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

For a long time both high Pb-Sn and eutectic 63Sn-37Pb have been the suitable materials for the interconnection of electronic components due to their low cost and physical properties, 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. The Sn-Ag alloys are expected to be one of the best candidates as lead-free solders. Furthermore, from a reliability point of view, there has been interest in the 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 degreesC to 150 degreesC. The steady-state strain rates spanned seven orders of magnitude ranging from 10(-11) s(-1) to 10(-4)s(-1). The scope of 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 degreesC to 125 degreesC and -55 degreesC to 150 degreesC. The deterioration (characterized by the electrical resistance and SEM studies) will be described. Furthermore, it will be 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.