Mechanical Shock and Vibration Fatigue Behavior of Environmentally-Benign PB-Free Solders in Electronic Packaging

Arizona State University, Scottsdale AZ

Investigators

Abstract

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.