TECHNICAL: With the increasing focus on developing environmentally-benign electronic packages, Pb-free alloys have received a great deal of attention. The mechanical behavior of these alloys is extremely important because solder joints must retain their mechanical integrity under thermo-mechanical fatigue, creep, and mechanical shock and vibration fatigue. The latter has become an increasing problem in the industry. Relatively low cyclic stresses may be applied to electronic packages, particularly in automotive and aircraft applications, which results in isothermal fatigue. Fundamental issues related to this problem have received very little attention. To date, the understanding of mechanical shock is largely empirical. Typical testing involves dropping the modules on the ground, from a given height, and measuring the electrical resistance and qualitative appearance to determine whether the component has failed. An understanding of the stress and strain state in the package during mechanical shock is largely non-existent. Furthermore, the role of intermetallic thickness and solder microstructure on mechanical shock and vibration fatigue has not really been examined. A methodology for modeling the stress state under mechanical shock is urgently required. PIs will conduct a thorough study and analysis of the mechanical shock and vibration fatigue behavior of Sn-3.5Ag-0.7Cu and Sn-Ag solders with a comparison to pure Sn. The program will: (i) quantify the mechanical shock and vibration fatigue behavior using a novel, sophisticated system that enables application of realistic and controlled strain rates (10/s or higher) on single and multiple solder joints, as well as bulk solder, (ii) measure the strain distribution and evolution in a small-length scale solder joint, with fiducial marks micromachined by Focused Ion Beam (FIB), using a high speed camera and digital image correlation (DIC), (iii) understand the relationships between intermetallic thickness at the solder/joint interface and solder microstructure with mechanical shock and vibration fatigue resistance, (iv) model deformation of solder joints during mechanical shock and vibration, using multi-scale numerical techniques, to obtain a fundamental understanding of microstructural and geometric effects on solder deformation. NON-TECHNICAL: Pb-free solders are of importance because of the environmentally-benign nature of these materials. The research program will yield a thorough understanding of mechanical shock and vibration fatigue damage in Pb-free solders. It will also provide the semiconductor industry with a quantitative understanding of high strain rate deformation in these materials. The research program will include substantial interaction between university and industry. While research on solders has been extensive over the last several years, education of students in this area has not received the same attention. An integrated education outreach program that combines: (a) contributions to the development of a new Master?s in Electronic Packaging Program at ASU, (b) project-oriented activities for students, and (c) industrial outreach, is planned.
With the increasing demand on performance from advanced electronic circuits, electronic packaging must be tailored to incorporate as many input/output interconnects as possible, in a limited amount of space. Solder balls connect the Si device to a flip-chip, as well as the flip-chip to the printed wiring board. Most solder balls are made of a eutectic Pb-Sn alloy, because of its low melting point, excellent wetting characteristics, and adequate creep and thermal fatigue strength. With the increasing focus on developing environmentally-benign electronic packages, Pb-free alloys have received a great deal of attention. The mechanical behavior of these alloys is extremely important because solder joints must retain their mechanical integrity under thermomechanical fatigue, creep, and mechanical shock and vibration fatigue. The latter has become an increasing problem in the industry. For example, mishandling of packages, during manufacture, assembly, or by the user may cause failure of solder joint. Relatively low cyclic stresses may be applied to electronic packages, particularly in automotive and aircraft applications which results in isothermal fatigue. In this program, fundamental issues related to this problem were addressed. The effect of microstructure on mechanical performance of these materials was investigated. Simulations were conducted to understand the precise nature of the deformation mechanisms, and how these were affected by the material's structure. This fundamental knowledge will go a long way to developing new devices with better mechanical shock resistance.
