Lifetime Modeling based on an Advanced Test Chip Configuration for Lead-free Solder Joints

This paper presents the results of thermo-mechanical lifetime tests. A novel test vehicle was employed to characterize solder joint reliability of miniaturized lead-free tin-silver-copper solder bumps. The test vehicle consists of a stiff ceramic chip (AI₂O₃) with four corner solder joints assembled on a FR-4 PWB. The distance between the solder joint centers is variable (d= 800/1600/2400/3200 mum). This allows creating four strain amplitudes in the solder joints by applying one temperature load profile. The ceramic chip was built up in thin film technology for creating an equal UBM (under bump metallurgy) on each bump interface. The influence of different pad finishes was investigated. Four-wire resistance measurement allows the control of the electrical failure for each single joint. During four different TCTs (temperature cycling tests) with a variation of the characteristic parameters (DeltaT, f, dT/dt) the resistance of the four corner bumps was measured and the detected failures were fitted to the Weibull equation. Additional to the electrical measurements cross-sections were analyzed in regular intervals to determine intermetallics formation, grain sizes and crack propagation. Based on the experimental results and the FEM simulation of the stressed bump geometry, the coefficients for the Coffin-Manson-equation were determined.