Analysis of the Micro-Mechanical Properties in Aged Lead-Free, Fine Pitch Flip Chip Joints

Abstract

This research seeks to characterize the micro-mechanical behavior of Sn-Ag-Cu solder bumps/joints generated by fine pitch flip chip assembly processes. The solder bumps and joints that were aged at either 80 °C or 150 °C for up to 440 hours (∼18 days); have been studied by an analysis using micro-shear testing and nano-indentation techniques. The shear test of the aged bumps showed a slight increase in shear strength after an initial period of aging (∼50 hours) as compared to the non-aged bumps, but a decrease after longer aging (e.g. 440 hours). A brittle Ag3Sn phase formed as large stick-like features in the body of bulk solder and near the interface of solder/UBM during the initial aging, and is attributed with the increase of shear strength, along with the refinement of the bump microstructure. However, as the time of aging extended, the solder bumps were softened due to grain growth and re-crystallization. It was found that the formation of brittle phases in the body of solder and along the interfaces caused localized stress concentration, which can significantly affect joint reliability. In addition, Nano-testing identified a large lamellar Au-rich structure, formed in the solder and interface of the solder/PCB in the joints after the aging process at 150 °C. These are believed to be detrimental to joint reliability.