Abstract
The formation of Ag3Sn plates in the Sn-Ag-Cu lead-free solder joints for two different Ag content solder balls was investigated in wafer level chip scale packages (WLCSPs). After an appropriate surface mount technology reflow process on a printed circuit board, samples were subjected to 150°C high-temperature storage (HTS), 1,000 h aging, or 1,000 cycles thermal cycling test (TCT). Sequentially, the cross-sectional analysis was scrutinized using a scanning electron microscope/energy dispersive spectrometer (SEM/EDX) to observe the metallurgical evolution of the amount of the Ag3Sn plates at the interface and the solder bulk itself. Pull and shear tests were also performed on samples. It was found that the interfacial intermetallic compound (IMC) thickness, the overall IMC area, and the numbers of Ag3Sn plates increase with increasing HTS and TCT cycles. The amount of large Ag3Sn plates found in the Sn-4.0Ag-0.5 Cu solder balls is much greater than that found in the Sn-2.6Ag-0.5Cu solder balls; however, no significant difference was found in the joint strength between two different Ag content solder joints.
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