Qualification of SnAg solder bumps for lead-free flip chip applications

Lead-free solder bumps as first level flip chip interconnections are still new to the semiconductor industry. Before products can confidently use this new material extensive qualification tests have to be performed to ensure that the bumps meet all requirements. The latter relate both to the subsequent flip chip assembly process and to the reliability of the interconnections under product specific operating conditions. Reliability tests have to take into account all kinds of stress that can lead to degradation of the solder bump interconnects and reduce their lifetime. The results obtained from these tests allow us to set the application range for the electroplated SnAg bumps that are investigated here. Bump quality and homogeneity was assessed on wafer level by optical inspection and shear tests. Reliability after various stress conditions, mainly determined by temperature and electric current, was checked on wafer level but mostly on the assembled system including chip pad and substrate pad. The assembled bumps were studied using electric resistance measurements and physical analysis. Results obtained from the various tests give evidence of the high maturity of these lead-free bumps, which would allow them to replace eutectic SnPb in most applications.