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

This research seeks to characterize the micro-mechanical behavior of Sn-Ag-Cu solder bumps/joints generated by fine feature flip chip fabrication and assembly processes. The bumps used for characterization were produced by stencil deposition of solder paste onto an electroless Nickel UBM, followed by a bump-forming reflow soldering process and the final assembly joints were then achieved by a subsequent reflow of die onto a fine feature Printed Circuit Board (PCB). The bumps and joints were aged at either 80°C or 150°C for up to 1.5 months and then analyzed by means of 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 (∼ 50h) as compared to as-manufactured bumps, but a decrease after longer aging (e.g. 440 h). A brittle Ag₃Sn phase formed as large lamellae in the solder and along the interface between the Cu on the PCB during the initial aging, and is attributed to 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 solder and along the interfaces caused localized stress concentration, which can significantly affect joint reliability. In addition, Nano-testing identified a lamellar Au-rich structure, formed in the solder and interface of the solder/PCB in the joints after the aging process. These are believed to be detrimental to joint reliability.