Mechanical fatigue assessment of SAC305 solder joints under harmonic vibrations

Abstract

Vibration-induced solder joint fatigue is a main reliability concern for aerospace and military industries whose electronic equipment used in the field is required to remain functional under such loading. Due to the RoHS directive which eventually will prevent lead from being utilized in electronic systems, there is a need for a better understanding of lead-free mechanical behavior under vibration conditions. This study reports the durability of Sn3.0Ag0.5Cu (SAC305) solder joints subjected to harmonic solicitations at three specific temperatures (-55°C, 20°C and 105°C). A test assembly is designed and consists in a single daisy-chained 1152 I/O ball grid array (FBGA1152) package assembled on a flame retardant (FR-4) printed circuit board (PCB). The vibration levels are imposed by a controlled deflection at the center of the board at its natural frequency. The electric continuity is monitored to determine the number of cycles to failure of each sample. Mode shape measurements with a scanning vibrometer are also conducted and correlated with Finite Element Analysis (FEA) to ensure accurate calculation of stress within the critical solder balls at the corners of the component. The failed specimens are then cross-sectioned in order to determine failure modes. A comparison of SAC305 durability with SnPb36Ag2 solder is given, along with a set of lifetime measurements for two complementary assemblies: 68 I/O Leadless Chip Carrier (LCC68) and 324 I/O Plastic Ball Grid Array (PBGA324). It turns out that SAC305 outperforms SnPb36Ag2 and the effect of temperature on the mechanical durability of SAC305 appears to be minor. Failure analysis points out different failure modes such as ductile and brittle cracks at the interface between the solder bulk and the component, along with pad cratering-induced copper trace failures. FEA calculations provide data to estimate the high cycle fatigue (HCF) behavior of SAC305 solder under harmonic vibrations.