Abstract
The present study details the microstructure evolution of the interfacial intermetallic compounds (IMCs) layer formed between the Sn-xAg-0.5Cu (x = 1, 3, and 4 wt.%) solder balls and electroless Ni-P layer, and their bond strength variation during aging. The interfacial IMCs layer in the as-reflowed specimens was only (Cu,Ni)6Sn5 for Sn-xAg-0.5Cu solders. The (Ni,Cu)3Sn4 IMCs layer formed when Sn-4Ag-0.5Cu and Sn-3Ag-0.5Cu solders were used as aging time increased. However, only (Cu,Ni)6Sn5 IMCs formed in Sn-1Ag-0.5Cu solders, when the aging time was extended beyond 1500 h. Two factors are expected to influence bond strength and fracture modes. One of the factors is that the interfacial (Ni,Cu)3Sn4 IMCs formed at the interface and the fact that fracture occurs along the interface. The other factor is Ag3Sn IMCs coarsening in the solder matrix, and fracture reveals the ductility of the solder balls. The above analysis indicates that during aging, the formation of interfacial (Ni,Cu)3Sn4 IMCs layers strongly influences the pull strength and the fracture behavior of a solder joint. This fact demonstrates that interfacial layers are key to understanding the changes in bonding strength. Additionally, comparison of the bond strength with various Sn-Ag-Cu lead-free solders for various Ag contents show that the Sn-1Ag-0.5Cu solder joint is not sensitive to extended aging time.
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