder that occurs during aging. Moreover, electronic devices, sometimes experience harsh environment applications including well drilling, geothermal energy, automotive power electronics, and aerospace engines, where solders are exposed to very high temperatures from T = 100 -200 °C. Also, solder joints in many electronic packages are exposed to temperature cycling environments. During high temperature dwell, solder joints go under isothermal aging phenomena. However, additional aging can occur during the ramp time. In real scenario, CTE mismatch occurs between die and PCB causes shear fatigue on solder joints and affects the reliability of the entire package. Mechanical properties of lead-free solders at elevated temperatures are limited. This research involves several projects to create a database of mechanical properties of bulk and real solder joints, its individual phases and the associated mechanical and microstructural changes of them under the exposure of harsh environments. In the first project, several SAC and SAC+X lead free solder alloys, recommended for high reliability applications have been chemically analyzed and then mechanically tested in order to determine the temperature dependent mechanical properties of these alloys. The alloys include SAC405, Cyclomax (SAC_Q), and Innolot. The mechanical behavior of these alloys have been explored at several extreme high temperatures from 100 to 200 °C. For each of 5 elevated temperatures (T = 100, 125, 150, 175, and 200 °C), creep tests were performed at two different stress level (σ = 10, and 15 MPa). For each alloy and testing temperature, secondary creep strain rate was measured and compared. In the second project, mechanical behavior of individual phases of solder joints (Intermetallics and β-Sn phase) was nano-mechanically characterized as a function of operating temperature condition using nanoindentation technique. At first phase, mechanical behaviors of IMC particles and layers, as well as β-Sn in SAC solder joints have been characterized using nanoindentation at room temperature. Solder joints were extracted from 14 x 14 mm PBGA assemblies (0.8 mm ball pitch, 0.46 mm ball diameter). Then, SAC BGA solder joints were aged for 6 months at T = 125 o C. Intermetallics formed in the bulk solder region, copper pad and SAC solder interface, and ENIG plating finish and SAC solder interface were then indented to measure their room temperature mechanical properties including the elastic modulus, hardness, and creep strain rate. At the second phase, same intermetallics were indented to measure their mechanical properties which includes the elastic modulus and hardness at elevated temperatures (50, 75, 100 and 125 o C) using a heating stage. A special high temperature stage and test protocol was used within the nanoindentation system to carefully control the testing temperature, and make the measurements insensitive to thermal drift problems. Time dependent deformation (creep) behaviors were also evaluated at 100 o C and then compared to room temperature results to see the effect of high temperature on the creep rate of IMCs. In third project, the aging induced variations of the mechanical properties of the β-Sn phase, and of Sn-Ag-Cu IMC particles in SAC solder joints have been explored using nanoindentation. SAC solder joints extracted from SuperBGA (SBGA) packages were aged for different time intervals (0, 1, 5, 10 and 30 days) at T = 125 °C. Nanoindentation test samples were prepared by cross sectioning the solder joints, and then molding them in epoxy and polishing them to prepare the joint surfaces for nanoindentation. Multiple β-Sn grains were identified in joints using optical polarized microscopy and IMCs were also observed. Individual β-Sn grains and IMC particles were then indented at room temperature to measure their mechanical properties (elastic modulus and hardness) and time dependent creep deformations. Properties measured at different aging time were then compared to explore aging induced degradations of the individual phases. In the fourth project, nanoindentation technique was used to understand the evolution of mechanical properties (modulus, hardness and creep behavior) of SAC305 BGA solder joints subjected to thermal cycling loading for various durations. In addition, microstructural changes in those joints that occur during thermal cycling were observed using both SEM and optical microscopy. BGA solder joint strip specimens were first prepared by cross sectioning BGA assemblies followed by surface polishing to facilitate SEM imaging and nanoindentation testing. The strip specimens were chosen to contain several single grain solder joints. This enabled large regions of solder material with equivalent mechanical behavior, which could then be indented several times after various durations of cycling. After preparation, the solder joint strip samples were thermally cycled from T = -40 to 125 o C in an environmental chamber. At various points in the cycling (e.g., after 0, 50, 100, 250, 500, 750, 1000 cycles), the package was taken out from the chamber, and nanoindentation was performed on each single grain joint to obtain the modulus, hardness, and creep behavior at 25 oC. This allowed the evolution of the mechanical properties with the duration of thermal cycling to be determined. Moreover, microstructural changes were also observed after various durations of cycling using optical microscopy. In the final project, evolution of mechanical behavior of the various materials within PBGA package assembly which includes die attachment adhesive, silicon die, and solder mask material for various durations of thermal cycling. Test specimens were first prepared by cross sectioning a PBGA package to reveal the different materials, followed by surface polishing to facilitate SEM imaging and nanoindentation testing. After preparation, the package samples were thermally cycled from T = −40 to 125 °C in an environmental chamber. At various points in the cycling (e.g., after 0, 50, 100, 250, 500, 750 and 1000 cycles), the package was taken out from the chamber, and nanoindentation was performed on above mentioned materials to observe evolution of mechanical behavior (modulus, hardness and creep) at room temperature (25 °C).