Reliability assessment of flip-chip assemblies with lead-free solder joints

Due to environmental awareness, and the health hazards involved in using lead in solders, large efforts to develop lead-free soldering have been made in recent years. Sn-Ag alloys are expected to be one of the best candidate lead-free solders. Furthermore, from a reliability viewpoint, there has been interest in improved thermal fatigue resistance of solder interconnects. In this study, two lead-free solder alloys (Sn96.5Ag3.5, Sn95.5Ag3.8Cu0.7) were investigated in comparison to lead-containing solder alloys (Sn63Pb37, Sn59Pb40Ag1). These investigations were focused on mechanical and physical properties (coefficient of thermal expansion, stress-strain curves at different strain-rates) as well as on the microstructural appearance of the solders. The mechanical and thermomechanical behavior of the solders were examined by TMA, DTMA, tensile tests, and creep tests. Constant-load creep tests were performed on the specimens at temperatures from 20°C to 150°C. Steady-state strain rates spanned seven orders of magnitude ranging from 10^-11 s^-1 to 10^-4 s^-1. The second step is a reliability study of flip-chip assemblies on FR-4 (high T_g material) with three different underfill materials and with Sn63Pb37, Sn96.5Ag3.5, and Sn95.5Ag4.0Cu0.5 bumps, undergoing thermal cycles from -55°C to 125°C and -55°C to 150°C. The deterioration (characterized by electrical resistance and SEM) are described. Furthermore, it is shown that the material parameters obtained from the tests will increase the precision of finite-element analysis for reliability studies of microelectronic packages with lead-free solder interconnects.