Growth of Intermetallic Compounds during Isothermal Annealing of a Sn-Ag-Cu Lead-free Solder

Research and development of lead-free solders has been accelerated by impending WEEE and RoHS legislative ban on lead (Pb) in electronic products by 1 st July 2006. Considerable research has been carried out on Sn-Ag-Cu lead-free solders, because it is generally noted that Sn-Ag-Cu solders possess mechanical properties better than those of conventional Pb-Sn solders. However, formation of complex intermetallic compound (IMC) layers may have detrimental effects during prolonged thermal aging in service. Therefore, the primary objective of the study was to characterize the effects of isothermal thermal aging on the formation of IMC in the solder, with special attention paid to the interface between the solder and Cu with Ni/Au plating. Controlled experiment was conducted to study the effect of aging temperature and time on the microstructures coarsening. One hour annealing at 60 °C and 125 °C was observed to produce no discernable change in the microstructure. However, annealing temperature at 190 °C resulted in coarsening of microstructure over time. Intermetallic growth measurement for Sn-Ag-Cu of lap shear solder/copper joint specimen subjected to isothermal aging at 125 °C, 150 °C, and 175 °C over 24, 72, 144 and 336 hours were also studied. Increase in either isothermal aging time or temperature led to an increase in large elongated and platelet Cu6Sn5 IMCs and ripening of interface (Cu,Ni)-Sn IMC layer. The growth of the interface IMC layer subjected to isothermal aging exhibits a linear relationship with respect to square root of aging time.

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