Mechanical fatigue assessment of SAC305 solder joints under harmonic and random 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 harsh loadings. 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) and random vibrations at ambient temperature (20°C). A test assembly was designed and consisted 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 were imposed by a controlled deflection at the center of the board at its natural frequency. The electric continuity was monitored to determine the number of cycles to failure of each sample. Mode shape measurements with a scanning vibrometer were also conducted and correlated with finite element analysis (FEA) to ensure accurate calculation of strain within the critical solder balls at the corners of the component. The failed specimens were 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). For the tested harmonic vibration levels, SAC305 outperforms SnPb36Ag2. Furthermore, the effect of temperature on the mechanical durability of SAC305 appears to be minor. Failure analysis pointed out different failure modes on PCB and component side, along with pad cratering and copper trace failures. FEA calculations allows the determination of the SAC305 fatigue curve to estimate the high cycle fatigue (HCF) behavior of SAC305 solder under harmonic vibrations. The random vibrations durability of SAC305 solder was assessed using the same test assembly (FBGA1152) which was subjected to three different levels of Power Spectral Density (PSD) at 20°C. The random vibrations tests were conducted within a frequency band ranging from 500 Hz to 900 Hz around the natural frequency. The chosen PSD levels applied were 0.04, 0.10 and 0.20 g2/Hz. Using power-law fitting, the results give a first estimation of the durability of SAC305 solder joints subjected to random vibrations.